PROGRAMA DE PÓS-GRADUAÇÃO EM CIÊNCIA DOS MATERIAIS 

 

 

 

 

 

 

 

 

 

“Study on sugar cane straw ash (SCSA) in alkali-

activated binders” 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

JOÃO CLÁUDIO BASSAN DE MORAES 

 

 

 

 

 
 

 

 

 

 

 

 

 

 

Ilha Solteira 

2017 
 

 



     
 

 
 

 

 

PROGRAMA DE PÓS-GRADUAÇÃO EM CIÊNCIA DOS MATERIAIS 

 

 

 

“Study on sugar cane straw ash (SCSA) in alkali-

activated binders” 
 

 

 

 

 

JOÃO CLÁUDIO BASSAN DE MORAES 

 

 

 

 

 
Supervisor: Prof. Dr. Jorge Luís Akasaki 

Co-Supervisor: Prof. Dr. Jordi Payá Bernabeu 
 

 

 

 

 

A thesis submitted to the Faculdade de 

Engenharia – Campus de Ilha Solteira/UNESP, 

for the degree of Doctor in Materials Science 

and Engineering. 

Area of knowledge: Materials Science and 

Engineering. 

 

 

 

 

 

 

 

 

 

Ilha Solteira 

2017 
 



     
 

 
 

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 



     
 

 
 

 

 
Prof. Dr. JORGE LUIS AKASAKI 
Departamento de Engenharia Civil / Faculdade de Engenharia de Ilha Solteira 

 
Prof. Dr. MAURO MITSUUCHI TASHIMA 
Departamento de Engenharia Civil / Faculdade de Engenharia de Ilha Solteira 

 
Profa. Dra. MARIA VICTORIA BORRACHERO ROSADO 
ICITECH – Instituto de Ciencia y Tecnología del Hormigón / Universitat Politécnica de Valencia 

 

 
Prof. Dr. EMÍLIO CÉSAR CAVALCANTE MELO DA SILVA 
CENPES – Centro de Pesquisa da Petrobras / Petróleo Brasileiro 
 

Ilha Solteira, 13 de novembro de 2017 
 
 

 

 

 



     
 

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I dedicate this thesis to my 

parents, Cássia Regina and 

João Batista, and my sister, 

Maria Júlia. 



     
 

 
 

ACKNOWLEDGMENTS 
 

I would like to thank my mother Cássia Regina, my father João Batista, my sister 

Maria Júlia, my aunt Claudia and Meggy to the support and love. 

I would like to thank the supervisors and friends, Jorge Akasaki and Jordi Payá, 

for the knowledge and the opportunity. 

I would like to thank the professor and friend Mauro Tashima for helping me in 

my scientific career and for sharing funny moments. 

I would like to than the professors and friends, José Luiz Melges, María Victoria 

Borrachero and José Monzó, for the support and knowledge to write the thesis. Thanks also 

to professor João Carlos Moraes for supporting the qualifying exam. Thanks to professors 

José Lollo, Rogério Rodrigues, Renato Bertolino, Tsunao and Haroldo Bernardes from 

Civil Engineering Department for the knowledge provided in my professional career. 

I would like to thank my friends from the Civil Engineering Graduate, Priscila, 

Heitor, Franciéli and Jônatas for the support and funny moments. 

I would like to thank all my friends from GIQUIMA, specially to Lourdes and 

Alba for the strong friendship and for making my year in Valencia very especial. Thanks 

also to Lucía, Ariel, Patrícia, Jesús, Amin, Yasna, Noelia, Clara, Edwin, Ricardo, Álvaro, 

Fausto and Manolo “Mister” Paredes. In addition, I would like to thank the researchers and 

workers from ICITECH for my great year in Valencia. 

I would like to thank all my friends from MAC group, João Victor, Adriana, Alex, 

Danilo, Alan and Thiago Trentin, for the funny and professional moments. 

I would like to thank the best neighborhood I ever had in Ilha Solteira: Bruna, 

Fernanda, Gabriel and Paula. 

I would like to thank my friends from Marília, Everton, Bia, Bruno and Renata, 

for the funny moments. 

I would like to thank my friends from Ilha Solteira for the supporting and taking 

care of me: Estéfani, folks from Equilbrium gym, Mariana Lopes and family, Fernando 

Tangerino and Angelo Doimo. 

I would like to thank my friends from Valencia for one of the best trips I have 

ever gone: Luigi, Jana, Francesco, Theresa, Lech and Iona. 



     
 

 
 

I would like to thank my virtual friends, but they were closer than ever: Kelly, 

Gisele and the FC Barcelona group (Caio, Fabielle, Edvande, Drielle, Filipe, Poly and 

Laiza). 

I would like to thank the folks who work in the Civil Engineering Departament: 

Sandra, José Carlos, Juliana, Gilson, Flavio, Ozias, Ademir and Marcelo. 

I would like to thank Jaira and Maria Gercy for taking care of me during these 

years. 

I would like to thank CNPq and CAPES for the scholarship. 

Finally, thanks to everyone who contributed directly or indirectly to this thesis. 

 

  



     
 

 
 

RESUMO 
 

Aglomerantes ativados alcalinamente (AAA) são obtidos da combinação de um precursor 

solido (geralmente um aluminosilicato) e uma solução alcalina de alta concentração. As 

vantagens de utilizar este novo tipo de aglomerante comparado ao cimento Portland, um 

aglomerante convencional, são as menores emissões de CO2, menor consumo de energia e 

a possibilidade de utilizar matérias prima renováveis e/ou resíduos. Neste sentido, este 

trabalho apresenta um novo resíduo da indústria da cada de açúcar: a folha de cana de 

açúcar. A folha apresenta um poder calorífico interessante; portanto, ela pode ser utilizada 

como biomassa para produzir energia através um processo de queima. Depois deste 

procedimento, é gerado um novo resíduo: a cinza de folha de cana de açúcar (CF). Esta 

cinza não apresenta uma destinação correta, então este trabalho tem como intenção utilizar 

esta cinza como material prima em AAA. A CF foi avaliada de duas formas: como 

precursor solido e como matéria prima para produzir a solução alcalina. No primeiro 

modo, a CF foi utilizada em sistemas combinados com a escória de alto forno (EAF) 

ativado com ambas soluções de NaOH e NaOH/silicato de sódio. No segundo modo, a CF 

foi utilizada como fonte de sílica para produzir a solução alcalina com o NaOH em AAA 

baseados em EAF. Os sistemas foram estudados através da resistência a compressão de 

argamassas e pelo estudo da microestrutura de pastas. Ensaios realizados para avaliar a 

microestrutura foram a difração de raios-X (DRX), espectroscopia de infravermelho por 

transformada de Fourier (EITF), análise termogravimétrica (ATG), microscopia eletrônica 

de varredura (MEV) e porosimetria por intrusão de mercúrio (PIM). Resultados dos 

ensaios mostraram que a CF melhorou as propriedades mecânicas dos AAA baseados em 

EAF nos dois modos, como precursor solido e como fonte de silício para a solução 

ativadora. Estes resultados permitem concluir que a CF pode ser utilizada em AAA, dando 

uma destinação ao resíduo. 

 

Palavras chave: Material alternativo. Material sustentável. Resistência à compressão. 

Estudos microestruturais. 

 
 

 



     
 

 
 

ABSTRACT 
 

Alkali-activated binders (AAB) are obtained from a combination of a solid precursor 

(generally an aluminosilicate) and a high concentrated alkaline solution. The advantages of 

using this new type of binder compared to the Portland cement, a conventional binder, are 

the less CO2 emissions, lower energy consumption and the possibility of using renewable 

and/or residues as raw materials. In this way, this work presents a new residue from the sugar 

cane industry: the sugar cane straw. The straw presents an interesting calorific value; 

therefore, it can be utilised as biomass to produce energy by a burning process. After this 

procedure, it is generated another waste: the sugar cane straw ash (SCSA). This ash does not 

have an appropriate destination, then this work intends to utilise this ash as raw material in 

AAB. SCSA was evaluated in both ways: as solid precursor and as raw material to produce 

the alkaline solution. In the first way, SCSA was utilised in combined systems with blast 

furnace slag BFS activated with both NaOH and NaOH/sodium silicate solutions. In the 

second one, the SCSA was utilised as silica source to produce the alkaline solution with 

NaOH in BFS-based systems. These systems were assessed by the compressive strength of 

mortars and by microstructural studies on pastes. Tests carried out to assesses their 

microstructure were X-ray diffraction (XRD), Fourier transform infrared spectroscopy 

(FTIR), thermogravimetric analysis (TGA), field emission scanning electron microscopy 

(FESEM), and mercury intrusion porosimetry (MIP). Results of the tests showed that the 

SCSA improved the mechanical properties of BFS-based AAB in both methods, as solid 

precursor and as silica source to produce the activating solution. These results allow to 

conclude that the SCSA can be utilised in AAB, giving it a suitable destination. 

 

Keywords: Alternative material. Sustainable material. Compressive strength. 

Microstructural studies. 
  



     
 

 
 

FIGURES LIST 
   

Figure 1.1 Experimental program of the present doctoral thesis 18 

Figure 2.1 Scheme of types of binders related to the Ca, Al and M+ content, 

emphasizing the classification of alkali-activated binders and 

geopolymers (PROVIS, 2014) 

20 

Figure 2.2 Development in the accumulated publications number in 

Scopus/Elsevier database of the words “alkali-activated binders” 

(dotted line) and “geopolymer” (solid line) presented in title, 

abstract or keywords (PACHECO-TORGAL et al., 2015) 

21 

Figure 2.3 Proposed model to formation of BFS-based alkali-activated 

binders: a) role from the alkalis in the reaction (being R+ the 

alkaline cation), and b) gel formation (GARCIA-LODEIRO et al., 

2015) 

23 

Figure 2.4 A general view from the N-A-S-H gels produced in the alkaline 

activation of binders with low Ca-content (GARCIA-LODEIRO et 

al., 2015) 

24 

Figure 2.5 Reaction mechanism from the first step named destruction-

coagulation: a) dissolution of Si-O-Si bonds, and b) dissolution of 

Si-O-Al bonds (GARCIA-LODEIRO et al., 2015) 

25 

Figure 2.6 Reaction mechanism from the second step named coagulation-

condensation (GARCIA-LODEIRO et al., 2015) 

26 

Figure 2.7 Illustration of the gel formation in alkali-activated binders with low 

Ca-content (GARCIA-LODEIRO et al., 2015) 

27 

Figure 2.8 Studies on influence of sodium silicate in the compressive strength 

of BFS-based AAB: a) phosphorous BFS; and b) Acid, neutral and 

basic BFS (SHI et al., 2006) 

29 

Figure 2.9 Compressive strength results of combined systems composed by 

blast furnace slag and red clay brick waste (RCBW/BFS) after 28 

days of curing at room temperature (RAKHIMOVA and 

RAKHIMOV, 2015) 

30 

Figure 2.10 Compressive strength results of binary systems composed by 

metakaolin and fly ash (MK/FA) after 7 and 28 days of curing at 

room temperature (ZHANG et al., 2014) 

33 

Figure 2.11 Compressive strength evolution of FA/POFA specimens studied in 

high temperatures (adapted from RANJBAR et al., 2014) 

35 

Figure 2.12 Comparison between CO2 footprints of concretes based on alkali-

activated binders and OPC, highlighting the emissions from the 

alkaline activator (adapted from Turner; Collins, 2013) 

37 

Figure 2.13 Compressive strength versus reflux time of FCC-based alkali 

activated binders prepared with different siliceous sources 

combined with NaOH solutions: G-RHA (ground rice husk ash), 

O-RHA (original rice husk ash), quartz, control (commercial 

waterglass solution). Key: full line for compressive strength and 

dotted line for flexural strength (Bouzón et al., 2014) 

38 

Figure 2.14 Compressive strength of concrete manufactured with fly ash and 

palm oil fuel ash (BA geopolymer concrete) compared to one of 

Portland cement after sulfuric acid attack (ARIFFIN et al., 2013) 

39 



     
 

 
 

 

Figure 2.15 Compressive strength after temperature treatment of OPC mixtures 

and alkali-activated binders of solid precursors with a) high Ca 

content and b) low Ca content (TURKER et al., 2016; DUAN et 

al., 2015) 

41 

Figure 2.16 Mechanized harvesting of the sugar cane (UNICA, 2016) 42 

Figure 2.17 Electrical conductivity of hydrated lime/RHA and hydrated 

lime/SCSA suspensions (VILLAR-COCIÑA, 2002) 

43 

Figure 2.18 Compressive strength of mortars cured after 60 days (GUZMÁN et 

al., 2011) 

44 

Figure 3.1 Loss of electrical conductivity (Lc) of suspensions studied at (a) 60 

°C; (b) 50 °C; (c) 40 °C; and (d) 30 °C 

62 

Figure 3.2 pH variation (ΔpH) of the CH/SCSA suspensions at each 

temperature studied 

63 

Figure 3.3 DTG curves of the solid part of the suspensions after electrical 

conductivity measurements at 40 °C. 

64 

Figure 3.4 FTIR spectra of 3:7 and 5:5 CH/SCSA pastes after 1, 3, 7, and 28 

days of curing at 40 °C 

66 

Figure 3.5 DTG curves of 5:5 and 3:7 CH/SCSA pastes from 1 to 28 days of 

curing at 40 °C 

68 

Figure 3.6 SEM images of 3:7 CH/SCSA paste after 28 days of curing cured 

at 40 °C 

70 

Figure 4.1 X-Ray Diffraction of SCSA 76 

Figure 4.2 SEM images of SCSA 77 

Figure 4.3 Compressive strength of Portland cement mortars 80 

Figure 4.4 Compressive strength of alkaline activated mortars 81 

Figure 5.1 Calculated γ factor for specimens with BFS/SCSA ratios of 85/15, 

75/25, 67/33 and 50/50, in the ε value range of 0-0.75, after: a) 3 

days of curing, b) 7 days of curing, c) 28 days of curing and d) 90 

days of curing (at 25ºC) 

91 

Figure 5.2 DTG curves for the N-100/0 (a), SS50-100/0 (b), N-75/25 (c), 

SS50-75/25 (d), N-50/50 (e) and SS50-50/50 (f) pastes cured for 7, 

28 and 90 days at 25ºC 

92 

Figure 5.3 FTIR spectra for the N-100/0 (a), SS50-100/0 (b), N-75/25 (c), 

SS50-75/25 (d), N-50/50 (e) and SS50-S50 (f) pastes cured for 7, 

28 and 90 days at 25ºC 

94 

Figure 5.4 XRD patterns for the raw materials, BFS and SCSA, and for the N-

100/0 and N-50/50 pastes, cured for 28 days at 25ºC. (Keys: Q: 

Quartz; C: Calcite; W: Wollastonite; N: Termonatrite; T: 

Hydrotalcite; K: Katoite; S: Stratlingite; H: Hydrosodalite; P: 

Hydrated Nepheline) 

95 

Figure 5.5 FESEM images of N-100/0 (a and b) and N-50/50 (c and d) after 

28 days of curing at 25ºC. 

96 

Figure 6.1 Values of the φ ratio for mortars containing SCSA at 7, 28 and 90 

days of curing 

112 

 

 

 

 



     
 

 
 

 

Figure 6.2 XRD patterns of the raw materials (BFS and SCSA) and the pastes 

28-0-100/0, 28-0-75/25, 28-0.75-100/0 and 28-0.75-75/25 after 90 

days of curing (Key: Q: quartz; C: calcite, H: hydrotalcite; K, 

katoite; C-S-H: calcium silicate hydrate) 

113 

Figure 6.3 FTIR spectra of the raw materials (BFS and SCSA) and the pastes 

28-0-100/0, 28-0-75/25, 28-0.75-100/0 and 28-0.75-75/25 after 7, 

28 and 90 days of curing 

114 

Figure 6.4 DTG curves of the pastes 28-0-100/0, 28-0-75/25, 28-0.75-100/0 

and 28-0.75-75/25 after 7, 28 and 90 days of curing 

117 

Figure 6.5 Relationships between thermogravimetric mass losses for pastes 

(Pg, 35-250ºC; and Pt, 35-1000ºC) and the compressive strength of 

mortars 

118 

Figure 6.6 MIP curves of the pastes 28-0-100/0, 28-0-75/25, 28-0.75-100/0 

and 28-0.75-75/25 after 90 days of curing: a) differential, and b) 

accumulated distribution 

119 

Figure 6.7 FESEM micrographs of BFS activated paste with NaOH (28-0-

100/0): a) general view of the paste with an unreacted BFS particle 

(spot A) and gel (spot B); b) detailed view of the gel; c) in-lens 

micrograph showing BFS unreacted particle (spot A) and main gel 

(spot B); d) enlarged zone from c), in which a denser gel is shown 

(spot C) 

120 

Figure 6.8 FESEM micrographs of BFS/SCSA activated paste with NaOH 

(28-0-75/25): a) general view of the paste; b) general view of 

formed gels (spot D shows a porous gel, and spot E, a compacted 

gel); c) detailed view of both gels; d) in-lens view (lighter area for 

porous gel, and darker are for compacted gel) 

122 

Figure 6.9 FESEM micrographs of BFS activated paste with NaOH and 

sodium silicate (28-0.75-100/0): a) general view of the paste with 

some gel formation (spot F shows a porous gel, and spot G, a 

compacted gel); b) detailed view of formed gels; c) detailed view 

of sheet-like crystals (spot H, stratlingite); d) pirssonite crystals 

surrounded by gels 

123 

Figure 6.10 FESEM micrographs of BFS/SCSA activated paste with NaOH 

and sodium silicate (28-0.75-75/25): a) general view of the paste 

with some gel formation (spot J) and a quartz particle (spot K); b) 

detailed view of compacted gel; c) detailed view of gel; d) 

pirssonite crystals surrounded by gels. 

124 

Figure 7.1 Influence of the H2O/Na2O ratio by line adjustment on BFS/SCSA 

proportions of 100/0 (a), 90/10 (b), 80/20 (c) and 70/30 (d) after 3, 

7, 28 and 90 days of curing at 25 ºC 

137 

Figure 7.2 σSCSA factor for SCSA percentage replacements of 10, 20 and 30% 

after: a) 3; b) 7; c) 28; and d) 90 days of curing at 25 ºC 

140 

Figure 7.3 Best BFS/SCSA proportion for the H2O/Na2O ratios of: a) 11.1; b) 

13.9; and c) 18.5 after 3, 7, 28 and 90 days of curing at 25 ºC 

141 

 

 

 

 



     
 

 
 

 

 

Figure 7.4 Non-linear fitting surface of compressive strength values 

considering all the results after 90 days of curing. Points linked by 

a solid curve represent the optimum percentage of SCSA in the 

mixture. 

142 

Figure 7.5 X-ray diffraction patterns of the raw material and pastes after 90 

days of curing: a) BFS and SCSA (raw materials), 19-100/00 and 

19-80/20 (pastes); b) BFS and SCSA (raw materials), 19-80/20, 14-

80/20 and 11-80/20 (pastes). Key: Q: quartz; C-S-H: semi-

crystalline C-S-H; H: hydrotalcite; K: katoite; F: faujasite; C: 

calcite; S: hydrosodalite 

144 

Figure 7.6 FTIR spectra of the raw materials (BFS and SCSA) and pastes 

cured for: a and d) 7 days; b and e) 28 days; and c and f) 90 days at 

25 ºC. Study on the effect of SCSA content (see a, b and c graphs). 

Study on the effect of the sodium hydroxide concentration (see d, e 

and f graphs) 

146 

Figure 7.7 DTG curves of the pastes: 19-100/0, 19-90/10, 19-80/20 and 19-

70/30 cured after a) 7 days, b) 28 days and c) 90 days; and 19-

80/20, 14-80/20 and 11-80/20 cured after d) 7 days, e) 28 days and 

f) 90 days. 

149 

Figure 7.8 FESEM micrographs of the pastes: a) 19-100/0; and b) 19-80/20 

cured after 90 days 

151 

Figure 7.9 FESEM micrographs of the pastes: a) 19-80/20; b) 14-80/20; and 

c) 11-80/20 cured after 90 days 

152 

Figure 8.1 Temperature evolution with time in the dissolution of sodium 

hydroxide (32.4 g) in water (202.5 g) inside the thermal bottle 

165 

Figure 8.2 Compressive strength values of mortars (cured at 65 ºC for 3 days) 

with activator prepared by thermal bottle treatment in different 

reaction time (τ) of 0, 6, 24 and 48 hours 

169 

Figure 8.3 XRD patterns of raw materials (SCSA and BFS) and pastes 

(SCSA-1.46-24, SCSA-1.46-0, NH-0 and SS-1.46) cured after 3 

days at 65 ºC. Keys: Q: quartz; C: calcite; H: hydrotalcite; F: 

faujasite; C-S-H: calcium silicate hydrate; K: katoite. 

171 

Figure 8.4 FTIR spectra of raw materials (SCSA and BFS) and pastes (SCSA-

1.46-24, SCSA-1.46-0, NH-0 and SS-1.46) cured after 3 days at 65 

ºC 

172 

Figure 8.5 DTG curves of pastes (SCSA-1.46-24, SCSA-1.46-0, NH-0 and 

SS-1.46) cured after 3 days at 65 ºC (numbers close to main peaks 

are in ºC) 

173 

Figure 8.6 FESEM micrographs of the paste SCSA-1.46-24 174 

Figure 8.7 FESEM micrographs of the paste SCSA-1.46-0 175 

Figure 8.8 Compressive strength values of mortars (cured at 65 ºC for 3 days) 

with activator prepared by thermal bottle treatment in different 

SiO2/Na2O molar ratio (ε) of 0 (only NaOH), 0.73, 1.09, 1.46 and 

1.82 

176 

 

 

 



     
 

 
 

 

 

 

Figure 8.9 FESEM micrographs of the paste NH-0 178 

Figure 8.10 Comparison of compressive strength values of mortars (cured at 

65 ºC for 3 days and 20 ºC for 28 days) obtained from SCSA, SS 

and RHA 

179 

Figure 8.11 FESEM micrographs of the paste SS-1.46 181 
 

  



     
 

 
 

TABLES LIST 
   

Table 3.1 Chemical composition of SCSA by percentage 60 

Table 3.2 Mass loss related to the dehydration of pozzolanic reaction 

products (PP) and calcium hydroxide (PCH), and lime fixation of 

CH:SCSA pastes 

68 

Table 4.1 Chemical composition of blast furnace slag (BFS) and sugar cane 

straw ash (SCSA) 

78 

Table 5.1 Chemical composition of SCSA and BFS by weight percentage 87 

Table 5.2 Specimens’ names, compressive strength of mortars (MPa) and 

their standard deviations 

91 

Table 5.3 Mass losses for the N-100/0, N-75/25, N-50/50, SS50-100/0, SS50-

75/25 and SS50-50/50) pastes cured for 7, 28 and 90 days at 25ºC 

in defined temperatures ranges of TGA (35-180ºC, 180-250ºC and 

250-600ºC) 

93 

Table 6.1 Chemical characterisation of the solid precursors utilised in this 

paper (BFS and SCSA) 

105 

Table 6.2 Mixture dosage and specimen´s names and tests carried out: 

compressive strength (Rc), X-ray diffraction (XRD), Fourier 

transform infrared spectroscopy (FTIR), Thermogravimetric 

analyses (TGA), Mercury intrusion porosimetry (MIP) and Field 

emission scanning electron microscopy (FESEM) 

106 

Table 6.3 Compressive strength of mortars (MPa) and their standard 

deviations 

110 

Table 6.4 Mass losses (%) for the pastes 28-0-100/0, 28-0-75/25, 28-0.75-

100/0 and 28-0.75-75/25 after 7, 28 and 90 days of curing at 

different temperature ranges (35-250ºC, 450-650ºC, and total mass 

loss 35-1000ºC) 

117 

Table 6.5 MIP results of the pastes 28-0-100/0, 28-0-75/25, 28-0.75-100/0 

and 28-0.75-75/25 after 90 days of curing 

119 

Table 7.1 Chemical composition of the raw materials sugar cane straw ash 

(SCSA) and blast furnace slag (BFS) in weight percentages 

134 

Table 7.2 Specimen names, mixture designs (H2O/Na2O and BFS/SCSA 

ratios) and tests performed: compressive strength (Rc), X-ray 

diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), 

thermogravimetric analysis (TGA) and field emission scanning 

electron microscopy (FESEM) 

136 

Table 7.3 Slope values from the graphic compressive strength versus 

H2O/Na2O ratio for the four BFS/SCSA proportions 

138 

Table 7.4 Mass losses from TGA curves in the 35–250 ºC range (P35-200ºC) 

and the total mass loss in 35–1000 ºC range (PT) in pastes cured for 

7, 28 and 90 days at 25 ºC 

149 

Table 8.1 Chemical composition of BFS, SCSA and RHA in wt% 164 

Table 8.2 Mixtures and tests carried out 168 

Table 8.3 Mass losses (%) of the pastes SCSA-1.46-24, SCSA-1.46-0, NaOH 

and SS-1.46 cured after 3 days at 65 ºC in the test temperature 

intervals of 35-300 ºC (P35-300) and 300-1000 ºC (P300-1000) 

173 

 



     
 

 
 

TABLE OF CONTENTS 

   

1 INTRODUCTION 16 

 1.1 OBJECTIVES 17 

 2.1 THESIS STRUCTURE 17 

2 STATE OF THE ART 19 

 2.1 ALKALI-ACTIVATED BINDERS 19 

  2.1.1   Definition 19 

  2.1.2 History 20 

  2.1.3 Comparisons to the Portland cement 21 

  2.1.4 Alkali-activated binders reaction products 22 

  2.1.5 Solid precursor  27 

  2.1.6  Alkaline Activators 35 

  2.1.7 Durability 38 

 2.2 SUGAR CANE STRAW ASH 42 

 REFERENCES 44 

3 POZZOLANIC REACTIVITY STUDIES ON SUGAR CANE 

STRAW ASH 

55 

 3.1 INTRODUCTION 56 

 3.2 MATERIALS AND METHODS 57 

  3.2.1 Materials 57 

  3.2.2 Equipment 58 

 3.3 RESULTS AND DISCUSSION 59 

  3.3.1 Sugar Cane Straw Ash Characterization 59 

  3.3.2 Electrical Conductivity Measurements 60 

  3.3.3 Fourier Transformed Infrared Spectroscopy 65 

  3.3.4 Thermogravimetric Analysis 66 

  3.3.5 Comparison of Results from Different Techniques 69 

  3.3.6 Scanning Electron Microscopy 69 

 3.4 CONCLUSIONS 70 

 REFERENCES 71 

4 PRELIMINARIES STUDIES ON SUGAR CANE STRAW ASH 

(SCSA) IN MORTARS OF PORTLAND CEMENT/SCSA AND 

BLAST-FURNACE SLAG/SCSA 

74 

 4.1 INTRODUCTION 75 

 4.2 EXPERIMENTAL 76 

  4.2.1 Materials and Equipment 76 

  4.2.2 Experimental Procedures 78 

   4.2.2.1 Portland cement mortars 78 

   4.2.2.2 Alkali-activated mortars 78 

 4.3 RESULTS AND DISCUSSIONS 79 

  4.3.1 Compressive Strength of Portland Cement Mortars 79 

  4.3.2 Compressive Strength of Alkaline Activated mortars 80 

 4.3 CONCLUSIONS 81 

 REFERENCES 81 

 



     
 

 
 

5 SUGAR CANE STRAW ASH AS SOLID PRECURSOR IN 

ALKALI-ACTIVATED BINDERS BASED ON BLAST 

FURNACE SLAG: SOLUTIONS WITH [N+] OF 8 MOL.KG-1 

AND SIO2/NA2O RATIOS OF 0-0.75 

84 

 5.1 INTRODUCTION 85 

 5.2 MATERIALS AND METHODS 86 

  5.2.1 Materials and Equipment 86 

  5.2.2 Alkali activated binder dosage 88 

 5.3 RESULTS AND DISCUSSION 88 

 5.4 CONCLUSIONS 96 

 REFERENCES 97 

6 SUGAR CANE STRAW ASH AS SOLID PRECURSOR IN 

ALKALI-ACTIVATED BINDERS BASED ON BLAST 

FURNACE SLAG: SOLUTIONS WITH [N+] OF 4 MOL.KG-1 

AND SIO2/NA2O RATIOS OF 0-0.75 

100 

 6.1 INTRODUCTION 101 

 6.2 MATERIALS AND METHODS 104 

  6.2.1 Materials 104 

  6.2.2 Alkali activated binders’ dosage and preparation 105 

  6.2.3 Test procedures for pastes and mortars 105 

 6.3 RESULTS AND DISCUSSION 107 

  6.3.1 Compressive strength of mortars 107 

  6.3.2 Microstructural studies 112 

 6.3 CONCLUSIONS 124 

 REFERENCES 126 

7 SUGAR CANE STRAW ASH AS SOLID PRECURSOR IN 

ALKALI-ACTIVATED BINDERS BASED ON BLAST 

FURNACE SLAG: SOLUTIONS WITH [N+] OF 6-10 MOL.KG-1 

129 

 7.1 INTRODUCTION 130 

 7.2 MATERIALS AND METHODS 133 

  7.2.1 Materials 133 

  7.2.2 Design of alkali-activated binders 134 

  7.2.3 Tests carried out for pastes and mortars 134 

 7.3 RESULTS AND DISCUSSION 137 

  7.3.1 Compressive strength of mortars 137 

  7.3.2 Microstructural studies 143 

   7.3.2.1  X-ray diffraction (XRD) 143 

   7.3.2.2 Fourier transform infrared spectroscopy (FTIR) 144 

   7.3.2.3 Thermogravimetric analysis (TGA) 147 

   7.3.2.4 Field emission scanning electron microscopy (FESEM) and Energy-

dispersive X-ray spectroscopy (EDS) 

150 

 7.4 CONCLUSIONS 152 

8 SUGAR CANE STRAW ASH AS SILICA SOURCE TO 

PRODUCE THE ACTIVATING SOLUTION IN ALKALI-

ACTIVATED BINDERS BASED ON BLAST FURNACE SLAG 

160 

 8.1 INTRODUCTION 160 

 8.2 EXPERIMENTAL 162 

  8.2.1 Materials and Equipment 162 



     
 

 
 

  8.2.2 Alkali-activated materials preparation 164 

  8.2.3 Tests procedures 166 

  8.2.3 Alkali-activated materials studies 166 

 8.3 RESULTS AND DISCUSSION 168 

  8.3.1 Effect of the thermal bottle time (τ) for SCSA/NaOH suspensions 

(Section 1) 

168 

  8.3.2 Effect of the SiO2/Na2O molar ratio (ε) for SCSA/NaOH 

suspensions (Section 2) 

176 

  8.3.3 Comparison of SCSA to others silica sources (Section 3) 178 

 8.4 CONCLUSIONS 181 

 REFERENCES 182 

9 GENERAL CONCLUSIONS 185 

10 PROPOSALS FOR FUTURE STUDIES 187 
 

 
 



     
 

16 
 

1 INTRODUCTION 

 

 
Alkali-activated binders (AAB) are the new trend in the research of building 

construction materials. This type of material is obtained when a silico-aluminous material 

(also known as solid precursor) is combined with a high concentrated alkaline solution 

(activating solutuin) in appropriate proportions. The studies on this type of binder started in 

the last century with the purpose to reduce the Portland cement consumption. Authors 

highlighted some advantages of alkali-activated binders’ mixtures when compared to the 

Portland cement: similar or higher compressive strength, improved durability and, mainly, 

less CO2 emissions and energy consumption. However, these new kinds of binders present 

the desavantages of handling difficult due the high alkalinity, and the several proportions 

and factors to analyse that influence the mechanical properties of the material (for example, 

the solid precursor and alkaline solution compositions). 

Recent studies focused on the obtainment of new materials source (solid precursors 

and alkaline solution) for alkali-activated binders’ production. The usual materials used as 

solids precursors are the blast-furnace slag, fly ash and metakaolin. About the alkaline 

sources in the preparation of the activating solution, hydroxides and silicates are the most 

common activators. New materials that are being researched in the preparation of alkali-

activated binders are, in majority, residues from industry, agro-industry and building 

construction. This thesis presents a new agro-industry residue from sugar cane production in 

order to obtain an alkali-activated binder: the sugar cane straw ash (SCSA). 

The issue of sugar cane starts in the increase of its production in the last years in 

Brazil due the production of alcohol and sugar: in only ten years (from 2004/2005 to 

2014/2015), the increase was 64%. The state of Sao Paulo, where this research takes place, 

is the major sugar cane production in Brazil, which represents over than 50% of total 

production. Another important issue of the sugar cane is the harvesting process. Some years 

ago, the sugar cane harvesting used to be performed by a burning process in the cultivation 

area. However, an Agro-environmental protocol was signed to put an end on this burning 

procedure, making that the mechanized harvesting gains in importance on this scenario. In 

this type of harvesting, is generated a by-product that is composed by the dried and green 

leaves, which are mostly left on the cultivation field: the sugar cane straw. Due its interesting 



     
 

17 
 

calorific value, which is compared to the sugar cane bagasse, authors are studying new 

methods to collect and generate energy from this by-product. After this process to obtain 

energy from the straw, an ash is obtained: the sugar cane straw ash (SCSA). This residue 

does not present a suitable valorisation, and similar to other ashes from agro-industry (rice 

husk ash and sugar cane bagasse ash), it can be utilised in the building construction in alkali-

activated binders.  

 

 2.1  OBJECTIVES 

 

The main objective of this study was to assess the sugar cane straw ash (SCSA) 

behaviour in alkali-activated binders.  

The specific objectives were:  

I) Assess the SCSA reactivty in mixtures of the ash with calcium hydroxide; 

II Evaluate the SCSA as solid precursor in alkali-activated binders based on blast-

furnace slag and; 

III) Analyse the SCSA potential as silica source to produce the activating solution. 

 

 2.2  THESIS STRUCTURE 

 

The SCSA was obtained from an auto-combustion process of the straw, and it was 

characterized chemically and physically. After, SCSA reactivity was evaluated in calcium 

hydroxide (CH) pastes by several experimental techniques. Confirmed SCSA good 

reactivity in these studies, the ash was assessed as solid precursor in binary systems of alkali-

activated binders with blast furnace slag (BFS). In the last part of the study, the alkaline 

solution was prepared by NaOH combined with the SCSA as silicon source in order to 

replace the use of silicates. The solid precursor utilised in this last work was also the blast 

furnace slag. Figure 1.1 shows the steps of thesis. 

The thesis was divided in 10 chapters, including the Introduction. The Chapter 2 is 

a State of the Art about alkali-activated binders and sugar cane straw ash. Chapter 3 shows 

reactivity studies on SCSA in mixtures with calcium hydroxide. Chapter 4 presents the first 

applications of SCSA in alkali-activated binders. In the Chapters 5, 6 and 7 it is presented 

studies of SCSA as solid precursor in alkali-activated binders based on BFS. Chapter 8 



     
 

18 
 

shows the SCSA as silica source to produce the alkaline solution in BFS-based AAB. 

Finally, Chapter 9 and 10 presents the general conclusions and proposal for future works.  

 

Figure 1.1 – Experimental program of the present doctoral thesis  

  



     
 

19 
 

2 STATE OF THE ART 

 

 
This chapter introduces concepts of alkali-activated binders, empathising 

definitions, history, comparisons to Portland cement, reaction products, solids precursors, 

alkaline activator solution and durability. Afterwards, a general view of the background and 

importance on the SCSA as a construction material is presented, completing this chapter of 

literature review. 

 

 2.1  ALKALI-ACTIVATED BINDERS 

 

  2.1.1  Definition 

 

Alkali-activated binders are obtained when an alkaline solution is combined with 

an amorphous silico-aluminous source (solid precursor), resulting in a material with 

cementing properties (PACHECO-TORGAL et al., 2015). These types of binders have 

several denominations as, for example, inorganic polymers, alkali-activated cements, and it 

is also widely called by “geopolymers”. However, Provis (2014) differs the meaning of 

alkali-activated binders from geopolymers in the following points. Alkali-activated binders, 

according to the author, is a combination of a silicate fine solid and an activating alkali metal 

(in the solid or dissolved forms). That fine solid can be a calcium silicate (as blast furnace 

slag), or an aluminosilicate (as fly ash and metakaolin). The alkali metal can be obtained 

from alkali hydroxides, carbonates, silicates, sulphates, aluminates and others sources that 

can increase the pH and dissolve quicker the fine solid. The author excludes, in his definition, 

the lime-pozzolan systems and cases that water without the alkaline activator is utilised to 

make the reaction possible. In the other hand, geopolymer is obtained from the combination 

of exclusively aluminosilicates (low Ca content) with hydroxides/silicates as alkaline 

activating solution. From this point of view, any geopolymer can be an alkali-activated 

binder, but not vice versa. Figure 2.1 illustrate that distinction clearly and compared to others 

type of binders. In this thesis, the term alkali-activated binders will be adopted to describe 

that type of construction material.  

 



     
 

20 
 

Figure 2.1 – Scheme of types of binders related to the Ca, Al and M+ content, 

emphasizing the classification of alkali-activated binders and geopolymers (PROVIS, 

2014) 

 

 

  2.1.2 History 

 

Before to present studies related to alkali-activated binders, this topic shows some 

important steps of history from alkali-activated binders. The first record of these materials 

was in 1908 by a patent of Kuhl called “Slag cement and process of making the same”. The 

researcher combined slag with alkalis sulphate/carbonate, and described its performance as 

“fully equal to the best Portland cements”. Purdon, in 1940, published a study of slag 

activated by sodium hydroxide. These studies compared the new binder to the Portland 

cement mixtures, obtaining similar compressive strength, lower solubility and heat 

evolution. In the 1950s, Glukhovsky, in Eastern Europe, studied the binders used in the 

ancient Egypt and Rome. These binders were produced by aluminosilicates with low calcium 

content, which were denominated as “soil cements” by the author. The next important point 

was in the 1970s, where the French Joseph Davidovits created the name “geopolymer”, and 

has patented several formulations of aluminosilicates. In the 80s and 90s, several works 

about alkali-activated binders took place in the most important scientific journals in the 

world. However, only after the 2000s, the number of these publications increased drastically 

(Figure 2.2). The most discussed topics in these recent studies are related to the 

microstructural properties and characterization, new raw materials and alkaline activators, 

durability, environmental and others issues (BERNAL et al., 2013; PROVIS, 2014; 

PACHECO-TORGAL et al., 2015). 

 



     
 

21 
 

Figure 2.2 – Development in the accumulated publications number in Scopus/Elsevier 

database of the words “alkali-activated binders” (dotted line) and “geopolymer” (solid 

line) presented in title, abstract or keywords (PACHECO-TORGAL et al., 2015) 

 

 

  2.1.3  Comparisons to the Portland cement 

 

It is known that Portland cement is the most traditional binder utilized nowadays in 

building construction, and the main purpose to study the alkali-activated binders is to replace 

it for several reasons. The main advantages of these new binders compared to that 

conventional binder are: similar or higher strength; resistance to fire and low thermal 

conductivity; resistance to acid and chemical attack; lower degradation by alkali-silica 

reaction; good volumetric stability after hardening; adhesion to cement, ceramic, glass and 

metallic substrates; lower permeability and cost; among others advantages (PROVIS; van 

DEVENTER, 2014). However, the most important note in the literature is the environmental 

advantages of alkali-activated binders compared to the Portland cement (DUXSON et al., 

2007). 

In the Portland cement production, it is emitted 0.66-0.82 ton of CO2 in atmosphere 

per 1 ton of binder manufactured. This number put this binder production responsible of 5-

8% of CO2 emissions in the world. Another problem is the use of 2.8-ton non-renewable raw 

materials (limestone and clay) per 1 ton of Portland cement obtained (GUO et al., 2010). 

They are heated over than 1400 ºC within a rotating kiln, which requires a high-energy 

consumption to obtain the final product (van DEVENTER et al., 2012; TURNER; 

COLLINS, 2013). All this information motivated the research of new possibilities do reduce 

the Portland cement consumption, being one of them the alkali-activated binders. Some of 

these new materials can release only 0.184 ton of CO2 per ton of binder produced, 



     
 

22 
 

representing a reduction in 50-70% when compared to Portland cement (DAVIDOVITS, 

2002; McLELLAN et al., 2011). About the energy consumption, that type of binder can 

reduce it in 70% (DAVIDOVITS, 2001). In general, the materials used in alkali-activated 

binders were residues as, for example, fly ash (FA) and fluid catalyst cracking (FCC), which 

means that the use of these type of binders are environmental friendly for two reasons: the 

no use of non-renewable raw materials and valorisation of wastes. 

 

  2.1.4  Alkali-activated binders reaction products 

 

The results of the combination between the solid precursor and the alkaline solution 

are, primary, a gel (amorphous phase) and, secondary, the zeolites (crystalline phase). 

Authors divides the studies on reaction products formed in the alkali-activated reaction by 

the calcium content of the solid precursor in three groups: low (metakolin and fly ash class 

F), medium (fly ash class C and some combination of binders) and high (blast-furnace slag). 

The solid precursors with high and low Ca content are that present more information about 

the reaction products (PROVIS, 2014; PACHECO-TORGAL et al., 2015), whereas the 

medium presents fewer studies in understating their reaction products.     

 

I) High Ca-content 

 

This group of reaction products, also known as (Na,K)2O-CaO-Al2O3-SiO2-H2O 

system, is usually obtained from a binder richer of calcium and silicon (SiO2 + CaO > 70%). 

The information of this sub-topic is based on the alkali-activated binders obtained by the 

activation of the blast-furnace slag, which is the main solid precursor researched in this 

group. The main reaction product of activating BFS is the C-A-S-H (calcium silicate 

hydrate) gel, which is similar to the gel obtained from the Portland cement reaction 

(GARCIA-LODEIRO et al., 2015). Secondary products are also obtained in the reaction: 

AFm (as stranglingite), hydrocalcite (due the presence of MgO on the BFS) and zeolites 

(BERNAL et al., 2014; GARCIA-LODEIRO et al., 2015).  

The development of the BFS-based AAB structure is divided in four steps 

(BERNAL et al., 2014): 

- Dissolution of glassy part from the solid precursor;  



     
 

23 
 

- Nucleation and growth from the first solid phases; 

- Interactions and mechanical binding at the boundaries of the products formed; 

- The reactions continue by the chemical equilibrium and diffusion of reactive 

species through the reaction products formed in older curing time. 

The model proposed by these steps is presented in Figure 2.3a. In this model, the 

alkaline cation, represented as R+, works as a catalyser in the beginning of the reaction, and 

are replaced by Ca+ in a cationic exchange in the sequence of the reaction. However, the 

alkaline cation returns to participate of the gel structure with the advance of the reaction. Of 

course, the anions (SiO4
4- and AlO4

5-) also presents an important function, especially in the 

early ages, as showed in the Figure 2.3b (GARCIA-LODEIRO et al., 2015). The presence 

of highly condensed anions favours the increase of compressive strength (FERNÁNDEZ-

JIMÉNEZ; PUERTAS, 2003). 

 

Figure 2.3 – Proposed model to formation of BFS-based alkali-activated binders: a) role 

from the alkalis in the reaction (being R+ the alkaline cation), and b) gel formation 

(GARCIA-LODEIRO et al., 2015) 

 

 

Aluminium has an important role in the alkali-activated binders. The aluminium 

tetrahedra replaces silicon tetrahedra and permits longer linear chains and inter-chain of Si-

O-Al, making that C-S-H gels become two-dimensional C-A-S-H gels. These formed gels 

are directly influenced by the alkaline activator used in the mixture. The role of alkalis (as 

Na, K…) is to neutralise the charge balance of the Si replacement by Al (GARCIA-

LODEIRO et al., 2015). The activators that are being largely utilised to study the gel 

formation are the sodium hydroxide and silicate. In general, the presence of silicates, when 

compared to only hydroxide, make the Ca/Si relation lower (BERNAL et al., 2014) and a 

higher presence of aluminium in the gel (GARCIA-LODEIRO et al., 2015). The presence of 

sodium turns the C-A-S-H gel into a (N,C)-A-S-H gel (PROVIS et al., 2015; GARCIA-



     
 

24 
 

LODEIRO et al., 2015). Some authors are trying to describe how the sodium is incorporated 

to the gel structure by crosslinking, non-crosslinking or replacing of cations (Ca2+ by Na+), 

and it is not a fully resolved issue (BERNAL et al., 2014; GARCIA-LODEIRO et al., 2015; 

PROVIS et al., 2015). The understating on AAB-based on solid precursors of high Ca-

content increase in the last years by publication of several studies, however, some gel 

formation issues and the lack of study on other sources of solid precursor should be more 

studied to real understand the behaviour of this group of reaction products. 

 

II) Low Ca-content 

 

The reaction products studied in this group, also named as (Na,K)2O-Al2O3-SiO2-

H2O system, are obtained from binders composed basically from silicon and aluminium. In 

this group, the most used solid precursors are fly ash and metakaolin, and the studies on their 

reaction products will be focused in this topic. The main reaction product from alkaline 

activation of these binders is the Me-A-S-H (alkaline aluminosilicate hydrate, where Me can 

be Na, K…) gel. This gel is a three-dimensional inorganic alkaline polymer aluminosilicate, 

with highly disordered and crosslinked structure (Figure 2.4). The silicon and aluminium are 

found in the tetrahedral coordination (SiO4
4- and AlO4

5-, respectively), where the alkaline 

cation balances the extra negative charge from the aluminium tetrahedra replacing the silicon 

tetrahedra. The secondary products are the zeolites, as faujasite, hydroxysodalite, zeolite P, 

zeolite Y, and others. These formed products depend on the alkaline activator and curing 

conditions (PROVIS et al., 2014; GARCIA-LODEIRO et al., 2015).  

 

Figure 2.4 – A general view from the N-A-S-H gels produced in the alkaline activation 

of binders with low Ca-content (GARCIA-LODEIRO et al., 2015) 

 



     
 

25 
 

 

The reaction mechanism of these AAB products from fly ash and metakaolin 

activation is presented by three steps: destruction-coagulation, coagulation-condensation 

and condensation-crystallisation (GARCIA-LODEIRO et al., 2015). 

 

- Destruction-coagulation: In the beginning, OH- ions from the alkaline activator 

break the bonds siloxane (Si-O-Si). These breaks are possible due the weakening of siloxane 

by the redistribution of the electronic density from the ions. Then, it is produced silanol (-

Si-OH) and sialate (-Si-O-) groups. The cation from the alkaline activator (Na+, K+…) 

neutralises the negative charge from de sialates, yielding -Si-O--Na+. In this form, the 

reversion to siloxane is hindered. This information is summarized in Figure 2.5a. The Si-O-

Al bonds are also affected by the OH- ions. The aluminium species is dissolved and form 

complex species, mainly aluminium tetrahedra (Al(OH)4
-) (Figure 2.5b). 

 

Figure 2.5 – Reaction mechanism from the first step named destruction-coagulation: a) 

dissolution of Si-O-Si bonds, and b) dissolution of Si-O-Al bonds (GARCIA-LODEIRO 

et al., 2015) 

 

 

- Coagulation-condensation: In this step, with the accumulation of ionic species 

produced in the last step, is favoured the contact between them and polycondensation starts, 

generating coagulated structures. Silica monomers inter-react and form dimers, trimers, etc. 

and forming polymers. The aluminium behaves in the same way, where the aluminium 

tetrahedra replaces the silicon tetrahedra in the structure. The alkaline metal works as a 

structural component in this step, differently from the first one when worked as catalyst. 

This information is represented in the Figure 2.6. 

 



     
 

26 
 

 

Figure 2.6 – Reaction mechanism from the second step named coagulation-condensation 

(GARCIA-LODEIRO et al., 2015) 

 

 

- Condensation-crystallisation: At this final stage, the increase of coagulation 

favours the precipitation of the reaction products. Their composition depends on the solid 

precursor, alkaline activator and the curing conditions. 

Another model proposed to alkaline activation of only fly ash is showed in the 

Figure 2.7. In this model, firstly occurs the dissolution of the solid precursor. In the next 

step, it is highlighted that the first precipitated reaction products are richer in aluminium than 

silicon. This behaviour is explained due the weaker Al-O bond compared to Si-O bond, 

which makes that the former dissolves quicker than the latter. After, more Si-O dissolves 

and raises the concertation of silicon in the gels, producing a richer Si gel. The structural 

reorganisation and polymerisation of gels determine composition of the polymer, and defines 

the structure and, consequently, the mechanical behaviour of the alkali-activated binder 

(GARCIA-LODEIRO et al., 2015). Recent studies focused on the gel formed in these 

reactions. The improve in the experimental techniques, as nuclear magnetic resonance 

(NMR), attenuated total reflectance (ATR) and Fourier transform infrared spectroscopy 

(FTIR), permit to understand better the reactions products. In the next years, due the several 

raw materials (solid precursors and alkaline activator) that are utilised in alkali-activated 

binders, the researches in nanostructure and microstructural characterisation increased in 

numbers and probably will continuous in this way (PALOMO et al., 2014; PROVIS et al., 

2015). 

 

 

 



     
 

27 
 

Figure 2.7 – Illustration of the gel formation in alkali-activated binders with low Ca-

content (GARCIA-LODEIRO et al., 2015) 

 

 

III) Medium Ca-content 

 

This third and last group is a combination of the two last ones discussed. They are 

also known as hybrid alkaline cement and are obtained by the alkaline activation of materials 

(or combination of materials) with SiO2, Al2O3 and CaO > 20%. Examples of binders that 

produce a hybrid cement are the fly ash Class C, and combination of OPC with slag and/or 

fly ash. The reaction product from their alkaline activation is really complex due the non-

uniformity of the raw materials, where can be generated the C-A-S-H, (N,C)-A-S-H and N-

A-S-H gels. Stability of these gels are directly related to the pH from the phases obtained: 

several studies on the pH showed that it favours some type of gel formation. Aluminium and 

alkali also have an important role in the gel formation, since they can increase the crosslink 

of the gels. As can be seen, the reaction products of this group present less information when 

compared to a high and low Ca-content due its complexity, and the based information is 

mostly empirical (PROVIS; BERNAL, 2014; GARCIA-LODEIRO et al., 2015). 

 

  2.1.5  Solid Precursor  

 

Solid precursor is one of the raw materials to produce an alkali-activated binder, 

where that is basically a fine power composed by silicon and aluminium (and calcium in 

some cases). The main researched solid precursors and ones that will be discussed are blast-

furnace slag, fly ash and metakaolin (PACHECO-TORGAL et al., 2008). After, a general 

view of others materials (mainly residues) that are being researched to produce alkali-

activated binders. In addition, in these topics, will be showed some studies about combined 



     
 

28 
 

systems (the use of two or more solid precursors), which improve the mechanical properties 

of alkali-activated binders when compared to which presents only one solid precursor 

(YUSUF et al., 2014).  

 

I)  Blast-furnace slag (BFS) 

 

As showed previously, the first solid precursor that was studied in alkali-activated 

binders is the blast-furnace slag (BFS) in 1908. This material is a by-product of iron 

production, and has the presence of calcium, magnesium, silicon and aluminium in its 

composition. There are several studies focused on properties of alkali-activated binders 

based on BFS, mainly issues about microstructure, fresh properties, mechanical properties 

and durability (SHI et al., 2006).  

The main reaction product obtained from the alkaline activation of BFS is the C-A-

S-H and (Me,C)-A-S-H gels (being Me a N, K…), which its formation was already discussed 

for solid precursors with high Ca content (PROVIS et al., 2014). The characteristic of this 

gel is related to several factors (activating solution, curing conditions, etc.), and are directly 

linked to the mechanical properties. Improving in these properties on BFS based-AAB was 

previously studied with the use of waterglass combined with a metal hydroxide in the 

alkaline solution preparation (Figure 2.8) (BROUGH; ATKINSON, 2002; SHI et al., 2006). 

However, recent studies are highlighting the advantages of using combined systems in order 

to improve the mechanical properties (RAKHIMOVA and RAKHIMOV, 2015). Some of 

materials used in combined systems studied in the last years are red clay brick waste 

(RAKHIMOVA and RAKHIMOV, 2015), palm oil fuel ash (YUSUF et al., 2014), silica 

fume (HEIKAL et al., 2014) and sugar cane bagasse ash (PEREIRA et al., 2015).  

 

 

 

 

 

 

 

 



     
 

29 
 

Figure 2.8 – Studies on influence of sodium silicate in the compressive strength of BFS-

based AAB: a) phosphorous BFS; and b) Acid, neutral and basic BFS (SHI et al., 2006) 

 

 

The red clay brick waste (RCBW) is a residue from construction and demolition 

industries that presents, in its chemical composition, SiO2 and Al2O3 of 70 and 10%, 

respectively (by mass). The authors concluded that RCBW can replace the BFS until 60% 

from compressive strength studies, where the optimum stands on the range 20-40% (Figure 

2.9). The justifying of the good results using this waste is due to the interaction of 

aluminosilicates and feldspars from RCBW, and also the presence of amorphous silica that 

forms hydrates (RAKHIMOVA and RAKHIMOV, 2015). About the palm oil fuel ash 

(POFA), this is a residue from burning process of the palm empty fruit brunches, palm fibres 

and palm kernel shell, and the resultant ash is composed by mainly silica (45%) with a high 

loss on ignition (21%). Studies on this material concluded that the UPOFA (POFA after a 

treatment) presents an optimum BFS replacement of 20%, which reaches 50% more 

compressive strength than the control. The authors justify this best replacement percentage 

by the optimum Ca concentration in the mixture, more Al reactive sourced by the BFS, and 

the increasing in the amorphousness of the reaction products (YUSUF et al., 2014). Silica 

fume (SF) is a well-known pozzolanic material. It is a by-product from the silicon or silicon 

alloys, and presents higher than 95% of high-reactivity amorphous silica. In this study in 

alkali-activated materials with BFS, the authors replaced this binder by 8% of SF. The 

authors explained that the higher surface area of SF acts as nucleating agents for formation 

of C-S-H gel, which increases the compressive strength (HEIKAL et al., 2014). Finally, a 

residue from sugar cane production, the sugar cane bagasse ash (SCBA), was assessed in 

alkali-activated binders with interesting results. SCBA replaced BFS in the range of 0-50% 

by mass, where the authors discovered that the optimum replacement percentage was the 



     
 

30 
 

25%. In this percentage of SCBA, the compressive strength presented similar values to the 

control. The authors concluded that the presence of the sugar cane waste did not cause 

change in the formed gels from alkali-activated reaction (PEREIRA et al., 2015). The 

materials studied in binary systems with BFS are mainly composed by silica. All promising 

results from those studies shows the use of SCSA (which are also composed by silica) an 

interesting way in combined systems with BFS.  

 

Figure 2.9 – Compressive strength results of combined systems composed by blast 

furnace slag and red clay brick waste (RCBW/BFS) after 28 days of curing at room 

temperature (RAKHIMOVA and RAKHIMOV, 2015) 

 

 

II) Metakaolin (MK) 

 

Metakaolin is a pozzolanic material obtained from the burning process of the kaolin 

(clay) in the temperature range of 600-850 ºC. This material is basically composed by silica 

and alumina with high degree of amorphous phase and surface area. The first use of 

metakaolin in alkali-activated binders was by Davidovits in 1979, when the denomination 

geopolymer was proposed. In MK-based AAB, the main reaction product is the Me-A-S-H 

gel (being Me a Na, K…), where its structure was already discussed for solid precursors with 

low Ca content. Some studies about influences on this gel formation, and consequently in 

the mechanical properties, is showed in this topic (PACHECO-TORGAL et al., 2008; 

RASHAD, 2013a; RASHAD, 2013b). 



     
 

31 
 

The first influence presented is the alkaline solution. In general, the mechanical 

properties of MK-based AAB with the combination of alkaline metal silicate/hydroxide is 

higher than one with only the alkaline metal hydroxide (OZER; SOYER-UZUM, 2015; 

TYPPAYASAM et al., 2016). About the alkaline metal concentration, there are studies with 

sodium and potassium, being the former with more works about it. In sodium hydroxide, the 

Na+ concentration studied is in the range or 6-15 M (ZHANG et al., 2012; YUSUF et al., 

2014; OZER; SOYER-UZUM, 2015; CHENG et al., 2015), whereas, for potassium 

hydroxide, the K+ concentration is found in the range of 5-40 M (TAWFIK, A., 2016; 

TYPPAYASAM et al., 2016). From these studies, the optimum concentration stands in the 

range of 8-12M (which can vary according to origin of studied metakaolin). For 

concentrations of the alkaline metal that is outside from this range, it is not able to balance 

completely the negative charge of the AlO4
- groups, justifying the worst results in 

mechanical properties (DUXSON et al., 2005; DUSXON et al., 2007). In addition, for lower 

concentrations than this range, there is no sufficient alkaline metal to dissolve the metakaolin 

particles to form polymerized and strength network (WANG et al., 2005). Another factor 

that influences in the properties of MK-based AAB is the curing temperature. Some authors 

studied the compressive strength in the range of 20-105°C (PELLISER et al., 2013; 

ARELLANO-AGUILAR et al., 2014; BING-HUI et al., 2014; CATAURO et al., 2014; 

TAWFIK et al., 2016; TIPPAYASAM et al., 2016). These studies showed that the higher 

temperatures (until 80 ºC) accelerates the compressive strength development in the first 

hours when compared to room temperature, however, they became similar after 7-14 days. 

Now about temperatures above of 80 ºC, this caused a decrease in the compressive strength, 

which was justified by the authors for too rapid setting speed of the matrix that restrained 

the transformation into a strength structure (BING-HUI et al., 2014). Finally, an important 

factor that affect the mechanical properties of MK-based AAB is the Si/Al proportion of the 

mixture (DUXSON et al., 2005; RASHAD, 2013a; OZER; SOYER-UZUM, 2015). The 

silica and alumina sources are the metakaolin and the waterglass. The chemical composition 

of the metakaolin cannot be varied in a study; therefore, the Si/Al is varied from the amount 

of waterglass in the activating solution. Authors concluded that the ideal Si/Al amount is in 

the range 1.40 to 2.20. In this interval, the microstructure appears glassy, more homogenous, 

with regular pores. With less Si/Al relation, is showed there is high formation of zeolite 



     
 

32 
 

structures. For Si/Al higher than this interval, there are presence of unreacted particles and 

isolated porous. 

To end this topic, some studies of combined systems with metakaolin is presented. 

The aim of use another binder with the metakaolin is to improve its mechanical properties 

of the final product (ZHANG et al., 2014). The others binders that were used in combined 

systems are: fly ash (RAJAMMA et al., 2012; ZHANG et al., 2014; PAPA et al., 2014), red 

mud (HAJJAJI et al., 2013; KAYA; SOYER-UZUN, 2016), boiler slag and rice husk ash 

(VILLAQUIRÁN-CAICEDO; GUITIÉRREZ, 2015) waste catalyst (CHENG et al., 2015) 

and palm oil fuel ash (YUSUF et al., 2014). 

The studies on fly ash replacing partially metakaolin showed interesting results 

(Figure 2.10). Authors achieved 15% more compressive strength than the control with a 10% 

replacement percentage. They give the credits for this improvement due the increase in the 

reaction extent measured by isothermal calorimetry studies (ZHANG et al., 2014). In studies 

of metakaolin/red mud, the researchers did not find significantly change in the mechanical 

properties after 28 days of curing until 40% of replacement percentage (HAJJAJI et al., 

2013). When the metakaolin is utilised in a ternary system with boiler slag and rice husk ash, 

authors achieved improvement of 122% in the compressive strength after 180 days of curing. 

Now about the waste catalyst replacing partially the metakaolin, researchers found that the 

optimum replacement percentage was 10% after a compressive strength test. They justify 

this result by TGA studies, where this replacement percentage presented higher mass loss in 

the range of 50-300 ºC than the control, indicating more reaction products formed (CHENG 

et al., 2015). Palm oil fuel ash was utilised replacing partially the metakaolin until 70%. The 

results showed that there is a reduction in the compressive strength of 30% (YUSUF et al., 

2014). These results show that is possible to utilise the metakaolin in combined systems with 

other type of binders.  

 

 

 

 

 

 

 



     
 

33 
 

Figure 2.10 – Compressive strength results of binary systems composed by metakaolin 

and fly ash (MK/FA) after 7 and 28 days of curing at room temperature (ZHANG et al., 

2014) 

 

 

III) Fly Ash (FA) 

 

Fly ash is a by-product from the energy production by coal burning in thermal 

plants. The worldwide production is around of 1 billion ton per year, which gives it as one 

of the most abundant anthropogenic materials (RASHAD, 2014; YAO et al., 2015). The fly 

ash can be classified in two groups: the Class F and the Class C. The Class F is the fly ash 

that presents the chemical composition composed by silica and alumina as major oxides. In 

the other hand, for Class C, there is a considerable quantity of calcium oxide, which gives it 

a hydraulic characteristic (ASTM, 2015). In this topic, only will be discussed the Fly Ash 

Class F, which presents better results in alkali-activated studies. This ash presents a glassy 

aluminosilicate structure with spherical particles, which results in less water demand in 

mixtures than metakaolin (van DEVENTER et al., 2012). The first study on fly ash in alkali-

activated binders was published in 1999 (PALOMO et al., 1999). As the metakaolin and 

others aluminosilicates materials, the activation of the fly ash generates a Me-A-S-H (being 

Me a Na, K…) gel. The factors that influences the gel formation, and consequently the 

mechanical properties, are similar to was discussed previously: metal concentration, curing 

temperature and SiO2/Na2O relation. 



     
 

34 
 

The alkaline metal most utilised in the recent years to activating the fly ash is the 

sodium by combination of sodium hydroxide and sodium silicate. As the others two solid 

precursors discussed, fly ash shows better results when activated with waterglass. Authors 

reported concentrations of this alkaline metal in the range or 3-15M, where these studies 

showed that the optimum Na+ concentration are in the range of 9-12 M by compressive 

strength tests. About the curing temperature, fly ash of Class F is activated only at high 

temperatures (in the range of 45-115 ºC) due its slow reactivity. The optimum curing 

temperature of fly ash-based AAB from these studies is in the range of 85-115 ºC, where a 

study reaches 120 MPa after 24 hours of curing at 115 ºC. The SiO2/Na2O relation has an 

important role in the compressive strength development following these works. This relation 

was studied in the range of 0-10, where better results are obtained in the range 0.8-1.3 (RYU 

et al., 2013; GORHAN; KURKLU, 2014; ZHANG et al., 2014; ATIS et al., 2015; CHI, 

2015; GUNASEKARA et al., 2015; KAZEMIAN et al., 2015; NIKOLIC et al., 2015).  

As the other two solid precursors studied, fly ash was also utilised in AAB 

combined systems with another binders: sugar cane bagasse ash (CASTALDELLI et al., 

2016), palm oil fuel ash (RANJBAR et al., 2014; NADZIRI et al., 2017), blast-furnace slag 

(MARJANOVIC et al., 2015; DEB et al., 2014), metakaolin and silica fume (RASHAD, 

2014).  

In studies on binary system of fly ash and sugar cane bagasse ash (SCBA), authors 

determined that the best FA/SCBA relation is 75/25. The authors highlighted the importance 

of the SCBA in the compressive strength results in the early curing time at 20 ºC. When fly 

ash was studied with blast-furnace slag, authors detected a compressive strength gain with 

the increase of this binder. They correlate the compressive strength with the gel composition, 

Ca/Si and Al/Si relations, where obtained optimum values of these rates of 0.5 and 0.2, 

respectively (FA/BFS = 25/75). Palm oil fuel ash showed good results when combined with 

fly ash on high temperature studies. Authors heated samples until 1000 ºC and assessed their 

compressive strengths. The specimens with more palm oil fuel ash in the composition 

presented less compressive strength loss (Figure 2.11). In studies on fly ash with metakaolin, 

authors concluded that the most beneficial effect was the increase in the compressive 

strength. Silica fume also presented good results when combined with fly ash, showing gains 

in the compressive strength until 7% replacement. From these studies with different binders 



     
 

35 
 

combined with fly ash, highlight for the sugar cane bagasse ash, which is an agro-industrial 

residue as sugar cane straw ash and presented interesting results. 

 

Figure 2.11 – Compressive strength evolution of FA/POFA specimens studied in high 

temperatures (adapted from RANJBAR et al., 2014) 

 

 

IV) Other materials 

 

This topic is destined to other materials that are being studied in alkali-activated 

binders. The list is large, but some of new binders are the being researched recently are: fluid 

catalytic cracking catalyst residue (TASHIMA et al., 2012a; TASHIMA et al., 2013a), red 

clay brick waste (REIG et al., 2013a), ceramics wastes (REIG et al., 2013b; REIG et al., 

2017), red mud (HE et al., 2013) and vitreous calcium aluminosilicate (TASHIMA et al. 

2012b; TASHIMA et al., 2013b). Those studies focused on best alkaline metal molar 

concentration and SiO2/Me2O relation in order to achieve highest mechanical properties. 

 

  2.1.6  Alkaline Activators 

 

Alkaline solution has an important role in the alkali-activated binders’ reaction. 

They are responsible for the high pH conditions that is necessary to activate the solid 

precursor (PROVIS; van DEVENTER, 2009). There are six groups that are usually used as 

alkaline source (SHI et al., 2006): 



     
 

36 
 

 

a) Caustic alkalis (MeOH, being Me a Na, K, Li…); 

b) Silicates (Me2O . nSiO2); 

c) Aluminates (Me2O . nAl2O3); 

d) Aluminosilicates (M2O . Al2O3 . (2-6)SiO2) 

e) Non-silicate weak acid salts (Me2CO3); 

f) Non-silicate strong acid salts: Me2SO4. 

 

From these activators listed, the most commons are the use of sodium hydroxide 

(NaOH), potassium hydroxide (KOH) and sodium silicate (Na2O . nSiO2). The sodium-

based alkaline activator is cheaper and more available than the potassium-based, 

consequently, is the most used in alkali-activated binders (SHI et al., 2006). In the other 

hand, the advantages of potassium-based are the improved phase behaviour (including the 

less rapid formation of zeolites) and rheology (PROVIS, 2009). The use of silicates 

combined with hydroxides improve the mechanical properties than those based only by 

hydroxides (SHI et al., 2006; PROVIS, 2009). 

The main advantage from alkali-activated binders over than OPC blends is the less 

greenhouse gas emission, where the alkaline solution has an important role in releasing this 

gas. Authors who studied CO2 footprints of those binders (TURNER; COLLINS, 2013; 

MELLADO et al., 2014) showed that the most pollutant raw material is the alkaline 

activator, mainly the waterglass (silicate) (Figure 2.12). The challenge is to increase even 

more the environmentally characteristic of alkali-activate binders, hence, new ways to 

replace the use of conventional alkaline source are being researched in the last years. 

 

 

 

 

 

 

 



     
 

37 
 

Figure 2.12 – Comparison between CO2 footprints of concretes based on alkali-activated 

binders and OPC, highlighting the emissions from the alkaline activator (adapted from 

Turner; Collins, 2013) 

 

 

In a study on using potassium-rich biomass ashes to activate metakaolin, authors 

achieved until 40 MPa after 3 days of curing (PEYS et al., 2016). The evolution of reaction 

was assessed by calorimetry, attenuated total reflectance Fourier-transformed infrared 

spectroscopy (ATR-FTIR) and electron probe micro-analysis (EPMA). These tests showed 

that the biomass ashes reacted with the metakaolin and formed gels. The successful of those 

biomass ashes is due to the high pH (> 13) and over 30% of K2O in their composition. A 

study tried do replace the use of silicates by adding rice husk ash (RHA) in activating 

solution with NaOH by a refluxing dissolution method (BOUZÓN et al., 2014). Results on 

compressive strength of mortars using FCC as solid precursor showed similar values to a 

control using sodium silicate after one day of curing, where both reached over than 40 MPa 

(Figure 2.13). The results were justified due the high dissolution of the amorphous silica in 

the sodium hydroxide solution. In the same line in order to replace the use of silicates, a 

glass waste was also assessed in alkaline solution preparation. Results showed compressive 

strength fewer than the control with waterglass, however, higher than only activated with 

NaOH/Na2CO3 (PUERTAS; TORRES-CARRASCO, 2014). More recently, sugar cane 

bagasse ash was evaluated as silica source in the production of activating solution 

(TCHAKOUTÉ et al., 2017). This new solution presented similar results in compressive 



     
 

38 
 

strength tests than the waste glass and rice husk ash in metakaolin-based AAM. These results 

showed that the alkaline activator influences the mechanical properties of the alkali-activated 

binders. 

 

Figure 2.13 – Compressive strength versus reflux time of FCC-based alkali activated 

binders prepared with different siliceous sources combined with NaOH solutions: G-

RHA (ground rice husk ash), O-RHA (original rice husk ash), quartz, control 

(commercial waterglass solution). Key: full line for compressive strength and dotted line 

for flexural strength (Bouzón et al., 2014) 

 

 

  2.1.7  Durability 

 

As showed previously, alkali-activated binders can present high compressive 

strength. However, only this mechanical property is not sufficient, because a construction 

material should be not only strength, but also durable. Therefore, durability tests on alkali-

activated binders are an important issue. These binders were not adopted yet due the lack of 

full characterization and performance under physicomechanical and chemical conditions 

(BERNAL; PROVIS, 2014). Therefore, several studies on durability of this type of binder 

were carried out recently, and it was compared to Portland cement mixtures (MELLADO et 

al., 2017). 

Durability (and compressive strength) of alkali-activated binders is directly linked 

to their microstructure. Hence, studying the products formed and the porosity of these 

binders help to understand the durability results of alkali-activated binders. The 

microstructural of an AAB can vary according to the solid precursor, alkaline solution, 

solid/liquid ratio, paste content, curing conditions and among others parameters (BERNAL; 



     
 

39 
 

PROVIS, 2014). Then, an overview about durability tests and their consequence on alkali-

activated binders’ microstructure will be presented. These studies are based on acid attack, 

sulphate attack, chloride penetration, temperature resistance, carbonation, alternate extreme 

cycles, and alkali-silica reaction. 

Alkali-activated binders present good resistance to acid attack when compared to 

Portland cement mixtures. This is explained by the higher alkalinity of the structure and 

lower CaO amount in those binders (ABORA et al., 2014; BASCAREVIC, 2015). In 

general, studies on blast furnace slag, fly ash and metakaolin show less compressive strength 

and mass losses when compared to Portland cement mixtures (PACHECO-TORGAL et al., 

2012; HOSSAIN et al., 2015; DUAN et al., 2015). About studies on binary systems, AAB 

based on blast-furnace slag and sugar cane bagasse ash (BFS/SCBA) were assessed in acid 

attack tests were compared to a control mixture composed by only Portland cement. The 

alkali-activated binder presented a loss in the compressive strength similar for hydrochloride 

acid and lower for acetic acid when compared to the control one (PEREIRA et al., 2015). In 

another study of binary systems composed by fly ash and palm oil fuel ash in acid attack, 

the alkali-activated binders also showed lower compressive strength loss than the Portland 

cement after an immersion of 18 months in sulfuric acid (Figure 2.14) (ARIFFIN et al., 

2013). 

 

Figure 2.14 – Compressive strength of concrete manufactured with fly ash and palm oil 

fuel ash (BA geopolymer concrete) compared to one of Portland cement after sulfuric 

acid attack (ARIFFIN et al., 2013) 

 

 

Sulphate attack impact depends on the salt that alkali-activated binders are exposed. 

The most used sulphates are the NaSO4 and MgSO4. Whereas MgSO4 causes a cation-



     
 

40 
 

exchange mechanism degrading the gel structure (mainly in Ca-rich structures), leading to 

formation of M-S-H gels and precipitation of gypsum (in Ca-rich binders), NaSO4 do not 

lead any degradation and can promotes the structural evolution of the alkali-activated binders 

(ISMAIL et al., 2013; BERNAL; PROVIS, 2014). Blast-furnace slag, fly ash and metakaolin 

specimens were tested in sulphate attack and their showed better results than the Portland 

cement samples for presenting less expansion and compressive strength loss 

(CHINDAPRASIRT; CHALEE, 2014; HOSSAIN et al., 2015). In studies on binary systems 

of BFS/SCBA, MgSO4 resulted in more compressive strength and mass losses than the 

NaSO4. When compared to the Portland cement mixtures, the alkali-activated binders 

presented lower compressive strength loss in both sulphates attack (PEREIRA et al., 2015).  

Chloride ion penetration is also an important durability study. If is aiming the use 

of alkali-activated binders as a structure material, they will be used as a reinforced concrete. 

As know that the chloride ion can induce to corrosion of the steel bars by an electrolytic 

action; therefore, a low permeability of this ion is required for a durable structure 

(PACHECO-TORGAL et al., 2012; HOSSAIN et al., 2015). The chloride ion penetration is 

directly related to the porosity of the material, where authors reported that alkali-activated 

binders presents lower permeability than the Portland cement ones (BERNAL; PROVIS, 

2014). In AAB combined systems containing fly ash and blast furnace slag, authors obtained 

lower Cl- concentration in chloride penetration test when compared to a one-system binder 

(ZHU et al., 2014). 

Studies on alkali-activated binders treated at high temperatures are also found in 

bibliography. Authors found optimistic results in alkali-activated binders until 1200 ºC due 

their high stability (PACHECO-TORGAL et al., 2012). When compared to OPC, the alkali-

activated binders produced by solid precursors with low Ca content (as fly ash and 

metakaolin) show better results than the high Ca content (blast furnace slag). The first class 

of these binders presents higher stability in the compressive strength with the increase of the 

temperature treatment (MARTIN et al., 2015; DUAN et al., 2015), whereas the second class 

showed similar results to OPC (TURKER et al., 2016) (Figure 2.15). Authors justified the 

first comparison (first class and OPC) due the pore structure of the AAB be able to absorb 

the stress of water pressure that tends to evaporate at higher temperatures, whereas the OPC 

pore structure is not able to do it under similar conditions. In addition, XRD analysis 

presented a halo from the gel structure of AAB after the treatment at 800 ºC. In the other 



     
 

41 
 

hand, the similar results by the second class is verified in TGA studies, where both, AAB 

and OPC specimens, presented a significant reduction in the mass loss related to the hydrate 

products that are responsible for the compressive strength. Binary systems of AAB were also 

studied in temperature treatment by the combination of metakaolin and fly ash, where 

authors achieved an optimum proportion of 50/50 by mass after expose specimens until 800 

ºC (ZHANG et al., 2014). The influence of the activator was also evaluated in thermal tests. 

Whereas AAB produced by sodium as alkali activator presented deterioration and loss in the 

compressive strength after 800 ºC, potassium-based AAB presented problems only after 

1000 ºC (PANIAS et al., 2015). 

 

Figure 2.15 – Compressive strength after temperature treatment of OPC mixtures and 

alkali-activated binders of solid precursors with a) high Ca content and b) low Ca 

content (TURKER et al., 2016; DUAN et al., 2015) 

 

 

Carbonation is also an important issue studied for alkali-activated binders. In 

general, it depends on the type of the solid precursor, the nature/concentration of the alkaline 

solution and, consequently, the porous size from the final binder. In normal conditions, AAB 

presents similar or lower carbonation rates than the OPC mixtures (BERNAL; PROVIS, 

2014; HOSSAIN et al., 2015). There are studies on the behaviour of alkali-activated binders 

at alternate extreme cycles, as the freeze-thaw. Authors showed that alkali-activated binders 

presented better performance than OPC mixtures, where the former can endure more and 

presents less compressive strength loss in those type of durability tests. Alkali-silica 

reactions still a complex issue for alkali-activated binders due few studies, whereas for OPC 

presents several studies. In some of those studies, alkali-activated binders showed to be less 



     
 

42 
 

susceptive when compared to OPC mixtures. The presence of calcium has an important role 

in the expansion of the binders (PACHECO-TORGAL et al., 2012). 

As showed in the last paragraphs, there are several durability studies reported on 

alkali-activated binders in the last years. However, more studies must be done to know the 

real limits of these new type of binders, since there are many types of solid precursors and 

alkaline solutions being researched. 

 

 2.2 SUGAR CANE STRAW ASH (SCSA) 

 

Sugar cane straw ash is obtained after a burning process of the straw. The interesting 

in the sugar cane straw arises a few years ago in Brazil for two reasons: the increase in sugar 

cane production and the mechanized harvesting.  

The cultivation of sugar cane increased in the last years in Brazil mainly to obtain 

ethanol, and put the country as the major producer in the world (FAOSTAT, 2016). The 

production in 2004/2005 was 385.2 million tons, and increased to 632 million tons in 

2014/2015, showing an increase of 64% in only ten years (UNICA, 2016). Some years ago, 

the sugar cane used to be burned in open-air, which facilitated the process of harvesting. Due 

to environmental and health risks, a Brazilian law nº 11.241/2002 prohibited gradually this 

harvesting mode until 2031. However, an Agro-environmental Protocol was assigned by 

stakeholders of sugar cane production chain reduces this deadline to 2018 (RIBEIRO; 

FICARELLI, 2010; LEAL et al., 2013a). With these law and protocol, the mechanized 

harvesting gained importance as a method to collect the sugar cane (Figure 2.16).  

 

Figure 2.16 – Mechanized harvesting of the sugar cane (UNICA, 2016) 

 

 



     
 

43 
 

After harvesting the sugar cane with these machines, is obtained a by-product that 

are composed by green and dry leaves, which is called by sugar cane straw. The amount of 

straw obtained is 140 kg per 1000 kg of sugar cane harvested (BNDES; CGEE, 2008; LEAL 

et al., 2013a). Authors stated that some part of this straw should be in the field to protect the 

soil, however, a part can be used as biomass to generate energy, as the bagasse are being 

used nowadays (LEAL et al., 2013b). Many studies are being carried out in order to collect 

the straw from the field (LEMOS et al., 2014) and to obtain energy from the straw due its 

interesting calorific value and economic point of view (LOMBARDI et al., 2012; MORAES 

et al., 2012; MESA-PÉREZ et al., 2013; ALVES et al., 2015). This process of energy 

generation by a burning process yields an inorganic ash composed mainly by silicon dioxide: 

the sugar cane straw ash (SCSA) (ALVES et al., 2015). The SCSA do not present a suitable 

valorisation and, considering others agro-industrial ashes destinations, it can be utilised in 

building construction (MADURWAR et al., 2013). 

The SCSA was already assessed previously by its potential reactivity and as a 

construction material, but not in alkali-activated binders. The reactivity of SCSA was 

evaluated by Villar-Cociña (2002) and Frías et al. (2005 and 2007) in systems of hydrated 

lime/SCSA. The researchers concluded that the SCSA presents high reactivity, and it can be 

compared to a well-known pozzolanic material: rice husk ash (RHA) (Figure 2.17). In other 

study, Calligaris et al. (2015) assessed the SCSA reactivity replacing partially the Portland 

cement in 20% by X-ray diffraction (XRD) analysis. Authors found pozzolanic behaviour 

due to crystals analysis on XRD after 90 days of curing: less Ca(OH)2 and higher ettringite 

(Aft) in pastes with presence of SCSA when compared to a control without the ash. 

 

Figure 2.17 – Electrical conductivity of hydrated lime/RHA and hydrated lime/SCSA 

suspensions (VILLAR-COCIÑA, 2002) 

 



     
 

44 
 

About studies with the application of SCSA as construction material, Rodrigues et 

al. (2013) assessed the ash in cement composites on mechanical and durability (accelerated 

ageing) tests. According to the authors, the SCSA presented pozzolanic characteristics and 

high porosity of these particles. The researchers obtained that the mechanical properties of 

specimens with SCSA presented similar values to the control. Regarding to the durability 

test, specimens with SCSA showed improved results than compared to the control one. By 

the results obtained, they concluded that the SCSA could replace partially the Portland 

cement by 20% in mass in these composites studied. In studies of Portland cement mortars, 

Oliveira et al. (2015) assessed a combined effect of sugar cane straw and bagasse ashes. The 

researches obtained that 20% replacement of Portland cement by SCSA presented similar 

values of compressive strength to the control. In another study of Portland cement mortars, 

Guzán et al. (2011) studied replacements by SCSA until 40%. The authors concluded that 

the optimum replacement percentage is in the range 10-20% after studies on compressive 

strength of 60 curing days (Figure 2.18). 

 

Figure 2.18 – Compressive strength of mortars cured after 60 days (GUZMÁN et al., 

2011) 

 

 

All these SCSA studies showed that the ash presents high reactivity and a potential 

to be utilised in alkali-activated binders. 

 

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