Influence of sugar cane straw ash in metakaolin-based geopolymers

The present investigation studied the use of agro-industrial waste, sugar cane straw ash (SCSA), to partially replace commercial metakaolin (MK) in geopolymers (0 %, 15 %, 30 %, and 45 % percentages of substitution by mass). The study presented solid precursors characterization, microstructural studies of pastes, and compressive strength of mortars assessed between 1 and 90 days of curing at 25 ºC. The chemical composition of MK was mainly based on SiO2 (54.1 wt%) and Al2O3 (35.9 wt%), and SCSA was mainly composed of SiO2 (58.6 wt%). Microstructural studies of pastes showed that occurred a formation of N-A-S-H amorphous reaction products and no crystalline phases were formed; SCSA contributed to the formation of geopolymer reaction products as a secondary role to the main reaction of MK; the presence of SCSA slightly increased the porosity and did not excessively modify the pore distribution; and the pastes with SCSA presented a heterogeneous microstructure. The compressive strength of mortars reached values over 50 MPa after 90 days of curing. Therefore, SCSA was a sustainable replacement for MK in producing geopolymers.