Valorization of oat husk ash in metakaolin-based geopolymer pastes

Abstract

Geopolymeric binders are materials with properties equivalent to Portland cement, which may present lower CO2 emissions associated with the production cycle. To produce sustainable geopolymeric binders, it is necessary to use raw materials with a low carbon footprint and energy consumption, characteristics found in biomass ash. In this sense, this work proposes the evaluation of oat husk ash (OHA) as a partial replacement material for metakaolin in the production of geopolymer paste. OHA has replaced metakaolin at levels up to 20%. In addition to routine microstructural (XRD and FTIR) and mechanical analyses, i) the pore size distribution by nitrogen adsorption in cubic samples, and ii) an extensive rheological investigation in which it was determined the model that best fits the analyzed matrix. Furthermore, the determination of CO2-e emission embodied energy and the cost of the pastes was carried out. The results showed that the insertion of 20 wt% of OHA i) increased the dynamic yield stress (40.2%) and the equivalent viscosity (13.7%) compared to plain paste, ii) reduced the cumulative heat after 168 h by 29.7%, which is related with the lower reactivity of OHA compared to MK. Nevertheless, iii) did not cause significant chemical and mineralogical changes, (iv) maintained compressive strength statistically equal to the control mixture, and v) reduced up to 34% and 23%, respectively, the embodied energy and the cost of producing the matrices. Thus, the investigation made possible the valorization of OHA for the production of technically and economically viable and environmentally friendly geopolymer matrices.