Geopolymer based on biomass ash from agricultural residues

The expansion of biomass power stations, while advancing carbon neutrality, generates abundant biomass ash waste whose disposal is a challenge. This study uses crop biomass ash (CBA) waste from a power plant to produce geopolymers. Geopolymers are an alternative for reducing CO2 emissions and resource extraction in construction. This paper studies geopolymers made with raw and alkali-fused CBA. A sodium silicate activator with varying Na+ and SiO2/Na2O ratios is tested to optimise geopolymerisation. A low fusion temperature (600°C) is used to preserve the environmental credentials of the geopolymers. The geopolymers made with (only) raw CBA have extremely low carbon emissions and environmental impact; they attained the lowest strengths (7.46 and 2.99 MPa-compressive and flexural respectively), a strength range suitable for most construction applications unless higher strength is required for load bearing purposes. Partly replacing the raw ash with slag raised their strength to over 25 MPa and 7 MPa-compressive and flexural respectively. The slag released active Si4+, Al3+and Ca2+ that participate in the polymerization reaction, forming cements that enhanced strength. Alkali-fusing the ash, even at low temperatures (600°C), enhances reactivity; it breaks down the silicates comprising the ash, providing active radicals that polymerise and form cementing gels that increase strength: the geopolymer made with (only) alkali-fused ash achieved high strengths of 39.35 and 10.74 MPa-compressive and flexural respectively. The geopolymers made with alkali-fused ash and slag consistently show outstanding performance (42–44 MPa compressive strength), and the slag stabilized the system against fluctuations in activator composition. The geopolymers have vitreous structures of cementing gels with high Si4+ and Na+ contents, consistent with an alkaline, silico-aluminate hydrate gel (N-(A)-S-H) which is a typical product of polymerisation. The FTIR results show ample Si-O-Si(Al) bonds in the geopolymer made with alkali-fused CBA. The presence of slag significantly increased the absorption intensity of these bonds, indicating the presence of more abundant C(N)-A-S-H which improved mechanical properties. Incorporating slag vitrifies the geopolymer structure significantly more than alkali-fusing the ash. However, there is no direct proportionality between increasing vitreousness and escalating strength as the crystalline geopolymer (made with alkali-fused CBA only) is nearly as strong as the vitreous, CBA-GGBS geopolymer (39 vs 44 MPa).