Physical, hydrolytic, and mechanical stability of alkali-activated fly ash-slag foam concrete

Foam concrete is prone to instability during mixing, casting, and curing. Besides preformed foam stability characteristics and volume, several geopolymer mix parameters, viz., precursor composition, silica modulus, alkali content, and water content, influence the overall foamed concrete stability, the understanding of which is still limited. This study evaluates the effect of the above parameters on the physical, hydrolytic, and mechanical stability of fly ash and slag-based foamed geopolymer. Physical stability was assessed by the design of experiments using a central composite design (inscribed), while systematic laboratory experiments assessed hydrolytic and mechanical stability characteristics. The foamed geopolymer behavior is sensitive to silica modulus and solid alkali-to-binder ratio, showing maximum interaction effects for producing stable foamed geopolymer for i) target to actual fresh density difference and ii) settlement along the height. The water-to-binder ratio was the most significant factor affecting the homogeneity across the cross-section along the height. Further, the alkali and slag content significantly affected the hydrolytic and mechanical stability characteristics. An optimum proportion of slag content, silica modulus, solid alkali-to-binder ratio, and water-to-binder ratio is suggested for the maximum benefit of fly ash and slag-based foamed geopolymer. This study provides a framework for comprehensive stability assessment of foamed geopolymers and can aid in developing sustainable lightweight materials for industrial applications.