Understanding the gel compatibility and thermal behavior of alkali activated Class F fly ash/ladle slag: The underlying role of Ca availability

The gel compatibility in calcium contained geopolymers remains a controversial topic. This work aims to clarify the role of Ca availability in determining the geopolymerization of alkali-activated ladle slag/Class F fly ash blends. The results show that the product layer wrapping around slag particles largely governs the Ca diffusion into the environment, enabling the development of two separated gels, namely C-(N)-A-(S)-H and N-(C)-A-S-H type gel. A dense matrix consisting of geopolymer gel and Ca-enriched gel is achieved with 8 % of Na2O while increasing Ms. intensifies the incompatibility between two gels, leading to microcrack formation and lowered mechanical strength. The competition between different reaction mechanisms of the two gels predominates the initial properties and thermal performance of the hybrid binder. A thermal degradation model of hybrid geopolymer co-existing geopolymeric gel and Ca-enriched gel is proposed to describe the influence of gel compatibility on the thermal behavior of Ca incorporated geopolymer.