Micro and macro properties of silico-aluminophosphate geopolymer: Role of incinerated sewage sludge ash (ISSA)

The mechanistic insights into silico-aluminophosphate (SAP) geopolymer formation remain an enigma. Here, we aim to elucidate this process and improve the early-age performance of SAP geopolymers by utilizing incinerated sewage sludge ash (ISSA). Interestingly, phosphoric acid-ISSA interaction released Ca²⁺ and Al³⁺ ions from ISSA faster than the dealumination of metakaolin. These ions agglutinated with the active soluble (PO₄)^3- units, evidenced by transmission electron microscopy (TEM), enhancing the early-age compressive strength of ISSA-incorporated SAP geopolymer binders. In contrast, the later strengths were relatively similar to the reference. Such binders had primary silico-alumino-phosphate (S-A-P) gels, possibly intimate with calcium-phosphate (C-P) and alumino-phosphate (A-P) gels, improving the micromechanical properties of the binder. The elastic moduli of these gels lay between 23–25 GPa, higher than conventional N-A-S-H gels. Furthermore, ISSA switched the chemistry of reaction products by favoring Al_IV-OP units over Al_VI-OP units in SAP geopolymers, even though the Si environment remained unchanged, as confirmed by NMR.