Influence of coal fly ash on the early performance enhancement and formation mechanisms of silico-aluminophosphate geopolymer

This study proposes the use of two types of coal fly-ash (CFA) (high- and low-calcium, HCFA and LCFA respectively) as additives in silico-aluminophosphate (SAP) geopolymers for early performance improvement. The chemistry of CFA in phosphate environment was clarified using CFA/orthophosphoric acid suspensions through XRD, SEM and NMR analyses. The results revealed that HCFA could seed the active calcium sources which induced the acid-base or/and hydrolysis reaction between the alkaline calcium species within HCFAs and the acidic phosphate species in the activator. The produced calcium phosphate compounds, e.g., brushite and monetite, hurried up the setting (initial and final setting were 18 and 26 min, respectively) and early strength development (1-d and 3-d strengths increased to 15 and 29 MPa, respectively) of the HCFA-SAP geopolymer relative to CFA-free SAP geopolymer. However, the incorporation of LCFA was found to improve the workability but weakened the setting and strength properties of the LCFA-SAP geopolymers.