Enhanced strength and accelerated setting of CaO-activated fly ash binders induced by aluminum sulfate hydrates for cementless brick production

Abstract

This study investigated the beneficial effects of added aluminum sulfate hydrate (Al2(SO4)3 & sdot;14-18H2O, hereafter referred to as "AS") on the hydration behavior, mechanical properties, and microstructure of CaO-activated low-calcium fly ash-based cementless binders. The incorporation of 10 wt% AS increased the compressive strength by nearly 2.5 times compared to the control, primarily due to accelerated ettringite formation and enhanced microstructural densification. Microstructural analyses confirmed the presence of ettringite and calcium aluminate hydrate (C-A-H) as key hydration products, with ettringite contributing significantly to matrix densification and microstructural refinement. In contrast, excessive AS substitution (15 wt %) led to monosulfate formation and pore coarsening. AS also induced rapid setting and reduced workability through early sulfate reactions, thereby allowing prompt demolding and a shorter production cycle. Bricks produced with the best mixture satisfied KS F 4004 specifications and exhibited safe heavy metal leaching levels according to the toxicity characteristic leaching procedure (TCLP). Overall, these findings demonstrate the potential of AS-modified binders as a sustainable and high-performance alternative for cementless brick production.