The application of polyaluminium sulfate as retarder for magnesium potassium phosphate cements (MKPC): Performance and mechanism

Magnesium potassium phosphate cement (MKPC) is a new special cementitious material hardened by an acid-base reaction to form phosphate hydrate gel, exhibiting high early strength and low shrinkage. However, rapid setting limits its workability and applications. While aluminum sulfate hexadecahydrate (Al 2 (SO 4 ) 3 ·16H 2 O) provides retardation, polyaluminium sulfate ([Al 2 (OH) m (SO 4 ) 3- ] n ) with complex inorganic polymer structure shows superior performance. This study investigates [Al 2 (OH) m (SO 4 ) 3- ] n retarding mechanism in MKPC. Compared to Al 2 (SO 4 ) 3 ·16H 2 O, [Al 2 (OH) m (SO 4 ) 3- ] n (Mg/PO 4 =1) significantly reduces hydration exothermic rate (w/c=0.3) and slows conductivity decline (w/c=100), confirming enhanced retardation. Key mechanisms include: (1) Formation of unstable early hydrates (cattiite, arcanite, phosphorrösslerite) delaying K-struvite precipitation; (2) Dense [Al 2 (OH) m (SO 4 ) 3- ] n polymer films coating MgO surfaces inhibiting ion diffusion. Critically, [Al 2 (OH) m (SO 4 ) 3- ] n promotes needle-like crystals forming interlocked microstructures (w/c=0.3), enhancing long-term mechanical properties despite retardation.