Improvement of early-age properties of silico-aluminophosphate geopolymer using dead burnt magnesia

This paper proposes the induction of an acid-base reaction during the formation of silico-aluminophosphate (SAP)geopolymer. Such reaction is promoted by dead burnt magnesia (DBM)and phosphate activating solution to synthesize a DBM-doped silico-aluminophosphate (DBM-SAP)geopolymer with enhanced early-age properties. The reaction mechanisms and chemistries between DBM and mono-aluminum phosphate (MAP)in aqueous solution were firstly investigated at four Mg/Al ratios. Conductivity and pH evolutions of the solution systems, in addition to X-ray diffraction (XRD)and scanning electron microscopy (SEM)results of the final precipitations indicated that the crystalline phase (i.e., Newberyite)started to precipitate at a pH of 3 approximately. Besides, the reaction between DBM and MAP produced an amorphous aluminum magnesium phosphate (Al₂O₃·3MgO·2P₂O₅)phase. The rate of such reaction was governed by the DBM/MAP ratio in the solution system. Setting time and early strength of the DBM-SAP geopolymer pastes with an optimal Mg/Al ratio (i.e., 4)in solution system were evaluated to confirm the acceleration effect of DBM-induced acid-base reaction. Experimental results showed that 20% incorporation of DBM enabled the preparation of an SAP geopolymer with an initial setting time of 8 min and 1 day compressive strength of 8.3 MPa. Phosphorrösslerite was detected in the 1d-cured geopolymer paste but disappeared after 3 days curing. Such reaction intermediate might contribute to the fast setting and achieving early strength.