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 (A1(2)O(3)center dot 3MgO center dot 2P(2)O(5)) 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. Phosphorrosslerite 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. (C) 2019 Elsevier Ltd. All rights reserved.