Preparation of sulphoaluminate-magnesium potassium phosphate cementitious composite material under low-temperature

Abstract

Although the novel sulphoaluminate-magnesium potassium phosphate cementitious composite (SAC-MKPC) has the advantages of rapid setting, high early strength, and excellent water resistance, MKPC factories traditionally use dead burnt magnesite as raw material, which requires a high consumption of primary energy sources. This research was mainly focused on investigating the feasibility of using magnesium sulphate, calcium oxide, and aluminium oxide in the preparation of the SAC-MKPC composite clinker at low-temperature. Two different CaO:Al2O3:MgSO4 molar ratios, namely 4:3:37.7 and 4:3:16.15, were calcined at temperatures ranging from 1200 to 1350 °C, with a spacing of 50 °C. The results showed that the composite clinker was successfully obtained by calcining a mixture of magnesium sulphate, calcium oxide, and aluminium oxide at 1250–1350 °C, which is 150–250 °C lower than the temperature used in the dead burnt magnesite production. In the clinker system, yeelimite, periclase, and anhydrite were the main mineral phases and the hydration products consisted mainly of K-struvite, ettringite, residual periclase, and yeelimite. The CaO:Al2O3:MgSO4 molar ratio yielding the best mechanical properties was 4:3:37.7. The compressive strength of the hydration products reached 31 and 70 MPa after 2 h and 1 day, respectively. The scanning electron micrographs of the hydration products showed a relatively dense structure, covered by ‘amorphous materials’ generated during the preparation process. The elemental composition of the amorphous materials included O, Mg, Al, P, S, K and Ca, and derived mostly from the cementation of MKPC and AFt.