Experimental research on mechanical properties and microstructure of magnesium phosphate cement-based high-strength concrete

Magnesium phosphate cements (MPCs) have attracted new interest in the construction industry due to their low carbon and excellent material properties. However, most of the current studies in the field of MPC only focus on the properties, hydration mechanism and microstructure of cement materials, while few studies related to the preparation and mechanical properties of MPC-based high-strength concrete (MPC-HSC) have been reported. This paper aims to investigate the effect of aggregate-cement ratio (A/C) and curing age on the workability, mechanical properties, microstructure and phase composition of MPC-HSC. Four MPC-HSC mixtures containing fly ash (FA) and metakaolin (MK) with different aggregate-cement ratios were prepared, and concrete specimens were tested for fluidity, setting time, cubic compressive strength (CCS), axial compressive strength (ACS), elasticity modulus (ES), flexural strength (FS), phase composition and microstructure at 1d, 3d, 7d and 28d, respectively. In addition, based on the experimental results, some empirical formulas were proposed to predict the strength properties. The test results indicated that the CCS, ACS and EM of MPC-HSC at different curing ages showed a trend of increasing and then decreasing with the increase of A/C. The A/C had no significant effect on the FS of MPC-HSC at 1d, 3d and 7d, but the FS at 28d increased slightly with increasing A/C. The mechanical properties of MPC-HSC developed rapidly at 1d and 3d, in which the 1-day and 3-day CCS were up to 50% and 70%–90% of the 28-day CCS, respectively. Furthermore, the excess aggregates could weaken the bond between the aggregate and the paste, thus reducing the strength of the MPCH-HSC.