Effect of slag on the shrinkage behavior and microstructural evolution of MgO-K2HPO4-SiO2 cement

Magnesium silicon potassium phosphate cement (MSPPC, MgO-SiO2-K2HPO4) exhibits high early strength and improved water resistance, but its shrinkage behavior remains a concern. This study investigated the effects of slag incorporation (0 to 30 wt%) on fresh properties, strength, internal relative humidity, shrinkage, and microstructural evolution of MSPPC. Increasing slag content reduced flowability from 170 mm for the control mixture without slag (M-SL0) to 125 mm for the mixture containing 30 wt% slag (M-SL30). The final setting time varied non-monotonically, increasing from 18.50 min for M-SL0 to 24.75 min for the mixture containing 10 wt% slag (M-SL10) and then decreasing to 19.75 and 18.00 min for the mixtures containing 20 wt% slag (M-SL20) and M-SL30, respectively. M-SL30 showed the lowest shrinkage strain within the investigated range and reached 96.8 MPa at 56 d. Microstructural analyses suggest that slag may regulate hydration and contribute to later-age densification.