Effect of magnesia on properties and microstructure of alkali-activated slag cement

The effect of magnesia burnt at 800-950℃ on the properties, especially the shrinkage, of alkali-activated slag cement (AASC) was experimentally studied. Experimental results show that, although adding 4%-8% lightly-burnt magnesia may shorten the setting time and slightly reduce the compressive strength of AASC, it can remarkably reduce the shrinkage of AASC. The results also show that the setting time of AASC with a certain amount of magnesia increases with the burning temperature, and that the flexural and compressive strengths of AASC decrease with the increase of the additive amount of magnesia. Generally, the adverse effect of magnesia decreases with the increase of the burning temperature, and the shrinkage-reducing effect of magnesia increases with the additive amount of magnesia. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses show that some magnesia particles in the hardened AASC paste at a 28-d age remained unhydrated, and that the compactness decreased a little as magnesia was added. We can also conclude that magnesia burnt at 850-950℃ can be used to reduce the shrinkage of AASC only when its additive amount does not exceed 8%; otherwise, the setting time may be too short, and the flexural and compressive strengths may severely decrease.