Effect of light-burnt dolomite and metakaolin on workability, mechanical properties and shrinkage of cement

In order to investigate the synergistic effect of light-burnt dolomite and metakaolin in cement, dolomite powder was calcined at 750℃, 800℃ and 850℃ for 1 hour to obtain light-burnt dolomite which is composed mainly of MgO and CaCO₃. Then, light-burnt dolomite and metakaolin were compounded in different mass ratios to replace 50% of cement by mass. The influences of light-burnt dolomite and metakaolin on the hydration heat, setting time, fluidity, strength and drying shrinkage of cement-based materials were investigated. Meanwhile, the hydration products and microstructure of the composite cement paste were analyzed by X-ray diffractometer (XRD), thermogravimetric analyzer (TG), scanning electron microscope (SEM) and low-field nuclear magnetic resonance (LF-NMR). The results showed that the co-addition of light-burnt dolomite and metakaolin accelerated the hydration of cement, shortened its setting time, reduced its fluidity and also decreased the total hydration heat release of blended cement. The combination of light-burnt dolomite and metakaolin decreased the early strength of cement mortar, but could effectively promote the development of later strength. Moreover, the later strength was increased with increasing the mass ratio of metakaolin to light-burnt dolomite in the blended cement. The 60-day compressive strengths of cement mortars blended with metakaolin and light-burnt dolomite (calcined at 750℃, 800℃, and 850℃) in a mass ratio of 4∶1 were 55.7 MPa, 55.4 MPa, and 55.2 MPa, respectively, which were 7.5 MPa, 7.2 MPa, and 7.0 MPa higher than those of pure cement mortar. This is due to the fact that light-burnt dolomite and metakaolin exhibit micro-filling effects, CaCO₃ in light-burnt dolomite can react with the active aluminum phases in metakaolin and cement to form carboaluminates, and metakaolin also undergoes a pozzolanic reaction with Ca(OH)₂ in the cement paste to generate C-(A)-S-H gel, thereby reducing the porosity and enhancing the later strengths of the composite cement mortars. In addition, MgO in light-burnt dolomite can hydrate to form Mg(OH)₂, leading to an increase in the solid phase volume and decreasing the drying shrinkage of cement mortar. Moreover, the drying shrinkage was gradually decreased with the increase of the light-burnt dolomite proportions. The 180-day drying shrinkages of cement mortars blended with light-burnt dolomite (calcined at 750℃, 800℃, and 850℃) and metakaolin in a mass ratio of 4∶1 were decreased by 28.1%, 31.1%, and 33.7%, respectively, compared with those of pure cement mortar.