Effects of Limestone Powder and Metakaolin on Strength and Hydration Assemblages of Blended Cements

To clarify the synergistic effect of limestone powder (LS) and metakaolin (MK) in cement, the strength of cement mortars blended with LS and MK at different mass ratios was investigated. The hydration products of ternary blend cement were examined by X-ray diffraction and thermogravimetry. The results indicate that when LS and MK are combined at a mass ratio of 1:1 and replaces 20% of cement, the mortar strength is slightly lower than that of blank sample at 3 d, but comparable or slight higher at 7 d and 28 d, respectively. At the replacement level of 30% and LS and MK combined at mass ratios of 1:2 and 2:1, two blend cement mortars give the similar strength at 3 d and 7 d, however, blend cement with a higher MK content (i.e., LS/MK mass ratio of 1:2) develops a comparable strength to the blank at 28 d, which is 6 MPa greater than that with a higher LS content (i.e., LS/MK mass ratio of 1:2). The incorporation of 40% of a combined LS and MK at a mass ratio of 3:1, the compressive strength decreases throughout the testing period, and the strength is only 54%, 55% and 75% of the strength of blank sample at 3, 7 d and 28 d, respectively. The combination of LS and MK can improve the hydration degree of cement and strengthen the reaction between CaCO₃ and Al components to form carboaluminate. The diffraction intensity of carboaluminate increases with the increases of the proportion of LS in blend cement and the curing period. An appropriate proportion of LS and MK can optimize the strengthening effect of carboaluminate formation and the pozzolanic reaction of MK, and promote the mortar strength development at different ages.