Drying shrinkage and microstructural evolution of concrete with high-volume and low-grade metakaolin

Low-grade metakaolin (LMK) are often neglected in concrete production due to their low reactivity. High-volume LMK was used to replace part of the cement to produce high-volume LMK concrete (HVMC), and its mechanical properties, volume stability and microstructure were tested. The research results show that the addition of 20% LMK increases the strength of concrete in the long term (60-180 d), and further increasing the content of LMK is not conducive to the improvement of the strength of HVMC. The addition of LMK reduces the drying shrinkage of the HVMC, and the drying shrinkage also decreases as the sample size increases. When 20% of LMK is applied, the 180-d drying shrinkage of HVMC is reduced by 39.6% relative to plain concrete. Using an appropriate content of LMK increases the compactness of the microstructure and improves the performance of ITZ. Meanwhile, an appropriate amount of LMK also has a positive effect on refining the pore structure, but too high a content will increase the volume of large-sized pores (> 10 nm) in the sample. As the content of LMK increases, the content of Ca(OH)2 decreases, and the content of SiO2 increases due to the lower purity of LMK.