Combination of ultrafine granulated blast furnace slag and metakaolin in low portland cement clinker cementitious mixtures

Effective utilization of supplementary cementitious materials to substitute high Portland cement contents in cementitious systems is a promising approach towards decarbonizing the cement and concrete industry. In this study, ultrafine ground granulated blast furnace slag (UFS) was combined with a locally available and medium-grade metakaolin (MK) to prepare low-Portland cement content binders. First, UFS was obtained from ultrafine grinding of a commercial slag, and analyzed for amorphous content, surface chemistry, and reactivity using R³ method. MK and UFS blends at MK/UFS ratios of 80/20, 70/30, and 60/40 were used to replace 50 and 60 wt% of Portland cement in mortars and pastes. The compressive strength of mortars was monitored from 1 to 91 d. The heat evolution of pastes was recorded by isothermal calorimetry, and the microstructure of selected pastes was analyzed through thermogravimetric analysis, quantitative X-ray diffraction, and scanning electron microscopy. The results showed that ultrafine grinding of slag induced changes in its size, surface area, amorphous content, and surface chemistry, leading to higher reactivity of UFS compared to slag. Using MK-UFS blends with MK/UFS of 70/30 and 60/40 in mortars provided comparable 3-d, and higher 28- and 91-d, compressive strengths compared to the mortar made with Portland cement only. Formation of more reaction products, such as calcium–(alumino)–silicate-hydrates (C–(A)–S–H) and carboaluminates, and refinement of the microstructure in the mixtures containing MK-UFS contributed to the enhanced strength development of these mixtures.