Influence of sulfate adjustment on the hydration kinetics and microstructural properties of LC3 binders produced with low- and high-grade kaolinitic clays

The utilization of kaolinitic clays as a source for producing SCMs may affect the hydration kinetics due to the presence of reactive aluminates, which may modify the hydration of the primary clinker phases. Therefore, the addition of an external sulfate source, such as gypsum, is critical to delay the onset of aluminates reaction. In this study, the effect of sulfate adjustment on LC3 systems containing calcined clays of varying kaolinite purities was investigated. Six different kaolinitic clays were collected from various sources, and from these, two clay samples which contained the lowest and the highest kaolinite purity levels were selected for further analysis. Different gypsum contents (low, medium, and high) were incorporated into the mixtures to evaluate the effect of different sulfation levels on the LC3 systems. A multi-criteria approach, including TGA, MIP, quantitative and qualitative XRD, semi-adiabatic calorimetry, and compressive strength testing, was employed to assess microstructural evolution and pore refinement. Furthermore, a semi-adiabatic calorimetry device was custom-built and calibrated for this purpose to assess the hydration rate and kinetics. The results indicate that the sulfate requirement is heavily dependent on the kaolinite purity of the initial clay. The sulfate optimization is significantly more critical in LC3 systems incorporating high-purity kaolinite clays compared to those with lower purity. The optimal sulfate content in LC3 systems can lead to a reduction in overall pore diameter and a denser microstructure, which further reduces permeability. In addition, under-sulfation can disrupt hydration at early ages, ultimately leading to reduced C-A-S-H gel formation.