Evaluating the effect of slag variability on the properties of slag-based cement grouts

Slag cements are used to design grouts with high water-to-cement ratio and low permeability, to improve the physical and mechanical properties of permeable soils. Because of the variable physico-chemical properties of slag, it can be challenging to predict their behavior. Most research on slag reactivity involves low water-to-binder ratios in concrete and strength. This study investigates grouts with higher water-to-binder ratio of 6, for water resistant barriers executed using soilmixing or self-hardening slurry cut-off wall techniques, where the main engineering property is the hydraulic conductivity. The effect of slag composition on grout properties was studied at two water-to-binder ratios (0.5 and 6) and varying slag proportions, focusing on compressive strength and hydraulic conductivity. The results showed that higher water-to-binder ratios influence hydration, phase formation, and pore distribution, preventing the formation of portlandite. Isothermal calorimetry revealed that the silicate peak in sulfate-rich slag-based grout, linked to C-S-H and Ca(OH)₂ formation, overlapped with the aluminate peak and appeared earlier, which was not observed at a water-to-binder ratio of 0.5. Higher water-to-binder ratio promoted the formation of ettringite due to high sulfate content, affecting compressive strength and pore distribution. Coarser pores in sulfate-rich slag-based grout led to low compressive strength and high permeability. In addition, results showed that the reactivity of slag could not be determined solely by its basicity index. The performance of slag-based grouts depends on sulfate content and short-term slag reactivity. However, the evaluation of long-term slag hydration is necessary to understand the potential influence of latent slag hydration on engineering properties and performance.