Effects of kaolinite and montmorillonite calcined clays on the sulfate balance, early hydration and artificial pore solution of limestone calcined clay cements

Abstract

Combining clinker with limestone and calcined clay has been one of the main strategies to reduce CO2 emissions in the cement production industry. Calcium sulfate is mainly used to control setting times. Compared to ordinary Portland cement, these blended cements exhibit accelerated sulfate depletion during hydration. This is highly influenced by the calcined clays' physical and chemical effects and methods for determining the appropriate sulfate content require further investigation. There is no unique or best method to ensure sulfate balancing, nor what effects different clay minerals (kaolinite vs. montmorillonite) have on this adjustment. Thus, this study aimed to assess methods for the optimum sulfate content determination, in terms of SO3total, of ternary cements composed of both calcined clays. In addition, their physical and chemical effects on the sulfate balance, early-age hydration, and artificial pore solution chemistry were evaluated. The impacts of the clays were compared with an inert material (quartz powder) in a reference cement. The experimental program was divided into two phases. In phase 1, sulfate optimization was carried out using isothermal calorimetry, compressive strength, thermogravimetric (TG) analysis, and chemical shrinkage (CS) analysis at different ages. In phase 2, the impact of fineness, dissolution, and possible adsorption capacity of the calcined clay minerals were evaluated by isothermal calorimetry, ICP-OES and zeta potential, respectively. Phase 1 demonstrated that there is no single best technique to predict sulfate optimization, especially for LC³s, but combining several techniques can assist in more coherent decision-making. A careful combination of calorimetry results with compressive strength can be useful at early ages. TG results do not relate well to the other techniques but are useful for comparative reactivity between cements. As well as CS, since it is not sensitive to detect changes between total SO3 contents but indicates the reactivity of cement pastes as a function of reduction of volume paste. Also, due to chemical and physical effects, the use of kaolinite clay should be more careful, as there was the highest increase in sulfate demand in all analyses. Phase 2 emphasized the physical and chemical effects of calcined clays in LC³ and alkaline solutions on the sulfate balance. The results suggested that during hydration, sulfate depletion kinetics tend to be similar between kaolinite and montmorillonite clays with fineness, but, due to the availability of Al ions for dissolution, the sulfate demand was higher for the cements with kaolinite clay. On the other hand, the physicochemical characteristics of calcined montmorillonite clays (such as availability of Si and Al ions, zeta potential) had little impact on sulfate demand even with increasing fineness.