Rheology of calcined illitic and smectitic clays in cement-free aqueous solutions with a focus on ion adsorption and superplasticizer interaction

Calcined clays (CCs) are a promising supplementary cementitious material to advance decarbonization of cement, even though they are a challenge in rheology. The study focuses on the interaction of CCs, pore solution ions, and superplasticizer to improve rheology with CC. This was done with four illitic and smectitic dominated CCs in cement-free aqueous solutions. Rheometry, infrared spectroscopy, electroacoustic measurements, X-ray diffraction, and water absorption capacity measurements were conducted to investigate superplasticizer intercalation and adsorption, hydration, surface charge, ion adsorption, and rheology. Yield stress increased in synthetic cement pore solution in comparison to deionized water. This was not attributed to hydration products or a change in pH value, but explained by Derjaguin–Landau–Verwey–Overbeek theory (DLVO) due to changed zeta potential and ion concentration. CCs can adsorb anionic superplasticizers onto their negatively charged surface. With calcium addition, their surface charge polarity was reversed, and superplasticizer adsorption increased drastically. Superplasticizer demand seemed independent of sulfate addition, which is contradictory with cement systems. Superplasticizer demand was twice as high for illitic CCs compared to smectitic CCs. Intercalation of superplasticizers and swelling of calcined smectite can be excluded. The results confirmed BET specific surface area as a generally good indicator for superplasticizer demand of CCs, but it is not sufficient for differentiation within similar mineralogy. These findings help to further establish CCs further as an eco-friendly but also workable supplementary cementitious material. • Calcined smectites: no swelling and no intercalation of superplasticizers. • Calcined clays can adsorb anionic superplasticizers on negative charged surface. • Calcium adsorption on calcined clay surfaces increases superplasticizer adsorption. • Yield stress of calcined clay in different solutions is dependent on zeta potential. • Mineralogy and BET surface area are key parameters for superplasticizer adsorption.