Quantifying the impact of montmorillonite on water demand and polycarboxylate superplasticizer efficiency in cement pastes

The presence of montmorillonite (MMT) as an impurity in aggregates and limestone diminishes the plasticization efficacy of polycarboxylate ether (PCE) superplasticizers in fresh cement pastes. Despite extensive research efforts to elucidate the mechanisms behind the reduced efficacy of PCE in cementitious systems with MMT and to design tailored PCE molecules with enhanced MMT tolerance, quantitative insights into PCE behavior, specifically surface adsorption and intercalation, within cement pastes containing MMT remain ambiguous. In this work, a delayed addition method was employed to investigate how two PCEs with different side-chain lengths (P-1000 and P-3000) influence the flowability of cement-MMT pastes through quantification of their adsorption and intercalation behavior. The results indicate that approximately 1 g of MMT necessitates an additional 3 g of water to achieve the equivalent fluidity as the plain mixture without MMT. The maximum adsorption of PCE on MMT in cement-MMT pastes was approximately 35 mg/g, below the threshold (∼40 mg/g) required for intercalation. This demonstrates that the reduction in fluidity primarily arises from the extensive surface adsorption driven by the high specific surface area of MMT, which decreases the availability of PCE for effective dispersion of cement particles.