HEMC modifications to tricalcium silicate hydration: Changes in kinetics and nanostructure examined by time-resolved high energy X-ray scattering

Hydroxyethyl methyl cellulose ethers (HEMCs) are of increasing interest for their ability to modify rheology and workability of cementitious systems and most importantly with the increase in automation in cement industry. In this paper, the effect of two HEMCs - with similar molecular masses but different degrees of substitution (DS) and molar substitution (MS) - on the hydration of tricalcium silicate (C₃S) was examined by calorimetry, high energy X-ray scattering, and SEM up to 48 hours. The HEMCs decreased the secondary dissolution rate of C₃S and delayed the precipitation of hydration products. The dissolution rate of C₃S in the control sample, with no added HEMC, was ∼20 % and ∼ 13 % higher than that for samples with 0.2 and 0.45 % by mass of added HEMC, regardless of the DS and MS values. Portlandite precipitation was strongly delayed for the HEMC2 with the lower DS. It is proposed that interaction/adsorption of the HEMCs onto the surface of both hydrated particles and anhydrous C₃S is likely the primary source of their action in early hydration. In-situ X-ray pair distribution function (PDF) analysis of HEMC-modified pastes revealed that the atomic ordering in the C-S-H was unchanged when compared to the control sample over the period studied, thus suggesting that the interaction occurs on the surface of the phase. It is proposed that HEMC2 preferentially inhibits the formation/growth of C-S-H, whereas HEMC1 shows almost no impact on C-S-H. Portlandite crystals with differing morphologies occurred in the HEMC-modified and control pastes. HEMC1 inhibits crystal growth along [001] due to their preferential interaction/adsorption on (001) faces. HEMC polymer bridge/film formation was observed between the stacked layers of portlandite and on C-S-H in pastes containing HEMC1.