Influence of Calcium Silicate Hydrate Seeds Modified by Polycarboxylate Superplasticizer on Cement Hydration Process

To study the promotion process of cement hydration by polycarboxylate superplasticizers modified calcium silicate hydrate (C–S–H/PCE) and understand the mechanism of C–S–H/PCE seeds promotion of cement hydration. Acrylic acid-based polycarboxylate superplasticizer (PCE-1) and silane-based polycarboxylate superplasticizer (PCE-2) were used to surface modify the hydrothermal synthesized calcium silicate hydrate (C–S–H/Unmodified) seeds, resulting in the polycarboxylate superplasticizer modified C–S–H seeds (C–S–H/PCE-1 and C–S–H/PCE-2). Subsequently, these three C–S–H seeds were added to the cement paste. The compressive strength, phase composition, and heat release curve of cement paste and the surface tension of C–S–H seeds were tested. The results showed that the surface modification of PCE-1 and PCE-2 resulted in an increase in the surface contact angle of C–S–H seeds from 36.9° to 41.1° and 42.9°, resulting in lattice mismatch of the sedimentary phase (hydration product) on the surface of the substrate material (C–S–H seeds) and inhibiting the epitaxial growth of cement hydration products on the surface of C–S–H seeds. PCE modification reduced the promoting effect of C–S–H seeds on the hydration process of Portland cement. Moreover, the degree of reduction in the hydration promoting effect of C–S–H/PCE-2 on Portland cement is significantly greater than that of C–S–H/PCE-1 crystal nucleus. Tests of the adsorption equilibrium of PCE in the pore solution indicated that the C–S–H/PCE seeds did not directly introduce new crystal nucleation sites but exposed new C–S–H surfaces through the desorption of PCE on the surface of C–S–H/PCE seeds. The desorption degree of PCE on the surface of C–S–H/PCE seeds determined the promoting effect of C–S–H/PCE seeds on cement hydration.