Regulation mechanism of HPEG-type polycarboxylate superplasticizer side-chain length on dispersion, hydration, and volume stability of cement-based materials containing clay powder

Clay minerals in manufactured sand can consume polycarboxylate ether superplasticizers (PCEs) through external adsorption and, for expandable minerals, interlayer uptake. This study compares five HPEG-based PCEs with nominal polyoxyethylene (PEO) side-chain polymerization degrees of n = 32, 44, 53, 66, and 78. The HPEG-to-acrylic acid feed molar ratio was maintained at 1:4, while conversion, molecular weight, and dispersity varied among the products. Clay-free cement paste was used only as a flow reference, whereas adsorption, purified-montmorillonite intercalation, hydration response, electrical resistivity, compressive strength, and restrained cracking were evaluated for the cement-clay system. In the clay-free paste, initial flow increased from 295 to 388 mm as n increased. In the 95:5 cement-to-clay powder system, the highest absolute initial and 1 h flows were obtained at n = 66 (319 and 280 mm, respectively), whereas n = 78 retained only 58.0% of the corresponding clay-free initial flow. The operationally estimated clay-associated fraction of adsorbed PCE increased from 27.8% to 61.8%, and the TGA-estimated intercalated fraction in purified montmorillonite increased from 19.1% to 62.6%. The n = 66 PCE produced the highest measured mortar strengths (42.1 MPa at 7 d and 52.6 MPa at 28 d), while n = 53 delayed crack initiation to 115 h and limited the maximum crack width to 0.68 mm. These results indicate property-specific optima arising from a trade-off between steric dispersion and clay-associated PCE uptake. The conclusions are restricted to the tested PCE series, cement, clay source, and 95:5 powder composition.