Influence of HPMC chain length on rheological properties of cement paste

Four HPMC samples are synthesized with molecular weight ranging from 35 to 450 kDa. • Long-chain HPMC causes τ s to peak then drop, while τ d rises monotonically. • Longer-chain HPMC enhances flocculation but deteriorates floc structure. • τ s is governed by competing polymer-bridging and steric-hindrance effects. • τ d increases with HPMC dosage and chain length via stronger polymer interactions. Hydroxypropyl methylcellulose (HPMC), one of the most commonly used viscosity-modifying admixtures (VMAs), is extensively employed in fresh cement-based materials. Indeed, the chain length of HPMC greatly influences the rheological behavior of cement pastes. Here, four HPMC samples with distinct molecular weights ( M w ) from 35 to 450 kDa were chemically synthesized to systematically elucidate how chain length governs the rheology of cement pastes and to reveal the underlying mechanisms. Results show that HPMC with shorter chains (H1, H2, and H3 with M w  < 220 kDa) induces a monotonic increase in both τ s and τ d with increasing dosage. In contrast, long-chain HPMC (H4 with M w  = 450 kDa) causes τ s to rise from 29 to 114 Pa and then decrease beyond 0.3%, reaching 59 Pa at 0.5%, while τ d continues to increase from 45 to 148 Pa. This divergence in τ s suggests a dual role of adsorbed HPMC: longer chains (H4) may simultaneously intensify interparticle attraction through polymer bridging and enlarge particle separation via steric hindrance, the latter eventually overriding the former at higher dosages. The continuous increase in τ d is attributed to the strengthened polymer-polymer interactions with increasing chain length and concentration, promoting the formation of a more rigid polymer network in the interstitial solution that resists applied shear stress. Overall, this work establishes a systematic mechanistic framework describing how HPMC chain length modulates the rheology of cement paste.