Abstract
Temperature-induced instability in early-age rheology poses a major challenge to the pumpability and application of robotic plastering mortars. This study systematically investigates the temperature-dependent effects of a high-viscosity (75,000 mPa·s) hydroxyethyl methyl cellulose (HEMC) on the rheological properties and early microstructural evolution of mortars at 5 °C, 20 °C, and 40 °C. Mortars with HEMC dosages from 0 to 0.25 wt% were tested using rheological measurements, ultrasonic pulse velocity (UPV), and complementary microstructural analyses (XRD, FTIR, and SEM-EDS). Results show that HEMC reduced the initial static yield stress while monotonically increasing plastic viscosity, with the thickening effect more pronounced at higher temperatures. Notably, at 40 °C, the initial plastic viscosity of a 0.25% HEMC mix reached 14.4 Pa·s, a 133% increase compared to the control group. HEMC also effectively retarded the time-dependent increase in yield stress and stabilized plastic viscosity, thereby mitigating the adverse influence of elevated temperature. UPV confirmed that HEMC delayed microstructural formation, in agreement with the observed retardation of hydration reactions. At 40 °C, a 0.10% HEMC dosage postponed the percolation threshold from 67 min to 150 min, highlighting its strong retardation effect. Microstructural tests further revealed that HEMC delayed CH formation, refined C-S-H gels, and reduced the crystallinity of AFt, supporting the rheological and ultrasonic findings. A statistically significant, moderate-to-strong correlation (r = 0.88, R(2) = 0.77, p < 0.001) was established between static yield stress and UPV, indicating that macroscopic rheological resistance responds to microstructural evolution. Based on these results, the recommended HEMC dosages to achieve stable rheological performance are 0.05-0.10% at 5 °C, 0.10-0.15% at 20 °C, and 0.15-0.20% at 40 °C.