Hydroxypropyl methylcellulose (HPMC) is typically restricted to low dosages (≤0.3%) to maintain workability and bulk compressive strength. This study investigates the functional trade-offs and microstructural mechanisms of high HPMC dosages (0.5%, 0.75%, or 1.0% by weight of cement) to develop energy-efficient, high-adhesion mortar matrices. The cement mortar incorporates a 1:3 cement-to-sand ratio by weight and a water-to-cement ratio of 0.5. Experimental evaluations demonstrate that high HPMC dosages lower bulk density and compressive strength (reduction between 17% and 30%); however, the use of high HPMC dosages significantly enhances interfacial and thermal performance. Notably, incorporating 0.75% HPMC yields a 64% reduction in thermal conductivity to 0.79 W/m.°C versus 2.18 W/m.°C for control mortar, while 1.0% HPMC increases pull-off strength by 176% (2.15 MPa) and shear strength by 167% (0.4 MPa). Additionally, the use of HPMC caused an increase in the initial setting time and the formation of a thick layer on the surface; the absorption of cement mortar increased with an increasing dosage of HPMC, with a 71% increase in the absorption for cement mortar with 1% HPMC compared to the control mortar. Scanning Electron Microscopy (SEM) observations revealed progressive changes in the mortar matrix morphology with increasing HPMC dosage, including changes in the appearance and distribution of pores. These observations are qualitatively consistent with the measured changes in density, water absorption, mechanical properties, and thermal conductivity. These findings confirm that high-dosage HPMC mortars, especially with 0.75%, provide an optimal balance for energy-efficient exterior renders and high-adhesion repair coatings where thermal resistance and bonding performance take priority over bulk compressive capacity.
