Capillary water absorption is an important parameter to evaluate the durability of cement mortars. For testing the capillary water absorption of cement mortars, drying is a necessary and crucial procedure. However, different standards have different requests on drying temperatures for doing the tests. Since the calcium aluminate cement-hemihydrate gypsum (CAC-C \( {\bar{\text{S}}} \) H0.5) mortars are temperature-sensitive, three temperatures of 20 °C, 40 °C and 70 °C were selected to study the effect of hydroxypropyl methyl cellulose (HPMC) on the capillary water absorption of these mortars. The influence of temperature on the capillary water absorption of the mortars was analyzed from the perspectives of pore structure and hydration. The results show that when the viscosity of HPMC is the same, the higher the dosage of HPMC used, the higher the capillary water absorption of mortars. When the dosage of HPMC is the same, the higher the viscosity of HPMC, the greater the capillary water absorption of mortars. The addition of HPMC can significantly increase the total porosity of mortars, especially the volume of the large pores. However, pore structure is not the main factor influencing the capillary water absorption of HPMC-modified CAC-C \( {\bar{\text{S}}}\) H0.5 mortar. When the drying temperature rises to 40 °C, a small amount of AFt is decomposed. When the drying temperature reaches 70 °C, most of AFt is carbonized to form hemicarboaluminate and AH3, and the latter is the key to refining the pore structure of mortars. The temperature change causes a phase transition of the hydration products, thereby altering the pore structure. 40 °C is suggested as an appropriate drying temperature for the capillary water absorption test of the investigated mortars.
