Surface Roughness and Contact Angle of Calcium Sulfoaluminate Cement Mortar for Optimal Hydrophobic-Agent Content

In this study, polydimethylsiloxane (PDMS) and tetraethoxysilane, commonly used in water-repellent coatings, were directly incorporated into calcium sulfoaluminate (CSA) cement mortar to create a cement composite with water-repellent properties. The mixing ratio of the water-repellent solution was prepared by varying the content of PDMS from 25% to 90%, and the water-repellent ability was assessed by incorporating 0.5% of the cement weight into the mortar. The degree of degradation of water-repellent performance after surface abrasion was also evaluated in actual concrete applications. The contact angle of the conventional test material was 40.9° but the contact angle of the mortar with a water-repellent solution containing 70–90% PDMS was more than 140°, indicating that the solution’s effectiveness was sufficient. Although the contact angle decreased slightly after abrasion, the contact angle of the 70–90% PDMS was more than 120°, while the other variables remained unchanged or increased slightly. The surface roughness was measured using a three-dimensional laser microscope and compared with the water droplet contact angle experimental results. It is shown that the influence of the distribution of microstructural irregularities and surface energy control on surface hydrophobicity is stronger than that of the maximum or average height of the surface roughness.