The aim of the research presented in the paper was to evaluate the feasibility of using hydrophobising agents based on organosilicon compounds for surface protection on the lightweight mortars with pumice aggregate. The work discusses the issues related to absorptivity and wettability of the surface layer of mortars. The experimental part pertains to the physical and mechanical properties of pumice-modified mortars and the influence of hydrophobising agents on the contact angle and surface free energy depending on the humidity of the substrate. The contact angle of lightweight mortars (θ_w) was determined in the function of moisture using a single measurement liquid. This provided a basis for calculating the surface free energy with Neumann method, characterizing the wettability and adhesion of mortars. The structure of polisiloxane film was presented by means of scanning electron microscopy. The studies enabled to determine the hydrophobisation efficiency of heat-insulating mortars with pumice aggregate at various degrees of humidity of the substrate: 0%; 1%; 3%; 6%. The obtained results confirmed the possibility of producing pumice-modified mortars along with proper surface protection against moisture. The acceptable level of moisture content at which the hydrophobisation is still effective was defined as well.
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