Abstract The use of industrial by-products, wastes and residues in the production of building materials as a practice to enhance constructions’ sustainability (provided that the products are harmless and effective) is constantly gaining ground in the light of the urgent need to minimize the environmental impact of the construction industry. The focus of this work lies on the experimental investigation of hygrothermal properties of mortars that contain perlite by-products. Two types of fine perlite by-products are included in the study. Three lime-based mortars with a high content of by-products and providing satisfactory additional properties are examined. The compositions of these mortars differ regarding various aspects (by-product used, proportions of constituents, etc.), as they were designed to be used as bedding mortars and renders. The examined hygrothermal properties are the thermal conductivity and resistance to water vapor diffusion of the mortars. Additionally, the porosity and specific gravity of these materials are determined. The physical properties, as well as the thermal conductivity coefficient are defined at two different time points (28 and 90 days of hardening). The experimental procedures applied, and the derived results are systematically presented and discussed. The discussion focuses both on the comparative evaluation of the behavior of the three mortars, as well as on its variation over time – where applicable. The results indicate that the behavior of the studied materials with respect to the examined properties presents some variations, which seem to be more pronounced when different perlite by-products are used. Specifically, the composition containing the finer and more reactive perlite by-product is characterized by the lowest thermal conductivity and the highest resistance to water vapor diffusion. Furthermore, it is noted that the thermal conductivity of all the examined mortars decreases, by varying percentages, over time (from 28 to 90 days of hardening).

Highlights • Hygrothermal properties of 3 mortars containing perlite by-products are examined. • Thermal conductivity coefficients (λ) and water vapor diffusion resistance factors (μ) are derived. • The perlite by-product’s type (fineness, composition) affects the mortars’ λ and μ values. • The three examined mortars’ λ values decrease over time (% decrease varies among them). • The mortars’ μ values, although lying close, present differentiations of varying height.