Mechanical and Hygrothermal Evaluation of Eco-Friendly Cement Mortar Incorporating Upcycled Low-Density Polyethylene

This study examined the mechanical and hygrothermal response of cement mortars incorporating recycled low-density polyethylene (LDPE) as a partial replacement of fine aggregate at levels of 2.5%, 5.0%, and 7.5% by weight of sand. A reference mixture without polymer was also prepared. The experimental program included determining density, workability, and compressive strength at 7 and 28 days. Based on the mechanical results, the formulation containing 2.5% LDPE was selected for further hygrothermal assessment in 1:5 scale chambers coated with conventional and modified mortars. LDPE incorporation progressively decreased density and generally reduced compressive strength relative to the control mixture, although a non-monotonic 28-day response was observed between the 2.5% and 5.0% LDPE mixtures. At 28 days, the reference mortar reached 5.243 MPa, whereas the mixture with 2.5% LDPE attained 1.461 MPa. Hygrothermal monitoring showed no substantial improvement in indoor temperature regulation; however, the LDPE-modified coating maintained higher indoor relative humidity than the conventional system during the test period. These results indicate that recycled LDPE can be incorporated into cement-based mortars at low replacement levels for exploratory non-structural applications. However, the resulting decrease in mechanical strength restricts its wider applicability and highlights the necessity for further verification to satisfy performance standards tailored to specific applications.