Use of recycled plastic bottle caps (RPBC) as partial sand replacement in cement mortars: Mechanical and CO₂ emissions assessment

Abstract

This study evaluates the use of recycled plastic bottle caps (RPBC), composed of high-density polyethylene (HDPE), as a partial replacement for natural sand in cement mortars. Mixes with varying RPBC contents (0 %, 10 %, 25 %, 50 %, 75 %, and 100 %) were developed to analyze their mechanical, microstructural, and environmental behavior. Tests included compressive strength, splitting tensile strength, and ultrasonic pulse velocity (UPV) at 7, 28, and 56 days, along with scanning electron microscopy (SEM) analysis. Additionally, CO₂ emissions associated with each mix were estimated, considering the material content and energy consumption during RPBC crushing, under two emission scenarios for sand. Results showed that incorporating up to 25 % RPBC does not significantly affect the mechanical properties of the mortar. However, a considerable decrease in mechanical performance was observed at replacement levels above 50 %, attributed to the low adhesion between recycled HDPE and the cementitious matrix, as well as its hydrophobic nature, which leads to increased porosity and a higher effective water-to-cement ratio. From an environmental standpoint, a progressive reduction in CO₂ emissions was observed with increasing RPBC content. Nevertheless, the eco-efficiency indicator (kg CO₂/(m³·MPa)) revealed that structural/environmental efficiency decreases for replacements above 50 %. The use of RPBC is technically and environmentally viable up to 25 %, offering a sustainable option for revalorizing plastic waste in construction.