Recent work demonstrated 50:50 sand-recycled polycarbonate (rPC) composites have an average compressive strength of 71MPa, which dramatically exceeds the average offered by commercial concrete (23.3-30.2MPa). Due to the promising technical viability of replacing carbon-intensive concrete with recycled sand plastic composites, this study analyzes the cradle-to-gate environmental impacts with a life cycle assessment (LCA). 50:50 sand-to-plastic composites at different sample sizes were fabricated and the electricity consumption monitored. Cumulative energy demand and IPCC global warming potential 100a were evaluated to quantify energy consumption and greenhouse gas emission associated with sand-plastic brick and two types of concrete, spanning the lifecycle from raw materials extraction to use phase. The results showed that at small sizes using Ontario grid electricity, the composites are more carbon intensive than concrete, but as samples increase to standard brick scale rPC composite bricks demonstrate significantly lower environmental impact, emitting 96% less CO₂/cm³ than sand-virgin PC(vPC) composite, 45% less than ordinary concrete, and 54% less than frost-resistant concrete. Energy sourcing has a significant influence on emissions. Sand-rPC composite achieves a 67%-98% lower carbon footprint compared to sand-vPC composite and a 3%-98% reduction compared to both types of concrete in different production rate. Recycling global polycarbonate production for use in sand-rPC composite although small compared to the total market could annually displace approximately 26 Mt of concrete, saving of 4.5-5.4 Mt of CO₂ emissions. The results showed twin problems of carbon emissions form concrete and poor plastic recycling could be partially solved with sand-rPC building material composites to replace concrete.