Fly ash, waste plastic/rubber, and natural fibre-reinforced cement composite bricks: State-of-the-art review and experimental investigation of mechanical, thermal, and acoustic performance

Fired-clay brick is a major source of greenhouse gases and topsoil loss, while conventional cement-sand bricks offer poor insulating properties for both thermal and acoustic applications. In addition to a literature review on the recent advancements in sustainable cement-based composites, with special emphasis on cementitious composites using waste polymers, natural fibres, and fly ash, improvement in durability and life cycle analysis, this paper focuses on an investigation into the production and performance of environmentally friendly cement-based composite bricks. The prepared composites consist of the use of fly ash as a partial substitution of cement, waste plastic or rubber crumb as a partial substitution of fine aggregate, and natural fibres such as coir, jute or sisal as reinforcing material. Five different compositions of bricks (M0-M4) have been considered, out of which composition M4 contains 20 wt% of fly ash, 20 wt% of plastic/rubber crumb and 1.5 wt% of sisal fibre. Mechanical, physical, thermal, and acoustic properties were evaluated and compared with fired-clay and plain cement–sand bricks. The optimised composite achieved a compressive strength of 31.0 MPa and flexural strength of 6.1 MPa, substantially exceeding conventional bricks. Thermal conductivity was reduced to 0.39 W/(m·K), while the sound absorption coefficient reached 0.58 at 1 kHz, indicating superior insulation performance. The brick also exhibited lower density (1620 kg/m³) and reduced water absorption (8.9%). Furthermore, the formulation is estimated to lower embodied carbon by about 80% and material cost by 36% relative to fired-clay bricks. The results demonstrate a viable low-carbon masonry unit with enhanced mechanical, thermal, and acoustic performance.