Experimental Study on Cement Brick Using Low-Density Polyethylene (LDPE) Plastics and Biochar

Concrete bricks are fundamental building blocks in global construction, but their production relies heavily on cement and sand, materials associated with substantial carbon emissions and environmental degradation. The cement industry alone contributes approximately 8% of global anthropogenic CO? emissions. Furthermore, the extensive extraction of natural sand leads to ecological imbalance and resource scarcity. In parallel, the accumulation of plastic waste, particularly Low-Density Polyethylene (LDPE) from packaging materials, presents a severe environmental threat due to its non-biodegradable nature. Similarly, agricultural waste often ends up in landfills or is burned openly, contributing to air pollution. Integrating these waste streams into construction materials offers a promising dual solution: diverting waste from landfills and reducing the consumption of virgin resources. Biochar, a carbon-rich solid produced from the pyrolysis of biomass, can act as a supplementary cementitious material and a carbon sink. LDPE plastic, when shredded, can replace a portion of fine aggregates, introducing properties like reduced density and enhanced toughness. This study explores the synergistic use of these two waste materials in cement brick production, evaluating their impact on the brick's key engineering properties to determine optimal mix proportions for sustainable construction.

LITERATURE REVIEW

The pursuit of sustainable alternatives in construction has led to significant research on incorporating industrial and domestic waste into concrete and masonry products. Rahman et al. (2020) investigated the use of LDPE as a sand replacement (5–15%), finding that 5–10% replacement improved toughness and impact resistance, though compressive strength decreased beyond 10% due to weak bonding with the cement matrix.  Frigione (2010) emphasized that LDPE inclusion reduces concrete's unit weight and enhances its thermal insulation capacity.  Ismail & Al-Hashmi (2008) highlighted enhanced ductility but also noted increased porosity with higher plastic content. On the use of biochar, Gupta et al. (2021) demonstrated that replacing cement with 5–10% biochar improved compressive strength and water retention due to its fine particle size and internal curing ability.  Tan et al. (2018) reported improved thermal insulation and moisture regulation in biochar-modified concrete.  Lehmann & Joseph (2015) detailed the carbon sequestration potential of biochar, which can significantly lower the embodied carbon of construction materials. The combined use of LDPE and biochar is a novel approach.  Ahmed et al. (2019) proposed that blending plastic and biochar could yield composites with improved thermal properties and durability.  Ramesh & Kumar (2021) produced eco-bricks using 10% biochar and 5% LDPE, reporting compressive strengths close to traditional clay bricks and suggesting a complementary relationship where biochar mitigates the weaknesses introduced by LDPE. This study builds upon these findings by systematically evaluating a range of replacement values for both LDPE and biochar in cement bricks, providing a comprehensive analysis of their combined effect on structural and durability properties.

Relevant conflicts of interest/financial disclosures: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

MATERIALS AND METHODS

MATERIALS

The materials used in this experimental investigation were sourced locally to emphasize practicality and are listed in Table 1.

Table 1: Materials and their specifications.

The success of integrating waste materials into construction elements hinges on the careful selection and preparation of constituents. The following materials were meticulously sourced and processed for this study:

1. Cement: Ordinary Portland Cement (OPC) of 43-grade, conforming to IS 8112:1989, was used as the primary binding agent for its consistent quality and widespread availability.
2. Fine Aggregate: Naturally available river sand, confirming to Grading Zone-II as per IS 383:1970, was used. The sand was air-dried to remove surface moisture, which could alter the water-cement ratio, and then sieved to remove any oversized particles or organic matter.
3. LDPE Plastic Waste: Post-consumer LDPE waste, primarily from packaging films and bags, was collected from college and institutional campuses. The collected plastic was thoroughly cleaned with water to remove dirt and adhesives, air-dried, and then mechanically shredded into small, irregular flakes ranging from 3–10 mm in size to facilitate mixing and interlocking within the cement matrix.
4. Biochar: Biochar was produced locally using a controlled pyrolysis process. A mixture of common agricultural residues—sugarcane bagasse, mixed vegetable waste, and nutshells—was carbonized in a low-oxygen environment at temperatures between 300–600°C. The resulting biochar was then crushed and sieved through a 2.36 mm IS sieve to achieve a fine, consistent particle size comparable to cement, ensuring it could act as a effective filler and partial cement replacement.
5. Water: Potable water, free from impurities, acids, and alkalis that could interfere with the hydration process of cement, was used throughout the mixing process.

• Biochar Preparation