Waste Meets Bio-Based Resources: Sustainable Cement Blocks with Recycled Aggregates and Hemp Residues

The construction sector is increasingly embracing circular economy principles through the use of recycled and bio-based materials in building products. In this context, this study investigates the development and experimental characterization of sustainable cement-based blocks produced with recycled construction and demolition waste aggregates and hemp agricultural by-products.

Four different mixtures were designed and tested: a reference mixture with natural coarse aggregates (NCAs), a mixture with 100% recycled coarse aggregates (RCAs), and two innovative mixtures incorporating untreated hemp fibers and hurds as partial volumetric replacement of recycled aggregates, with hemp contents of 4% and 8% (RCA_4%H and RCA_8%H, respectively). Hemp materials were sourced from local Italian cultivations and used in their untreated state after being cut to a length of 2–4 cm. Recycled aggregates were obtained from mixed construction and demolition waste.

To evaluate the mechanical performance of the proposed materials, compressive tests were carried out on cubic specimens, while indirect tensile tests were performed on cylindrical specimens. In addition, the density of each mixture was determined.

The experimental results showed that replacing natural aggregates with recycled aggregates reduced compressive strength from 19.5 MPa to 13.7 MPa while decreasing density by approximately 14%. The incorporation of hemp by-products led to a further reduction in density while maintaining compressive strengths comparable to those of the recycled aggregate mixture. Conversely, hemp addition improved the tensile performance, increasing the indirect tensile strength from 1.7 MPa for the RCA mixture to 2.0 MPa and 2.1 MPa for the mixtures containing 4% and 8% hemp, respectively.

The findings highlight the potential of combining recycled aggregates and hemp agricultural residues to produce lightweight and environmentally sustainable cement-based blocks, promoting waste valorization and resource efficiency while maintaining satisfactory mechanical performance for non-structural building applications.