Recycled concrete paving block waste as a selected sustainable substitute for natural aggregate in cement composites

This study examines the impact of replacing natural aggregates with second-quality sorted concrete paving block (CPB) aggregates in mortars and concretes. Substitution levels varied from 0 % to 100 % for fine aggregates in mortars and 50 % for fine and/or coarse aggregates in concretes. The research analysed the properties of the waste material, including particle size distribution, porosity, water absorption, and density. For mortars and concretes, the evaluated parameters encompassed workability, consistency, density, compressive strength, electrical resistivity, and microstructure. The findings indicate that mortars incorporating CPB aggregates necessitate higher dosages of superplasticizers to achieve desired workability. This requirement is attributed to the increased cement content per unit volume and the finer particle size of the recycled aggregates. Early compressive strength of mortars peaks at a 50 % recycled aggregate content due to a reduced effective water-to-cement ratio, while 100 % substitution of fine aggregate leads to a significant decrease in compressive strength. Concretes with recycled aggregates exhibit comparable or superior early compressive strength relative to reference concrete, but after 28 days, all concretes with recycled aggregates display lower strength. The 28-day compressive strengths are 65.9 MPa, 64.8 MPa, and 62.3 MPa for 50 % replacement by fine, coarse, and a combination of both aggregates, respectively. These results surpass those achieved in similar studies. Durability assessments suggest that concrete mixtures with CPB exhibit trends comparable to reference concrete, indicating moderate resistance to chloride penetration. Overall, the findings suggest that utilizing sorted CPB aggregates is a viable approach to replacing natural aggregates in mortars and concretes.