Experimental investigation on physical and mechanical properties of excavated soil- and fine recycled concrete aggregate-based unfired clay bricks containing compound additives

The excavated soil waste and the demolished concrete can be used as the source of natural aggregates’ replacement in concrete. With this purpose, this paper presents two orthogonal experiments (i.e., compression and flexure) to examine the feasibility of jointly using excavated soil and fine recycled concrete aggregate (FRCA) to manufacture unfired clay bricks (UCBs). The water-to-cement ratio (w/c), the cement-to-excavated soil ratio (c/s), the FRCA-to-excavated soil ratio (a/s), and the compound additive content (AC) were selected as the variables in the mix proportion. The test results show that excavated soil & FRCA-based UCBs exhibit the favourable water absorption, bulk density, compressive strength and flexural strength. From the perspective of physical and mechanical properties, the optimal mix proportion is the case of w/c= 0.65, a/s = 0.5, c/s = 0.3, AC= 5%. A grey relational analysis was performed to evaluate the parametric sensitivity of physical properties to four factors. The result revealed that the water absorption is the most sensitive to a/s, and the bulk density is the most sensitive to c/s; in contrast, both of them have the least sensitivity to AC. The prediction models of compressive strength and flexural strength were developed to gain the confidence of recycling excavated soil and FRCA in UCBs. A Scanning Electron Microscope (SEM) was carried out to investigate the mechanism of strength increase of excavated soil & FRCA-based UCBs with different mix proportions, and the result demonstrated that the primary reason of the strength increase is owning to the formation of more hydration products and the tighter internal structure.