Novel process for valorising excavated clay to produce light weight aggregates by addition of carbonaceous residue, cement and CO₂ curing

This study explores the potential of valorising excavated soft clay as a construction material through the development of novel soft clay aggregates (SCCAs) by the incorporation of a suitable side stream carbonaceous residue from methane pyrolysis. The SCCAs were developed by mixing clay, the carbonaceous residue, and cement at a ratio of 3:1:1 through a cold-bonded aggregate pelletisation process. Furthermore, SCCAs were cured under a 100% CO₂ atmosphere for 8 h to precipitate mineralized CaCO₃. The analytical investigation revealed that the carbonaceous residue constituted fibrous nanotube-like structures that facilitated an increase in CaCO₃ precipitation by 70% with respect to the carbonated aggregates not containing the carbonaceous residue. This calcite precipitation and densification resulted in a point load compressive strength (1.3 MPa) of medium-sized (≥15 mm) SCCAs, equivalent to those of commercially used expanded glass foam aggregates. A proof-of-concept investigation of the cast concrete blocks in which SCCAs replaced 20 vol% of the natural aggregate showed a 7-day compressive strength exceeding 22 MPa, indicating their potential for use in masonry construction in accordance with Eurocode 6. Carbon footprint analysis highlighted that even considering the additional emissions related to the manufacturing steps, concrete manufactured using the aggregate formulation proposed in this study would present a 15% lower footprint, due to the carbon sequestered in the carbonaceous residue. The results of the study hence show the potential application of SCCAs as backfill materials or even for the manufacturing of medium-strength concrete products.