Achieving 100% use of recycled concrete in new concrete

The potential for reusing recycled concrete aggregates and their residual powder particles in fresh concrete mixtures is mainly limited by the high porosity and high-water demand of the adhered mortar and the non-reactivity of the recycled powder. Conventional valorisation techniques are designed for a single class of materials; be that coarse aggregate, fine aggregate or recycled powder. This work proposes a two-pronged structural and mineralogical framework for valorisation: 1) a full-fraction recycling pathway using 100% of the generated material streams (coarse aggregate, fine aggregate and powder) from crushed end-of-life concrete and 2) the engineering of high-performance supplementary cementitious materials from recycled concrete powder by sequential chemical-thermal processing.
In the initial phase of the research, a laboratory scale waste concrete was processed using two-pass jaw crushing, combined with a dry, 30-minute rubbing cycle to remove loose attached mortar below a critical 44% threshold without producing industrial wastewater. Subsequently, all size fractions were then enhanced using forced accelerated carbonation (35°C, 20% CO2, 65% RH) to achieve permanent carbon dioxide overused. The carbonation process effectively decreased the porosity of the attached mortar, resulting in a clear linear decrease in aggregate water absorption in relation to the adhered paste thickness. The integration of 100% raw, non-carbonated RCA reduced the concrete mix slump from 14 cm to 8.5 cm, attributable to particle angularity and rapid water absorption. In contrast, employing 100% carbonated coarse- and fine-RCA mitigated the decline in workability, preserving a slump of 12.5 cm. Concrete formulated with 100% raw recycled concrete aggregate attained a 28-day compressive strength of 44.8 MPa, while the carbonated alternatives (cRAC) reached 48.8 MPa. When combined with a 0.2% superplasticiser, the cRAC mixture attained a compressive strength of 50.9 MPa, markedly surpassing the natural aggregate control mixture (43.7 MPa) owing to a greatly densified interfacial transition zone. The sub-150-micron powder was effectively incorporated into the system. Replacing 10% of Portland cement with raw RCP resulted in a 3 MPa reduction in strength; however, substituting cement with 10% carbonated powder (cRCP) mixed with 100% cRCA improved the 28-day compressive strength to 50.46 MPa. This verifies that cRCP operates efficiently as an active binding phase through the synergistic nucleation characteristics of precipitated calcite and the pozzolanic activity of the aluminosilicate gel.
In the subsequent phase of the research, raw RCP (RCPw) was valorised via a sequence of carbonation, thermal activation, and composite treatments. Phase characterisation by 29Si NMR, XRD, FTIR, and TGA demonstrated that the raw powder is mainly chemically inert, functioning mainly as a physical filler, whereas forced carbonation converts the calcium-silicate-hydrate (C-S-H) matrix into an amorphous silica gel and calcite. Thermal treatment at 500°C de-polymerizes siloxane chains, while increasing the temperature to 800°C results in nearly full conversion to reactive monomeric silicate minerals (beta C2S). Importantly, a sequential carbonation followed by heating protocol (RCPch800) establishes an alternative decomposition route, eliminating the amorphous C-S-H phase in advance to prevent the high-temperature generation of non-hydraulic crystalline rankinite, thus maximising pure dicalcium silicate yields; Q0 achieving 100% intensity. RCPw produced a substandard 28-day Strength Activity Index (SAI) of 60.37%, but the RCPch800 powders satisfied ASTM C311 standards with a 28-day SAI of 95.81%. Isothermal calorimetry, however, revealed a "kinetic delaying" passivation effect that restricted the 28-day degree of hydration to around 69%, caused by the substantial early water absorption of highly reactive free lime, which formed a dense portlandite layer over the remaining cement grains. The rapid pre-hydration interaction demonstrated that typical R3 reactivity tests (ASTM C1897) are inappropriate for treated RCPs, as they produce spurious heat release and skew baseline slurry computations. This research connects molecular phase changes with macro-structural performance, offering a scalable engineering foundation for achieving complete circularity in concrete production.