Blended cements, where Portland cement clinker is partially replaced by supplementary cementitious materials (SCMs), provide the most feasible route for reducing carbon dioxide emissions associated with concrete production. However, lowering the clinker content can lead to an increasing risk of neutralisation of the concrete pore solution and potential reinforcement corrosion due to carbonation. carbonation of concrete with SCMs differs from carbonation of concrete solely based on Portland cement (PC). This is a consequence of the differences in the hydrate phase assemblage and pore solution chemistry, as well as the pore structure and transport properties, when varying the binder composition, age and curing conditions of the concretes. The carbonation mechanism and kinetics also depend on the saturation degree of the concrete and CO₂ partial pressure which in turn depends on exposure conditions (e.g. relative humidity, volume, and duration of water in contact with the concrete surface and temperature conditions). This in turn influence the microstructural changes identified upon carbonation. This literature review, prepared by members of RILEM technical committee 281-CCC carbonation of concrete with supplementary cementitious materials, working groups 1 and 2, elucidates the effect of numerous SCM characteristics, exposure environments and curing conditions on the carbonation mechanism, kinetics and structural alterations in cementitious systems containing SCMs.

This novel study was triggered by a lack in the international literature of the simultaneous use of ground recycled concrete (GRC) as a cement replacement and mixed recycled aggregate as part of the granular skeleton in recycled concrete. It explores the thermal behaviour of concrete mixes bearing 10 wt% or 25 wt% GRC as a cement replacement and 25 wt% or 50 wt% mixed recycled aggregate (MRA) sourced from construction and demolition waste (CDW). The experimental programme conducted assessed concrete’s dry density, open porosity, electrical and thermal conductivity and specific heat capacity. The findings showed that the use of 10% and 25% GRC, in conjunction with 50% MRA, reduced thermal conductivity by 7.9% to 11.8% and raised specific heat capacity by 6.0% to 9.1% relative to concrete with 100% natural aggregate (NA). A cross-property analysis revealed that improved thermal performance was linearly related to lower density and higher porosity. The results also support the conclusion that these new recycled aggregate concrete mixes are more energy-efficient construction materials than conventional concrete.

Concrete is the most widely used construction material in the world; as such, the best way to promote a more sustainable construction industry is to improve the environmental performance of this material. Since cement production is the main source of the high environmental impact of concrete, due to the high calcination temperature that clinker requires, replacing this binder with recycled cement would allow for the establishment of a new concrete design with a much lower ecological footprint. This research intends to analyse the mechanical performance of mortars with recycled cement and fly ash. Mixes with two replacement ratios of recycled cement (5% and 10%) were studied separately or in combination with fly ash (10% and 20%). An exhaustive experimental programme was designed to assess the variation in air content, density, compressive and flexural strengths, modulus of elasticity, and ultrasonic pulse velocity. The results suggest that the simultaneous use of recycled cement and fly ash improves the mechanical performance of mortars relative to those with recycled cement only or fly ash only.

This research study analysed the effect of adding fine—fMRA (0.25% and 50%)—and coarse—cMRA (0%, 25% and 50%)—mixed recycled aggregate both individually and simultaneously in the development of sustainable recycled concretes that require a lower consumption of natural resources. For this purpose, we first conducted a physical and mechanical characterisation of the new recycled raw materials and then analysed the effect of its addition on fresh and hardened new concretes. The results highlight that the addition of fMRA and/or cMRA does not cause a loss of workability in the new concrete but does increase the amount of entrained air. Regarding compressive strength, we observed that fMRA and/or cMRA cause a maximum increase of +12.4% compared with conventional concrete. Tensile strength increases with the addition of fMRA (between 8.7% and 5.5%) and decreases with the use of either cMRA or fMRA + cMRA (between 4.6% and 7%). The addition of fMRA mitigates the adverse effect that using cMRA has on tensile strength. Regarding watertightness, all designed concretes have a structure that is impermeable to water. Lastly, the results show the feasibility of using these concretes to design elements with a characteristic strength of 25 MPa and that the optimal percentage of fMRA replacement is 25%.

Acceptance by the construction industry of recycled concrete as a sustainable alternative material is contingent upon a reliable assessment of its permeability to corrosive agents. This study analyses the transport mechanisms associated with chloride (Cl⁻), oxygen (O₂) and carbon dioxide (CO₂) ions in concrete with cement made with 10% or 25% ground recycled concrete (GRC) separately or in combination with 50% mixed recycled aggregate (MRA). The findings show that, irrespective of aggregate type, concrete with GRC exhibited lower resistance to ingress than conventional concrete due to its greater porosity. Nonetheless, O₂ permeability was consistently below 4.5 × 10⁻¹⁷ m² and CO₂ penetration, under 4 mm/year^0.5, indicative of concrete with high quality. Resistance to CO₂ and Cl⁻ penetration in the materials with 10% GRC was similar to the values observed in conventional concrete. On the other hand, the incorporation of 25% GRC increased the penetration of CO₂ and Cl^- by 106% and 38%, respectively. Further to those findings in normal carbonation environments, reinforcement passivity would be guaranteed in such recycled materials over a 100 year service life.