Mineralization and utilization of CO2 in construction and demolition wastes recycling for building materials: A systematic review of recycled concrete aggregate and recycled hardened cement powder

Mechanistic study on the promotion of Ca2+ leaching in steel slag through high-temperature solid waste modification

Indirect carbonation of steel slag is an effective method for CO2 storage, reducing emissions, and promoting cleaner production in the steel industry. However, challenges remain, such as low Ca2+ leaching rates and slag management complexities arising from variations in mineral compositions. To address this, a high-temperature modification process is proposed to alter the mineral composition and facilitate the synergistic utilization of calcium and iron. This study delves into the effects of various solid waste modifications on the leaching of Ca2+ and the total iron content within steel slag. Results show that high-basicity modified slag forms Ca2(Al, Fe)2O5, reducing calcium leaching. Low-alkalinity modified slag produces calcium-rich aluminum minerals and also reduces the leaching of Ca2+ ions. At a basicity of 2.5, coal gangue, fly ash, and blast slag achieve maximum Ca2+ leaching rates of 88.93%, 89.46%, and 90.17%, respectively, with corresponding total iron contents of 41.46%, 37.72%, and 35.29%. Upgraded coal gangue exhibits a 50.02% increase in calcium leaching and a 15.58% increase in total iron content compared to the original slag. This enhances CO2 fixation and iron resource utilization. Overall, the proposed indirect carbonation and iron enrichment modification offer a novel approach for the resource utilization and environmental stability of steel slag.

A comparative life cycle assessment on recycled concrete aggregates modified by accelerated carbonation treatment and traditional methods

Recycling of construction and demolition wastes contributes to achieve carbon summit and carbon neutrality early in the construction industry. Accelerated carbonation is a promising new technology for enhancing the properties of recycled concrete aggregates (RCAs) as well as mitigating global warming. This study performed a comparative life cycle assessment on RCAs modified by accelerated carbonation treatment and traditional methods. The effect of different treatment methods on environmental impacts of concrete was evaluated. The key contributors of environmental impacts for concrete incorporating carbonated RCAs were identified. Moreover, a sensitivity analysis on the transport distance of concrete incorporating carbonated RCAs was conducted. Results demonstrated that incorporating carbonated RCAs could significantly reduce the energy demand, environmental impacts and environmental cost compared with natural aggregate concrete. Accelerated carbonation treatment exhibited greater potential than the normal two-stage crushing and heating treatment in mitigating environmental burden, especially for the global warming potential. Cement production and transportation were the primary contributors to environmental impacts of concrete incorporating carbonated RCAs. Sensitivity analysis indicated incorporating carbonated RCAs as alternatives of natural aggregates contributes to lower the environmental impacts of concrete when the natural aggregates are far from urban areas while the recycling center is near the city.

Experimental Study on the Application of Recycled Concrete Waste Powder in Alkali-Activated Foamed Concrete

The alkali-activated cementitious material was prepared by partially replacing slag with recycled concrete powder (RCP). The influence of RCP substitution rates (10%, 20%, 30%, 40%, and 50% mass fraction) on the performance of alkali-activated slag-RCP-based (AASR) foamed concrete was studied. The fluidity, water absorption, softening coefficient, compressive strength, flexural strength, drying shrinkage, thermal conductivity, and frost resistance of AASR foamed concrete were studied. The results show that the fluidity and softening coefficient of AASR foamed concrete decreases with the increase in RCP content, and the fluidity range is 230-270 mm. Due to the porous structure of the RCP, the water absorption of AASR increases. With the increase in the curing age, the strength of AASR foamed concrete increases. The addition of RCP reduced the mechanical properties of AASR foamed concrete. Compared with the control group, the compressive strength of AASR50 decreased by 66.7% at 28 days, and the flexural strength decreased by 61.5%. However, the 28 d compressive strength of AASR foamed concrete under all RCP replacement rates still meets the standard value (0.6 MPa). The addition of RCP effectively reduces the thermal conductivity of the AASR foamed concrete, and when the RCP content is 50%, the thermal conductivity is lowest, 0.119 W/(m·K); the drying shrinkage of the AASR foamed concrete can be improved by adding RCP, and the drying shrinkage value is lowest when the RCP is 30%, which is 14.7% lower than that of the control group. The frost resistance of AASR foamed concrete decreases with the increase in the RCP content. When the recycled micropowder content is 20-50% and after 25 freeze-thaw cycles, AASR foamed concrete has reached freeze-thaw damage.

Determinants of contractor's construction and demolition waste recycling intention in China: Integrating theory of planned behavior and norm activation model

The abundant generation of construction and demolition waste (CDW) poses a threat to the sustainable development in China and recycling plays a vital role in complying with circular economy zero-waste goal. In this study, we first investigate the determinants of contractor's intention to recycle CDW by establishing an integrative model of the theory of planned behavior (TPB) and the norm activation model (NAM) with rational and moral considerations. Based on the questionnaire data of 210 valid responses, structural equation modeling is employed to test the proposed hypotheses and analyze the integrative structural model. The results show that the integrative model with adequate reliability and validity fits the empirical data well and the explanation power is superior to that of initial TPB model and NAM model, demonstrating the appropriateness of merging TPB and NAM in the area of CDW recycling research. Moreover, it is found that personal norms are the most critical determinant enhancing the CDW recycling intention, followed by perceived behavioral control. Although subjective norms fail to directly affect the CDW recycling intention, they can significantly strengthen personal norms and perceived behavioral control. These findings provide useful insights for government to develop effective management strategies to motivate CDW recycling intention of contractors.

Influence of Mechanical Screened Recycled Coarse Aggregates on Properties of Self-Compacting Concrete

The use of recycled coarse aggregates (RA) in concrete is a sustainable alternative to non-renewable natural aggregate (NA) to fabricate concrete products using in concrete structures. However, the adhered mortar on the surface of RA would considerably impact the qualities of concrete products. As a practical treatment procedure, mechanical screening can remove the adhered mortar. This research aims to study the influence of mechanical screening on the fundamental properties of RA and the resulting self-compacting concrete (SCC). The RA were mechanically screened up to four times, and their physical properties including particle size distribution, water absorption, and crushing value were investigated. The properties of RA-SCC including workability, density, compressive and tensile strengths, modulus of elasticity, and microstructure were also examined. The results demonstrated that screening reduced the water absorption of RA from 6.26% to 5.33% and consequently enhanced the workability of RA-SCC. Furthermore, it was shown that increasing the screening up to twice improved the mechanical properties of concrete. In particular, screening increased the compressive strength of concrete by 15-35% compared to the concrete with unscreened RA. Similar improvements were found in tensile strength as well as the elastic modulus results. The microstructure of screened RA-SCC was comparable to that of the control concrete, showing minimal porosity and cracks along the interfacial transition zone. In conclusion, once or twice screening is recommended to the recycling facility plant to remove adequate amount of adhered mortar and fines while preventing damages to the RA. Improving the quality of RA via mechanical screening is one of the promising approaches to increase their potential for use in concrete, thereby reducing extraction of natural resources and promoting a circular economy.