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Due to the depletion of river sand, the construction industry is eager to develop upcycling techniques for transforming secondary by-products derived from construction and demolition (C&D) waste into quality fine aggregates. This paper presents a study of replacing river sand with enhanced recycled fine aggregate through a wet carbonation process developed by the authors previously. The fine recycled concrete aggregate (FRCA) ranging from 0.15 to 5 mm was prepared by demolishing a concrete with a known mixture design. After wet carbonation, the particle size, water absorption, and density of the FRCA were tested and compared with the original samples. The chemical characteristics of the original and carbonated FRCA (C-FRCA) were analyzed by a series of experiments. The results showed that (1) an increase of carbonation products and a significant reduction of hydration products; (2) microscopic observation of the C-FRCA showed a surface layer densified by calcite after wet carbonation; and (3) no significant strength loss were observed when replacing up to 50% river sand by C-FRCA in mortar specimens. The potential environmental and economic impacts were also analyzed.

The use of recycled coarse aggregate (RCA) produced from waste concrete in new concrete is an effective way to realize the recycling of construction and demolition waste. The utilization of accelerated carbonation to enhance RCA has attracted a lot attention because it can enhance performance of recycled aggregate concrete (RAC) and has potential for CO2 sequestration. This study systematically investigated the influences of the duration of pressurized carbonation on the properties of RCA, RCA-new mortar interface, and RAC. The results showed that water absorption of carbonated RCA (CRCA) decreased with the increase of carbonation duration. But the beneficial effect of carbonation was less obvious after 1-day carbonation because most of calcium hydroxide and Ca-rich calcium silicate hydrates in RCA had reacted with CO2 within 1 day. After using CRCA, the improvement in different performance indicators of RAC was dependent on different factors. The enhancement in compressive strength and chloride penetration resistance of RAC prepared with CRCA were dependent on the reduced porosity of CRCA. That was why they correlated with the water absorption of CRCA as carbonation duration increased. However, the decrease in initial absorption of water of RAC was mainly dependent on the denser surface layer of CRCA, rather than the reduced porosity of CRCA. This is because the denser surface layer can change water path from “penetrating through RCA” to “passing around RCA”. It is recommended to use 1-day carbonation to enhance RCA, which might have higher cost-effectiveness.

Highlights Carbon-negative particleboard is for the first time pioneered by incorporating 70 wt.% biochar. Biochar addition in MOSC systems reduced the thermal conductivity. Further incorporation of SCMs improves the mechanical properties and CO2 sequestration. Biochar particleboard can sequester 137 kg CO2 tonne−1 and yield potential profit of 92–116 USD m−3.

The inferior mechanical properties and durability of conventional lightweight concrete restrict its wider application. This study intended to design a novel lightweight ultra high-performance concrete (L-UHPC) using a mathematical approach, with a view to achieving low density and excellent performance. Two lightweight materials (i.e. micro-sized glass microsphere and expanded shale aggregates) were introduced in the mix design of L-UHPC. The statistical optimization via Central Composite Design was proved to be an effective method to produce the L-UHPC. The designed L-UHPC showed comparable or even superior functional and durability properties than normal-weight UHPC and high-performance cement mortar. The use of specified lightweight materials played prominent roles in ensuring the mechanical properties of L-UHPC. The high microhardness of paste matrix and the internal curing of lightweight aggregates contributed to the great durability of L-UHPC. The development of L-UHPC can provide a new solution for constructing high-performance lightweight structures and composites.

Optimization of pretreatment and various gas-solid interaction conditions is necessary for effective carbonation of recycled coarse aggregates derived from concrete wastes (RCA). This study demonstrated the use of linear weighted sum method to simultaneously optimize the pretreatment and carbonation test conditions. The optimization of the test conditions produced simplified, and eco-friendly pretreatments and drastically reduced the carbonation duration of RCA from 72 h to 3 weeks in previous studies to 18 h in this study. In addition, the optimum experimental conditions were validated by casting concrete using the carbonated RCAs. The optimized pretreatment and carbonation conditions were spraying Ca2+-rich wastewater at a level of 60% of the 24 h water absorption of RCA, 12 h air drying and 6 h carbonation at 60 °C. This study also demonstrated that wastewater derived from ready-mix concrete batching plants can be utilized for spraying the RCAs to improve carbonation efficiency. Also, the mechanical and durability properties of concrete prepared with RCAs produced from the optimized conventional carbonation and pressurized carbonation were similar. Therefore, optimized conventional carbonation can be utilized as an alternative to pressurized carbonation. The improved mechanical and durability properties of concrete prepared with carbonated RCAs were attributed to the improved physical properties of carbonated RCAs and improved microhardness of the interfacial transition zone of the new mortar. The simplified, faster, ecofriendly, optimized pretreatment and carbonation conditions opens a new vista for industrial carbonation applications and optimization of gas-solid interactions.

Stabilization/solidification (S/S) is a low-cost and high-efficiency remediation method for contaminated soils, however, conventional cement-based S/S method has environmental constraints and sustainability concerns. This study proposes a low-carbon, cement-free, clay-based approach for simultaneous S/S of As and Pb in the contaminated soil, and accordingly elucidates the chemical interactions between alkali-activated clay binders and potentially toxic elements. Quantitative X-ray diffraction and 27Al nuclear magnetic resonance analyses indicated that the addition of lime effectively activated the hydration of kaolinite clay, and the presence of limestone further enhanced the polymerization of hydrates. X-ray photoelectron spectroscopy showed that approximately 19% of As[III] was oxidized to As[V] in the alkali-activated clay system, which reduced toxicity and facilitated immobilization of As. During the cement-free S/S process, As and Pb consumed Ca(OH)2 and precipitated as Ca3(AsO4)2·4H2O and Pb3(NO3)(OH)5, respectively, accounting for the low leachability of As (7.0%) and Pb (5.4%). However, the reduced amount of Ca(OH)2 decreased the degree of hydration of clay minerals, and the pH buffering capacity of the contaminated soil hindered the pH increase. Sufficient dosage of lime was required for ensuring satisfactory solidification and contaminant immobilization of the clay-based S/S products. The leachability of As and Pb in high-Ca S/S treated soil samples was reduced by 96.2% and 98.8%, respectively. This is the first study developing a green and cement-free S/S of As- and Pb-contaminated soil using clay minerals as an environmentally compatible binding material. Keywords: Green/sustainable remediation, Low-carbon contaminant immobilization, Calcined clay, Alkali-activated materials, Waste valorization/recycling, Arsenic/lead leachability

Proper management of hazardous materials arouses widespread environmental concerns due to its enormous ecological and health impacts. The development of green stabilization/solidification (S/S) technology for resourceful utilization of hazardous materials, as well as the immobilization of potentially toxic elements is of great scientific interests. Cement-based S/S is often considered a low-cost and highly efficient technology, but the environmental sustainability of a broad spectrum of S/S technologies has yet to be evaluated. Therefore, this study assessed the environmental sustainability of S/S technologies for managing two common types of hazardous wastes, i.e., contaminated marine sediment and municipal solid waste incineration fly ash (MIFA) by using life cycle assessment (LCA). A total of 17 scenarios under three strategies for sediment and two strategies for MIFA S/S technologies were comprehensively evaluated. The LCA results identified the most preferable S/S technology in each strategy. In particular, Scenario 1 (mixture of sediment with a small percentage of ordinary Portland cement and incinerated sewage sludge ash) of Strategy 1 (use as fill materials) would be the preferred option, as it reduces about 54% and 70% global warming potential compared to those of Scenarios 2 and 3, respectively. This is the first initiative for evaluating the environmental impacts of a wide range of recently developed S/S technologies using green/alternative binders for diverting hazardous wastes from disposal. The results can serve as a decision support for the practical application of the environmentally friendly S/S technology for sustainable remediation.

Five recycled coarse aggregate (RCA) treatment techniques including flow-through carbonation, pressurized carbonation, wet carbonation, nano silica (NS) pre-spraying and combined pressurized carbonation with NS pre-spraying, were utilized to improve the performance of recycled aggregate concrete (RAC). The characteristics of the stress–strain curves of RACs including peak stress, peak strain, elastic modulus, ultimate strain and toughness were evaluated after using the above RCA treatment techniques. A theoretical model for natural aggregate concrete was used to analyse the stress–strain curve of RAC. Additionally, the carbonation resistance of RAC after using different RCA treatment techniques were investigated. The results showed that the calculated stress–strain curve of RAC based on the theoretical model matched well with the experimental results. Among the three types of carbonation techniques, pressurized carbonation caused the highest improvement in peak stress and elastic modulus of RAC, followed by flow-through carbonation, the last was wet carbonation. The NS pre-spraying method contributed to even higher improvement in peak stress and elastic modulus of RAC than the pressurized carbonation method. The combined pressurized carbonation with NS pre-spraying exhibited the highest enhancement of RAC because both the RCA and the new interface transition zone (ITZ) were improved. The carbonation resistance of RAC was improved after using all the studied RCA treatment techniques.

Concrete is the most used manufactured material in the world and certainly one of those having the most impact on the environment [...]