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Recycled aggregate concrete (RAC) has received huge amounts of attention in the past two decades. However, there are still many critical problems about fundamental behaviours of RAC, which need to be figured out and which otherwise limit its further sustainable popularisation. Therefore, this series of two overviews rethinks fundamental behaviours of RAC based on the latest literature. In this overview (part I), the strength development and its mechanism, as well as failure patterns of RAC under static and dynamic loading, are explored. Then, the influencing factors and prediction models of elastic modulus, shrinkage and creep of RAC are intensively discussed. It is found that, compared with natural aggregate concrete, the 28 day compressive strength of RAC is lower, whereas the long-term compressive strength of RAC may be higher, which partly depends on the strength of the parent concrete. Furthermore, because the elastic modulus of RAC is often decreased, the drying shrinkage and creep of RAC are always increased. However, the autogenous shrinkage of RAC can be decreased, and also generally develops for above 60 days. Finally, some models have been summarised to better predict the elastic modulus and long-term deformation of RAC.

Although recycled aggregate concrete (RAC) has been extensively studied, several critical problems with its fundamental behaviors still urgently need to be figured out. Therefore, this series of two reviews re-thinks fundamental behaviors of RAC based on the latest literature. In the last review (part I), the mechanical and deformation behaviors of RAC were intensively discussed. Here in part II, the relationships between the replacement ratio of recycled aggregates (RA) and the durability of RAC are further presented. It is found that, owing to the increase of total porosity, the resistance of RAC to freeze–thaw cycles, carbonation, and chloride ion penetration is usually decreased with the increasing replacement ratio of RA. As a result, the steel corrosion and corrosion-induced cracking in RAC are more serious than those in natural aggregate concrete (NAC). The prediction models for carbonation and chloride ion penetration of RAC and the corrosion-induced crack patterns are also presented. To address the degradation on the performance of RAC, several enhancement technologies such as the combined/optimisation pre-treatment on RA and fiber reinforcement are put forward. Finally, the prospects of the high-performance/efficiency methodologies and applications of RAC are also shown.

In the search for new sources of land supply, difficult ground or contamination issues may be encountered with potentially harmful trace elements existing in subsurface soils. On many occasions, these substances are naturally occurring and are not the result of anthropogenic contamination. This paper presents the multi-disciplinary efforts in characterising the spatial variability and evaluating the distributions of arsenic in the Kwu Tung North New Development Area (KTN NDA), and the soil stabilisation/solidification (S/S) works to further mitigate the associated health risks. Arsenic is a naturally occurring trace element that may pose a threat to human health and the ecosystem, and it is found in large variations of concentration at the KTN NDA. Based on data from over 1,300 samples obtained across the site, probabilistic estimates were made regarding the distributions of arsenic-containing soil, and soil volumes that may require S/S treatment. Through this probabilistic approach, the volume estimates are presented with various levels of confidence, and subsequent S/S treatment works can be planned in a time-efficient and cost-effective way according to the tolerable risk levels and availability of resources. Successful application of the approach at the KTN NDA demonstrates its applicability for other projects with geogenic trace elements in soil.

Biochar from bio-waste pyrolysis presents excellent CO2 sequestration capacity. This study innovated the design of cement-bonded particleboards utilizing a substantial amount of 50–70 wt.% pre-soaked biochar to render the products carbon-negative. We investigated the roles of biochar in magnesium oxysulfate cement (MOSC) system and demonstrated good mechanical and functional properties of biochar cement particleboards. In the presence of biochar, the amounts of hydration products were enriched in the cement systems as illustrated by the thermogravimetric analyses (TGA) and X-ray diffraction (XRD). We further incorporated supplementary cementitious materials (SCMs) and generated 5 Mg(OH)2⋅MgSO4·7H2O (5–1–7) phase in the MOSC system. As a result, our designs of biochar particleboards satisfied the standard requirements for flexural strength (> 5.5 MPa) and thickness swelling (2 emissions of the particleboards (i.e., carbon-negative) via life cycle assessment. Noticeable economic profits could also be accomplished for the biochar particleboards. For instance, the 50BC-MOSC bonded particleboard (with 50 wt.% pre-soaked biochar as aggregate, 50 wt.% MOSC as binder) with promising mechanical properties could store 137 kg CO2 tonne−1 and yield an overall economic profit of 92 to 116 USD m−3 depending on the carbon prices in different countries. In summary, our new designs of carbon-negative biochar particleboards could curtail carbon emissions in the construction materials and promote the realization of carbon neutrality and circular economy. Graphical Abstract

Modification of biochar by low-cost iron sources has gained increasing attention to improve pollutants removal performance and reduce production costs compared to conventional chemical modifications. While such iron sources generally have complex compositions, their effects on properties of the iron-biochar composite are not well investigated. This study produced an iron-biochar (RBC) composite from co-pyrolysis of incinerated sewage sludge ash (ISSA) and peanut shell, and examined the role of silica with widespread existence in ISSA and other low-cost iron sources on properties of the iron-biochar composite relevant to As(III)/As(V) removal. Silica was found to react with iron during the pyrolysis process at 850 °C and formed iron silicon at the expense of producing zero valent iron and Fe

This work investigated the strength degradation mechanisms of seawater ordinary Portland cement (OPC) pastes. Two types of specimens were prepared ((i) OSD sample (i.e. OPC was mixed with seawater, and then cured in deionized (DI) water) and (ii) ODD sample (i.e. OPC was mixed with DI water, and then cured in DI water, as the reference system)). The use of seawater in preparing OPC pastes effectively increased the hydration rate and early-age mechanical strength, but lowered the mechanical strength at the later age. The higher hydration degree and larger amounts of carbonates with a smaller crystal size enabled the seawater OPC pastes to exhibit a higher early-age mechanical strength, increasing by 13.0%–17.0% compared with the DI water OPC pastes. While the formation of Friedel’s salt and the formation of calcium-silicate-hydrate (C-S-H) gel with a lower polymerization degree and mean molecular chain length resulted in the deterioration of the pore structure and negatively affected the later-age strength development of the seawater OPC paste, decreasing by 5.8%–11.9% compared with the DI water OPC pastes.

The current work investigates the influence of seawater on morphological properties during the hydration process of tricalcium silicate (C3S) at 3, 7, 14, and 28 days to better understand the effec...

Construction and demolition (C&D) waste accounts for a large proportion of solid waste, thus effective C&D waste management is essential for promoting sustainable development. In Hong Kong, the government has promulgated a series of policies to encourage related stakeholders to implement effective C&D waste management. However, the overall status remains not satisfactory and has a great potential for further improvement. In this circumstance, the aim of this study is proposing solutions for further improvement of effective C&D waste management in Hong Kong. Based on a comprehensive content analysis of seven selected countries and two rounds of focus group meetings with local experienced professionals, prevailing C&D waste management policies and successful practices were identified and their feasibility of implementation in Hong Kong was discussed. Results revealed that non‐inert waste recycling is a key for improving the C&D waste management in Hong Kong to a higher level. A “3R + I” principle was suggested to be implemented in Hong Kong instead of the currently employed “3R” principle. Four solutions, namely promulgation of more specialized regulations, adoption of advanced recycling technologies, development of mature recycling markets, and implementation of high landfilling costs, were identified as the key directions for future improvement of C&D waste management in Hong Kong. The identified C&D waste management policies and the research methods used in this study can be employed in other countries or regions which are at different C&D waste management levels and economic status to form their own applicable policy framework.

Efficient depolymerization of lignocellulosic biomass is a prerequisite for sugar production and its subsequent upgradation to fuels and chemicals. Organic carbonate solvents, i.e., propylene carbonate (PC), ethylene carbonate (EC), and dimethyl carbonate (DMC), which are low in toxicity and biodegradable, were investigated as "green"co-solvents (PC/H2O, EC/H2O, DMC/H2O, solvent ratio 1:1) for depolymerization of cellulosic paper towel waste. PC/H2O and EC/H2O enhanced the depolymerization of paper towel waste and improved the total sugar yield (up to ∼25 C mol %) compared to H2O only (up to ∼11 C mol %) under mild reaction conditions (130 °C, 20 min). The higher performance of PC/H2O and EC/H2O can be attributed to higher availability of reactive protons in the catalytic system that facilitates efficient acid hydrolysis of recalcitrant cellulosic fibers. Moreover, a substantial buildup of in-vessel pressure by CO2 release during the microwave-assisted reaction because of decomposition of PC or EC might have accelerated the conversion of paper towel wastes. PC and EC are prospective solvents for lignocellulosic biomass conversion considering their green features and notable catalytic performance, which have a good potential for substituting conventional organic solvents such as dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) that are often considered hazardous in terms of health, safety, and environmental implications.

This study investigated the compressive strength, drying shrinkage and alkali-silica reaction (ASR) expansion of alkali-activated mortars using sand or glass cullet (GC) as aggregates and using glass powder (GP) to partially replace ground granulated furnace slag (GGBS) as the precursor. For mortars using GC as the aggregate, the replacement of GGBS by GP decreased the compressive strength. The mortars using sand as aggregate showed severe drying shrinkage and the replacement of sand by GC could significantly decrease the drying shrinkage. The addition of calcium aluminium cement (CAC) could further suppress the drying shrinkage. The alkali-activated GGBS mortars using GC as the aggregate showed large expansion after alkaline immersion. When using GP to partially replace GGBS, the expansion was significantly decreased. The replacement of 15% GGBS by CAC could further decrease the expansion. The optimal alkali-activated cement (AAC) mixture proportion developed in this study was the mortars incorporating 15% of CAC, 10% of GGBS and 75% of GP as the compressive strength, expansion and drying shrinkage of this mixture could meet the stipulated mechanical and durability requirements for partition wall applications.

Incinerated sewage sludge ash (ISSA) is produced worldwide where incineration is used to manage sewage sludge. ISSA contains significant amounts of phosphorus (P) with high recovery potential, and ...