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Polyvinyl chloride (PVC) is one of the most widely used plastics in the world. Due to the generation of corrosive HCl and chlorine-containing organic pollutants during the recycling process, waste PVC can cause great environmental risks. At the same time, a large number of waste concrete in the construction industry have not been effectively utilized. Subcritical water (SubCW) has showed good prospect for treatment of PVC due to that dechlorination can be carried out with lower energy consumption. In this study, a high-efficiency and low-temperature SubCW treatment for dechlorination of PVC using waste concrete as an enhancer was proposed. The effects of temperature, residence time, solid-to-liquid ratio and PVC-to-Concrete ratio on dechlorination efficiency, chlorine distribution and weight loss ratio were studied. The PVC dechlorination efficiency could be enhanced to as high as 95.04% by introducing waste concrete. Significant dechlorination occurred under the conditions of 220 ℃, 60 min, solid-to-liquid ratio of 1:30 g/mL, and PVC-to-Concrete ratio of 4:1. Under the optimal conditions, all the removed chlorine from PVC was transferred into the liquid phase in the form of inorganic chlorine. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), and elemental analysis were used to analyze the PVC dechlorination mechanism. The dechlorination pathway of PVC was found to include both direct dehydrochlorination and hydroxyl substitution. The enhancement mechanism of the waste concrete on the PVC dechlorination could be attributed to the rapid capture ability of CaCO 3 for HCl. The SubCW-Concrete process proposed in this study was beneficial to both the low-temperature dechlorination of PVC and the resource utilization of waste concrete. [Display omitted] [ABSTRACT FROM AUTHOR]

Plant biomass has been widely used as an exogenous carbon source for constructed wetlands to increase nitrogen removal efficiency, but a convenient and efficient addition strategy is lacking. In this study, a new strategy was developed and tested in full-scale horizontal subsurface flow constructed wetlands, i.e., designing a decomposition zone in front of the constructed wetland, adding plant biomass directly to the decomposition zone, and re-adding the refractory parts to the decomposition zone after removal from the decomposition zone and treatment with alkaline leachate. Adding straw significantly increased the average total nitrogen removal efficiency from 36.0% to 44.9% without a significant increase in the chemical oxygen demand concentration. The refractory straw significantly increased the total nitrogen removal efficiency after it was removed, treated with alkaline leachate, and re-added to the system. Every 1 g of raw straw or refractory straw removed 0.088 g or 0.038 g of nitrate, respectively. The relative abundances of Bacillacea, Gemmatimonadaceae , Blastocatellaceae , and Caldilineaceae , which are associated with denitrification, increased after straw addition. The relative abundances of the functional groups of the bacterial community associated with organic matter degradation increased with straw addition, and the functional groups of the bacterial communities associated with nitrogen cycling were positively correlated with the nitrogen concentration. The strategy developed in this study is convenient and efficient for increasing the nitrogen removal efficiency by increasing plant biomass and can be applied in several scenarios. [Display omitted] • Plant biomass addition requires a decomposition zone in constructed wetlands. • Adding refractory straw after alkaline treatment boosts denitrification. • Every 1 g of wheat straw removed 0.099 g nitrate after two additions. • Straw increases the relative abundance of some denitrification bacteria. [ABSTRACT FROM AUTHOR]

• Carbonate binder was prepared by dehydrating waste hydrated cement paste. • Carbonate binder possesses high mechanical properties and CO 2 capture capacity. • Major phase β-C 2 S in carbonate binder forms calcite and aragonite after carbonation. A novel carbonate binder for utilization of CO 2 was prepared with dehydration of waste hydrated cement paste. The effect of water-solid ratios (w/s) varying from 0.175 to 0.275 on carbonation properties of the carbonate binder was evaluated and its phase composition and microstructure before and after carbonation were characterized. The results showed that the compressive strength of carbonate binder exposed to a CO 2 -rich environment for 2 h maximized at 82.6MPa with w/s of 0.225 while the CO 2 uptake was 27.2% which were on a par with that of carbonated pure dicalcium silicate (C 2 S) and carbonated steel slag. Analysis of phase composition and microstructure of the carbonate binder before and after carbonation revealed that the major phase beta C 2 S (β-C 2 S) in the dehydrated waste hydrated cement paste transformed into granular texture carbonation products (calcite and aragonite) with a size of less than 1μm. The carbonation products closely packed to form a large number of mesoporous which result in high compressive strength. The total CO 2 uptake of carbonate binder in the whole preparation process was equal to that of the waste hydrated cement paste but it processed higher mechanical properties. It will provide a novel method to prepare high carbonation reactivity cementitious materials and improve the recycling rate of waste concrete which are benefit to utilization of CO 2 , energy conservation and environmental protection. [ABSTRACT FROM AUTHOR]

Concrete has been a common building material widely used for decades to build the infrastructure. As a results, considerable amount of waste concrete is produced which is hazardous to the environment. At present, recycling waste concrete is limited to the aggregates for non-structural applications. The low reactivity of waste concrete powder (WCP) limits its use as a binder. Therefore, this study is conducted to enhance the reactivity of WCP to increase its viability as an alkali activated binder. The study was conducted in two phases. In Phase I, the WCP was thermally treated at temperatures ranging between 600 °C and 800 °C and the impact of this calcination on the physiochemical characteristics of thermally treated waste concrete powder (TWCP) was studied. The phase transformation after calcination was analyzed by observing the properties of WCP and TWCP using scanning electron microscope (SEM), X-ray diffractometer (XRD), Fourier transformed infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) techniques. The physical changes were also studied by comparing the density, specific surface area and particle size distribution in WCP and TWCP. It was found that the thermal treatment of WCP resulted in the decomposition of several phases along with a change in quartz structure. Also with temperature increase, the increase in density and particles fineness was observed, which was significant when thermal treatment was performed at 800 °C. The alkali activated WCP and TWCP was prepared to explore the microstructure and strength. The reactivity and strength of TWCP was superior to that of WCP. However, the highest strength obtained after alkali activation of TWCP was less than 10 MPa which is not optimal for a primary binder. Therefore, in Phase II of this study, the performance of TWCP in a binary cement-based mortar was explored by blending it with 0–50 wt% ground granulated blast furnace slag (GGBS) and the series of experiments including compressive strength, flexural strength and drying shrinkage was carried out to characterize the properties of composites. The superior strength and improved drying shrinkage obtained by mortars comprising TWCP-800 were due to the combination of filler effects and increased reactivity. No significant change in the properties of mortar containing 10 wt% TWCP-800 was observed. Study reveals that the thermal treatment of WCP at 800 °C is optimal and a sustainable approach to producing a supplementary binder for alkali activated materials. [ABSTRACT FROM AUTHOR]

• A mix design method for SCRAC which targets slump-flow and strength is proposed. • The proposed method avoids tedious adjustments and laboratory re-testing. • Effectiveness of the proposed method was verified by the tests of 11 mix proportions. • The formula for predicting the cubic strength of SCRAC has generally good accuracy. • The proposed method can also be used to prepare SCRAC with RCA and punching residues. If waste concrete and excavated engineering soil can be recycled into self-compacting concrete, it could not only alleviate the shortage of natural resources but also realize the sustainable development of self-compacting concrete technology. Therefore, a novel method is proposed for designing self-compacting recycled aggregate concrete (SCRAC) mix. The method is developed by target slump-flow and target compressive strength, avoiding tedious adjustments and laboratory re-testing at the concrete level. The proposed method first determines the contents of natural and recycled coarse aggregate based on the target slump-flow and those of natural and recycled sand are then obtained. Then, the required effective water-cement ratio is calculated based on the target compressive strength. The required admixture content is determined based on the paste rheology. Eleven mixtures designed in this manner were cast and tested. The tested variables included the target slump-flow, target compressive strength, replacement ratio of recycled coarse aggregate (RCA) and recycled sand (RS) from the excavated engineering soil, and coarse aggregate size. The results showed that the proposed method was applicable to SCRAC with a 600–800 mm target slump-flow, 40–60 MPa target compressive strength, 0–100% RCA, 0 and 100% RS, and 5–10, 5–16, and 5–20 mm coarse aggregate sizes. Finally, SCRAC using the RCA and punching residues was successfully fabricated to further verify the feasibility of the proposed method. The proposed method provides design guidance for self-compacting concrete containing RCA and RS obtained from excavated engineering soil. [ABSTRACT FROM AUTHOR]

Highlights • Creep tests on RACFST with recycled aggregates obtained from different parent waste material. • Creep model for RACFST accounting for the influence of the strength of the parent concrete. • Influence of the strength of the parent waste concrete on the creep behaviour of the RACFST columns. Abstract This paper intends to investigate how the creep behaviour of recycled aggregate concrete filled steel tubes (RACFST) is affected by the strength of the parent waste concrete that used to produce the recycled coarse aggregates (RCAs). Long-term tests were performed on ten RACFST members. A creep model was suggested to account for the influence of the strength of the parent waste concrete. Based on an extensive parametric study, it was found that with the strength of the parent waste concrete varying between 35 MPa and 70 MPa, a maximum of 37% difference can be achieved in the long-term deformations of the RACFST members. [ABSTRACT FROM AUTHOR]

Mineral sequestration stabilizes carbon dioxide in carbonate form, its thermodynamic ground state. In the case of waste concrete, it produces precipitated calcium carbonate, a commodity. We propose an indirect, aqueous process consisting of six steps: (i) recovery of waste cement (primarily calcium silicate hydrate) from waste concrete, (ii) leaching of calcium from waste cement using hydrochloric acid, (iii) purification of the leachate through alkalinity addition, (iv) precipitation of calcium carbonate via aqueous reaction with sodium carbonate, four of which were investigated here. The omissions include regeneration of hydrochloric acid and sodium hydroxide solutions from sodium chloride solution through bipolar membrane electrodialysis and absorption of CO 2 by sodium hydroxide solution. Hydrochloric acid was capable of leaching all the calcium ions present in the fines when the stoichiometric amount was added. Results showed up to 70% of the total calcium ions recovered were associated with waste cement with the remainder from limestone in the aggregates. A bright, high-purity PCC was produced after purification to pH ∼11. The high-purity PCC particles were larger than what is typically used in paper manufacturing (∼2 μm) with metastable morphology. The particle size was reduced to ∼2 μm when the purification stopped at pH 9.0 and a magnesium containing PCC was produced; the morphology remained stable in de-ionized water for 24 h. [ABSTRACT FROM AUTHOR]

The use of CO 2 mineral carbonation represents an attractive approach to recycling waste concrete. In this study, the effect of gas pressure, Liquid/Solid (L/S) ratio (w/w), Gas/Liquid (G/L) ratio (v/v) and reaction time for CO 2 sequestration were investigated. While carbonation of such matrix is already described, this study opens new insights in concrete carbonation. To increase the reactivity potential of concrete, the fine fraction (<500 μm), which contains mostly cement paste, was separated from the inert coarse aggregates. Separation was conducted by crushing and sieving. The ground fine concrete fraction showed enhanced reactivity with 75% of CO 2 removed (corresponding to 0.057 g CO 2 removed/sample) compared to that of raw concrete, with 54% of CO 2 removed (corresponding to 0.034 g CO 2 removed/sample). Tests were conducted under 144 psi of gas pressure (9.93 Bars) at ambient temperature for 10 min. On the other hand, the resulting aggregates fraction have an improved potential recycling value. The new proposed approach allows better carbonation efficiency and increases the overall valuation of waste concrete. [ABSTRACT FROM AUTHOR]

A mass of construction and demolition (C&D) waste are generated in Shanghai every year, and it has become a serious environment problem. Reclaiming the waste concrete to produce recycled aggregate (RA) and recycled aggregate concrete (RAC) is an effective method to reduce the C&D waste. This paper develops a reclamation chain of waste concrete based on the researches and practices in Shanghai. C&D waste management, waste concrete disposition, RA production and RAC preparation are discussed respectively. In addition, technical suggestions are also given according to the findings in practical engineering, which aims to optimize the reclamation chain. The results show that the properties of RA and RAC can well meet the requirement of design and practical application through a series of technical measures. The reclamation chain of waste concrete is necessary and appropriate for Shanghai, which provides more opportunities for the wider application of RA and RAC, and it shows a favorable environmental benefit. [ABSTRACT FROM AUTHOR]

The emerging sustainable development movement in the construction industry requires the recycling of waste materials to reduce the negative environmental impact of construction activities. In many countries, old concrete, a major waste stream generated from the demolition of obsolete buildings/structures, is being recycled. However, for various reasons, progress toward concrete recycling varies from country to country. This paper discusses the current status of concrete recycling in the U.S. construction industry based on results from a two-part questionnaire survey. The first part of the survey collects information on the recycling practices of surveyed concrete companies. The second part adapts questions from a study conducted in Australia and Japan to examine the awareness, benefits, difficulties, and recommended methods related to concrete recycling. The findings showed that although recycling old concrete is common in the U.S., its application is mostly limited to backfill and pavement base; using waste concrete in new concrete production is not widely applied. There are also similarities and differences in the perceptions of concrete recycling between U.S. concrete companies and their counterparts in Australia and Japan. [ABSTRACT FROM AUTHOR]

With the goal of achieving sustainable development, many studies have been performed on approaches to reducing environmental load and cutting construction-related waste and greenhouse gas emissions. For this reason, this study aims to reduce carbon emissions and preserve the fixed materials such as limestone by utilizing the presumably high value-added cementitious powder of waste concrete, which is not currently being recycled. First, the chemical properties of cementitious powder were analyzed to manufacture recycled carbonic cement by deriving the optimal mix proportion of the materials using the industrial byproduct (former slag) and limestone as a property adjustment. To make a clinkering evaluation, Free CaO measurement, an XRD analysis, a thermal analysis, a compressive strength test and a flowability analysis were performed. Through this series of analyses, overall properties were found to be at 80% those of OPC or better. In addition, when using the cementitious powder as a substitute for limestone, it was discovered that the carbon emissions were reduced by up to 46%. However, as the amount of fine aggregate that was not separated from the cementitious powder due to its similar grain size and shape increased, the plasticity and the reduction in carbon emissions were radically decreased. Moreover, when the fine aggregate was mixed in the proportion of more than 30%, it is believed to be hardly materialized. Therefore, an effective separation technology of cementitious powder from fine aggregate should be researched and developed in the future. [ABSTRACT FROM AUTHOR]

The cement industry is currently faced with the great challenge of reducing its vast carbon footprint toward sustainability. With the findings on the rehydration behaviors of thermal-activated hardened cement paste, cement, the 'glue' of concrete, originally considered a non-renewable resource, now could complete a closed-loop of its whole life cycle. The up-to-date investigations on thermal activation, rehydration kinetics and microstructure phase development of thermal-activated recycled cement (RC) are summarized in this paper. Factors affecting the fresh properties (water demand and setting time), mechanical properties, volume stability (shrinkage and creep) and durability of RC paste and RC concrete are coherently analyzed. The factors considered include properties of mother concrete (degree of hydration, degree of carbonization, existence of SCMs, inert siliceous fines and fine aggregate powder), thermal activation methods (target temperature, heating rate and cooling rate), grinding method and particle size distribution of RC, and post-processing measures (adding superplasticizers, retarders and SCMs). An optimum method for producing RC from waste hardened cement paste was proposed based on the findings in this study to achieve similar properties as OPC. The findings confirm that RC could provide engineers with a novel and eco-efficient solution for the sustainable development of the construction industry. [Display omitted] [ABSTRACT FROM AUTHOR]

• Spark plasma sintering (SPS) of milled calcareous waste concrete was investigated. • Waste mortar powder (WMP) and waste coarse aggregate powder (WCAP) were prepared. • The flexural strength of the WMP and WCAP was higher than the ISO 13,006 standard. • Spurrite, gehlenite, wollastonite, larnite, and magnetite were synthesized by SPS. In this study, spark plasma sintering (SPS) was performed using calcareous waste concrete powder. In particular, we examined the sintering temperature for SPS. This improved the flexural strength and flexural modulus. Two types of calcareous waste concrete powders were prepared: waste mortar powder (WMP) and waste coarse aggregate powder (WCAP). The characteristics of the sintered bodies were evaluated using a three-point flexural test and powder X-ray diffraction (XRD). As the main crystalline component of the raw powder, WMP included calcite and quartz, and WCAP included calcite and hematite. Regarding the flexural strength of the sintered body, WMP had 52.6 MPa at a sintering temperature of 900 °C, and WCAP had 42.4 MPa at a sintering temperature of 850 °C. These values are higher than the value of 35 MPa of ISO 13,006 standard, which prescribes porcelain stoneware tiles. XRD analysis reveals the synthesis of spurrite, gehlenite, wollastonite, and larnite in WMP, and in WCAP, spurrite and magnetite are confirmed. The synthesis of these chemical compounds is attributed to the decarbonation reaction of calcite at 760 °C. Therefore, calcareous concrete can be used as a raw material for construction ceramics when applied to SPS. [ABSTRACT FROM AUTHOR]

Solid adsorbent materials, prepared from waste cement powder and concrete sludge were assessed for removal of arsenic in the form of arsenic (As(V)) from water. All the materials exhibited arsenic removal capacity when added to distilled water containing 10–700 mg/L arsenic. The arsenic removal isotherms were expressed by the Langmuir type equations, and the highest removal capacity was observed for the adsorbent prepared from concrete sludge with heat treatment at 105 °C, the maximum removal capacity being 175 mg-As(V)/g. Based on changes in arsenic and calcium ion concentrations, and solution pH, the removal mechanism for arsenic was considered to involve the precipitation of calcium arsenate, Ca 3 (AsO 4 ) 2 . The enhanced removal of arsenic for the adsorbent prepared from concrete sludge with heat treatment was thought to reflect ion exchange by ettringite. The prepared adsorbents, derived from waste cement and concrete using simple procedures, may offer a cost effective approach for arsenic removal and clean-up of contaminated waters, especially in developing countries. [ABSTRACT FROM AUTHOR]

Abstract: Recycling and utilization of waste concrete is a significant contribution to environment and sustainable development. In current practice, the recycling of waste concrete is mainly limited to the use of crushed aggregates in low-specification applications. Few researchers have investigated complete recycling of waste concrete. These complete recycling methods, however, need to re-clinker the hydrated cement using the standard cement kiln procedures and thus consume significant amount of energy and release large quantity of CO2. To completely recycle and utilize waste concrete in a sustainable and environmentally-friendly way, a method that does not need re-clinkering at high temperature should be used. This paper studies the production of geopolymeric binder from ground waste concrete (GWC) powder mixed with class F fly ash (FA), which can then be used with recycled concrete aggregates to produce new concrete. Specifically, the effect of composition and concentration of the alkaline solution and the content of GWC on the unconfined compressive strength (UCS) of the produced geopolymeric binder is investigated. SEM/EDX, XRD, and FTIR analyses are also performed to investigate the micro/nano-structure, morphology and phase/surface elemental compositions of the produced geopolymeric binder and the effect of calcium (Ca) on them. The results indicate that utilization of GWC together with FA can increase the UCS of the geopolymeric binder up to 50% GWC content. Further increase of GWC decreases the UCS of the geopolymeric binder. So with proper combination of GWC and FA, the geopolymeric binder with required strength can be produced. [Copyright &y& Elsevier]

The treatment of acidic groundwater generated from acid sulphate soil (ASS) terrain is a challenging environmental issue in coastal floodplains of Australia. In this study, a laboratory column experiment was conducted to assess the performance of waste concrete for treating the acidic ground-water leachate from ASS terrain of the Shoalhaven region of NSW. The groundwater was highly acidic (pH of 2.5-3.5) and contained elevated concentrations of iron (10-90 mg/L) and aluminium (30-45 mg/L). Passage of the acidic groundwater through the column filled with waste concrete resulted in a significant improvement in water quality. Reduction in the concentration of iron and aluminium to below detection limits and improvement of the pH from acidic to near-neutral (pH 6-8) were observed, along with a significant release of alkalinity over a six month period under controlled laboratory conditions. The results show that the working lifetime of waste concrete as the reactive media was governed primarily by the precipitation of secondary minerals despite the high acid neutralisation capacity of the waste concrete material. [ABSTRACT FROM AUTHOR]

Abstract: For producing high-quality recycled aggregates from waste concrete, the characteristics of waste concrete when subjected to breakage by impact were investigated under free-fall conditions at various heights. In general, a lump of waste concrete did not disintegrate by a single impact, but underwent abrasion and occasional chipping before eventually breaking into several pieces. Further, when the sample was pretreated by heat, the number of free falls required for disintegrative fracture reduced markedly. Moreover, the resulting recycled aggregates had less adherent cement mortar due to weakening of the bonding strength between the aggregates and the cement mortar. Therefore, the energy penalty resulting from preheating the sample could be partially compensated for by the production of high-quality recycled aggregates. These results suggest that preheating followed by gentle breakage through free fall is an efficient technique to produce high-quality recycled aggregates from waste concrete. [Copyright &y& Elsevier]

Abstract: This study was carried out to investigate the reaction between CO2 and materials that contain CaO under dry grinding. Chemical reagent CaO was used in this experiment, and waste concrete was also tested to examine the feasibility of CO2 sorption into it. Samples were ground in a CO2 atmosphere by a centrifugal ball mill. The reaction was measured with the constant volume method. The effects of amount of sample, the number and diameter of balls, the concentration of CO2 in the mixture of CO2 and air and the rotational speed on the CO2 sorption were examined. The amount of the CO2 sorption under grinding was larger than that without grinding. The grinding enhanced the reaction between CaO and CO2. The CO2 sorption steeply increased with time in the early stage of grinding. After that, it increased gradually. The CO2 reacted with the CaO at the surface layer of the newly exposed surface of the CaO particles during the grinding. The initial sorption rate of CO2 was related with the shear force. In the latter stage of grinding, the grinding process caused the CaO particles to agglomerate. As a result, the sorption of CO2 became slow. It was found that the waste concrete had high potential for sorption of CO2 by means of dry grinding. [Copyright &y& Elsevier]

• Carbonation is a promising technology to sequester CO 2 emissions. • The locational aspects and transportation can influence the carbonation value chains significantly. • Regionalizing the analysis is of importance in order to obtain accurate results. • The economic feasibility will play a major role in determining the extent of utilizing the potential capacities. • Tracking the changes in the construction industry should be considered as they can significantly influence the supply chain. Carbon capture and utilization (CCU) is an essential method to sequester unavoidable CO 2 emissions in regions with insufficient geological storage capacities. Nonetheless, there are several uncertainties and knowledge gaps in terms of the future value chains of some CCU technologies (e.g. carbonation). This paper analyzes the potentials of coupling CCU with the supply chains of the construction industry by means of carbonating the concrete products and waste concrete in the German federal state of North Rhine–Westphalia. Based on extensive data and statistical analyses, the locations and outputs of the concrete and recycling plants have been determined in order to quantify their CO 2 sequestration capacities. Location-allocation models have been applied to allocate the carbon sources to the potential carbon sinks and calculate the minimum transportation costs. The analysis shows that the total annual sequestration capacity is up to 1 Mt CO 2 with an average transportation distance of 37.4 km (8.3 EUR/ton). Nonetheless, some emission sources have a clear comparative advantage in terms of their proximity to the carbon sinks as the distance ranges between 0.7 km and 99.7 km. Also, some carbon sinks have a comparative advantage in terms of capacities and technology readiness levels. Therefore, the paper also presents models for the different products in order to display the potentials of each category separately and offer more flexibility to the stakeholders. [ABSTRACT FROM AUTHOR]

The effect of steam gasification conditions on products properties was investigated in a bubbling fluidized bed reactor, using larch wood as the starting material. For bed material effect, calcined limestone and calcined waste concrete gave high content of H2 and CO2, while silica sand provided the high content of CO. At 650°C, calcined limestone proved to be most effective for tar adsorption and showed high ability to adsorb CO2 in bed. At 750°C it could not capture CO2 but still gave the highest cold gas efficiency (% LHV) of 79.61%. Steam gasification gave higher amount of gas product and higher H2/CO ratio than those obtained with N2 pyrolysis. The combined use of calcined limestone and calcined waste concrete with equal proportion contributed relatively the same gas composition, gas yield and cold gas efficiency as those of calcined limestone, but showed less attrition, sintering, and agglomeration propensities similar to the use of calcined waste concrete alone. [Copyright &y& Elsevier]

To prevent rapid climate change, it will be necessary to reduce net anthropogenic CO2 emissions drastically. This likely will require imposition of a tax or tradable permit scheme that creates a subsidy for negative emissions. Here, we examine possible niche markets in the cement and steel industries where it is possible to generate a limited supply of negative emissions (carbon storage or sequestration) cost-effectively.Ca(OH)2 and CaO from steel slag or concrete waste can be dissolved in water and reacted with CO2 in ambient air to capture and store carbon safely and permanently in the form of stable carbonate minerals (CaCO3). The kinetics of Ca dissolution for various particle size fractions of ground steel slag and concrete were measured in batch experiments. The majority of available Ca was found to dissolve on a time scale of hours, which was taken to be sufficiently fast for use in an industrial process.An overview of the management options for steel slag and concrete waste is presented, which indicates how their use for carbon sequestration might be integrated into existing industrial processes. Use of the materials in a carbon sequestration scheme does not preclude subsequent use and is likely to add value by removing the undesirable qualities of water absorption and expansion from the products.Finally, an example scheme is presented which could be built and operated with current technology to sequester CO2 with steel slag or concrete waste. Numerical models and simple calculations are used to establish the feasibility and estimate the operating parameters of the scheme. The operating cost is estimated to be US$8/t-CO2 sequestered. The scheme would be important as an early application of technology for capturing CO2 directly from ambient air. [Copyright &y& Elsevier]

An environmentally friendly approach to the disposal of waste materials, a difficult issue to cope with in today''s world, would only be possible through a useful recycling process. For this reason, we suggest that clearing the debris from destroyed buildings in such a way as to obtain waste concrete aggregates (WCA) to be reused in concrete production could well be a partial solution to environmental pollution. For this study, the physical and mechanical properties along with their freeze–thaw durability of concrete produced with WCAs were investigated and test results presented. While experimenting with fresh and hardened concrete, mixtures containing recycled concrete aggregates in amounts of 30%, 50%, 70%, and 100% were prepared. Afterward, these mixtures underwent freeze–thaw cycles. As a result, we found out that C16-quality concrete could be produced using less then 30% C14-quality WCA. Moreover, it was observed that the unit weight, workability, and durability of the concretes produced through WCA decreased in inverse proportion to their endurance for freeze–thaw cycle. [Copyright &y& Elsevier]

• A dry treatment potential for practical recycling of waste aggregates were estimated. • We assessed quantitively the amount of cement pastes attached to such aggregates. • Relationship between absorption, porosity, and amount cement pastes was formulated. • We proposed reliable dry treatment process for obtaining better recycled aggregates. While most recycled aggregates are obtained through the wet-treatment process, the objective of the present study was to examine the effectiveness of a dry-treatment technique for removing the cement pastes attached to recycled aggregates obtained from radioactive waste concrete to improve the dismantling and recycling technologies used in nuclear power plants. The aggregates obtained from the waste concrete were dry-treated with calcination at 600 °C and triturated for 1–12 h. The amount of cement pastes attached to the recycled aggregates was quantitively estimated by immersing the aggregates in 20% sulfuric acid. The water absorption and oven-dry density of the dry-treated recycled coarse aggregates were compared with the quality requirements specified in KS F 2527 and JIS A 5021 for natural aggregates. Based on a regression analysis using the test results, simple equations were formulated to explain the relationships between the amount of attached cement pastes and the water absorption and porosity of the recycled coarse aggregates. The dry treatment involving calcination at 600 °C and 12-h trituration is promising for obtaining recycled coarse aggregates with high quality and a small amount of attached cement pastes. [ABSTRACT FROM AUTHOR]

• NO 2 sequestration by the waste concrete was investigated at temperatures relevant to cement kilns flue gas exhaust. • Our study found that waste concrete can be utilized for NO 2 mitigation in a cheap and sustainable manner. • XRD, TGA, DRIFTS, and BET results allowed us to propose a new mechanism for NO 2 interactions with concrete surfaces. Removing NO 2 from cement kilns can have tremendously beneficial effects on the environment and human health. Sequestering NO 2 in demolished concrete is an innovative, cost-effective, and sustainable approach to remove NO 2 flue-gas from cement kilns and other industrial plants to minimize their environmental impact. Another notable advantage of this approach was signified by our recent discovery of NO 2 sequestered Recycled Concrete Aggregate (NRCA) acting as a corrosion inhibitor when recycled backed into new concrete. This paper focuses on NO 2 sequestration by the waste concrete at elevated temperatures that are representative of those found in the cement kilns flue gas exhaust. The gas-phase uptake experiments were performed for 1, 2, and 20-years old concrete samples to reflect the variable age of NRCA. The mechanistic studies of NO 2 adsorption to concrete surfaces at 27 °C, 150 °C, and 250 °C temperatures that simulate flue gas temperatures were obtained by time-resolved Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFTs) and gas-phase analysis. The results showed that NO 2 sequestration increased with an increase in temperature. Most importantly, the 20-year-old concrete still had significant uptake capacity, which shows that aged waste concrete can be used to reduce air pollution and then recycled back into new concrete structures to prevent corrosion. These findings were also supported by TGA, BET, and XRD results. The XRD data indicated a presence of alite, belite, ettringite, portlandite, and dolomite, where the increased fractions of portlandite and hydrates were correlated to higher NO 2 uptake. Moreover, the BET results indicated notable changes in the microstructure of the concrete at elevated temperatures, which also contributed to changes in the NO 2 uptake capacity of concrete. [ABSTRACT FROM AUTHOR]

• Shear strength of 50 MPa concrete beams without stirrups is investigated. • Recycled concrete aggregates replaced natural coarse aggregates at rates from 10% to 100%. • Recycled aggregates did not affect failure mode and shear crack patterns. • Normalized shear strengths of RAC beams were higher than those of control NAC beams. • Shear calculations of ACI, CSA, EC2, MC2010 and MCFT are conservative for RAC beams. Crushed waste concrete is a viable source of recycled coarse aggregates (RCA) to produce concrete. The RCA generally has less favorable properties relative to natural coarse aggregates (NCA), which can affect the shear strength of the concrete. This paper reports the results of an experimental study of the shear behavior of seven shallow longitudinally reinforced beams containing seven ercentages of replacement (PR) of NCA with RCA ranging from 0 to 100%. The target cylinder compressive strength (f cyl ) of the concrete was 50 MPa and the longitudinal reinforcement ratio (ρ L) was about 1%. The results indicate that all the beams failed in shear. The use of RCA did not change their mode of failure and did not directly affect the characteristics of the critical diagonal failure cracks. In addition, it was found that, on average, the recycled aggregate concrete (RAC) beams had a normalized shear strength that was 12% higher than that of the natural aggregates concrete (NAC) control beam. The difference in strength amongst the RAC beams with various PR was also limited. It is to be noted that the literature reports numerous cases where the RAC beams resisted higher strengths than their NAC control beams. Finally, the observed ultimate strengths of the beams were compared to the shear strength calculations of the ACI, CSA, EC2, and MC2010 codes and the Modified Compression Field Theory (MCFT). The calculated strengths were found to be conservative for the RAC beams. [ABSTRACT FROM AUTHOR]

• CaCO 3 was obtained by indirect carbonation process and scaled-up from batch to pilot-scale. • The maximum amount of Ca was extracted from the waste concrete using 10 M HCl. • Waste concrete can be used as valued resource material for manufacture of high-purity CaCO 3. This research article explains the synthesis and scale-up of calcium carbonate (CaCO 3) from waste concrete as calcium-rich material by an inorganic carbonation process. The operating parameters include S/L ratio, HCl concentration, contact time, and extraction pH were investigated. The calcium hydroxide (Ca(OH) 2) was synthesized by reaction between calcium chloride (CaCl 2) and sodium hydroxide (NaOH), which induced the spontaneous reaction of CaCO 3 without additional energy consumption. The productivity of CaCO 3 was 1 kg/d in the laboratory scale experiment, and the CaCO 3 productivity was scale-up to 20 kg/d through pilot scale process by same way as the laboratory scale. The approximately 4800 g of CaCO 3 was produced and 2112 g of CO 2 was captured per each cycle operation. Consequently, considered power consumption, the estimated amount of reduced CO 2 was 465 g of CO 2 in the pilot-scale reactor per cycle and produced CaCO 3 with a purity of 99.0 %. [ABSTRACT FROM AUTHOR]

Urban mining from construction and demolition waste (CDW) is highly relevant for the circular economy ambitions of the European Union (EU). Given the large volumes involved, end-of-life (EoL) concrete is identified as one of the priority streams for CDW recycling in most EU countries, but it is currently largely downcycled or even landfilled. The European projects C2CA and VEEP have proposed several cost-effective technologies to recover EoL concrete for new concrete manufacturing. To understand the potential effects of large-scale implementation of those recycling technologies on the circular construction, this study deployed static material flow analysis (MFA) for a set of EoL concrete management scenarios in the Netherlands constructed by considering the development factors in two, technological and temporal dimensions. On the technological dimension, three treatment systems for EoL concrete management, namely: business-as-usual treatment, C2CA technological system and VEEP technological system were investigated. On the temporal dimension, 2015 was selected as the reference year, representing the current situation, and 2025 as the future year for the prospective analysis. The results show that the development of cost-effective technologies has the potential to improve the share of recycling (as opposed to downcycling) in the Netherlands from around 5% in 2015 up to 22%–32% in 2025. From the academic aspect, the presented work illustrates how the temporal dimension can be included in the static MFA study to explore the potential effects in the future. [ABSTRACT FROM AUTHOR]

• A novel Ca-based desulfurizing agent was developed to reduce SO 2 emission. • The desulfurizing agent was prepared from waste hardened cement mortar. • SO 2 was removed through a dissolution and precipitation process. • The SO 2 removal product could be used as raw material for cement production. Developing efficient low-cost absorbents has been recognized as a prerequisite for industrial application of wet flue gas desulfurization (WFGD). Herein, hardened cement mortar (HCM) particles developed from waste concrete blocks were used as an innovative absorbent for SO 2. The results show that the SO 2 in flue gas can be completely absorbed by the highly alkaline HCM slurry. Under optimum operating conditions, 0.8 g of SO 2 was retained by per gram of HCM. Under acid conditions produced upon dissolving SO 2 in water, the Ca-rich compounds in HCM particles can continuously release Ca2+ and OH− into the HCM slurry. The Ca2+ ions released can effectively combine with SO 3 2−, resulting in the absorption of SO 2 dissolved in water. The dissolution process of HCM particles is well described by the pseudo-second-order model. The desulphurization byproduct was characterized by X-Ray diffraction (XRD) analysis, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and energy dispersive spectrometry (EDS). The results show that the desulphurization product mainly consists of gypsum. The technology developed provides a type of new material for removing SO 2 in waste flue gas. It also offers an innovative solution for the disposal of waste concrete which is also a global environmental concern. [ABSTRACT FROM AUTHOR]

Waste concrete is one of the largest contributors to solid waste, but its high value-added utilization is still shortage of economic and effective ways. In this study, the Ca-rich cement mortar (CCM) developed from waste concrete was innovatively used to remove phosphorus from wastewater. The aims of this study were to identify the phosphorus removal mechanisms of CCM and determine its phosphorus removal capacity. Also, the results were used to evaluate the potential use of the CCM as an alternative material to reduce phosphorus pollution. The results show that the Ca2+ releasing capacity of the CCMs modified by heat treatment (800 °C for 20 min) or hydrochloric acid (5.0 mmol HCl for per gram of CCM) are significantly enhanced because of the formation of porous microstructure and highly soluble calcareous compound. The Ca2+ released combined with phosphorus under an alkaline condition (pH > 10) to form Ca 5 (PO 4) 3 (OH) precipitate, which was bound on the porous surface of the CCMs particles. The phosphorus binding capacity of modified CCM reached 100 mg P/g, and the phosphate concentration in wastewater can be reduced from 100 mg/L to below 0.1 mg/L within 30 min. The Ca2+ releasing process form CCM can be divided into two stages: a primarily drastic increasing period and a subsequent stable stage; and it can be described by pseudo-second-order kinetic model. The modified CCM developed from waste concrete blocks exhibited sufficient potential for phosphorus removal/recovery. • Novel phosphorus removal agent was developed from waste concrete blocks. • The hardened cement mortar was modified to improve phosphorus removal efficiency. • Ca2+ releasing process of cement mortar can be described by pseudo-2nd order model. • Phosphate in wastewater was precipitated in the form of Ca 5 (PO4) 3 (OH). [ABSTRACT FROM AUTHOR]

• Waste concrete powder was evaluated as a new phosphorus removal material. • Phosphorus removal efficiency of waste concrete fines varies with its particle size. • A" components separation" mechanism was discovered in waste concrete fines. • The mechanism of phosphorus removal by waste concrete powder was proposed. Reusing of fine particles of waste concrete (FPWC) is a disturbing problem for waste concrete recycling process. In this study, the FPWC developed from waste concrete was used as an innovative absorbent to remove phosphorus (P) from wastewater. The results showed, with the change of particle size of the FPWC, the maximum phosphorus binding capacity of the FPWC varies in the range of 1.07–4.96 mg P/g. The P-removal ability of FPWC increases as its particle size decreases, because of the FPWC with smaller particle size has more Ca-rich and porous hardened cement paste (HCP) powder. A "components separation" mechanism was proposed to explain the change of HCP powder content in FPWC with different particle size distribution. Both the Thomas and Yoon-Nelson model can be used to describe the P-removal behaviors of FPWC. The phosphorus removal mechanism is that the Ca2+ and OH− can release from HCP powder and forms a local alkaline condition with high Ca2+ concentration. The condition was beneficial to the formation of Ca 5 (PO4) 3 (OH) which could be attached on the surface of the FPWC. Based on the Fick's law, the Ca2+ release behavior form FPWC can be separated into two stages, and it could be described by pseudo-second order model. Totally, FPWC developed from waste concrete blocks exhibited sufficient potential in phosphate removal. [ABSTRACT FROM AUTHOR]