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Highlights This study fabricated composite materials using waste concrete powder (WCP) and linear low‐density polyethylene via injection molding to improve the resource value of polymer composites of waste concrete. The flexural strength and modulus of the composite material improved with increasing WCP content and did not decrease even at a WCP content of 60 wt%. Moreover, the effect of the WCP particle size on the mechanical properties of the composite was low for composites with a WCP content of 20 wt%. To investigate the effect of the WCP particle size on the mechanical properties of the composite material in detail, the von Mises stress distribution of the composite material with a WCP content of 20 wt% was analyzed using finite element method (FEM). The von Mises stress distributions of the composite materials were almost identical across all WCP particle sizes. The results from the experiment and FEM analysis suggested that the interfacial adhesion between the WCP and LLDPE is in good condition. Therefore, it can be concluded that injection molding using WCP and LLDPE is feasible and WCP shows promise as a reinforcement particle. Fabrication of plastic composite material using the WCP by injection molding. The flexural strength and modulus increased by adding the WCP. The effect of the WCP particle size on the mechanical properties was low. The Mises stress were almost identical at each WCP particle size. The WCP is promising reinforcement of flexural strength and modulus. [ABSTRACT FROM AUTHOR]

To solve the problems on resource utilization and environmental pollution of waste concrete and waste polypropylene (PP) plastics, the recycling of them into asphalt pavement is a feasible approach. Considering the high melting temperature of waste PP, this study adopted a thermal-and-mechanochemical method to convert waste PP into high-performance warm-mix asphalt modifiers (PPMs) through the hybrid use of dicumyl peroxide (DCP), maleic anhydride (MAH), and epoxidized soybean oil (ESO) for preparing an asphalt mixture (RCAAM) containing recycled concrete aggregate (RCA). For the prepared RCAAM containing PPMs, the mixing temperature was about 30 °C lower than that of the hot-mix RCAAM containing untreated PP. Further, the high-temperature property, low-temperature crack resistance, moisture-induced damage resistance, and fatigue resistance of the RCAAM were characterized. The results indicated that the maximum flexural strain of the RCAAM increased by 7.8~21.4% after using PPMs, while the sectional fractures of the asphalt binder were reduced after damaging at low temperature. The use of ESO in PPMs can promote the cohesion enhancement of the asphalt binder and also improve the high-temperature deformation resistance and fatigue performance of the RCAAM. Notably, the warm-mix epoxidized PPMA mixture worked better close to the hot-mix untreated PPMA mixture, even after the mixing temperature was reduced by 30 °C. [ABSTRACT FROM AUTHOR]

A novel multipoint titration coprecipitation method was used to prepare concrete-based layered double hydroxide (CLDH) to uptake phosphorus into slow-release phosphate fertilizer. Both the characteristics and the slow release of CLDH-P were investigated. Results showed that the multipoint titration method was efficient for coprecipitation. P-loaded CLDH under weak acid conditions (CLDH-P1) contained mainly Al-P and Ca-P (CaHPO4·2H2O, DCPD), while the content of Ca-P greatly increased in the product under weak alkaline conditions (CLDH-P2). The slow-release effects of the two samples in soil leaching columns were significantly better than triple superphosphate (TSP). CLDH-P1 was preferably used for mung bean growth promotion due to its better availability. The dry weight of CLDH-P1 mung bean increased by 33% in 40 days, which was obviously better than that of the TSP group. In addition to phosphorus, the nutrient elements Ca and Mg were also enriched in mung bean plants. The results revealed an economical and sustainable candidate for phosphorus recovery. [ABSTRACT FROM AUTHOR]

In this study, the effect of using waste granodiorite powder (WGP) as a cement replacement material in percentages ranging from 1 to 9% on various physical and mechanical properties of ordinary concrete was investigated. The resistance of produced concretes with WGP to the high temperature effects on the 28 days compressive strength were studied as well. Granodiorite is one of the well-known igneous rocks that have been used in previous research as a replacement for coarse or fine aggregates due to the hardness of its grains. However, rare studies have investigated its powder as a partial cement replacement material. Results showed the ability of investigated WGP with high surface area to act as a supplementary cementitious material that contributed to enhance the results of mechanical and physical properties of concrete. At traditional room temperature, the optimal replacement percentage was 7%, where the 28 days compressive and tensile strengths were improved by 28.3% and 17.3%, respectively. The optimal replacement ratio varied between 7 and 9% in the case of high temperatures, according to exposure time and temperature degree. Statistical and sensitivity analysis was conducted on 66 available compressive strength value represents all variables before and after elevated temperature exposure. While the regression equation showed good R2 value of 85.3%, sensitivity analysis indicated that compressive strength value is most sensitive to temperature, followed by WGP ratio and exposure time, with importance values of 56, 26, and 18 %, respectively. Results showed also that the setting times and consistency was decreased with increasing the replacement ratio with WGP, while the workability was slightly improved up to 5% replacement ratio. Furthermore, microstructure analysis showed that WGP can help to densify the concrete matrix due to its small size and ability to fill the interstitial voids in the concrete matrix. [ABSTRACT FROM AUTHOR]

Microwave heating, which is used for pre-treatment of concrete before it is comminuted, stands as a strong candidate for selective liberation of multiphase materials like concrete. This paper is concerned with the selective liberation of concrete's raw constituents (particularly aggregate) for recycling by considering the water content of concrete as a parameter of microwave heating for the first time. The deterioration law of the concrete's performance was characterized by the variation in the splitting tensile strength and relative dynamic modulus after heating by microwave at different water contents. Besides, tests were conducted to evaluate the performance of the interface transition zone (ITZ) between aggregate and mortar as well as to investigate the reasons for the stripping behavior of aggregate-mortar, which included the interface tensile strength test, temperature measurement, and porosity test. The deterioration law of splitting tensile strength and relative dynamic modulus revealed that the performance of concrete was subject to different degrees of damage depending on the water content. Furthermore, experimental results showed that interface bonding strength between aggregate and mortar was dramatically impaired, and a large temperature difference was generated between the aggregate and mortar during microwave heating. Meanwhile, the permeable pores increased considerably even when the specimens were dried. In the presence of water, the intactness of ITZ between aggregate and mortar was destroyed by microwave heating, and its performance was significantly lowered, which led to the occurrence of stripping behavior between aggregate and mortar. This was reaffirmed by the microstructure presented by scanning electron microscopy. Thus, the newly developed microwave pre-treatment improved by providing appropriate water contents for concrete corresponding to different strength grades is a promising method for recycling aggregate from waste concrete.

This study investigated the applicability of siliceous waste concrete powder (WCP‐S) as a reinforcing particle in plastic composite materials for effectively utilizing waste concrete. WCP‐S was obtained by milling waste concrete. Two types of resin materials, polypropylene (PP) and polyethylene (PE), were used as the matrix of the composite material. Composite materials were fabricated via compression molding, and the effect of the WCP‐S content on the mechanical properties of the composite materials was investigated. The flexural strength of the PP‐based material decreased with increasing WCP‐S content, whereas the flexural modulus increased. This result suggests insufficient interfacial adhesion between the matrix resin and WCP‐S in the plastic region with high flexural load. To improve this interfacial adhesion, we investigated the mechanical properties of the PE‐based composite material. The flexural strength remained almost constant on increasing the WCP‐S content in the PE‐based composite material, whereas the flexural strength increased. Therefore, WCP‐S is a promising reinforcement particle with an improved flexural modulus for PP or PE resins. Highlights: Plastic composite material was fabricated using WCP‐S.The flexural strength of PP‐based composite material decreased.The flexural strength of PE‐based composite material remained almost constant.Flexural modulus increased the PP‐ and PE‐based composite material.WCP‐S is a promising reinforcement particle for PP/PE resins. [ABSTRACT FROM AUTHOR]

Polymer concrete (PC) was developed at the end of the 1950s and gained popularity in the 1970s for precast parts, flimsy floor coverings, and repairs. Due to its superior performance over traditional Portland cement concrete, which offers many benefits, including mechanical properties, quick hardening, and durability. In this article, polymeric concrete was made using a mixture of sand and epoxy, and different proportions of sand were replaced with crushed concrete waste. This study found that the ideal ratio between resin and fine aggregate was 23% resin to 77% fine aggregate in terms of the total weight of the combination to get the best dispersion of fine aggregate. Waste concrete replaced 5, 10, 15, and 20% of aggregate in PC, respectively. It was further demonstrated that increased waste concrete aggregate content in PC increased the 28-day compressive strength by 7.7, 13.44, 16.8, and 18.97%, respectively; flexural strength increased by 16.68, 25.32, 37.16, and 47.71% at 28 days’ age; and direct tensile strength was higher than the reference mixture by values of 3.41, 17.21, 23.54, and 30.38% at 28 days age. The findings recommended using recycled fine aggregate on PC and suggested a 20% replacement ratio as an optimum percentage.

In order to achieve the reuse of waste concrete, the hydrothermal synthesis of low-temperature calcined calcium silica products with an ideal admixture of fly ash and waste concrete as raw materials was investigated and various properties were studied. The findings suggest that the optimal method involves adding 10% fly ash to waste concrete to lower the temperature at which calcium carbonate decomposes. The compressive strength of the specimens generally increases with increasing calcium–silicon ratio and pressure can reach up to 43.98 MPa. Nevertheless, the duration of holding requires adjustment in line with autoclave pressure: the higher the pressure, the shorter the holding time, and vice versa for lower pressure. Most of the specimens are water-resistant with softening coefficients above 0.6 and up to 0.91. The macroscopic strength is determined by the way in which the microstructure of the hydration products forms under different conditions. The optimum design for the experimental conditions should be that the pressure, holding time and calcium–silica ratio should be 1.0 MPa, 9 h and 1.0, respectively. Due to their potential for resource conservation and environmental improvement, autoclaved silicate materials manufactured from waste concrete may be a viable alternative as a green construction material. [ABSTRACT FROM AUTHOR]

The differences in physical properties, chemical properties, and mechanical properties between reclaimed concrete aggregate and natural aggregate are discussed in this paper. In this paper, the commonly used improvement techniques of recycled concrete aggregate are reviewed. Physical modification involves peeling the attached mortar layer using mechanical and thermodynamic means, including mechanical grinding and shaping, heat treatment, and microwave or electric pulse treatment. Chemical modification is based on the chemical reaction of some materials with recycled aggregate attached mortar, including acid treatment removal, water glass strengthening, carbonation strengthening, inorganic slurry strengthening, and polymer strengthening. Microbial modification is mainly based on the metabolic activity of specific microorganisms that induce carbon deposition modification. The results show that the reinforced technology of recycled aggregate has made some progress in improving the performance of recycled aggregate, but there are still some problems, such as inconsistent strengthening effects and the unstable compatibility of composite materials. In this paper, future research directions, such as the development of new strengthening materials and the integration of multi-functional strengthening technology, are described in order to provide some theoretical support for the utilization of recycled concrete aggregate. [ABSTRACT FROM AUTHOR]

Currently, the main material used in various construction materials is concrete, which is a composite material containing cement, water, and aggregate. These ingredients are materials derived from sometimes limited natural resources. In addition, economic growth today uses a variety of materials and generates a lot of waste that directly affects the environment. This research compared concrete mixes using the wastes of concrete piles as aggregate and replacing 10, 20, or 30 % (v/v) of fine aggregates with plastic waste (PW) from the recycling process, called Recycled Aggregate Concrete or RAC. Sensors were installed on the concrete surface of samples to determine the early movement of concrete particles under compressive load. It was found that the RAC developed greater compressive strength compared to ordinary concrete using natural ingredients. Increasing the proportion of fine aggregate replacement with plastic waste in RAC decreased the maximum compressive strength and the rate of compressive strength development. In a similar manner, the repeated use of RAC as aggregate affected the compressive strength of the concrete and the rate of development. Increasing plastic replacement in the recycled concrete had a more substantial impact on recycled concrete strength than increasing the number of recycle times. PW replacement of 10 % of the fine aggregates is recommended. [ABSTRACT FROM AUTHOR]

Nowadays, the construction industry is challenged not only by increasingly strict environmental regulations, but also by a shortage of raw materials and additives. It is critical to find new sources with which the circular economy and zero waste approach can be achieved. Promising candidates are alkali activated cements (AAC), which offer the potential to convert industrial wastes into higher added value products. The aim of the present research is to develop waste-based AAC foams with thermal insulation properties. During the experiments, pozzolanic materials (blast furnace slag, fly ash, and metakaolin) and waste concrete powder were used to produce first dense and then foamed structural materials. The effects of the concrete fractions, the relative proportions of each fraction, the liquid/solid ratio, and the amount of foaming agents on the physical properties were investigated. A correlation between macroscopic properties (strength, porosity, and thermal conductivity) and micro/macro structure was examined. It was found that concrete waste itself is suitable for the production of AACs, but when combined with other aluminosilicate source, the strength can be increased from 10 MPa up to 47 MPa. The thermal conductivity (0.049 W/mK) of the produced non-flammable foams is comparable to commercially available insulating materials. [ABSTRACT FROM AUTHOR]

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]

With the acceleration of urbanization, the production of waste concrete is getting higher and higher. A large amount of outdoor accumulations of waste concrete will leach heavy metals, not only causing harm to the soil, but also posing a risk to human health. Based on this, this paper systematically studies the basic physical properties and microstructure (XRF, XRD, and SEM–EDS) of outdoor natural accumulation waste concrete, and analyzes the heavy metals in waste concrete from the aspects of existing state, leaching mechanism, human health risk analysis, and summarized the direction of resource utilization of waste concrete, calculated the carbon emission reduction during recycling. The study found that heavy metals in waste concrete mainly exist in hydration products in the form of precipitation, adsorption, and replacement, summarized the leaching mechanism from the micro- and macro-aspects. The leaching mechanism of heavy metals can be assigned to chemical (mineral dissolution and effective amount of components) and physical (advection, surface erosion, and diffusion) processes from the macro-perspective. From the micro-analysis, it can be assigned to the following five processes: acid migrates from solution to liquid–solid surface, acid migration through leaching layer, rapid dissolution reaction controlled by diffusion at leaching boundary, heavy metal through leaching layer, and heavy metals through the solid/liquid surface to the solution. In addition, the concentration and the leaching rate of heavy metals in waste concrete were analyzed. It was found that the concentration of Cr was the highest reached to 4.7 mg/kg and the leaching rate of Cd was the highest, its leaching coefficient was calculated as a result of 1.713 × 10–6. However, there was no obvious regularity in the leaching of heavy metals in different accumulate particle sizes. Through the establishment of risk assessment system was found the concentration of heavy metals in waste concrete will not cause significant harm to human health. The effective limit of heavy metals after 3 months of accumulation of waste concrete was calculated as: Cr < 0.09 mg/kg, Cd < 0.00715 mg/kg, As < 0.392 mg/kg, and Pb < 0.732 mg/kg. And the carbon emission reduction of waste concrete recycling was calculated to be 28.764kgCO2/t. All the results of this study can promote the safe and environmentally friendly utilization of waste concrete. [ABSTRACT FROM AUTHOR]

This paper presents a study on the possibility of replacing natural coarse aggregates with recycled aggregates in concrete in terms of strength, namely compressive strength and flexural strength. The concrete was designed to have a 25MPa compressive strength and an 8cm slump. The replacement rates of natural aggregates with recycled coarse aggregates were 0%, 10%, 15%, and 20%. The test samples were compressed to determine their strength value after 28 days of curing. The research results give a more complete assessment of the efficiency of the use of recycled raw aggregates instead of natural aggregates in concrete. The results indicate that using recycled aggregates is feasible due to the small decrease in concrete strength. The experiment shows that up to 20% replacement, the recorded reduction in the strength of concrete is lower than 15%. However, it is indispensable that the strength of concrete should be enhanced when recycled aggregates are used. According to previous works, fiber reinforcement may be an effective solution. Therefore, to further develop research using recycled aggregates, it is necessary to consider using different fibers to strengthen the concrete. The fiber content can be used in the range of 0-1% of the weight of concrete. [ABSTRACT FROM AUTHOR]

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Enhancing the sequestration capacity of waste concrete is crucial for achieving carbon neutrality within the construction industry. Although existing studies primarily focus on theoretical analysis of concrete carbon sequestration, limited attention has been paid to explore the potential of waste concrete sequestration during stockpiling phase under varying environmental conditions. To fill this knowledge gap, we developed a CO 2 uptake calculation model tailored for the stockpiling phase of waste concrete. This model investigates the impact of crush size, stacking method and environmental conditions on the total carbon sequestration capacity and efficiency, identifying the most advantageous approach. Our findings reveal the following: (1) Increasing the crush size of waste concrete enhances its carbon sequestration capacity, albeit extends the sequestration duration. A crush size of 5-20 mm is deemed optimal for achieving the desired sequestration efficiency. (2) The optimal stacking method involves smaller piles with reduced radii and angles. (3) High temperatures and humidity levels accelerate the sequestration rate. Practical measures such as watering and covering can be employed to enhance carbon sequestration. (4) In 2021, China's waste concrete exhibited a declining sequestration potential from the southeast to the northwest and northeast regions. The maximum sequestration potential has the capacity to neutralize up to 4% of the carbon emissions generated by the construction industry in that year. This research provides a foundation for accurate assessment and the development of effective carbon sequestration strategies for waste concrete., Competing Interests: Declaration of conflicting interestsThe authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

This paper presents whether some minerals in waste concrete and ferronickel slag are available for hydroponics instead of commercial fertilizer. Waste concrete and ferronickel slag contain rich calcium, silica, and magnesium, which are well-known nutrients for plants. In this study, small-scaled hydroponic plant growth tests targeting romaine lettuce and spinach were conducted to verify this idea. The experimental conditions were designed to satisfy that the total amounts of minerals released from the wastes were more than the required amounts for growth of the vegetable plants. The released amounts of minerals were evaluated from the serial batch leaching test using the wastes, then the required amounts for growth were estimated by measuring mineral contents of plants sold in supermarkets. According to the experimental results, the calcium in waste concrete was effective for growth of the romaine lettuces. The magnesium in ferronickel slag improved not only the growth of the spinaches due to photosynthesis but also the uptake amount of the phosphorous, resulting in the reduction of operating costs, such as lighting electric power and nutritional resources for plant factories. Finally, the romaine lettuces and spinaches obtained from the experiments had little contents of toxic heavy metals. [ABSTRACT FROM AUTHOR]

High carbon emissions, shortages of natural aggregates and environmental pollution of waste concrete powder (WCP) have become open issues for the traditional concrete industry. Aggregates prepared by crushing and screening waste concrete usually possess poor mechanical properties. Meanwhile, the WCP cannot be effectively utilized. This paper proposes a novel approach based on microorganisms for strengthening mechanical properties and improving CO2 sequestration of 'newly' artificial aggregates prepared by cold-bonding pelletization of WCP. Specifically, the microorganisms enhanced the artificial aggregates, resulting in their apparent density, crushing strength and water absorption increasing to 2620 kg/m3, 9.1 MPa and 4.8%, respectively. With the increase of well-crystallized mineralization products, the artificial aggregates exhibited a denser microstructure where the porosity decreased from 20.9 to 13.9%. The CO2 fixation of artificial aggregates increased from 7.4 to 16.0 wt. % due to the existence of microorganisms. The compressive strength of concrete indicated that artificial aggregate could partially substitute the natural aggregates without affecting its strength, and a better substitution rate should be controlled within 50%. This method improves waste resource utilization and CO2 emission reduction, showing good potential for future applications. Preparation process of the artificial aggregate and its application. [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]

Recycling waste concrete has become a large problem in developing countries. The aim of this work is to provide guidance for screening concrete particles and improving screening efficiency. First, the elastoplastic collision model is established for calculating the coefficient of restitution for concrete particles with different compressive strengths. Then, a bar circular vibrating screen is applied to simulate the screening process of concrete particles by using the discrete element method (DEM). The optimal vibrating parameters, which contain amplitude, frequency and inclination angles, is analyzed for the representative concrete particles containing C15, C45 and C80 by comparing the screening efficiency. The results show that the optimal screening parameters of amplitude and frequency is smaller with the increase in the compressive strength of the concrete particles. Appropriately, the large inclination angle is suitable for screening fine concrete particles with a gap vibrating screen. This work should be helpful for the screening process of concrete waste particles and provides a theoretical basis and simulation case for screening and recycling other particles, such as sand, stone, iron ore and copper ore. In the screening processes of construction wastes, the optimal screening parameters can be selected quickly by calculating the coefficient of restitution and adopting the DEM simulation. [ABSTRACT FROM AUTHOR]

This study optimized a co-solvent free electrochemical method for biodiesel synthesis using graphite electrode and waste concrete heterogeneous catalyst. Various parameters were evaluated, including: applied voltage (9.6, 14.4, 19.2 V), catalyst particle size uniformity (unfiltered and filtered with 150 mesh), and reaction time (15, 30, 120, 240 min). The results obtained 100% FAME content and 78.51% of biodiesel yield that were achieved at 14.4 V within 30 min using filtered catalyst and cooking oil feedstock. However, a slight decline was observed with the use of waste cooking oil. This optimized method offers a reliable and simple condition for mass biodiesel production. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

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. [ABSTRACT FROM AUTHOR]

This paper presents the results of experimental studies which were carried out to investigate residual properties of heat-treated concrete mixes made with recycled aggregates by adding fly ash. Mixes made with natural aggregates were used as control mixes. Concrete cylinders made of both types of mixes were employed in the experimental programme. The cylinders were heat treated at temperature varying from 100°C to 1000°C in increments of 100°C and were tested in compression and tension to determine their respective residual strengths. The residual strengths of both types of mixes followed a similar pattern of reduction at higher temperature exposure. The residual compressive stress and elastic modulus became nearly 15% at 900°C and 700°C, respectively. A slightly higher mass loss was observed in RAC mixes due to the use of recycled aggregates which contained more free water and sustained larger thermal deformation on heating at high temperatures. [ABSTRACT FROM AUTHOR]

In this study, zeolites were synthesized from calcareous waste concrete powder (WCP) using a hydrothermal synthesis method. Effective utilization methods of waste concrete are required worldwide for prolonging the final dumping sites’ lifetime and for promoting a society that recycles. Calcareous concrete, which includes many calcium components, has been widely used. The WCP includes mainly calcite (at 66.9%) and quartz. First, we applied hydrothermal synthesis to raw WCP. However, X-ray diffraction analysis (XRD) revealed no zeolite synthesis. Next, we used the same approach on WCP-HCl (WCP with the calcium component removed by HCl treatment). For WCP-HCl, the synthesis of analcime-type zeolites was confirmed based on XRD and simple absorption tests, in which the WCP-HCl color changed from gray to blue upon its absorption of methylene blue. Finally, to determine the analcime synthesis requirements, we experimentally varied the synthesis. Analcime synthesis was observed at temperatures above 433 K after at least 12 h of heating and for NaOH solution concentrations above 2 mol/L.

Calcium carbonate precipitation and crystallization induced by urease enzyme to solidify soil is known as biocement technology. The uses of waste and cheap materials can make this technology more cost-effective and practical for applications. In this study, calcium ions were obtained by dissolving waste concretes in acidic liquid. Sand columns were treated by enzyme-induced carbonate precipitation (EICP) with either concrete-extracted calcium or reagent calcium for comparison. Compressive strengths, calcium carbonate contents, and microscopic analysis on the treated sand were carried out. It was found that the compressive strength of the former could reach 833 kPa in the dry state and 204 kPa in the wet state after 5 times of EICP treatment, both of which were higher than that of the latter. The calcium carbonate contents could reach 2–3% after 3–5 times of treatment. Based on the scanning electron microscope (SEM) and X-ray diffractometer (XRD) analyses, the crystal type of calcium carbonate produced in sand was calcite. The comparative results showed that the treatment effect using concrete-extracted calcium was similar or better than that using reagent calcium.

Effective waste concrete recycling is desirable from the viewpoints of environmental protection and extending the working lives of waste concrete final disposal sites. Recycled fine aggregate powders (RFAP) were obtained by milling waste concrete, and in this paper, we attempted to use RFAP as reinforcement particles in a polyethylene (PE) composite material. The PE powder and RFAP were blended together, and composites were fabricated using compression molding. Our results showed that the flexural strength and flexural modulus of the created composites improved with increased RFAP content. The RFAP dispersion state was honeycomb-like in the composite material, and from inspecting the specimen side view after three-point bending tests, it was apparent that crack propagation proceeded into the RFAP part of the composite, between PE particles. We then performed elastic stress analysis on the composites, in order to define the RFAP reinforcing behavior, using finite element analysis based on the homogenization method. As a result, it was revealed that the Mises stress decreased with increased RFAP content, confirming that there is a potential role for RFAP as reinforcement particles in PE-based composites. [ABSTRACT FROM AUTHOR]

Recycling waste concrete has become a large problem in developing countries. The aim of this work is to provide guidance for screening concrete particles and improving screening efficiency. First, the elastoplastic collision model is established for calculating the coefficient of restitution for concrete particles with different compressive strengths. Then, a bar circular vibrating screen is applied to simulate the screening process of concrete particles by using the discrete element method (DEM). The optimal vibrating parameters, which contain amplitude, frequency and inclination angles, is analyzed for the representative concrete particles containing C15, C45 and C80 by comparing the screening efficiency. The results show that the optimal screening parameters of amplitude and frequency is smaller with the increase in the compressive strength of the concrete particles. Appropriately, the large inclination angle is suitable for screening fine concrete particles with a gap vibrating screen. This work should be helpful for the screening process of concrete waste particles and provides a theoretical basis and simulation case for screening and recycling other particles, such as sand, stone, iron ore and copper ore. In the screening processes of construction wastes, the optimal screening parameters can be selected quickly by calculating the coefficient of restitution and adopting the DEM simulation.

This paper investigates the mechanical and shrinkage behaviour of concrete with recycled cement (RC) thermoactivated from waste cement paste and waste concrete. Overall, compared to ordinary Portland cement (OPC), for the same water/binder ratio, the mechanical strength and ultrasonic pulse velocity were not significantly influenced by the incorporation of RC. The elasticity modulus decreased with the addition of RC and the shrinkage tended to increase at high RC content. The incorporation of up to 15% RC allowed the production of workable concrete with identical shrinkage and similar to higher mechanical strength than concrete with only OPC. RC proved to be a very promising more eco-efficient supplementary cementitious material. [ABSTRACT FROM AUTHOR]

This study optimized a co-solvent free electrochemical method for biodiesel synthesis using graphite electrode and waste concrete heterogeneous catalyst. Various parameters were evaluated, including: applied voltage (9.6, 14.4, 19.2 V), catalyst particle size uniformity (unfiltered and filtered with 150 mesh), and reaction time (15, 30, 120, 240 min). The results obtained 100% FAME content and 78.51% of biodiesel yield that were achieved at 14.4 V within 30 min using filtered catalyst and cooking oil feedstock. However, a slight decline was observed with the use of waste cooking oil. This optimized method offers a reliable and simple condition for mass biodiesel production. [ABSTRACT FROM AUTHOR]

In this paper, waste concrete of different strength grades is used as a substitute for calcareous and siliceous materials. The effects of waste concrete with different strength grades, various raw material compositions, distinct calcination temperatures, burnability of cement clinker, lithofacies characteristics and microscopic morphology hydration products are studied systematically. The results demonstrate that waste concrete is conducive to the low temperature formation of clinker mineral phase, which promotes the firing of clinker and the incorporation of low-strength grade waste concrete benefits the calcination of raw material more obviously. Under the same conditions, the content of f-CaO in recycled cement clinker shows an upward trend with the increase of the strength grade of KH and waste concrete. Meanwhile, if the amount of waste concrete exceeds 40%, the stability of recycled cement would be poor. Alite and Belite crystals are formed with obvious characteristic peaks in the waste concrete calcined regenerated cement clinker of different strength grades. However, each sample group has different degrees of over-burning phenomenon, it is difficult to identify the adhesion between A and B minerals in clinker. The regenerated cement hydration products have different degrees of C-S-H gel and ettringite formation, the hydration products of recycled cement prepared by waste concrete with low strength grade are more compact. [ABSTRACT FROM AUTHOR]

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.

The present study provides a correlation and regression analysis of lightweight waste aggregate concretes with varying degrees of fibre reinforcement. The concrete mix contains pre-soaked red ceramic waste aggregate, expanded clay coarse aggregate and Portland cement. Copper-coated crimped steel fibre was used as the reinforcement. The experimental results included properties measured by destructive test methods—compressive strength, splitting tensile strength, static modulus of elasticity, the limit of proportionality, shear strength; and by non-destructive test methods—dynamic modulus of elasticity and surface electrical resistivity. These properties were analysed to study the relevancy and significance between non-destructive and destructive methods of measurement in the case of different amounts of fibre. The results show differences in the degree of fit to the linear and quadratic regression curves for pairs of destructive and non-destructive test results. As expected, the linear relationship can be applied in a few cases, but the quadratic curve must be used for a few pairs. Another observation is that it is not possible to neglect the amount of fibre in the correlation analyses of the measured properties.

Quarry aggregate reserves are depleting rapidly within Australia and the rest of the world due to an increasing demand for aggregates driven by expansion in construction. The annual production of premix concrete in Australia is approximately 30 million cubic meters, while 3–5% of concrete delivered to site remains unused and is disposed of in landfill or crushing plants. The production of coarse aggregates using this waste concrete is potentially a sustainable approach to reduce environmental and economic impact. A testing program has been conducted to investigate mechanical performance and permeation characteristics of concrete produced using a novel manufactured coarse aggregate recycled directly from fresh premix concrete. The recycled coarse aggregate (RCA) concrete satisfied the specified 28-day design strength of 25 MPa and 40 MPa at 28 days and a mean compressive strength of 60 MPa at 90 days. Aggregate grading was observed to determine strength development, while low water absorption, low drying shrinkage, and higher packing density indicate that the RCA concrete is a high-quality material with a dense pore structure. The rough fracture surface of the aggregate increased the bond between C-S-H gel matrix and RCA at the interfacial transition zone. Furthermore, a good correlation was observed between compressive strength and all other mechanical properties displayed by the quarried aggregate concrete. The application of design equations as stated in Australian standards were observed to provide a conservative design for RCA concrete structures based on the mechanical properties. [ABSTRACT FROM AUTHOR]

Waste concrete is a key component of construction and demolition (C&D) waste produced in billions of tons. Exploring new technology for recycling waste concrete has become a global concern. Meanwhile, phosphorus (P) removal from wastewater consumes lots of natural minerals, leading to a heavy burden on the environment. In this study, the cement paste powder (HCPP) was used to remove phosphorus from wastewater. The results indicate that both HCPP and thermally modified HCPP (MHCPP) are effective phosphorus removal materials, with a maximum P-binding capacity of 3.9-mg P/g HCPP and 31.2-mg P/g MHCPP, respectively. The phosphorus removal mechanism of HCPP and MHCPP was also proposed: (1) Ca2+ and OH− can release from the surface of the HCPP or MHCPP to wastewater, forming a high-alkaline and Ca-rich solution; (2) hydrolysis of phosphorus species in the high-alkaline solution environment creates HPO42− species; (3) the HPO42− combines with Ca2+ and H2O, resulting in the formation of brushite; (4) the brushite precipitated from wastewater and adhered on the surface of the HCPP or the MHCPP particles. The study provides a new and low-cost material for treatment of phosphorus wastewater. Further, the study also offers a new approach for reusing of waste concrete fines. [ABSTRACT FROM AUTHOR]

This study investigated the feature of phosphorus uptake by low-cost waste concrete. Adsorption isotherms, metal dissolution, influence of P concentration and temperature, as well as adsorbent regeneration were investigated. Chemical extraction, SEM, XRD, FTIR, and XPS were employed to determine the products of P sequestration. Results demonstrated that phosphate adsorption fitted the Langmuir isotherm model well, with estimated maximum phosphate adsorption capacity of 80.5 mg/g (10 °C). Of adsorbed phosphate, 72.1% could be desorbed when 0.1 M citrate buffer was used as eluant, and waste concrete could be recovered and reused for 4 times by the combination of eluting and roasting. Mechanisms including Ca/alkali dissolution, surface adsorption, and chemical precipitation are involved in the sequestration of phosphorus from wastewater by waste concrete. Weakly adsorptive phosphorus and Ca-P precipitate were the main products. P concentration was the major factor that affected P removal capacity and the product types, while temperature had certain effect at low P concentration. The dominant product was weakly adsorptive phosphorus for low P concentration at low temperature, which was substituted by Ca-P precipitate as temperature or P concentration increased. The increase of P concentration assisted both the increase of P removal potential and the formation of Ca-P precipitate to crystal DCPD. [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]

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• 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]

This paper investigates the mechanical and shrinkage behaviour of concrete with recycled cement (RC) thermoactivated from waste cement paste and waste concrete. Overall, compared to ordinary Portland cement (OPC), for the same water/binder ratio, the mechanical strength and ultrasonic pulse velocity were not significantly influenced by the incorporation of RC. The elasticity modulus decreased with the addition of RC and the shrinkage tended to increase at high RC content. The incorporation of up to 15% RC allowed the production of workable concrete with identical shrinkage and similar to higher mechanical strength than concrete with only OPC. RC proved to be a very promising more eco-efficient supplementary cementitious material.

• 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]

Carbon capture and utilization technology is the research stream dedicated to mitigating the pressing effect of rising atmospheric carbon dioxide (CO 2). The present study investigates a potential environmentally conscious solvent to capture and utilize CO 2 using waste concrete and seawater under reactor conditions. Although seawater's CO 2 soubility is low due to salinity, waste concrete raises seawater's pH and alkalinity, acting as a feedstock for CO 2 dissolution and offsetting the adverse effects of salinity. To evaluate the performance of the novel natural seawater-concrete solutions for CO 2 capture, time-dependent pH changes of solutions exposed to CO 2 were measured in a microchannel using fluorescence microscopy. The concentration of dissolved CO 2 in the solution was derived from pH change, revealing a 4-fold increase in the total dissolved carbon from 0.034 to 0.13 M and a 57.54% increase in the CO 2 dissolution coefficient from 530 to 835 μm2/s in seawater upon concrete addition. Electrolysis further enhanced the CO 2 capture capacity of the seawater-concrete solution by increasing the pH, enabling the solid precipitation of carbonate minerals. Raman spectroscopy and scanning electron microscopy showed that electrolysis-driven precipitates are mainly amorphous calcium carbonates, useful building blocks for seashells and coral reefs. [Display omitted] • Waste concrete-based natural seawater solution is used for carbon capture and storage. • CO 2 dissolution is analyzed using high speed fluorescence microscopy, a microfluidic approach, and the rapid equilibrium constant theory • The addition of water concrete increases CO 2 dissolution in seawater from 1.49 g/L to 5.72 g/L (4 times increased) with a 57.54% higher dissolution rate. • With electrolysis, concrete based seawater solution was able to store the carbon in from of carbonate minerals (CaCO 3 and MgCO 3). [ABSTRACT FROM AUTHOR]

Quarry aggregate reserves are depleting rapidly within Australia and the rest of the world due to an increasing demand for aggregates driven by expansion in construction. The annual production of premix concrete in Australia is approximately 30 million cubic meters, while 3–5% of concrete delivered to site remains unused and is disposed of in landfill or crushing plants. The production of coarse aggregates using this waste concrete is potentially a sustainable approach to reduce environmental and economic impact. A testing program has been conducted to investigate mechanical performance and permeation characteristics of concrete produced using a novel manufactured coarse aggregate recycled directly from fresh premix concrete. The recycled coarse aggregate (RCA) concrete satisfied the specified 28-day design strength of 25 MPa and 40 MPa at 28 days and a mean compressive strength of 60 MPa at 90 days. Aggregate grading was observed to determine strength development, while low water absorption, low drying shrinkage, and higher packing density indicate that the RCA concrete is a high-quality material with a dense pore structure. The rough fracture surface of the aggregate increased the bond between C-S-H gel matrix and RCA at the interfacial transition zone. Furthermore, a good correlation was observed between compressive strength and all other mechanical properties displayed by the quarried aggregate concrete. The application of design equations as stated in Australian standards were observed to provide a conservative design for RCA concrete structures based on the mechanical properties.

Katı atıkların yönetimi, günümüzde gelişmiş toplumların en büyük sorunlarından biri haline gelmiştir. Yeşil bina konseptiyle birlikte atik malzemelerin geri dönüştürülerek yeniden kullanılması önem kazanmıştır. Sürdürülebilir hayatta inşaat yıkıntı atıklarının çevreye verdiği zarar geri kazanılmış agreganın kullanılmasını zorunlu kılmaktadır. Bu çalışmada, yaşı 7 gün ve sınıfı C30 olan beton atıklarından elde edilen iri ve ince agreganın geri dönüşüm agregası olarak betonda kullanım olanakları araştırılmıştır. Kırma agrega 0-4 ve 4-22.4 boyutlarında iki grup olarak kullanılmıştır. Kırma agrega grupları % 0, 10, 20, 30, 40, 50 ve 100 oranların da ağırlıkça azaltılarak yerine geri dönüşüm agregası ikame edilmiştir. Beton örneklerinin 28 ve 90 günlük basınç dayanımları, 28 günlük elastisite modülleri belirlenmiştir. Elde edilen deney sonuçlarına göre; geri dönüşüm agregasının beton üretiminde kullanılabileceği ve sürdürülebilir beton üretimine ışık tutacağı görülmüştür.

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]

Concrete is a major constituent of our world. Its contributes to building society but is also an important contributor to the global CO2 emissions. The combination of waste concrete recycling and greenhouse gas abatement is obviously an interesting approach. Mineral carbonation is the methodology that allows the use of calcium oxide within the concrete and transform it into carbonates with the CO2. Following previous results, carbonation experiments were performed using concrete paste extracted from a waste concrete sample after aggregate separation. The latter was performed after crushing and attrition followed by sieving to obtain three fractions. The coarser one composed of aggregates, the second of sand and the last, a fine powder of waste concrete paste (MCF). The MCF is then used in carbonation experiments in an 18.7 L stirred reactor with a diluted source of CO2 following previously optimized conditions. Different S/L ratios were experimented. The results show that 110 kg of CO2 can be stored per ton of MCF obtained after separation. Using the mass balance obtained from the experiments, an economic evaluation was performed on both aggregate separation and carbonation. While the first step can be profitable, using the MCF as a material for industrial flue gas abatement is less evident, both on the applicability and the feasibility. [ABSTRACT FROM AUTHOR]

There is a need to promote high-value added utilization of recycled aggregates, considering the aspect of effective use. It should be noted, however, that recycled fine aggregates are generally low in quality due to the presence of cement paste attached to the aggregate surface. Based on this, there have been studies, which aimed to improve the quality of recycled aggregates using mechanical abrasion methods of removing the cement paste based on the principles of crushing, grinding and abrasion and beneficiation method using heat or acid. Accordingly, this study was performed as part of the research to improve the quality of recycled fine aggregates with the aim to effectively remove cement paste using steel ball as mechanical method and acid as chemical method. The results of the experiment showed that the oven-dry density and absorption ratio obtained after the abrasion process using sulfuric acid solution were 2.51 g/cm3 and 2.3%, respectively. This evidenced the quality improvement of the recycled aggregates as they satisfied the quality criteria of over 2.2 g/cm3 and under 5%, respectively, for Class I concrete proposed in the quality standards for recycled aggregates as well as natural sand proposed in Korea Standard criteria of over 2.5 g/cm3 and under 3%. [ABSTRACT FROM AUTHOR]