Fine Concrete Research Articles

Sustainable Innovations in Construction: Enhancing Slag-Based Materials through Recycled Concrete Fines

Sustainable Innovations in Construction: Enhancing Slag-Based Materials through Recycled Concrete Fines

The Basic Properties of Lightweight Artificial Aggregates Made with Recycled Concrete Fines

The production of lightweight aggregate based on waste is an important step towards sustainable and ecological construction. It contributes to reducing the negative impact of the construction sector on the environment by reducing the consumption of natural raw materials and reducing waste of various origins, including rubble concrete. The physical and mechanical properties, including grain shape index, water absorption, bulk density, resistance to crushing, frost resistance, leachability of heavy metals, and porosity of lightweight artificial aggregate made from rubble concrete waste (KRC) were presented in the paper. The obtained test results prove that the proposed artificial aggregate has similar water absorption and bulk density and even better frost and crushing resistance than artificial aggregates available on the market. Due to its properties, it can be used for lightweight concrete, gardening, or as a separating layer in home sewage treatment plants.

Enhancing Sustainable Construction Practices: Utilizing Heat-Treated Recycled Concrete Fines for Improving Slag-Based Geopolymer Materials

Enhancing Sustainable Construction Practices: Utilizing Heat-Treated Recycled Concrete Fines for Improving Slag-Based Geopolymer Materials

Mechanochemical carbonation of recycled concrete fines: Towards a high-efficiency recycling and CO2 sequestration

The relative slow carbonation efficiency for conventional wet and dry carbonation of recycled concrete fines (RCF) limits its resource industrial utilization. In this study, an innovative mechanochemical carbonation (MC) method was developed. The carbonation kinetics, phase assemblage and microstructure evolution of RCF during the MC process were extensively examined. The results exhibited a substantial enhancement in the carbonation efficiency and CO2 utilization rate, as evidenced by achieving a notable carbonation degree within 10 min. This accomplishment surpassed what could be achieved even after a prolonged 2 h period of wet carbonation, and the CO2 uptake capacity and utilization rate achieved via MC reached >0.3 g-CO2/g-RCF and 80 %, respectively. The superior performance of MC was ascribed to the influence of mechanochemical effects. These effects contributed to the refinement in the geometrical characteristics of RCF, exfoliation of the passivating layers, and facilitation of CO2 dissolution, which favored the structural disintegration of RCF and carbonation progress. Another distinctive aspect of MC treatment was the production of a greater proportion of metastable CC characterized by reduced crystalline size, which was attributed to modifications in the carbonation environment and the structural alterations induced by mechanochemical effects. Moreover, the precipitation of silica gels commenced at approximately 4 min in the MC process, a notably earlier onset when compared with wet carbonation; additionally, a greater abundance of silica gels was observed in the current MC procedure, resulting from the higher carbonation degree caused by mechanochemical effects. The encouraging conclusions in the present work validated the feasibility of producing carbonated RCF more efficiently and paved the way for future industrial practice.

Recycled Concrete Fines as a Supplementary Cementitious Material: Mechanical Performances, Hydration, and Microstructures in Cementitious Systems

Using recycled concrete fines (RFs) as a supplementary cementitious material in blended cement systems offers potential benefits for the construction industry. This study investigated the mechanical properties, hydration development, and microstructural evolutions of blended Portland cement systems incorporating different particle-size distributions of untreated RFs. The results highlighted that finer RFs improved the compressive strength of mortar and influenced hydration by promoting monocarbonate formation. The incorporation of RFs optimizes the pore structure in the early stage but increases porosity. RFs affected the distribution of capillary pores and created a rougher and more complex surface compared to inert mineral admixtures. Furthermore, the particle size of RFs impacted the fractal dimension of multi-scale pore sizes.

Development of Ultra-High Performance Concrete (UHPC) matrix based on recycled concrete fines subjected to coupling curing of microwave and wet carbonation

The energy consumption of the wet carbonation method is increased by water bath heating, which is not conducive to the sustainable development of wet carbonation for Recycled Concrete Fines (RCF) treatment. In this study, a microwave wet carbonation method targeting low energy consumption and high carbonation rate is developed. Microwave heating is used instead of water bath heating to reduce the energy consumption of wet carbonation and prepare microwave carbonation recycled concrete fines (MCRCF). The results show that under the reaction condition of 80 °C, microwave reduces the activation energy of wet carbonation reaction through non-thermal effect and promotes the formation of aragonite and calcite grains. When preparing 50 g MCRCF and heating to 80 °C, microwave heating can reduce energy consumption by 26.6 %. At the same time, the CO2 absorption rates of microwave wet carbonation and pressured carbonation are 19.4 and 6.4 g/(min· kg), respectively. Therefore, microwave wet carbonation has stronger energy efficiency advantages. Additionally, the effect of MCRCF on the properties of UHPC matrix is studied. The results show that when the replacement rate of MCRCF is about 50 %, the 28d average compressive strength of the UHPC matrix increases slightly, and the development of the interface transition zone and the hydration process are promoted. This study provides a new idea for clean and efficient treatment method of RCF and verifies the feasibility of using microwaves for wet carbonation.

Highly reactive carbonated recycled concrete fines prepared via mechanochemical carbonation: Influence on the early performance of cement composites

Carbonated recycled concrete fines (RCF) possess the potential to serve as supplementary cementitious materials (SCMs). However, calcium carbonate (Cc) formed during traditional carbonation methods generally shows low reactivity. Moreover, the remaining calcium silicate hydrate (C-S-H) with a low Ca/Si ratio exhibits enhanced resistance to decalcification, leading to the formation of lower content of silica gels. These factors limit the reactivity and utilization of carbonated RCF in cementitious systems. In the present work, mechanochemistry was introduced into the RCF carbonation process, a technique named mechanochemical carbonation (MC), to enhance the reactivity of carbonated RCF. This study delved into the influence of MC-treated RCF (MCR) on the early performance of cement composites. The findings reveal that MCR exerted a more pronounced promotional effect on cement hydration, as demonstrated by the earlier appearance of hydration peaks for silicates and aluminates, coupled with increased peak intensity. This then resulted in a notable enhancement in early compressive strength. Specifically, with 10 wt% MCR, the compressive strength increased by approximately 19.9 % and 24.7 % at 1 d and 3 d, respectively. In comparison, samples incorporating wet carbonated RCF (WCR) demonstrated lower enhancements, with increases of only 4.5 % and 12.0 % at 1 d and 3 d, respectively. The enhanced performance of MCR can be attributed to the increased formation of nano-sized amorphous silica gels and metastable Cc. These components exhibit elevated reactivity and contribute to seeding effects, thereby promoting the overall reactivity and strength development of the cementitious system. These findings underscore the promising application potential of MC in transforming RCF into highly reactive SCMs.

Development of sustainable alkali-activated slag bricks with high content of carbonated recycled concrete fines: Strength, microstructure and environmental benefit

This research presents a novel method of recycling construction waste while capturing CO2 through the formation of mineral carbon. The technology uses carbonated recycled concrete fines (CRCF) as the primary raw material to produce sustainable, high-performance alkali bricks with high replacement rates. Firstly, a general assessment of the physical and chemical properties of CRCF was carried out, which revealed the formation of silica gel encapsulating the surface of CRCF during carbonation. Subsequently, alkali bricks were prepared utilizing CRCF. The study investigated the impact of various mix proportions, water-to-cement ratios, and manufacturing process parameters on brick strength. Additionally, an in-depth analysis of reaction products and brick microstructures was carried out. The results showed that the bricks produced from CRCF were stronger than those from RCF, indicating enhanced reactivity after carbonation. Finally, an analysis demonstrated that utilizing RCF from construction waste can significantly reduce carbon dioxide emissions, thereby mitigating the environmental impact associated with cement production.

Carbonation of Recycled Concrete Aggregates for New Concrete and Concrete Fines to Make Cement-Free Hollow Blocks

Mineral carbonation provides a way to increase the recycling of concrete waste in added-value products, and contributes to the principles of the circular economy. At present, most concrete waste is still downcycled. The high water absorption of recycled concrete aggregates, among other factors, impedes their recycling in the concrete industry. The quality of coarse recycled concrete aggregates (RCA) can, however, be enhanced by carbonation. Even when starting with high-grade RCA obtained from a selective demolition process, the carbonation process can decrease the water absorption of the RCA to as low as 3.0%. Concrete with a 50% replacement rate of carbonated RCA can be produced without a significant compressive strength reduction. The research further shows that carbonation can be performed at atmospheric pressure and low CO2 concentrations (e.g., 10%). The recycled concrete fines (RCF, 0–4 mm) in combination with 25% stainless steel slag were used to make zero-cement hollow blocks (39 × 19 × 9 cm) by carbonation curing without using any hydraulic binder. The hollow blocks have a compressive strength of 15.4 MPa at the lab scale. Both technologies were demonstrated on a pilot scale. In both processes, CO2 is immobilized in the resulting construction product. The developed production processes use less primary raw materials and cause less greenhouse-gas emissions than the production of traditional concrete products.

Carbonation Behaviors of Recycled Concrete Fines Containing Supplementary Cementitious Materials: Compositions, Microstructures, and Mechanisms

Carbonation Behaviors of Recycled Concrete Fines Containing Supplementary Cementitious Materials: Compositions, Microstructures, and Mechanisms

Exploring Porosity’s Role in Stiffening Waste Concrete Conglomerates Synthesized with Microbial Calcite: A Micromechanical Analysis

Abstract Amidst environmental challenges, this study utilizes the innovative method of microbially-induced calcite precipitation (MICP) to explore its application in recycling waste concrete fines (WCF). Specifically, we scrutinize the role of porosity in the mechanical behavior of MICP-cemented WCF conglomerates. Through a series of indentation tests, the study provides force-displacement relationships from which the stiffness of the samples is calculated. The Young’s modulus of the samples is then estimated using Sneddon’s solution, which assumes a conical indenter and homogeneous elastic behavior. Subsequently, the Mori–Tanaka scheme with Eshelby’s solution is employed to predict the Young’s modulus of the samples with different porosity, allowing a comparison with the calculated stiffness. This work contributes to the understanding of the role of porosity in the mechanical behavior of MICP-cemented WCF specimens and offers a pathway for optimizing the production process of such materials. Graphical Abstract

Ureolytic bacteria-assisted recycling of waste concrete fines

This study investigates the potential of microbially-induced calcite precipitation (MICP) in converting waste concrete fines (WCF), an underutilized byproduct generated during the crushing of old concrete, into construction material. WCF samples treated with Sporosarcina pasteurii DSM 33 for periods ranging from 14 to 90 days exhibited up to an 11% increase in CaCO3 content, compared to a control group treated with a saline solution. The formation of CaCO3 crystals also resulted in a 15% reduction in porosity and, most importantly, enhanced the integrity of the WCF conglomerates, reaching 20% of the stiffness of autoclaved aerated concrete. These results suggest that MICP could be used for the valorization of WCF, aligning with sustainability and circular economy goals.

Production of sustainable lime-based brick using carbonated recycled concrete fines: Mechanical, mineralogical and microstructure properties

A novel method for reusing construction waste and simultaneously sequestering CO2 in the form of mineral carbon is presented in this study. The method involves the production of construction bricks using waste concrete fines treated with carbonation. Initially, waste concrete is crushed and ground into recycled concrete fines (RCF), which are then subjected to a carbonation process to obtain carbonated recycled concrete fines (CRCF). Comprehensive characterization using XRD, TGA, FTIR, and zeta potential measurements reveals that CRCF exhibits a microstructure featuring silica-coated calcium carbonate particles on its surface, imparting significant reactivity to CRCF. In this investigation, RCF and CRCF are separately mixed with Ca(OH)2, molded, and compressed into bricks, with performance testing conducted under various material ratios and manufacturing conditions. The results indicate that bricks made with CRCF exhibit mechanical properties exceeding those of RCF-based bricks by more than two-fold. This improvement can be attributed to the refined pore structure observed in CRCF bricks. The highly reactive silica gel present on the surface of CRCF reacts with Ca(OH)2 to form C-S-H, providing a rationale for the superior strength displayed by CRCF bricks.

Enhanced H2 gas production from steam gasification of a winery waste through CO2 capture by waste concrete fines and use of alkali catalysts

This study focused on production of a high yield of hydrogen as a clean energy source from a winery waste, through steam gasification. The experiments were conducted in a fixed bed system and a thermal analysis-mass spectrometer unit. Quarry dust waste from cement industry was used to capture carbon dioxide emissions and alkali carbonates of K, Li and Na were used as catalysts. Based on the optimum conditions derived in a previous work, the effects of sorbent/biomass ratio, catalyst loading and temperature on conversion, gas composition and quality, cold gas efficiency and yield of high purity hydrogen were investigated. The amount of carbon dioxide captured up to 700 °C was 92–97 % and at 750 °C about 83 %. At Ca/C = 1 the molar fraction of hydrogen in the product gas was 74.8 %. Sodium carbonate exhibited a better overall catalytic activity at a loading of 20 % wt. In this case, hydrogen yield and concentration were 4.7 m3/kgchar and 95.9 %, respectively and char conversion 98.1 % on a dry ash-free basis. The proposed method was proved advantageous for high purity hydrogen gas production and environmental management of wastes.

Enhancing the treatment efficiency of recycled concrete fines with aqueous carbonation

Recycled concrete fines (RCFs, 0.3 mm ≪ 2.36 mm) are the fine fractions of waste concrete generated during the recycling process. RCFs contain less natural aggregate and more residual cement paste than their coarser counterparts, thus more prone to carbonation. Although carbonation is beneficial for the microstructure and reactivity of RCFs by refining the pores and changing the phase compositions of the residual cement paste, carbonation rate is usually too slow for practical applications. In this study, two critical modifications (i.e., manipulating the initial carbonate concentrations and using elevating temperatures) were applied to aqueous carbonation of RCFs, aiming to improve the carbonation efficiency. Particularly, the rate of calcium carbonate (Cc) precipitation, the advance of Cc deposition front, the mineralogy, the development of microstructure, and the aqueous chemistry during carbonation were investigated using techniques of multiple scales. Based on the results, the rate of Cc precipitation and advancement of the Cc deposition front was significantly improved. The pore structure was drastically refined. Using elevated temperatures not only activated carbonate transport but also enhanced silicate polymerization, leading to the formation of a larger amount of silica gel as well as the increase in Q4/Q3 ratio, thus improving the reactivity of RCFs. A lab-scale production of carbonated RCFs was conducted, and the carbonation products were used for preparing cement mortars. Based on the compressive strength results, the carbonation treatment duration for RCFs could be reasonably reduced to hours and even minutes.

Mechanism underlying early hydration kinetics of carbonated recycled concrete fines-ordinary portland cement (CRCF-OPC) paste

Aiming to promote applications of carbonated recycled concrete fines (CRCF) as a low-carbon alternative to ordinary Portland cement (OPC), this study explored the effect of CRCF on the early hydration kinetics of OPC paste. The results show that CRCF is not only a highly active pozzolanic material but also an effective accelerator affecting the early hydration kinetics of OPC paste, thereby leading to an increased early compressive strength and a comparable 28-d compressive strength, even at a replacement ratio of 20%. Although the rheological properties and workability are negatively affected due to the high surface area and fine particle size of CRCF, the amorphous silica and silica-alumina gels and calcite present in CRCF facilitate initial ions dissolution and provide additional active nucleation sites for hydrates. As a result, the microstructure is densified due to the formation of additional C–S–H gels with high Si/Ca and Al/Ca ratios and carboaluminate phases, along with the inert filler effect of unreacted calcite particles. The incorporation of 20% CRCF to replace OPC can achieve a CO2 emissions reduction up to 25%. These indicate that the CRCF has a promising application scenario in transforming wasted concrete into high-value industrial products by CO2 activation.

Simultaneous removal of fluoride and phosphate from semiconductor wastewater via chemical precipitation of calcium fluoride and hydroxyapatite using byproduct of recycled aggregate

Semiconductor wastewater with high concentrations of fluoride and phosphate is an environmental issue that cannot be ignored. Moreover, the byproduct of recycled aggregates, concrete fines, cannot be reused in concrete manufacturing, which is a key issue to address for the sustainable development of the concrete industry. The objective of this study was to tackle the crucial environmental issues of these two industries by developing concrete fines as an alternative material to treat semiconductor wastewater. The chemical precipitation of calcium fluoride and hydroxyapatite in the presence of concrete fines was determined as the mechanism underpinning the removal of fluoride and phosphate in wastewater. Owing to the wide range of contaminant concentration and solution pH and the possibility of multi-stage treatment, the effects of the initial contaminant concentration (F: 100–1000 mg/L; P: 20–200 mg/L) and solution pH (pH: 2–7) on the removal reactions were determined. The highest F and P removal percentages were more than 99%, and the final F and P concentrations met the effluent standard (F: 15 mg/L, P: 1.3 mg/L). The removal reactions of F and P are generally in competition, and the removal of F has priority over the removal of P. The pseudo-second-order model can describe the kinetics of the removal reactions well. The formation of fluorapatite can reduce the F concentration below the concentration achievable by CaF2 precipitation alone. Furthermore, using the byproduct of recycled aggregates instead of conventional chemicals to treat semiconductor wastewater is promising in terms of reducing CO2 emissions, and prospective applications are discussed. This study can lead to the development of a sustainable and clean process for semiconductor wastewater treatment using byproducts from the concrete industry.

Recycling and valorization of hydrated cement blends in mortars via semi-dry carbonation – The role of waste glass, granulated blast furnace slag and fly ash

The recycling technology of demolished concrete wastes in building materials manufacture provides a possibility of achieving sustainable constructions. Carbonation pre-treatment helps to enhance the performance of recycled concrete fines effectively. To investigate the effects of semi-dry carbonation on the utilization of recycled concrete fines containing supplementary cementitious materials (SCMs) in new concrete production, recycled blended cement paste powder (RBCPs) containing granulated blast furnace slag (GBS), fly ash (FA), or recycled glass powder (RGP) were prepared in the lab. The performance of mortars produced by carbonated recycled blended cement paste powder (C-RBCPs) and RBCPs blended cement were evaluated. C-RBCPs exhibited superior performance in workability and mechanical properties of prepared mortars compared to carbonated OPC powder. The results indicated that the RBCPs exhibited a lower carbonation degree (less than 53.4%) and CO2 uptake compared to OPC samples due to the lack of available Ca in the originally blended cement after incorporation of GBS, FA and RGP. As the main carbonation products, calcite, aragonite, and vaterite were all observed in C-RBCPs, the category of SCMs exhibited visible influences on the crystal format of calcium carbonate, as well as the reactivity of C-RBCPs in blended cement mortars. Overall, the semi-dry carbonation can promote the reactivity of C-RBCPs efficiently. The effects of the SCMs category should be considered in the recycling processes of cement wastes. The application of GBS, FA, and RGP in blended cement can help to achieve excellent sustainability of C-RBCPs blended mortars after semi-dry carbonation pre-treatment..

Total recycling of concrete waste using accelerated carbonation: A review

Our world is facing critical environmental challenges due to the overexploitation of natural resources and the extensive release of CO2. Meanwhile, huge amounts of concrete waste are generated due to accelerated urbanization and redevelopment. Carbonated concrete waste can thus both be used as alternative materials to aggregates and cement, and a carbon sink to reduce CO2 emission. In this paper, different carbonation approaches and their modified techniques to treat recycled coarse aggregates (RCAs), recycled concrete fines (RCFs), and recycled concrete powders (RCPs) are critically reviewed and discussed. The results suggest that carbonation effectively strengthens RCAs by densifying microstructures and improving physical properties through the precipitation of calcium carbonate (Cc) that fills the pores. Meanwhile, the carbonation reaction also develops surface reactivities on the recycled aggregates due to silica gel and Cc formation, which is particularly beneficial for RCFs because of the high surface area. Re-structuring and re-activation of RCPs through carbonation transforms the waste hydrated cement powders into novel supplementary cementitious materials. In addition to wet carbonation, which has a higher efficiency than dry carbonation, other modified carbonation techniques with the addition of limewater, magnesium ions, ammonium ions, and the use of elevated temperatures are found to produce RCAs/RCFs/RCPs with better qualities, and facilitate the formation of other high-value products such as nano-silica gel, micro-fiber, and vaterite, etc. Apart from their practical uses in conventional concrete, potential applications of the carbonation products in ultra-high-performance concrete, automated construction, and self-cleaning concrete were also briefly introduced. More importantly, the key issues in upscaling the carbonation technologies and limitations in current research are pointed out. Overall, carbonation could not only facilitate the re-circulating of concrete waste but also help reduce CO2 emissions and achieve carbon neutrality by permanently sequestrating CO2.

Bacterially induced calcite formation at the surface of recycled concrete

The construction industry is one of the main sources of greenhouse gas emissions, and portland cement production is responsible for approximately 8 % of anthropogenic CO2 emissions. Microbially induced calcium precipitation (MICP) has the potential to partially replace cement or modify the properties of materials that would otherwise not find use in construction, for example, in concrete recycling. MICP might be an environmentally friendly method to improve the properties of recycled aggregates and form conglomerates from the finest fractions. In this paper, factors influencing MICP’s ability to solidify recycled concrete fines are thoroughly investigated. Calcium carbonate precipitate crystals produced by the bacterium Sporosarcina pasteurii were analyzed using scanning electron microscopy and energy dispersive spectroscopy.