Recycled Fines Research Articles

Low-carbon cementitious composite incorporated with biochar and recycled fines suitable for 3D printing applications: hydration, shrinkage and early-age performance

In recent years, the construction industry has witnessed significant advancements in concrete technology, particularly with the integration of 3D printing in cement-based materials. While this innovation offers promising opportunities for the sector, the high binder and fine particle content of 3D printing mixes presents a substantial environmental challenge due to their considerable carbon footprint. To mitigate this impact, strategies often involve substituting portions of the binder or aggregate with waste materials. This article presents a comparative analysis of two promising approaches to reducing the carbon footprint of 3D printing concrete mixes by partially replacing cement with biochar and recycled fines. The study examines the effects of these materials on the rheological properties and early-age hydration processes of the 3D printing mix. A reference mix (REF) was established, followed by the development of eight additional mixes, with four incorporating biochar and four incorporating recycled fines, each replacing 1.25%, 2.5%, 5%, and 10% of the cement volume. The findings indicate that recycled fines have a neutral effect on the spread flow diameter of the mixture but increase initial deformations in printed elements. Conversely, biochar, due to its water absorption capacity, reduces fluidity, enhancing buildability by enabling faster printing with minimal initial deformation. Additionally, replacing up to 2.5 vol.% of cement with either material accelerates the normalized heat flow, contributing to a quicker gain in mechanical properties. However, biochar increases shrinkage deformations within the first 12 hours, while recycled fines mitigate them. Importantly, replacing up to 10 vol.% of cement with these materials does not significantly compromise early compressive strength.

Optimizing foaming agent concentration and recycled fine aggregate content to enhance mechanical and durable properties of foam concrete mixes

This research explores the impact of using recycled fine aggregates from construction and demolition wastes (CD-RFA) through the replacement of natural sand in proportion of 0 %, 10 %, 30 %, 50 %, 70 %, and 100 % for making foam concrete mixes. Protein based foaming agent was diluted with water in ratios of 1:20, 1:40 and 1:60. It was found that with the increase in the proportion of recycled fines, the porosity, sorptivity and water absorption of foam concrete mixes increases significantly. At the same time, a downward trend in the mechanical characteristics was noted. However, the obtained values of compressive strength, flexural strength, and split tensile strength of 22.35, 5.68 and 2.27 MPa were found to be well within the ACI 523 R 2014 recommended ranges of 8–22, 3–6.4 and 1–2.2 MPa, respectively. Hardened density, abrasion resistance, and resistance to sulphates and chlorides attack showed perceptible decline. The present study recommends the replacement of natural sand up to 50 % with CD-RFA. The foam concrete mix prepared using recycled fines will help clearance of a large quantities of CD wastes, thereby benefiting the environment and ecology.

Performance of fly ash/GGBFS based geopolymer concrete with recycled fine and coarse aggregates at hot and ambient curing

The incorporation of recycled aggregates in geopolymer concrete (GPC) exhibits higher environmental sustainability due to its ability to utilize industrial by-products and construction and demolition (C&D) waste. This study investigates the combined effect of fine and coarse recycled aggregates on the behavior of GPC. In this research, fly ash (FA) and ground granulated blast furnace slag (GGBFS) were used as binder, whereas sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) were used as alkaline activators. Recycled fine aggregates (RFA) were used at 30%, 60% and 100% replacement of natural fine aggregates (NFA) and recycled coarse aggregates (RCA) were used at 0%, 50% and 100% replacement of natural coarse aggregates (NCA). To study the effect of curing temperature, two types of curing regimes, i.e. hot and ambient curing, were employed. Performance of developed GPC was evaluated in terms of workability, mechanical properties (compressive strength, splitting tensile strength, and flexural strength) and durability properties such as water absorption, sorptivity, resistance against acid attack, resistance against marine environment and electrical resistivity. It was found that the behavior of GPC is improved when the samples were cured at high temperature for 24 h, as compared to the samples cured under ambient curing conditions. Furthermore, NFA can be replaced up to 100% with NCA, and up to 60% in the presence of 50% RCA and up to 30% in the presence of 100% RCA. It was also found that with an increase in the percentage of recycled aggregates, durability performance gradually declined. The results showed that GPC can be developed with inclusion of both recycled fine and coarse aggregates, which can help to preserve valuable natural resources, reduce CO2 emission, and dispose C&D waste.

Utilization of carbonated alkaline solid wastes in ordinary Portland cement-metakaolin-limestone ternary mixture: Underlying the role of low grade-sustainable calcium carbonate sources

Alkaline solid wastes are widely generated as by-products annually from construction demolition, steel making, and metal refining industries. Consequently, there is high pressure on their disposal and environmental protection. Carbonation treatment of alkaline solid wastes provides a potential way to valorize CO2 and produce low-grade artificial limestone particles. This study investigates the utilization of carbonated alkaline wastes, including recycled cement fines, steel slag, iron recycled steel slag, and magnesium slag in green ternary cement (limestone-metakaolin-cement system) production. The detailed characterization of carbonated alkaline solid wastes and the performance of metakaolin-cement based ternary systems were evaluated. The reaction mechanism of carbonated alkaline wastes in metakaokin-cement based binder is proposed. The results indicated carbonated alkaline solid waste powder containing relatively low calcite (14.4%–50.38%) and negative surface charges. In addition, the amount of reactive Si at Q3 and Q4 were observed. The precipitated calcite on the surface of particles resulted in the deterioration of the workability of ternary mixtures compared to natural limestone. However, the equivalent amount of CO3-AFm phases produced by precipitated calcium carbonate and additional hydrates from the reaction of amorphous silica in carbonated alkaline solid wastes contributed to a comparable mechanical performance (49.5 MPa–50.82 MPa) and compact microstructure of ternary binders. Carbonated alkaline solid wastes can be a sustainable component to replace natural limestone in green ternary cement production.

Properties of Geopolymer Mixtures Incorporating Recycled Ceramic Fines.

This research aimed to optimize the production conditions for geopolymer matrices by investigating the combination of heat curing conditions and the incorporation of recycled ceramic fines (CFs) as a partial replacement material for fly ash (FA). The obtained physical and mechanical properties of the composites confirmed the positive impact resulting from increasing the curing temperature from 65 °C to 85 °C and using CFs in the amount of 37.5% as a replacement for FA. The results were from laboratory tests performed to evaluate compressive strength, bending strength, bulk density, and water absorption of the geopolymer mixes. In addition, microscopic observations and porosity assessment were also performed, which confirmed that a further increase in the replacement of FA by CFs causes an increase in the porosity of the mixes and, thus, a decrease in all the assessed properties that are relevant to their practical use.

Properties of fiber incorporated concrete blocks manufactured using recycled aggregates

The construction and demolition industry generates a significant quantity of concrete waste, presenting an environmental challenge. The concrete waste generated can be processed to produce Recycled Aggregates (RA) of various sizes. Utilization of Recycled Aggregates (RA) as a substitute to conventional aggregates in concrete has captured considerable attention in the past few years, owing to its promising environmental and economic advantages. However, the combined utilization of recycled fine and coarse aggregate in the production of concrete for low-strength application has not been adequately explored. In this article, an attempt is made to investigate the characteristics of concrete blocks made with RA and polypropylene fiber (PF) are investigated for different cement content. Cement and PF content varied from 8 to 12% and 0% to 2% respectively in production of concrete blocks using Recycled Fine Aggregates (RFA) and Recycled Coarse Aggregates (RCA) at different replacement intervals. Water absorption of blocks manufactured across all replacement intervals of RA was less than 10%. Blocks containing 75% RFA and 25% RCA resulted in improved compressive strength of the order more than 3.8 MPa. Rate of improvement in compressive strength of block was 11% to 20% and 6.5% to 8.2% when the fiber dosage was increased from 0.5% to 1% and 1% to 2% respectively. The optimal fiber dosage was found to be 1%, beyond which no notable improvement in mechanical properties of blocks was observed. Use of RA in concrete blocks reduced embodied energy by 19% to 24% for varying cement content from 8 – 12%. Cost of blocks was found to be reduced by 10 – 15% when made with PF dosage of 0 to 2% with 8% cement content.

Behavior of large-scale LRS FRP-confined steel-reinforced recycled aggregate concrete square columns under axial compression

The combination of environmentally friendly large rupture strain fiber-reinforced polymers (LRS FRPs) and recycled fine and coarse aggregates presents a promising alternative to overcome the scarcity of natural river sand and natural gravels. Recycled aggregate concrete (RAC) can obtain sufficient ductility and strength by the confinement of LRS FRPs, and these two green materials are particularly suited to sustainable construction. However, existing research on LRS FRP-confined RAC is very limited and primarily focused on small-scale specimens. The objective of this paper is to investigate the axial compression performance of large-scale steel-reinforced RAC square columns wrapped with LRS FRP and promote the practical application of this sustainable structural system. The experimental variables include the effective confinement stiffness ratio (ρk,eff) and the type of LRS FRP (i.e., polyethylene naphthalate (PEN) FRP and polyethylene terephthalate (PET) FRP). Test results showed that the application of RAC had a negligible effect on the axial compression behavior of large-scale LRS FRP-confined square columns. The behavior of the test columns was primarily influenced by ρk,eff. Finally, evaluations were conducted on the existing models of LRS FRP-confined concrete with square cross-sections, based on the compilation of available test results from large-scale specimens.

Treated recycled coarse aggregate concrete: A new mixture proportioning method based on packing densities

In this paper, recycled coarse and fine aggregate collected from concrete rubble are treated with acid to remove the adhered cement mortar on the aggregates. These treated recycled aggregates both in fine and coarse aggregates are used to prepare concrete with water-cement ratio of 0.55 based on newly developed mix design method using packing densities. In this study two different recycled coarse aggregate sizes of 10 mm (40 % by mass fraction) and 20 mm (60 % by mass fraction) are used for various replacement levels of natural aggregates. The ratios of aggregates mixture of CA20, CA10 and FA are 38.4:25.6:36.0 by mass. For various packing densities, the recycled aggregates content is evaluated. Quantities for recycled aggregate concrete mixes made with 100 % recycled fine aggregate (RFA) and various replacement percentages of recycled coarse aggregate (RCA) for 0.55 w/c ratio are arrived at. Workability of concrete made with 100 % RFA in terms of slump (mm) can be enhanced by adding 5 % GGBS by mass of cement. Compressive strengths of concrete made with 0 to 100 % replacement of treated recycled fine and coarse aggregate ranges from 27.21 to 19.05 MPa. But when cement or silica fume is added to the recycled aggregate concrete then the compressive strengths are comparable to that of concrete made with natural aggregates.

Upcycling of demolition material from concrete and brick for the production of cold-bound, alkali-activated lightweight aggregates

This paper deals with the production of artificial aggregates based on the recycled fine fraction (≤ 4 mm) from construction demolition waste. Concrete powder, brick powder and their combination were used to produce aggregates through pelletisation using sodium silicate solution as an activator. For all aggregates, efficiency, bulk crushing resistance, particle density, water absorption and loose bulk density were evaluated. The bulk crushing resistance was evaluated for samples cured with different methods. A higher proportion of concrete powder increased the strength and density. The aggregates were successfully formed with bulk crushing resistance and particle densities in the range of lightweight aggregates.Graphical abstract

Use of recycled fine aggregates and recycled powders in sustainable recycled concrete

The shortage of sand and gravel resources has become a global problem. Processing construction and demolition wastes (CDW) into recycled materials and using them to prepare new concrete is conducive to the sustainable development of the construction industry. In terms of recycled coarse aggregate (RCA), relatively mature technical accumulation and industrialization have been formed, but the utilization technology of recycled fine fractions (RF), accounting for more than 40%, is still scarce. In this study, the feasible use of RF, including recycled fine aggregates (RFA) and recycled powders (RP), in recycled concrete was studied from the perspective of fresh properties (empirical and rheological). The results showed that the reduction in the slump was more prominent in Series with RFA than RP. The highest reduction was observed around 31.71% (30% RP) in Series I at 30mins; and 60.98% (50% RFA) in Series II in mix with 50% RFA, which is due to higher friction and higher interlocking behavior due to their rough surface and higher amount of attached mortar. The mix with RP showed the most significant increase in the static viscosity of 271% at 30% RP and 149% with 100% RFA replacement at 30 min of the time interval. Static yield stress increases greater with RP, which may have caused slump expansion to decrease over time. The friction from RFA's rough surface texture and the mix's extra SP and water content enhance dynamic yield stress and decrease viscosity. The study found that the incorporation of RP and FA in the mixes resulted in a noteworthy reduction in carbon emissions compared to the control mix. Specifically, RPC2 and RPC3 exhibited a decrease of 24.35% and 25.00%, respectively. It is also seen that using recycled fines can lead to stable, low-carbon concrete for leading to a sustainable automated construction industry.

Revisiting the carbonation of recycled concrete fine: A pH-cycle carbonation method

Recycled concrete fine (RCF) has been demonstrated as a promising CO2 absorbent owing to its high-calcium and high-alkali contents. For the high-efficiency utilization of bubbled CO2 for the carbonation of RCF, this paper revisits the carbonation process by proposing a pH-cycle carbonation method. The carbonation of RCF was suspended when the pH of RCF suspension dropped to 7, and the subsequent carbonation was started when the pH reached constant values. The effect of the pH-cycle carbonation method on the performance of RCF-added mortars was investigated by preparing mortars using RCF suspension with different carbonation cycles. Experimental results showed that RCF spontaneously dissolved and released Ca2+ to the suspension during the intervals of carbonation cycles, which could save some CO2 that should be spent on attacking RCF in continuous carbonation approaches. In this method, the CO2 sequestration potential of RCF is approaching depletion by the bubble of 7 min CO2. The carbonation of RCF enabled the formation of CaCO3 crystals and nano-silica, leading to accelerated hydration of cement and refined pore structure of RCF-added mortars. The 28d compressive strength of RCF-added mortars was increased by 10.27–12.57% when RCF was subjected to different carbonation cycles. The two cycles of carbonation for RCF could be the optimal one.

Impacts of biomimetic self-healing of Lysinibacillus boronitolerans immobilized through recycled fine and coarse brick aggregates in concrete

Recycling waste bricks for concrete is cost-effective and eco-friendly but may impact strength and durability. A promising solution is biomimetic self-healing concrete that uses bacteria to repair cracks. The efficacy of this approach is contingent upon the bacterial strain, carrier medium, medium size, and immobilization utilized. Nevertheless, it remains uncertain how carrier size affects biomimetic self-healing concrete when the same carrier substance and immobilization technique are used. This study explored using Lysinibacillus boronitolerans (LBBT) to develop biomimetic self-healing concrete using recycled fine brick aggregate (RFBA) as a carrier medium. Recycled coarse brick aggregate (RCBA) was also employed for the relative evaluation of the impact of carrier size in biocrete. Results revealed that adding LBBT to concrete through RFBA resulted in significant improvements in strength and toughness. Compressive, split tensile, and flexural strength increased by 16.2%, 17.6%, and 22.0% respectively, while compressive and flexural toughness improved by 51.0% and 27.6%. The abundant RFBA particles ensured the even distribution of LBBT throughout the mixture. RCBA immobilization facilitated self-healing, achieving 0.6 mm of crack healing with 89.7% compression strength recovery and a 26.0% healing potential. The achievement can be attributed to the macro-voids and the stiffened structure of RCBA. Additionally, forensic investigations confirm the presence of microbial induced carbonate precipitation. Thus, RFBA is a promising immobilizer for LLBT, exhibiting a greater impact on mechanical performance due to its smaller particle size. In contrast, RCBA facilitates self-healing by entrapping the biomimetic agent, despite its lower abundance.

Experimental and numerical investigation of accelerated carbonation of recycled fines

The purpose of this study is to perform an experimental and numerical study of accelerated carbonation of recycled fines (RF) to assess its potential and kinetics of CO2 uptake and to estimate its interest in a solicitous field of construction environmental impact. First, a parametric study of accelerated carbonation is carried out on RF, varying the following parameters: water content (1, 7, and 25%), fine size (50 μm and 80 μm), carbonation duration (1, 3, 8, 14, 17, and 24 h), CO2 concentration (18 and 100%), and heating temperature of recycled fine (500 °C). Then, the experimental results are used to calibrate a numerical model to simulate the accelerated carbonation of RF in the second step. The experimental and numerical results show the influence of each parameter on the degree of carbonation of the RF. The optimal water content for CO2 uptake is 7%. The CO2 uptake of the finer RF (50 µm) is faster than that of the larger RF (80 µm).

Influences of Carbonated Recycled Concrete Fines on Cement Hydration

The preparation of recycled concrete aggregate generates a lot of fines, which are obstacles for implementing the recycled concrete aggregate. In this work, carbonation treatment is applied to improve the properties of recycled concrete fine, and the influences of carbonated recycled concrete fine (CRCF) on cement hydration process are evaluated. Both fresh and hardened properties of the cement paste samples replacing 0 to 30% of the CRCF are measured. The results reveal that the addition of CRCF obviously accelerates the hydration process of cement, especially during the early stage, and the initial and final setting times of the cement paste containing 30% CRCF are both reduced by approximately 25% compared to the control. The CRCF improves the strength gain of cement, and that influence becomes obvious with longer curing; the relative compressive strength of cement paste containing 30% CRCF is increased by 18% relative to the control after being cured for 28 days. At the same time, the early hydration of cement paste is accelerated with the addition of CRCF and the total hydration heat after 48 h of cement paste is significantly decreased with the addition of CRCF. Specifically, the total hydration heat after 48 h of cement paste with 30% CRCF is less than 50% of that with 0% CRCF. Besides that, CRCF consumes CH in cement paste and improves the pore structure of hardened cement paste. The morphology of hydrated samples shows that the shape of ettringite formed within the control sample with 0% CRCF is longer than those of the other ones formed in cement paste with CRCF, and the length decreases as the CRCF contents increase. In addition, the sample containing 30% CRCF does not show the particles, which means that CRCF reduces the ettringite forming in hardened paste samples. Thus, the findings from this work provide a better understanding of the field of waste concrete reuse.

Properties of green self-compacting concrete designed by particle packing density method

Since depletion of natural resources and the amount of construction and demolition waste have overcome the socially and environmentally acceptable level, the construction industry must address this issue and reduce its impact on the environment. A step towards sustainability in the construction industry is the application of recycled aggregates and supplementary cementitious materials as integral components of concretes, which provides conserving natural aggregates and waste reduction. This study adopts a holistic approach to producing green self-compacting concrete with the highest portion of recycled aggregate as a replacement for natural aggregate and fly ash as filler. Based on the particle packing density method, four series of self-compacting concrete were prepared: the first series was made with natural fine and coarse aggregate, the second series was made with fine natural aggregate and recycled coarse aggregate, the third with 50 % (by mass) of fine natural aggregate replaced by recycled fine aggregate and recycled coarse aggregate, and the fourth series completely with recycled fine and coarse aggregate. The content of fly ash remained constant. Regardless of the expected decrease of workability in a fresh state with the increase of the recycled aggregate content, all series exceeded the requirements set for the hardened structural concrete.

Roles of carbonated recycled fines and aggregates in hydration, microstructure and mechanical properties of concrete: A critical review

CO2 mineralisation by recycled concrete is a novel approach of carbon capture and utilisation to improve the sustainability of concrete products, which has attracted increasing attention in recent years. This paper presents a comprehensive review on carbonation mechanisms related to CO2 mineralisation and hydration, microstructure and multiscale mechanical properties of concrete containing carbonated recycled concrete fines and aggregates, with special focus on the chemistry-microstructure-property relationships. The carbonation kinetics of recycled fines in aqueous environment exhibit a typical particle size effect, while the high pozzolanic reactivity of carbonated recycled fines can facilitate the nucleation and stabilization of hydration gels and contribute to the rapid hydration of cement paste. Carbonated recycled aggregates are favourable in enhancing the micro-mechanical properties of interfacial transition zone in concrete through the synergic effects of physical interlocking and chemical bonding. Compared to concrete with plain recycled aggregates, up to 33.0%, 12.1% and 28.7% rises in compressive, splitting and flexural strengths of concrete can be achieved under 100% replacement of carbonated recycled aggregates. Due to the Stefan effect, concrete with carbonated recycled aggregates has higher dynamic peak stress but lower strain-rate sensitivity of elastic modulus than concrete with plain recycled aggregates. The global CO2 mitigation capacity of recycled concrete varies from region to region and thus reasonable transportation network and high-efficient carbonation process are essential to ensure a balance between the environmental and economic benefits. This review summarises the recent advances in the field and discusses some future research opportunities to develop low-carbon concrete with recycled concrete as a carbon sink.

Development of a sustainable alkali activated ultra-high performance concrete (A-UHPC) incorporating recycled concrete fines

This study assesses the properties of alkali activated ultra-high performance concrete (A-UHPC) prepared by using recycled concrete fines (RCF) to replace mineral powder in cementitious materials. To be specific, firstly, an optimized combination of low carbon cementitious mixture and aggregates skeleton is designed based on particle dense packing model. Then, the RCF is partly employed and its effect on the hydration kinetics and microstructure development of A-UHPC are tested. The results show that the A-UHPC can exhibit excellent fresh and hardened properties when the water-binder ratio range is 0.29–0.27 and the RCF content is 10–30%. In addition, under the same water-binder ratio and RCF content, the compressive strength of A-UHPC is about 120 MPa. In addition, the carbon emission of A-UHPC prepared in this paper is only 75% of that of ordinary alkali activated ultra-high performance concrete. In general, the newly developed A-UHPC in this study can effectively reuse the recycled fines from waste concrete and provide a new building material with advanced properties, which is in line with sustainable development and low carbon management.

Flexural capacity prediction of steel fiber reinforced concrete beam with recycled fine and coarse aggregate

In this study, the flexural performance of the steel fiber reinforced concrete beams with recycled fine and coarse aggregates was investigated through the experiments. The effects of steel fiber volume fraction, recycled fine and coarse aggregate replacement ratio, and concrete compressive strength grade on the flexural performance of steel fiber reinforced recycled aggregate concrete (SFRAC) beams were systematically studied in terms of failure mode, flexural capacity, cracking patterns, load-deflection response, and load-tensile reinforcement strain response. The results showed that the flexural capacity and deflection of beams increased and decreased with the increase of steel fiber volume fraction, respectively. The beam with 100% replacement ratio of recycled fine aggregate and 0.5% volume fraction of steel fiber presented the similar or even better flexural performance than that of the ordinary concrete beam. By adding 1.0% volume fraction of steel fiber, the flexural capacity of beam with both 100% replacement ratio of recycled fine and coarse aggregates was higher than that of the ordinary concrete beam. Based on the experimental results of SFRAC beams in this study and other literature, the effects of steel fiber volume fraction and recycled aggregate replacement ratio on the flexural capacity of beams were quantitatively analyzed. Lastly, a model for the prediction of SFRAC beam flexural capacity was proposed and verified.

Flexural strength of concrete with recycled fine and coarse crushed glass

Flexural strength of concrete with recycled fine and coarse crushed glass

The Effect of Fines on the Change of the Friction Coefficient of Recycled Linerboard during Calendering Treatment

When linerboard is manufactured by blending recycled OCC pulp with recycled pulp from aseptic carton, the results reported that factors other than surface roughness are related to the coefficient of friction. Calendering exacerbated the problem of a sharp decrease in coefficient of friction according to the mixing of aseptic carton pulp. Therefore, we tried to understand the aforementioned changes in the coefficient of friction by evaluating the effect of the surface residue of fines derived from aseptic carton on the sliding angle and surface roughness of aseptic carton-based recycled paper. Handsheets were made from recycled pulp extracted from aseptic cartons and then coated with fines slurries at 3% and 5% concentrations. It was confirmed that the higher the proportion of recycled pulp fines used in handsheet coating, the lower is the coefficient of friction of the handsheets. Coating the handsheets with a large amount of recycled pulp fines reduced friction after calendering. This study suggested that influence factors such as changes in actual contact area are essential to be investigated in controlling the friction coefficient of recycled paper.