Crushed Concrete Fines Research Articles

Developing circular concrete: Acid treatment of waste concrete fines

The development of circular concrete, to enable key components to be extracted and reused, is a key requirement to achieve sustainability in the built environment. Current industry practice for end-of-life concrete is best described as down-cycling because recycled concrete aggregate has limited use, with disposal of the associated crushed concrete fines. Acid treatment of waste concrete is being investigated to allow key concrete components to become circular and, in this work, the effect of acetic acid concentration, liquid/solid (L/S) ratio, reaction time and temperature on the leaching of waste concrete fines is reported. An acid concentration of 0.6 mol/L, an L/S ratio of 7 ml/g, and a reaction time of 6 h at ambient temperature allows clean sand to be extracted from concrete fines. This performs identically to new sand in mortar samples. We show for the first time that the dried and ground silica-rich residue produced by acid leaching has pozzolanic properties comparable to commercially available supplementary cementitious materials (SCM) such as blast furnace slag and coal fly ash. The potential for CO 2 sequestration using the Ca 2+ - rich leached solution to form CaCO 3 is calculated. The research shows that acid leaching of concrete fines can produce clean reusable sand, generates a viable SCM and sequester significant amounts of CO 2 by forming precipitated calcium carbonate.

Interactions of Cr3+, Ni2+, and Sr2+ with Crushed Concrete Fines

The underutilized cement-rich fine fraction of concrete-based demolition waste is a potential sorbent for aqueous metal ion contaminants. In this study, crushed concrete fines (CCF) were found to exclude 33.9 mg g−1 of Cr3+, 35.8 mg g−1 of Ni2+, and 7.16 mg g−1 of Sr2+ from ~1000 ppm single metal nitrate solutions (CCF:solution 25 mg cm−3) under static batch conditions at 20 °C after 3 weeks. The removal of Sr2+ followed a pseudo-second-order reaction (k2 = 3.1 × 10−4 g mg−1 min−1, R2 = 0.999), whereas a pseudo-first-order model described the removal of Cr3+ (k1 = 2.3 × 10−4 min−1, R2 = 0.998) and Ni2+ (k1 = 5.7 × 10−4 min−1, R2 = 0.991). In all cases, the principal mechanism of interaction was the alkali-mediated precipitation of solubility-limiting phases on the surface of the CCF. Four consecutive deionized water leaching procedures (CCF:water 0.1 g cm−3) liberated 0.53%, 0.88%, and 8.39% of the bound Cr3+, Ni2+, and Sr2+ species, respectively. These findings indicate that CCF are an effective sorbent for the immobilization and retention of aqueous Cr3+ and Ni2+ ions, although they are comparatively ineffectual in the removal and sustained exclusion of Sr2+ ions. As is commonly noted with Portland cement-based sorbents, slow removal kinetics, long equilibrium times, the associated release of Ca2+ ions, high pH, and the formation of loose floc may preclude these materials from conventional wastewater treatments. This notwithstanding, they are potentially suitable for incorporation into permeable reactive barriers for the containment of metal species in contaminated groundwaters, sediments, and soils.

Use of recycled concrete fines in cement and as aggregate

The research project focused on investigating and optimizing the processing and use of recycled crushed sand 0/4 from concrete demolition waste, as an alternative raw material in the cement and concrete industry. Crushed sand is produced during the processing of concrete demolition waste. The goal was to identify the optimum way of using the processed material along the entire process chain so that greenhouse gas emissions, waste volumes are reduced, and natural resources are conserved. Different samples of laboratory and real crushed concrete fines were collected and examined in relation to various possible applications in accordance with the applicable standards. Results highlight, that crushed concrete fines can be used in various applications in the concrete value-chain. However, for an optimal usage, additional processing is needed.

Interactions of Cd2+, Co2+ and MoO42− Ions with Crushed Concrete Fines

Construction and demolition activities generate approximately two thirds of the world’s waste, with concrete-based demolition material accounting for the largest proportion. Primary aggregates are recovered and reused, although the cement-rich fine fraction is underutilised. In this study, single metal batch sorption experiments confirmed that crushed concrete fines (CCF) are an effective sorbent for the maximum exclusion of 45.2 mg g−1 Cd2+, 38.4 mg g−1 Co2+ and 56.0 mg g−1 MoO42− ions from aqueous media. The principal mechanisms of sorption were determined, by scanning electron microscopy of the metal-laden CCF, to be co-precipitation with Ca2+ ions released from the cement to form solubility limiting phases. The removal of Co2+ and MoO42− ions followed a zero-order reaction and that of Cd2+ was best described by a pseudo-second-order model. The Langmuir model provided the most appropriate description of the steady state immobilisation of Cd2+ and Co2+, whereas the removal of MoO42− conformed to the Freundlich isotherm. Long equilibration times (>120 h), loose floc formation and high pH are likely to limit the use of CCF in many conventional wastewater treatment applications; although, these properties could be usefully exploited in reactive barriers for the management of contaminated soils, sediments and groundwater.

Decorative light transmitting concrete based on crushed concrete fines

The increasing demand for decorative materials among architects and designers has led to the development of various new building materials. The combination of types of functional significance was the reason for the creation of light transmitting concretes (known as LiTraCon), which are combined the light-transmitting and constructive functions. Such concrete can be used in the decorative finishing of internal surfaces and the creation of decorative products. The most common technology for obtaining light transmitting concrete based on the process of introducing oriented light-conducting fibers into the concrete mixture. This technology is complex and labor-intensive, which leads to increased cost of the material. The usage of translucent compounds instead of light transmitting fibers can enhance the mechanical properties of the material. The polymer resins introduced in the composition at formation of material interact with concrete mix and form a dense contact zone between the layers of the solidified compound and concrete. Also, introduction of composition of mechanically and chemically activated crushed concrete fines and silica fume to the complex binder could lead to further decreasing of the cost of light transmitting concrete and to improve the process of hardening of concrete mix without loss of the physical properties. Developed light transmitting concrete works as two-layered system, both components of it has similar stress and bending strength, with a dense contact zone between layers. Aforementioned complex binder was chosen to form a concrete matrix and various types of acrylic-based and epoxy-based resins were chosen to form a translucent layer. To find out the influence of admixtures on the properties of light transmitting concrete, the experimental study of physical and mechanical properties and structure of the material were conducted. The influence of type and specific surface of crushed concrete fines and silica fume on the structure and quality of decorative surface was examined. The compressive strength of concrete and adhesion between layers of concrete and translucent compound were found using the standard testing methods. The structure of concrete was examined using the methods of electron microscopy and X-ray analysis. The study of the properties of concrete shows that developed material has a compressive strength of 30 Mpa, strong adhesion of layers (destruction of material due tests goes through the matrix of concrete) and sufficient light transmitting ability.

Usage of Crushed Concrete Fines in Decorative Concrete

The article is devoted to the questions of usage of crushed concrete fines from concrete scrap for the production of high-quality decorative composite materials based on mixed binder. The main problem in the application of crushed concrete in the manufacture of decorative concrete products is extremely low decorative properties of crushed concrete fines itself, as well as concrete products based on them. However, crushed concrete fines could have a positive impact on the structure of the concrete matrix and could improve the environmental and economic characteristics of the concrete products. Dust fraction of crushed concrete fines contains non-hydrated cement grains, which can be opened in screening process due to the low strength of the contact zone between the hydrated and non-hydrated cement. In addition, the screening process could increase activity of the crushed concrete fines, so it can be used as a fine aggregate and filler for concrete mixes. Previous studies have shown that the effect of the usage of the crushed concrete fines is small and does not allow to obtain concrete products with high strength. However, it is possible to improve the efficiency of the crushed concrete fines as a filler due to the complex of measures prior to mixing. Such measures may include a preliminary mechanochemical activation of the binder (cement binder, iron oxide pigment, silica fume and crushed concrete fines), as well as the usage of polycarboxylate superplasticizers. The development of specific surface area of activated crushed concrete fines ensures strong adhesion between grains of binder and filler during the formation of cement stone matrix. The particle size distribution of the crushed concrete fines could achieve the densest structure of cement stone matrix and improve its resistance to environmental effects. The authors examined the mechanisms of structure of concrete products with crushed concrete fines as a filler. The results of studies of the properties of the crushed concrete fines were provided. It is shown that the admixture of the crushed concrete fines has little effect on the colour characteristics of the decorative concrete products. The preferred options to improve the surfaces of decorative concrete are also proposed.

Application of response surface methodology and semi-mechanistic model to optimize fluoride removal using crushed concrete in a fixed-bed column

ABSTRACTThe aim of this study was to investigate the removal of fluoride from aqueous solutions by using crushed concrete fines as a filter medium under varying conditions of pH 3–7, flow rate of 0.3–0.7 mL/min, and filter depth of 10–20 cm. The performance of fixed-bed columns was evaluated on the basis of the removal ratio (Re), uptake capacity (qe), degree of sorbent used (DoSU), and sorbent usage rate (SUR) obtained from breakthrough curves (BTCs). Three widely used semi-mechanistic models, that is, Bohart-Adams, Thomas, and Yoon-Nelson models, were applied to simulate the BTCs and to derive the design parameters. The Box-Behnken design of response surface methodology (RSM) was used to elucidate the individual and interactive effects of the three operational parameters on the column performance and to optimize these parameters. The results demonstrated that pH is the most important factor in the performance of fluoride removal by a fixed-bed column. The flow rate had a significant negative influence on Re and DoSU, and the effect of filter depth was observed only in the regression model for DoSU. Statistical analysis indicated that the model attained from the RSM study is suitable for describing the semi-mechanistic model parameters.

The concrete-based high performance decorative material for severe climatic conditions

This paper presents experimental investigations on the development of high performance colored concrete-based decorative materials for regions with severe climatic conditions. The main idea of the research is the creation of concrete-based material with admixtures, consisting of mechanically activated crushed concrete fines. In this context, the influence of mechanic-chemical activation of admixture and coarse aggregate to the properties of decorative concrete were examined. Firstly, main features of the severe environment, which affects the decorative exterior materials, and methods of protection against them are shown. The optimized concrete mixes and their main properties are presented based on analysis of these features. The main areas of the usage of designed concrete-based materials are specified in the Siberian and Far East regions of Russian Federation.

Recycling of End of Life Concrete Fines into Hardened Cement and Clean Sand

One of the massive by-products of concrete to concrete recycling is the crushed concrete fines, that is often 0 - 4mm. Although the construction sector is to some extent familiar with the utilization of the recycled coarse fraction (>4 mm), at present there is no high-quality application for fines due to its moisturized and contaminated nature. Here we present an effective recycling process on lab scale to separate the cementitious powder from the sandy part in the crushed concrete fines and deliver attractive products with the minimum amount of contaminants. For this study, a lab scale Heating-Air classification system was designed and constructed. A combination of heat and air classification, resulted in a proper separation of finer fraction (0 - 0.250 mm), from coarser fractions. Heating of the materials was followed by ball milling to enhance the liberation of the cementitious fraction. Experiments were carried out at different heating temperatures and milling durations. Experimental results show that by heating the materials to 500℃ for 30 seconds, the required time of ball milling is diminished by a factor of three and the quality of the recycling products satisfies well the market demand. In addition, the removal of contaminants is complete at 500℃. The amount of CaO in the recovered finer fraction from the recycling process is comparable with the amount of CaO in low-quality limestone. By using this fraction in the cement kiln as the replacement of limestone, the release of the chemically bound CO2 could be reduced by a factor of three.

Reuse of recycled crushed concrete fines as mineral addition in cementitious materials

Environmental issues have led European stakeholders to join around an ambitious project, the reuse 70 % of inert wastes from construction and demolition by 2020. Among available solutions, the reuse of fine recycled aggregates as substitute of natural sand is somewhat problematic due to their high water absorption. The aim of the present research is to study a new way of recovery for fine recycled aggregates. We propose to use them as mineral addition, after a process of crushing and sieving resulting in powder (particles diameter lower than 80 µm). For this purpose, Recycled Crushed Concrete Fines (RCCF) were prepared from a 5 years old concrete and characterized by a physico-chemical approach. Attention was paid to assess the content of anhydrous cement. Mortars with different substitution levels of Portland cement by RCCF (Recycled Crushed Concrete Fines) or limestone filler were also studied at both fresh and harden states. The studied RCCF were found to have low content of anhydrous cement and thus low hydraulic properties. However, RCCF (Recycled Crushed Concrete Fines) was found to play a similar role than limestone filler on cement hydration. Portland cement could be substituted by RCCF (Recycled Crushed Concrete Fines) up to 25 % without altering properties of mortars. These results are encouraging and lead us to continue our research with inert wastes from real demolition sites.

Interactions of aqueous Cu 2+, Zn 2+ and Pb 2+ ions with crushed concrete fines

The crushing of reclaimed concrete-based demolition waste to produce recycled aggregate gives rise to a large volume of cement-rich fine material for which market development would be beneficial. It was envisaged that this fine fraction may prove to be an effective sorbent for aqueous heavy metal species by virtue of its ion exchangeable phases and high pH. A batch sorption study confirmed that crushed concrete, in the particle size range 1–2 mm, successfully excluded Cu 2+ (35 mg g −1), Zn 2+ (33 mg g −1) and Pb 2+ (37 mg g −1) from aqueous media. Subsequent distilled water leaching of the metal-laden concrete particles indicated that 1.9, 0.9 and 0.2% of the bound metals, Cu 2+, Zn 2+ and Pb 2+, respectively, were readily soluble. Scanning electron microscopy revealed that the removal of Cu 2+ and Zn 2+ arose from surface precipitation reactions, whereas, the principal mechanism of uptake of Pb 2+ was found to be by diffusion into the cement matrix. The metal ion removal efficiency of crushed concrete fines is compared with those of other low cost sorbents and potential applications which may exploit this sorptive property are also discussed.

Recycling concrete as fine aggregate in concrete

Increasing shortage of natural aggregates for concrete in urban areas leads to a search for aggregates from new sources. This paper examines the use of crushed concrete fines (CCF) produced from ‘waste’ concrete as fine aggregate in concrete. Tests were carried out to study the properties of crushed concrete fines and the properties of concrete incorporating them. The results show that the modulus of elasticity, pulse velocity, and the long-term properties such as drying shrinkage and creep are significantly affected. It has been found that the detrimental effects of using crushed concrete fines in concrete can be mitigated by a partial replacement of crushed concrete fines with pulverised fuel ash (pfa).

Crushed Concrete Fines Recycled for Soil Modification Purposes

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