Construction And Demolition Waste Research Articles

Performance of foam concrete developed from construction and demolition waste

Unconscious industrialization, urbanization and consumption of natural resources pose a serious threat to the environment. The threat can be reduced by upcycling construction and demolition waste (CDW) in the production of foam concrete, which is a special type of concrete used especially for thermal insulation purposes. This study aims to use of CDW as a source of fine aggregate (0-4 mm) in the production of foam concrete mixtures. A series of mixtures with target density values between 500 and 900 kg/m3 were designed with various recycled aggregate and foam contents to investigate the physical, mechanical and thermal properties of the mixtures. Findings of the study reveal that mixtures with an elevated foam content possess a greater number of larger pores, which, as observed through microscopy, form interconnected structures that are porous. On the other hand, adding more fine recycled material (FRM) makes the matrix denser, which reduces the overall porosity and increases the packing density. In contrast to increased content of FRM, correlation coefficient of (-0.77) reveal a more powerful inverse relationship between risen level of foam content (-0.67) and the reduction ratio of fresh-to-dry density and the increase in compressive strength from 7 to 28 days. It is also known that mixes with more foam have better thermal insulation because they trap more air, while mixes with more FRM show an increase in thermal conductivity. The sensitivity analysis showed that changes in thermal conductivity are almost three times as sensitive for FRM content, which means they have a bigger effect than changes in foam content. Employing the Taguchi method, (R_0.714) emerges as the most efficient tested combination indicative of superior overall performance. Notably, the Taguchi method predicts the untested combination (0.100-F/C ratio + 0.714-R/C ratio) as potentially more efficient, suggesting avenues for further exploration and optimization. This study furnishes valuable insights into sustainable construction practices by reutilizing CDW in foam concrete production, thereby contributing to both environmental conservation and enhanced structural outcomes.

Combined Application of CDWs as Precursors and Aggregates in Geopolymer Composites: A Comprehensive Rheological Analysis

The study of rheological characterization of construction and demolition waste (CDW)-based geopolymeric materials has gained interest due to digital innovation in construction and the increased focus on sustainable materials. Understanding the rheological behavior of these materials is essential due to their rapid geopolymeric reaction kinetics and viscosity behavior. This study investigated the impact of various CDW-based fine recycled aggregates, including crushed concrete aggregates (CRC), crushed ceramic-tile aggregates (CTA), and crushed brick aggregates (CBA), on the rheological behavior of geopolymer mortars (GPMs). Natural sand (NSA) was utilized as a reference for comparison purposes. In addition to CDW-aggregates, the GPMs were also made from optimized CDW-based precursors of brick waste powder (BP), ceramic-tile waste powder (TP), or concrete waste powder (CP). As a result, twelve different GPMs were considered, each containing 100% individual CRC, CTA, and CBA fine aggregates, blended with BP, TP, and CP precursors. The study evaluated the relationship between the design variable of SiO2/Al2O3 molar ratio as well as the packing densities (PD) of GPM systems and their rheological characteristics, such as flow, apparent and plastic viscosity, yield and shear stresses, thixotropy and shear strains employing the compressible packing method. The results indicate that all GPMs possess shear-thinning or pseudoplastic characteristics. Enhanced SiO2/Al2O3 molar ratio generally resulted in increased rheological parameters. However, using TP as a precursor and CBA as fine aggregates in GPM matrices led to significant enhancement in rheological parameters due to the higher specific surface area of TP and the greater irregularity, convexity, and absorption capacity of CBA particles. Notably, although the enhanced non-colloidal frictional interactions and colloidal flocculation of CRC, CTA, and CBA resulted in reduced flow tendency of GPMs, they have been found to improve the thixotropy during the initial geopolymeric gelation stage.

An evaluation model for city-scale construction and demolition waste management effectiveness: A case study in China

The growing generation of construction and demolition waste (CDW) has emerged as a prominent challenge on global environmental agendas. However, the effectiveness of CDW management (CDWM) strategies varies among cities. Existing literature predominantly evaluates the effectiveness of CDWM at the project level, offering a localized perspective that fails to capture a city’s comprehensive CDWM profile. This localized focus has certain limitations. To fill this gap in city-scale evaluations, this study introduces a novel model for assessing CDWM effectiveness at the municipal level. An empirical investigation was conducted across 11 cities within the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) to operationalize this model. The model defines five distinct levels of CDWM effectiveness. Findings indicate that Hong Kong consistently achieves the highest level (level I), while the majority of cities fall within levels III and IV. This pattern suggests that CDWM effectiveness in the GBA is moderately developed, with uneven progress in CDW management outcomes and supporting systems. Essentially, there is a lack of synchronous development of CDWM results and guarantee systems. The proposed evaluation model enriches existing CDWM research field and offers a framework that may inform future studies in other countries.

Examining the Challenges for Circular Economy Implementation in Construction and Demolition Waste Management: A Comprehensive Review Using Systematic Methods

The construction industry accounts for approximately one-third of the total waste generation globally. With the United Nations projecting a population increase of 2 billion within the next three decades, there is a heightened demand for building stock, generating unprecedented volumes of construction and demolition waste (CDW). To combat this, circular economy strategies are purported to help alleviate the prevailing situation. But a number of challenges are jeopardizing their implementation in the construction industry and preventing from achieving the UN Sustainable Development Goals, net zero carbon and zero avoidable waste targets. This paper systematically analyzes 54 research articles, published in the past decade within major peer-reviewed English-language scholarly publications in the form of a systematic research review. In doing so, it aims to identify and classify the challenges that prevent improved CDW management by assimilating previous research results in support of a circular economy. The classification and analysis using a PESTLE model offers insights into gaps and differences between categories, as well as regions and countries. This initial step could contribute to a better understanding of these barriers, along with associated solutions, which could result in a significant reduction on the impact of construction activities, therefore facilitating the development of an effective circular economy in the sector.

Optimal utilization of low-quality construction waste and industrial byproducts in sustainable recycled concrete

Construction and demolition waste (CDW) contains abundant low-quality recycled brick aggregates (RBA) that can potentially substitute natural aggregates (NA) in concrete production. However, recycled brick aggregate concrete (RBAC) shows inferior mechanical and durability performance compared to natural aggregate concrete (NAC), posing barriers to RBAC widespread adoption. This study investigates the combined influence of supplementary cementitious materials (SCMs) derived from industrial byproducts and hooked-end steel fibers (HSF) on properties of concrete with 0%, 50% and 100% RBAs from CDW as replacement for natural coarse aggregates (NCA). Addition of 0.5% HSF and 20%, 10%, and 30% fly ash (FA), silica fume (SF), and ground granulated blast furnace slag (GGBS) as cement replacement, respectively, were considered. Effect of different SCMs and HSF on compressive strength, splitting tensile strength, flexural strength, ultrasonic pulse velocity (UPV), water absorption (WA), freeze-thaw (FT) and acid attack resistance of concrete were studied. Results indicated that individual incorporation of HSF showed more contribution towards mechanical performance while combined incorporation of SCMs and HSF enhanced both mechanical and durability properties, along with reduction of up to 15.6% in embodied carbon of RBAC. Microstructural examination showed pore refinement and interfacial transition zone densification in RBAC with SCMs and HSF. This study demonstrates that sustainable RBAC with excellent mechanical and durability requirements could be produced using demolished brick waste and industrial byproducts.

Engineering and microstructural properties of self-compacting concrete containing coarse recycled concrete aggregate

In this paper, the possibility of utilizing coarse recycled concrete (CRCA) aggregate obtained from a Construction and Demolition Waste (CDW) Plant in Delhi for making a 60 MPa Self compacted concrete (SCC) was evaluated. The CRCA was used in as-collected condition and was not processed any further. Aggregate Packing (bulk) Density (APD) method was adopted to obtain an aggregate mixture exhibiting maximum bulk density/least void content (45%) with which the SCC-CRCA mixture was prepared. In addition, SCC was also made using aggregate mixtures in which the NCA was replaced with CRCA at 0%, 20%, and 100 % (of the total coarse aggregate content) by weight. The cement, fly ash, silica fume, and water were kept constant for all SCC mixtures. The effect of CRCA on the flow behavior, mechanical strength, shrinkage characteristics, and microstructure properties of SCC mixtures were evaluated. The test results indicated that the SCC mixtures made with CRCA up to 45% replacement can be used for structural concrete which is higher than that recommended in Indian (20 %) and International specifications (35 %) for traditionally vibrated (conventional) concrete.

Optimizing mechanical performance of geopolymers produced from construction and demolition waste: A comparative study of materials from different origins

Construction and demolition wastes (CDW) are promising sources for geopolymer production; however, the great variability of CDWs arising from their origin causes an abstention from implementation. As one of the key challenges in the CDW management and recycling practices, origin-depended variations in the waste materials and their reflection on the final product performance can be overcomed through in-depth waste characterization and design optimization. To this end, this paper aims to characterize CDWs collected from five different demolition zones with regard to their physical-, chemical-properties and the mechanical performance of geopolymer pastes produced with them. The compressive strength of geopolymer pastes was found to be significantly influenced by the chemical composition, crystalline nature, and fineness of CDWs. Optimization of precursor characteristics resulted in consistent mechanical performance ranging from 54.5 to 68.0 MPa after curing at 115 °C for 48 hours, regardless of the CDW origin. The aluminosilicate and calcium content of the precursor heavily influenced the ultimate gel structure in a positive manner, which determined material strength. The optimized use of CDWs from the same source increased the compressive strength of geopolymer pastes by up to 64%.

A study on the influencing parameters in developing construction and demolition waste-based geopolymer concretes and their sustainability assessment

Construction and demolition waste (CDW) has been recently identified as a potential aluminosilicate source for geopolymers. However, the available research has mainly focused on developing CDW-based geopolymer pastes and mortars, while studies on geopolymer concretes sourced from CDW have been very limited. Thus, the current study aimed at experimentally identifying different CDW materials suitable for producing geopolymer concretes. Additionally, the study analysed the mechanical, microstructural, and environmental properties of CDW- based geopolymer concrete produced. In this regard, the effect of relevant parameters on the compressive strength development of CDW-based geopolymer concretes was comprehensively investigated, including those related to precursor types/fineness, alkali activator solution, aggregate type/size and curing regimes. Microstructural analyses were also conducted on the selected samples (100% brick waste, 100% tile waste, 100% concrete waste and 75% brick waste + 25% GGBS). Finally, the environmental impact of geopolymer concrete was assessed and compared with similar traditional concrete. Results showed that employing CDWs alone is not suitable to achieve sufficient strengths under all curing regimes. However, the inclusion of 25% GGBS significantly improved the strength performance of CDW-based geopolymer concrete, in comparison to other supplementary cementitious materials (SCMs) such as Class-C fly ash and calcium hydroxide. The particle size of CDWs and concentration of alkaline activators highly affect the performance of CDW-based geopolymer concretes. Utilization of CDWs with particles finer than 75 μm and high concentrations of NaOH (12 M) is recommended to achieve good performance. The results also indicate that almost similar energy is needed for producing CDW-based geopolymer and OPC-based traditional concrete, whereas a huge reduction in CO2 emission (∼40%) was estimated in the case of geopolymers. The outcomes of the current study are expected to contribute to the advancement of geopolymer concrete derived from CDW in addition to providing valuable insights into this type of concrete for practitioners and academics.

Compositional Differences in Construction and Demolition Wastes (CDWs) for Geopolymer Mortars: A Comparative Study Using Different Precursors and Alkaline Reagents

In the view of the recycling and upscaling processes of waste materials, three different precursors, namely metakaolin, fly ash and volcanic ash, were mixed with Na- or K-silicate to produce binders aimed for the synthesis of geopolymer mortars based on construction and demolition wastes (CDWs). These later, used as aggregates in amount of 50 wt.%, were sampled in two geologically different Italian areas. A comparative study was carried out through a multidisciplinary approach using mineralogical–chemical analyses and physical–mechanical tests for the characterization of six binders and twelve mortars. The aim was to verify the effects of CDW interactions on binders as well as the extent of their compositional influences on the final properties. The chemical and mineralogical results evidenced strong compositional differences among the CDWs, differently influencing the physical–mechanical performances (i.e., compressive strength, density, water absorption and porosity) of the mortar samples. Regardless of the types of precursors and CDWs used, a better influence of K-silicate than sodium on the synthetised samples was observed. Furthermore, the higher versatility of metakaolin mortars with any type of CDW used was noted. Contrary, fly ash and volcanic ash mortars showed better properties with CDWs based on their high silica content and volcanic minerals. The study highlighted the critical roles of the CDW composition and precursor selection in mortar production. It confirmed that CDWs can be recycled for geopolymeric synthesis through proper characterisation and binder selection. Optimising these parameters allows for the successful integration of CDWs into geopolymeric materials. This process supports the advancement of a circular economy in the construction industry.

Mechanical and Durability Characterization of Hybrid Recycled Aggregate Concrete.

The recycling of construction and demolition waste (CDW) for the extraction of recycled concrete aggregates (RCAs) to be used to produce recycled aggregate concrete (RAC) is widely acknowledged internationally. However, CDW not only contains concrete debris but may also contain burnt clay bricks. The recycling of such CDW without the segregation of different components would result in recycled aggregates having different proportions of concrete and brick aggregates. The utilization of these aggregates in concrete requires a detailed investigation of their mechanical and durability properties. In this regard, the present study focused on investigating the mechanical and durability properties of hybrid recycled aggregate concrete (HRAC) made by the 100% replacing of natural aggregates with recycled brick (RBAs) and RCA in hybrid form. The partial replacement of cement with fly ash was also considered to reduce the corban footprint of concrete. An extensive experimental program was designed and carried out in two phases. In the first phase, a total of 48 concrete mixes containing coarse RBA and RCA in mono and hybrid forms were prepared and tested for their compressive strength. The test results indicated that the compressive strength of HRAC is greatly affected by the proportion of coarse RBA and RCA. In the second phase, based on the results of the first phase, eight concrete mixes with the most critical proportions of RBA and RCA in hybrid form were selected to evaluate their mechanical and durability performance. In addition, four mixes with natural aggregates were also prepared for comparison purposes. To evaluate the mechanical properties of the concrete mixes, compressive strength and modulus of rupture (MOR) tests were performed, while for the evaluation of durability properties, water absorption and behavior after exposure to aggressive conditions of acidic and brine solutions were studied. The results revealed that a 20% replacement of cement with fly ash resulted in acceptable mechanical and durability properties of HRAC intended to be used for making concrete bricks or pavers.

Environmental and Economic Viability of Using Concrete Block Wastes from a Concrete Production Plant as Recycled Coarse Aggregates.

The construction sector must incorporate the circular economy to improve sustainability and efficiency. The use of recycled aggregates (RAs) as a substitute for natural aggregates (NAs) is currently being investigated and is expected to yield considerable benefits in the future. The objective of this research is to evaluate the environmental and economic benefits of using recycled coarse aggregates (RCAs) in different 1 m3 samples of concrete, substituting the natural coarse aggregate (NCAs) with RCAs in different percentages. RCAs generally come from the treatment of construction and demolition wastes (CDWs). However, in this research, the RCAs are the concrete block wastes (CBWs) generated by a concrete production plant. Among the most notable results is that compared to concrete with no RCAs, using alternatives in which RCAs have replaced 50% of the NCAs leads to an average decrease in impact category statistics of -3.30%. In contrast to the existing literature on the subject, the process of producing RCAs generated efficiency improvements in categories such as abiotic depletion of fossil fuels (-58.72%) and global warming potential (-85.13%). This is because the transport process, a key factor in determining the viability of using RAs instead of NAs, was eliminated. In economic terms, there is a slight decrease in the financial cost of producing 1 m3 of concrete as the quantity of RCAs increases. The maximum decrease was 0.23€/m3 in the samples studied. Combining both the environmental and economic aspects resulted in a reduction factor of 0.420 g of CO2/€cent, which means fewer CO2 emissions per unit cost when using RCAs. In conclusion, these results are intended to further knowledge in the field of using RAs instead of NAs in order to help the sector achieve sustainability and find an alternative use for a particular type of business waste.

Valorization of spent Zn-C battery and construction and demolition waste (CDW): Fabrication and characterization of geopolymer composites for radiation shielding applications

There is a serious threat of pollution due to the high waste of energy and materials worldwide that needs to be decreased. One way is to reduce waste or use them in secondary processes. New eco-friendly geopolymer binders were fabricated using metakaolin, CDW, and battery waste (BW) powders as starting materials and a mixture of potassium silicate and sodium hydroxide as activation solutions. 5 formulations with BW powder replacements ranging from 5 to 40 wt% were prepared and subjected to heat curing at 80 °C for 24 h. The effect of Zn-C battery waste content on the phase composition, densification, microstructure, mechanical, and gamma shielding properties of the obtained geopolymer composites was investigated. The results revealed that incorporating battery waste reduced the density and mechanical strength of the geopolymer composites. Additionally, microstructural analysis indicated that synthesized geopolymers consisted of unreacted CDW and BW particles embedded in a geopolymer matrix with a low degree of compactness. The effect of BW replacement on the shielding capacity was also evaluated experimentally and theoretically. The gamma transmission experiments proved that the addition of BW caused a reduction in the radiation shielding ability of the composites when they were exposed to 137Cs and 60Co gamma-ray sources. Finally, the variation of the main attenuation parameters (MAC, HVL, EBF, and EABF) with photon energy was also obtained theoretically. The outcomes of this investigation can provide a basis for the design of new CDW-based geopolymers for radiation shielding applications in the context of a circular economy.

3D-printable construction and demolition waste-based geopolymer: Investigating the effects of additives on engineering properties

This paper focuses on the investigating the effects of various additives on the engineering properties of construction and demolition waste (CDW)-based geopolymer mortars. In this study, low-activity CDW-based precursors, specifically brick waste (BW) and concrete waste (CW), were employed. Additionally, industrial waste-based precursors such as slag (S) and silica fume (SF) were utilized to enhance the overall composition. To facilitate activation, a 5 M solution of NaOH was used. To address the efflorescence issue prevalent in geopolymer mixtures and to eliminate potential cracks that may arise due to the dimensional stability problem of 3D-printed CDW-based geopolymer, various kind of additives including calcite, polypropylene fiber, nano SiO2, Al2O3, and TiO2, carbon nano tubes (CNTs), graphene nanoplatelet (GNP), methyl cellulose (MC), Na2SiO3, calcium oxide (CaO), calcium aluminate cement (CAC), and Ca(OH)2 were incorporated into the matrix. The research utilized a comprehensive testing methodology, allowing for a detailed exploration of various properties and the performance of the geopolymer mixtures. The results indicated that nano SiO2 and Al2O3, Na2SiO3, BW, and CAC were recognized for their positive impact on improving the mechanical properties of the geopolymer. Notably, the geopolymer exhibited an impressive compressive strength of 29.8 MPa and a flexural strength of 5.4 MPa following a 28-day ambient curing (23 ± 2 °C and 50 ± 5% relative humidity). Furthermore, the inclusion of nano SiO2 and Al2O3, GNP, MC, Na2SiO3, CaO, CAC, and Ca(OH)2 proved effective in preventing efflorescence, while MC, Na2SiO3, CaO, CAC, and Ca(OH)2 successfully mitigated cracks formation. The results also demonstrated that the CDW-based geopolymer mixture is suitable for 3D printing, displaying satisfactory buildability, shape retention, and extrudability. A thorough testing approach highlighted the effectiveness of additives in shaping the properties of CDW-based geopolymer, positioning them as promising for improvement and addressing challenges.

Impact of Recycled Concrete and Brick Aggregates on the Flexural and Bond Performance of Reinforced Concrete

The construction industry strongly relies on concrete and clay bricks for various applications. The escalating demand for these materials, driven by rapid population growth, has led to resource depletion and increased construction and demolition waste (CDW). Recycling CDW into construction materials, particularly in the form of recycled concrete aggregates (RCAs) and recycled brick aggregates (RBAs), has emerged as a promising solution. This study deals with the structural performance of concrete incorporating RCAs and RBAs. The experimental program encompasses material characterization, concrete mix design, and several tests to assess density, compressive strength, bond behavior, and flexural properties. The results indicate that the replacement of fine natural aggregate (NA) with fine RCAs or RBAs has a negligible impact on density, while the partial replacement of coarse NAs with RAs yields modest reductions in compressive strength. Notably, the bond strength between steel rebar and concrete is influenced by the type and content of RA, with specimens containing RCAs exhibiting a higher bond strength than those with RBAs. Empirical models used to predict bond strength generally align with experimental results, with conservative predictions by some models, such as ACI 318, and overestimation by others, such as models proposed by AS-3600 and CEB-FIB. The flexural tests of beams highlight the variation in stiffness and load-bearing capacity with the proportion of NAs replaced by RAs. While beams with 50% NA replacement demonstrate comparable performance to control beams, those with 100% RA replacement exhibit lower cracking and yielding stiffness. Cracking patterns in beams with RAs differ from control beams, with RA-containing beams showing more cracks and an altered crack distribution. The findings underscore the feasibility of using recycled aggregates in construction, with partial NA replacement offering a balance between sustainable material usage and desired structural properties.

Characterization of CDW types by NIR spectroscopy: Towards an automatic selection of recycled aggregates

The petrographic and mechanical heterogeneity of construction and demolition waste (CDW) strongly affects the physic-mechanical performance of new concrete and mortar mixes (RAC: recycled aggregate concrete and RAM: recycled aggregate mortar) made with these recycled aggregates (RA). It is thus extremely important to quickly discriminate and sort them in groups of materials with homogeneous petrography, similarly to primary natural raw materials. Eighteen CDW samples, divided in six groups (natural stones, concrete, bricks, perforated bricks, roof tiles and tiles), from the Abruzzo region (central Italy) are analysed by both micro-X-ray fluorescence energy-dispersive spectroscopy (XRF-EDS) and hyperspectral imaging (HSI), in particular in the short wavelength infrared range (SWIR). Every SWIR spectrum is characterized by three regions of absorption at the following wavelength ranges: 1330–1680 (I), 1830–2140 (II) and 2140–2400 nm (III), displaying the lowest, highest and intermediate intensities, respectively. The first and second wavelength regions are linked to O–H and H–O–H motions, whereas the highest wavelength range is linked to atoms bonded to the O–H environment and/or to the carbonate group. According to previous analysis and to the micro-XRF-EDS surface chemistry, the amount of CaCO3 is rich to very rich for concrete and natural stones, intermediate to absent for masonries and absent for tiles. The intensity or area (%) of the absorption component vibrating at about 2345 nm is directly proportional to the amount of CaO (CaCO3 and CO2), and inversely to SiO2 and Al2O3. Therefore, SWIR can rapidly detect and separate CDW as a function of their carbonate to aluminosilicate contents, i.e. concrete and natural stones made with limestone vs masonries and tiles. The attained outcomes are of paramount importance either for routinely screening or in post-disaster circumstances.

Taking spontaneous plants as a natural strategy for vegetation restoration in construction and demolition waste landfills: a case study in Suzhou, China.

Construction and demolition waste (CDW) landfills around the city have caused serious damage to the ecological environment and menaced the public health. Restoration of closed CDW landfills is critical to compensate for the degraded ecosystem and ensure safety in further development and utilization. Vegetation restoration is an essential part of the restoration of CDW landfills, in which the use of spontaneous plants is the foundation of the nature-based strategy. In this study, Fenghuangshan CDW landfill in Suzhou, China, was selected as the research site, and the species composition and diversity of the spontaneous plants were analyzed. Moreover, the types of habitats and growth indexes of 8 species with high frequency and 18 species with medium frequency in the CDW landfill were investigated, and a comprehensive evaluation of growth rate and expansion capacity of the 26 species was conducted. The results showed that, herbs were the main type of the spontaneous plants in the CDW landfill. The species and quantities of the spontaneous plants in the CDW landfill were obviously fewer than those in the surrounding areas of the CDW landfill, and the Shannon-Wiener index and Pielou index of the spontaneous plants were lower compared with the surrounding areas of the CDW landfill. Meanwhile, the differences of dominant families and the distribution of origins, life forms and growth types between these two fields were insignificant. The heliophilous and drought tolerance species were widely distributed in the CDW landfill while the shade-tolerant or hygrophilous species were few. The relatively large comprehensive evaluation indexes of Elymus dahuricus, Daucus carota, Sonchus asper, Geranium carolinianum, Rumex acetosa, Metaplexis japonica, Carex breviculmis, Erigeron canadensis, Trigonotis peduncularis, Lamium amplexicaule reflected their high growth rates and strong expansion capacity, demonstrating their great potentiality in the vegetation restoration of CDW landfills as indispensable components of the nature-based solution.

Investigating the Determinants of Construction Stakeholders’ Intention to Use Construction and Demolition Waste Recycling Products Based on the S-O-R Model in China

In China, the annual generation of construction and demolition waste (CDW) has been steadily increasing, accompanied by a generally low recycling rate. To promote sustainable development, there is an urgent need to enhance the recycling of CDW. This paper aims to investigate the determinants of construction stakeholders’ intention to use CDW recycling products in China. The stimulus–organism–response (S-O-R) model, integrating the technological–organizational–environmental (TOE) framework, personal perceptions, personal traits, and the intention to use, was chosen as our theoretical model. Through an analysis of 272 valid questionnaires, the partial least squares structural equation modeling (PLS-SEM) was utilized to evaluate the model and test the proposed hypotheses. The results indicated that personal traits are the most influential factor shaping construction stakeholders’ intention to use, followed by personal perceptions, while external stimuli exert no direct significant impact on the intention to use. Nevertheless, personal traits and personal perceptions play a significant mediating role in the relationship between external stimuli and the intention to use, forming a noteworthy serial chain mediation. The research findings imply that in China, bolstering personal traits plays a critical role in guiding and promoting the intention to use CDW recycling products.

Sustainable urban development: Evaluating the potential of mineral-based construction and demolition waste recycling in emerging economies

Widespread illegal disposal of surging construction and demolition waste (CDW) is a prominent threat to recycling, leading to resource wastage and environmental issues. The lack of data is a major barrier to designing an effective waste management system in emerging economies. Employing a mixed-method approach, a cross-sectional case study was conducted on an Indian city to assess the existing system and investigate the root causes of low recycling rates and limited demand for recycled materials. The study finds that (a) illegal disposal is prevalent due to the "end-of-pipe" approach of the waste management system, (b) policy enforcement on waste generators is weak, (c) low-rise residential buildings are the major contributor of waste, (d) low demand for recycled material is due to lack of awareness, marketing, incentives, and high costs, (e) transportation costs are 50 % higher than the recycled material cost, and (f) lack of segregation leading to the production of low-value materials. All these factors make recycling unattractive and the circular economy (CE) unfeasible. Stakeholder awareness, a centralized trading platform, and integrated waste management systems are essential to promote recycling and advance SDGs 9, 11, and 12, focusing on industry, sustainable cities, and responsible consumption and production.

Optimization and mechanism analysis of a compound additive for unfired bricks made of construction and demolition wastes

Construction and demolition waste (CDW) was fully exploited to prepare high-strength and low-cost unfired bricks. A compound additive consisting of sodium silicate, microsilica powder, an early-strength water reducer, and wood fiber was incorporated into the bricks. Tests (compressive tests, freeze–thaw cycle tests, and scanning electron microscopy (SEM)) were carried out to determine the effect of the contents of the additive components on the properties (the strength, softening coefficient, freezing resistance, hydration products, and microscopic morphology) of unfired bricks of different curing ages. The experimental results were used to determine the optimum ratio of the components and the hydration mechanism. The optimized compound additive considerably improved the mechanical properties and crack resistance of the bricks, where the optimum content was found to be only 3.15% of the CDW dry mass. Compared with unfired bricks with no additives, unfired bricks with the optimized compound additive exhibited increases in the 1- and 28-day compressive strengths and softening coefficient of up to 66.8%, 65.9%, and 8.46%, respectively (corresponding to values of 8.46 MPa, 29.36 MPa, and 0.934, respectively) and a decrease in the freeze–thaw strength loss rate of 61.38%. Incorporating the compound additive into the unfired bricks considerably reduced the environmental impact. The SEM micrographs showed that the compound additive increased the silicon-to-calcium ratio and workability of the preparation mixture, increased the hydration rate, promoted the conversion of calcium hydroxide in the product to a C–S–H gel, and enhanced the density and strength of the hydration product.

Sensor-based characterization of construction and demolition waste at high occupancy densities using synthetic training data and deep learning.

Sensor-based monitoring of construction and demolition waste (CDW) streams plays an important role in recycling (RC). Extracted knowledge about the composition of a material stream helps identifying RC paths, optimizing processing plants and form the basis for sorting. To enable economical use, it is necessary to ensure robust detection of individual objects even with high material throughput. Conventional algorithms struggle with resulting high occupancy densities and object overlap, making deep learning object detection methods more promising. In this study, different deep learning architectures for object detection (Region-based CNN/Region-based Convolutional Neural Network (Faster R-CNN), You only look once (YOLOv3), Single Shot MultiBox Detector (SSD)) are investigated with respect to their suitability for CDW characterization. A mixture of brick and sand-lime brick is considered as an exemplary waste stream. Particular attention is paid to detection performance with increasing occupancy density and particle overlap. A method for the generation of synthetic training images is presented, which avoids time-consuming manual labelling. By testing the models trained on synthetic data on real images, the success of the method is demonstrated. Requirements for synthetic training data composition, potential improvements and simplifications of different architecture approaches are discussed based on the characteristic of the detection task. In addition, the required inference time of the presented models is investigated to ensure their suitability for use under real-time conditions.