Search Results (417)

Three-dimensional concrete printing (3DCP) has emerged as a promising digital construction technology that reduces material waste, eliminates formwork, and enables complex geometries. However, its sustainability remains constrained by the extensive use of ordinary Portland cement (OPC) and natural aggregates. This review comprehensively evaluates waste utilization in extrusion-based 3D printed concrete, classifying applications into three pathways: cement replacement in OPC-based systems, waste-derived precursors in alkali-activated/geopolymer binders, and fine aggregate replacement. Industrial, agricultural, and marine wastes are assessed regarding their effects on rheology, printability, mechanical performance, interlayer bonding, and durability. The reviewed literature investigated waste incorporation levels reaching up to 50% for cement replacement, up to 70% for alkali-activated/geopolymer systems, and up to 100% for aggregate replacement, depending on the material type and application pathway. Industrial wastes, particularly fly ash, slag, silica fume, and metakaolin, represent the most mature materials and generally improve printability and long-term performance. Agricultural and marine wastes show promising sustainability potential but remain insufficiently investigated. Despite encouraging laboratory-scale results, challenges related to material variability, early-age performance, standardization, and scalability continue to limit practical implementation. The review identifies critical research gaps and outlines future directions for developing sustainable and field-ready 3DCP technologies. Full article

This study investigates the valorization of construction and demolition (C&D) waste streams and an industrial by-product for sustainable concrete production. Recycled concrete aggregates (RCA) and recycled brick aggregates (RBA), derived from C&D wastes, together with pelletized recycled fly ash aggregates (FAA) produced from thermal power plant fly ash, were used as total replacements for natural coarse aggregates. Six concrete mixtures were prepared at a constant water-to-cement ratio of 0.50 using untreated and slag slurry–treated aggregates. A slag slurry-based two-stage mixing approach (TSMA), incorporating ground granulated blast furnace slag (GGBFS), was applied as a practical and potentially scalable treatment method to enhance aggregate quality and interfacial bonding. The results show that complete replacement of natural aggregates reduced fresh concrete unit weight by up to 17%, while meeting the minimum compressive strength requirements for structural applications. Slag slurry treatment led to statistically significant improvements in mechanical properties, reduced variability, and enhanced overall reliability. In addition, widely used code-based prediction models (TS500, ACI, Eurocode-2, NZS 3101-1:2006, and CSA A23.3-04), originally developed for conventional concrete, were evaluated for their applicability in estimating key mechanical properties of recycled and by-product aggregate concretes, and alternative regression-based models were developed to improve prediction accuracy. Overall, the findings demonstrate the potential for effective utilization of C&D wastes and industrial by-products in structural concrete, contributing to resource efficiency and reduced reliance on natural aggregates. Full article

Concrete contributes significantly to global CO₂ emissions due to high energy demand for cement production. This research integrates multiple advanced ensemble ML-based prediction models by combining experimental evaluation, explainable framework, and life cycle sustainability analysis for SCM (supplementary cementitious materials)-incorporated concrete mixtures. A comprehensive experimental program was conducted to evaluate the compressive and tensile strength of concrete revealing that the hybrid mix of GF4 with a 40% replacement level of cement with fly ash (FA) and ground granulated blast furnace slag (GGBFS) exhibited optimum synergistic performance due to balanced hydration kinetics and improved microstructure characteristics. For computational model development, a k-fold cross validation technique was deployed to evaluate robustness across multiple data partitions and to control overfitting in models. Model performance was assessed through multiple metrics including R², RMSE, and MAE with particular emphasis on the gap between training and testing performance. The best performing model was optimized using Particle Swarm Optimization (PSO) and Bayesian Optimization (BO) techniques providing an additional safeguard against overfitting. Shapley Additive Explanation (SHAP) interpretation revealed w/b ratio and curing age as key parameters for compressive strength, while fine aggregate content and curing age influenced tensile strength. For compressive strength, XGBoost model performed well with an R² value of 0.879 which was increased to 0.918 with the PSO optimization technique. For tensile strength, the Gradient Boosting model was selected with an R² value of 0.840 which was optimized to 0.879 after the PSO optimization technique. Moreover, life cycle assessment was performed to evaluate the environmental impacts in terms of embodied carbon and energy associated with concrete mixes. The hybrid GF4 mix demonstrated a 36% reduction in embodied carbon compared to the control mix, indicating strong potential for low carbon concrete applications. This integrated research contributes to the advancement of green construction practices and supports global efforts to reduce atmospheric impacts through the circular use of industrial byproducts. Full article

This work investigates Ferro-Rock concrete as a carbon-negative alternative to ordinary Portland cement (OPC), which accounts for 5–9% of global CO₂ emissions, and evaluates its viability as a sustainable construction material. Ferro-Rock is an iron-based binder comprising recycled iron powder, fly ash, metakaolin, limestone powder, and oxalic acid. This is enhanced by a carbonation reaction in which iron particles react with CO₂ and water to form iron (II) carbonate (FeCO₃), the main binding phase, thereby locking in atmospheric CO₂. The experimental program was divided into two groups. Group 1 studied 100% Ferro-Rock binders with different types of aggregate, specimen sizes, and CO₂ curing periods (0–6 days) with a new locally manufactured stainless steel curing chamber that provided a controlled CO₂ environment of 99.9% and 1.2–1.5 bar gauge pressure. Group 2 investigated Ferro-Rock as a partial cement replacement at 0%, 5%, 10%, 15% and 20% levels of substitution with 5% increments. The 7 and 28 days of compressive, flexural and indirect tensile strengths were determined. The results showed the Ferro-Rock with 100% iron ductile waste aggregates (Mix F4) achieved a 28-day compressive strength of 5.5 MPa, 37.5% higher than the standard Ferro-Rock reference mix. The optimum replacement range of Group 2 was 5–10% with an increase in compressive strength by 5–10%, flexural strength by 11%, and indirect tensile strength by 16% over the OPC control. When replacement exceeded 25%, the bonding was weakened, and all strength measures decreased significantly, reaching a 46% reduction in compressive strength at 50% substitution. Scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) microstructural analysis verified the gradual formation of the iron carbonate crystalline phase and provided mechanistic insights into the observed strength trends. Fully cured Ferro-Rock specimens sequestered as much as 11% CO₂ by weight, with a verifiably carbon-negative profile that no OPC-based system can match. Full article

The increasing generation of ceramic waste from manufacturing defects, construction activities, and demolition operations poses significant environmental and waste management challenges worldwide. This study presents a preliminary investigation into the incorporation of ceramic waste aggregates (CW) as partial and full replacement for natural coarse aggregates in fly ash-based geopolymer concrete (GPC) under water-curing conditions. Five mix compositions were prepared with ceramic waste aggregate replacement levels of 0%, 20%, 40%, 60%, and 100%. Fresh and hardened properties were evaluated using flow table and early-age compressive strength tests at 7 and 14 days. The 20% replacement mix achieved the best compressive strength value of 5.52 MPa at 14 days, slightly exceeding the control GPC mix (5.09 MPa) among the limited mixtures investigated in this preliminary study. However, higher replacement levels resulted in reduced compressive strength, which may be associated with increased porosity, weaker aggregate–matrix bonding, and limitations related to the adopted water-curing regime. Workability remained within acceptable flow ranges for most mixes, although reduced flowability was observed for the 40% replacement. The comparatively low strength values obtained across all mixtures may largely be associated with the absence of heat curing and the inclusion of additional water to improve workability, both of which likely limited the geopolymerization efficiency. Based on the comparatively low compressive strength values obtained, the investigated mixtures, in their current form, are only suitable for low-strength or non-structural applications rather than structural concrete applications. Overall, this study provides preliminary insights into the influence of ceramic waste coarse aggregates on the workability and early-age compressive strength behavior of fly ash-based geopolymer concrete under the adopted experimental conditions. Further optimization of the curing regimes, mix design parameters, and long-term mechanical and durability performance is necessary before broader engineering applicability can be established. Full article

Ecological concrete (EC) is a promising material that offers both effective surface protection in engineering and contributes to ecological restoration. However, it remains a significant challenge to improve bending resistance and tensile strength simultaneously, particularly for slope ecological restoration, where these properties are essential. This study employs an orthogonal experimental design to evaluate the effects of various factors, including water–cement ratio, coarse aggregate, fly ash, and metakaolin on the performance of EC, with the goal of determining an optimal mix ratio that satisfies both porosity and compressive strength requirements. The results indicate that, when the water–cement ratio is 0.30, the coarse aggregate particle size is 3–6 mm, the fly ash content is 15%, and metakaolin content is 10%, the EC achieves superior performance with a compressive strength of 18.3 MPa and a porosity of 29%. Then, a flexible ecological concrete blanket (FECB) is subsequently proposed utilizing this optimized EC mix. The FECB demonstrates excellent bending performance and a tensile strength of 4.2 MPa. This innovative FECB not only expands the application potential of EC in engineering but also provides a promising solution for future slope surface protection materials. Full article

This study investigates the influence of fly ash (FA) content and aggregate type on the mechanical performance and environmental efficiency of concrete. Twelve concrete mixtures were prepared using limestone and basalt aggregates, with FA replacement levels of 0%, 15%, and 35% and water-to-binder (w/b) ratios of 0.4 and 0.7. Compressive strength (CS), the modulus of elasticity (MoE), water absorption, and freeze–thaw resistance were measured. Basalt aggregates enhanced the CS and MoE while reducing water absorption and freeze–thaw deterioration compared to limestone. Although a higher FA content lowered early-age strength and stiffness, it contributed to long-term improvements and greater eco-efficiency. A new MoE prediction model incorporating CS, unit weight, aggregate type, and FA content demonstrated better accuracy than current standards. Assessment of binder usage and CO₂ intensity confirmed that all mixtures remained below the average literature values. The optimal combination was achieved with basalt aggregates, a high FA content, a low w/b ratio, and extended curing, highlighting strategies for sustainable concrete production. Full article

Low tensile strength (brittleness) is a significant drawback of lightweight aggregate concrete, as it significantly limits its application. The parameters can be improved by using dispersed reinforcement. For the purpose of the study, two fractions of high-strength lightweight aggregate were used. It was produced by sintering waste material from power plants and cogeneration plants (e.g., fly ash). Hook-shaped steel fibres were used as the reinforcement. The presented tension test results apply to lightweight fibre-reinforced concrete, i.e., flexural tensile strength, splitting tensile strength and residual flexural tensile strength compared to lightweight non-reinforced concrete. It also refers to the analysis of fibre distribution using computer tomography and the microstructure of the fibre–cement slurry contact zone. The test results revealed that steel fibres are distributed correctly in lightweight concrete, creating effective reinforcement for the brittle cement matrix. The experimental work was supported by numerical simulations based on the Finite Element Method (FEM). A lightweight concrete structure with volumetric content and steel fibre distribution identical to those used in the experiment was modelled. This way, the numerical simulations were verified. The confirmation of the numerical model’s reliability shall help engineers develop the material’s strength at the product design stage. The optimisation shall be possible owing to the easy application of the fibres’ variable configuration, given their share and orientation. As a result of combining experimental tests with numerical simulations, the paper evaluates the influence of steel fibres on the strength of lightweight concrete. Ansys Workbench software was used to model a three-point bending test on lightweight concrete beams. A Menetrey–Willam constitutive model was selected to represent the mechanical behaviour of fibre-reinforced concrete; the model assumed material hardening/softening. Simulations yielded numerical responses similar to the experimental results, confirming the model’s ability to capture the fibre reinforcement’s influence on the forms of destruction. Full article

Geopolymer concretes are increasingly regarded as advanced construction materials for applications requiring high thermal and chemical resistance. This article is a continuation of previously published research and focuses on the mechanical behaviour of geopolymer concretes containing aggregates made of foamed geopolymers and lightweight mineral aggregates, such as expanded clay and perlite, intended for use in chimney flue components. The aim of the study was to determine the influence of lightweight aggregates on the relationship between thermal insulation and the strength parameters of geopolymer concretes intended for use at elevated temperatures. Foamed geopolymer aggregates were produced by a controlled chemical foaming process, followed by grinding to specific grain sizes, yielding highly porous aggregates with low thermal conductivity, reaching approximately 0.075–0.099 W/(m·K). These aggregates were used as lightweight fillers in geopolymer concretes based on class F fly ash activated with alkaline solutions. The resulting composites were designed to combine low density and high thermal insulation with adequate mechanical strength. The mechanical properties of the developed concretes were assessed on the basis of compressive strength tests on cubic specimens and tensile strength in beam bending tests, carried out in accordance with standards. The results presented confirm that the use of foamed geopolymer aggregates enables a simultaneous increase in thermal insulation and the design of ultra-lightweight structural elements with sufficient load-bearing capacity for chimney systems (including suspended ones). This combination of low thermal conductivity, reduced mass, and appropriate mechanical properties makes geopolymer concretes with lightweight mineral and geopolymer aggregates a promising alternative to traditional ceramic materials. Full article

This study investigates the combined effect of incorporating recycled concrete aggregates (RCAs) and glass fibers (GFs) on the properties of geopolymer concrete. The precursor binder consisted of a blend of ground granulated blast furnace slag and fly ash. Furthermore, two types of GFs (i.e., short and long) were incorporated, either individually or in hybrid combinations, to enhance the performance of the concrete. Experimental results revealed that replacing natural aggregates (NAs) with RCAs in geopolymer concrete production had no tangible impact on workability but resulted in a slight reduction in the density, ultrasonic pulse velocity, and bulk resistivity. Similarly, the compressive strength and modulus of elasticity decreased by up to 18 and 57%, respectively. Meanwhile, the addition of GFs, particularly in hybrid configurations, effectively mitigated these reductions. Among the hybrid mixtures, a short-to-long fiber ratio (A:B) of 1:3 yielded the most significant improvements of the physical, mechanical, and durability properties, with increases of up to 16%, 91%, and 61%, respectively. Several correlation equations were established to describe the relationships between the physical, mechanical, and durability properties of GF-reinforced geopolymer concrete and were compared with existing codified models. The outcomes provide critical insights into the synergistic roles of RCA and GFs in tailoring high-performance, eco-efficient concrete systems. This research supports the advancement of sustainable concrete production and promotes the broader structural adoption of geopolymer technologies. Full article

Natural pumice can reduce the self-weight of concrete, but its high porosity, high water absorption, and weak interfacial bonding tend to limit the strength and durability of lightweight aggregate concrete. To address this issue, this study proposes a method for preparing and applying reinforced pumice lightweight aggregates, namely, using nano-SiO₂-modified fly ash to construct a nanocomposite material at the micro-interface for the reinforcement treatment of natural pumice aggregates, and reveals the mechanism by which this treatment enhances the performance of lightweight aggregate concrete. Through aggregate performance tests, compressive strength tests, XRD, SEM, and freeze–thaw cycle tests, the effects of the reinforced pumice aggregate on the performance of lightweight concrete were systematically investigated. The results show that after the reinforcement treatment, the water absorption of the pumice aggregate decreases by 17.6%, and the cylinder compressive strength increases by 34.3%. As the replacement ratio of reinforced pumice increases, both the early-age and later-age compressive strengths of the concrete continuously improve. When all the pumice aggregate is reinforced, the 3 d and 28 d compressive strengths increase by 35.1% and 33.44%, respectively. Meanwhile, the reinforced pumice effectively improves the interfacial bonding between the aggregate and the cement paste, reducing the width of the interfacial transition zone by 32%, enhancing the matrix compactness, and delaying crack propagation. The study demonstrates that the reinforced pumice aggregate possesses favorable characteristics, not only effectively improving the mechanical properties and freeze–thaw resistance of lightweight concrete but also providing a new technical pathway for the high-performance utilization of porous lightweight aggregates, offering a reference for the resource utilization of industrial solid waste and engineering applications in cold regions. Full article

The growing demand for sustainable construction has highlighted the need to effectively utilize solid waste materials in concrete production, yet achieving satisfactory workability, strength, and durability simultaneously remains challenging. A multi-parameter mix-design methodology is proposed for solid-waste-based self-compacting concrete (SCC). This method couples minimum water demand, control of paste film thickness, and multi-performance balancing. The ternary solid-waste powder system (silica fume, fly ash, and supersulfated solid-waste-based cement) was first optimized through minimizing water demand to achieve maximum packing density. The resulting composition was then blended with varying dosages of ordinary Portland cement (OPC) to form the final cementitious binder. Aggregate gradation was proportioned to minimize voids, and paste volume was determined using an equivalent-paste-film-thickness model. Under comparable mixture conditions, SCC with OPC contents of 70–40 wt.% and paste film thicknesses of 2.0–2.6 mm was evaluated for fresh performance, compressive strength, freeze–thaw resistance, and material cost. Mixtures with a paste film thickness of 2.4 or 2.6 mm satisfied the self-compactability criterion—the mix with 50 wt.% OPC and a paste film thickness of 2.4 mm showed the best overall performance balance, achieving higher 28 d strength than higher-OPC mixtures while improving freeze–thaw resistance and reducing cost. Results from TGA, XRD, ATR–FTIR, and SEM–EDS analyses indicated enhanced calcium hydroxide (CH) consumption, increased formation of C-(A)-S-H and ettringite, and a denser interfacial transition zone (ITZ), supporting the proposed multi-objective design approach. While the framework was validated for a specific ternary binder system, it provides a reproducible proportioning strategy applicable to a broader range of solid-waste-based concrete systems, with potential for extension to other waste streams and exposure conditions, thus supporting the development of more resource-efficient and environmentally sustainable concrete. Full article

Geopolymer concrete (GPC) has emerged as a promising low-carbon alternative to ordinary Portland cement (OPC), yet wider adoption is limited by the lack of standardised mix-design procedures. Precursor, activator, curing, and aggregates strongly interact to affect properties, but findings are scattered and hard to generalise. This review consolidates and normalises published findings to clarify how key parameters-precursor type, activator dosage and concentration, activator-to-binder ratio, curing temperature, and aggregate gradation-control fresh and hardened performance. Overall trends indicate that calcium-rich systems enhance early strength by 80–100% at typical replacement levels; while optimum activator conditions of 12 M NaOH and sodium silicate/sodium hydroxide ratio = 2.5 commonly improve strength by 40–60% relative to sub-optimal ratios; alkaline activator-to-binder ratios of 0.4–0.7 provide the most practical strength-workability balance. Heat curing at 80–100 °C significantly accelerates early-age property development by 50–200% compared to ambient curing, depending on duration and activator chemistry. A target-strength mix design is demonstrated through a 40 MPa case study. Using a compiled dataset for fly ash (FA)-based GPC, a modulus–strength framework is proposed; common OPC code equations over-predict elastic modulus for 15–50 MPa, and calibration yields a conservative, code-compatible relation: E = $2.75\sqrt{f{c}_{MPa}}$. Key limitations are highlighted, including variability in raw materials and durability uncertainties, and future directions are proposed toward performance-based design and standardisation to support structural use of GPC in sustainable infrastructure. Full article

To reduce the production cost of ultra-high performance concrete (UHPC), this study incorporated polyacrylonitrile (PAN) fibers and coarse aggregates (CA) to develop a novel UHPC with both excellent performance and reduced cost. A two-stage mortar-concrete design approach was employed to optimize the UHPC mix proportion. First, the mortar matrix was preliminarily optimized based on particle packing theory, and its strength development mechanism was analyzed. Subsequently, response surface methodology was applied to systematically investigate the effects of PAN fiber content, CA content, and superplasticizer (SP) dosage on the slump flow, compressive strength, flexural strength, indirect tensile strength, freeze–thaw resistance, and dynamic mechanical properties of UHPC. The entropy weight method was then adopted to determine the optimal mix proportion, followed by cost estimation. The results indicate that the optimal mortar matrix composition consists of 61.4% cement, 15% silica fume, and 23.6% fly ash, achieving a flow spread of 246 mm, a compressive strength of 117.2 MPa, and a flexural strength of 24.9 MPa. When the PAN fiber content, CA content, and SP dosage were 0.5%, 20%, and 3.8%, respectively, the prepared PAN-CA UHPC(PCUHPC) exhibited the best overall performance. Compared with conventional UHPC, the material cost was reduced by 81.7%, and the compressive strength-normalized cost decreased by 75.4%. The UHPC developed in this study, characterized by outstanding performance and significant cost advantages, provides a feasible solution and theoretical support for broader engineering applications. Full article

This study evaluates the feasibility of utilizing manufactured sand as a full or partial replacement for river sand in mass concrete production, motivated by the growing scarcity of natural river sand and stringent environmental regulations on mining. The influence of the manufactured sand replacement level, water-to-cement ratio, and fly ash content on key properties including workability, mechanical strength, early-age shrinkage, and thermal stress was systematically investigated. The results demonstrate that, while the incorporation of manufactured sand marginally impairs workability, it contributes to an improved particle size distribution of the fine aggregate. At 100% replacement, the 56-day compressive, flexural, and tensile strengths, as well as the elastic modulus of manufactured sand concrete, exceed those of river sand concrete, accompanied by a notable reduction in early-age shrinkage. A decrease in the water–binder ratio enhances mechanical performance but concurrently elevates the risk of cracking due to the increased autogenous shrinkage and adiabatic temperature rise associated with a higher cement content. The addition of an optimal amount of fly ash (e.g., 25%) effectively improves both workability and mechanical properties while substantially mitigating hydration heat, thereby reducing temperature differentials and the associated cracking risks. Microscopic analysis reveals that unhydrated particles, including fly ash and quartz, may act as initial defects within the microstructure. Overall, the replacement of river sand with manufactured sand in mass concrete is technically feasible, and an appropriate mix design optimization can achieve a desirable balance between performance and crack resistance. Full article