Search Results (89)

Extensive research has addressed concrete deterioration and its countermeasures; however, studies on responsive repair methods and materials remain comparatively limited and less systematic. In this study, six mixtures of repair mortars (RMs) were formulated using aluminate-based binders, specifically calcium sulfoaluminate (CSA) and amorphous calcium aluminate (ACA) cements. The experiment evaluated the mechanical properties and freeze–thaw resistance of these mortars. To accelerate hydration, a controlled amount of anhydrite gypsum was incorporated into each mixture. The fluidity and setting time of fresh RMs were measured, whereas the compressive strength, flexural strength, and ultrasonic pulse velocity (UPV) of hardened RMs were evaluated at 1, 7, and 28 days. In addition, freeze–thaw resistance was assessed as per ASTM C666 by determining the relative dynamic modulus of elasticity. Additionally, the hydration products and microstructural characteristics of paste specimens were qualitatively analyzed. The mechanical performance, including strength and UPV, and freeze–thaw resistance of RMs containing ACA were superior to those of RMs containing CSA. In particular, compared to the CSA-containing specimens exposed to freeze–thaw action were significantly deteriorated, the ACA-containing specimens showed excellent resistance with relatively less cracking and spalling. This may imply that ACA is effective as rapid repair materials for concrete structures in cold regions. Microstructural observations revealed variations in hydration products depending on the aluminate binder employed, which significantly influenced the mechanical and durability properties of the RMs. These results may aid the selection of optimal repair materials for deteriorated concrete structures. Full article

The study examines how crushed and sieved concrete rubble—recycled concrete fines (RCF) and the ways of their reactivity activation—affect processing, mix design, and properties of cement-based concrete. Based on the relationship to mass loss during crushing, the compressive strength of the concrete fines processed from rubble was initially determined. The morphology of the particles as well as the chemical and mineralogical composition of RCF were ascertained using XRD, SEM, and EDS characterization tests. Certain RCF surface area (fineness) and type of treatment are associated with specific pozzolanic activity of RCF. Using the approaches of factorial experimental design, tests were planned by varying six factors: RCF specific surface area, RCF content, thermal treatment temperature of RCF, cement content, superplasticizer dosage, and hardening accelerator (Na₂SiF₆) content in concrete containing RCF. Statistical processing of the research results data provided adequate polynomial regression models for the water demand of the concrete and the compressive strength of hardened concrete at 7 and 28 days. The models were quantitatively analyzed to evaluate the influence of the studied factors on the output parameters and to rank them according to their impact. The greatest increase in water demand was attributed to cement content change, in particular above 400 kg/m³, and to RCF content. It was established that the addition of a superplasticizer compensated for additional water demand and the reduction in compressive strength caused by partial replacement of cement with RCF. Increasing the specific surface area of RCF up to a specific surface area of 250 m²/kg improved compressive strength but further grinding caused strength reduction due to increased water demand. The positive effect of the superplasticizer on RCF-modified concrete strength was enhanced by the introduction of a chemical activator (hardening accelerator) and thermal treatment of RCF. The obtained models of water demand and compressive strength of concrete with RCF can be applied for the optimization of the mix design. This paper proposes a method of mix design and provides an example of calculation. Full article

In this article, composite binders based on industrial waste—phosphogypsum, granular phosphoric slag, and burnt barium carbonate tailings––are investigated. It was found that the optimal composition (65% slag, 20% phosphogypsum, 15% tailings) provides compressive strength up to 31.1 MPa after steaming, which corresponds to grade M300 cement. Replacing natural gypsum with phosphogypsum increases strength by 5–10%, and using waste reduces cost by 20–25% compared to traditional binders. This technology eliminates the need for high-temperature firing, reducing energy consumption by 40–50%. Neutralization of harmful impurities of phosphogypsum with oxides of MgO and CaO reduces the ecotoxicity of the material by 70–80%. It is shown that hydrothermal treatment accelerates hardening, providing 90% of brand strength in 28 days. The developed binders are promising for the production of building blocks, road surfaces, and land reclamation. Full article

This study presents the results of research on the use of Portland cement as a binder for producing semi-permanent molds intended for large-scale castings made from complex alloyed steels. Based on the conducted experiments, the optimal composition of a molding mixture based on Portland cement was determined to manufacture large molds with high operational performance. The technological properties of the mixtures were investigated, focusing on the flowability, sedimentation stability, and strength after curing. The recommended mixture composition is as follows: Portland cement—18.75%; sand—56.5%; quartz powder—25%; water—25%. To accelerate the hardening process, the use of curing accelerators is advised. The most effective additives are a 9% aluminum nitrate solution at 0.6–1.5% by weight or sodium aluminate at 3–4%. This composition ensures the required strength within a short curing time. A specific thermal treatment regime is also recommended to further stabilize the mold structure: heating to 450 °C at a rate of 75 °C per hour, holding for 2 h, followed by controlled cooling together with the furnace. Full article

This study presents a detailed and comprehensive investigation into the macro-scale performance, strength gain mechanisms, environment and economic performance of graphene oxide (GO)-enhanced low-emission concrete. A comprehensive experimental program evaluated fresh and hardened properties, including slump retention, bleeding, air content, compressive, flexural, and tensile strength, drying shrinkage, and elastic modulus. Scanning Electron Microscopy (SEM), energy-dispersive spectroscopy (EDS), Thermogravimetric analysis (TGA) and proton nuclear magnetic resonance (¹H-NMR) was employed to examine microstructural evolution and early age water retention, confirming GO’s role in accelerating cement hydration and promoting C-S-H formation. Optimal performance was achieved at 0.05% GO (by binder weight), resulting in a 25% increase in 28-day compressive strength without compromising workability. This outcome is attributed to a tailored, non-invasive mixing strategy, wherein GO was pre-dispersed during synthesis and subsequently blended without the use of invasive mixing methods such as high shear mixing or ultrasonication. Fourier-transform infrared (FTIR) spectroscopy further validated the chemical compatibility of GO and PCE and confirmed the compatibility and efficiency of the admixture. Sustainability metrics, including embodied carbon and strength-normalized cost indices (USD/MPa), indicated that, although GO increased material cost, the overall cost-performance ratio remained competitive at breakeven GO prices. Enhanced efficiency also led to lower net embodied CO₂ emissions. By integrating mechanical, microstructural, and environmental analyses, this study demonstrates GO’s multifunctional benefits and provides a robust basis for its industrial implementation in sustainable infrastructure. Full article

Cement production significantly contributes to CO₂ emissions (8% of worldwide CO₂ emissions) and global warming, accelerating climate change and increasing air pollution, which harms ecosystems and human health. To this end, this research investigates the fresh and hardened properties of sustainable concrete fabricated with three different replacement percentages (0%, 5%, and 10% by weight) of ordinary Portland cement (OPC) using rice husk ash (RHA). The hardened properties were evaluated at 14, 28, 60, 90, and 120 days of water curing. In addition, data-based models were developed, validated, and optimised, and the models were compared with experimental results and validated with the literature findings. The outcomes reveal that the slump values increased (17% higher) with the increased content of RHA, which aligns with the lower temperatures (12% lower) of freshly mixed concrete with RHA than the control mix (100% OPC). The slopes of the stress–strain profiles decreased at early ages and improved at longer curing ages (more than 28 days), especially for mixes with 5% RHA. The compressive strength decreased slightly (18% at 28 days) with increased percentages of RHA, which was minimised with increased curing ages (8% at 90 days). The data-based model accurately predicted the stress–strain profiles (coefficient of determination, $R^2$ ≈ 0.9950–0.9993) and compressive strength at each curing age, including crack progression (i.e., highly nonlinear region) and validates its effectiveness. In contrast, the optimisation model shows excellent results, mirroring the experimental data throughout the profile. These outcomes indicate that the 10% RHA could potentially replace OPC due to its lower reduction in strength (8% at 90 days), which in turn lowers CO₂ emissions and promotes sustainability. Full article

In order to improve early-age strength, steam curing is mostly used for railway prefabricated components, which consumes a lot of energy and affects the durability of concrete. Synthetic calcium silicate hydrate (C-S-H) has an excellent early-age strength effect, which can improve the early-age strength of concrete and help to reduce the energy consumption of steam curing, but C-S-H will increase the shrinkage of concrete and affect the durability of concrete. In this work, C-S-H/SRPCA was synthesized using a shrinkage-reducing polycarboxylate superplasticizer (SRPCA) in order to increase the early-age strength and decrease the shrinkage of concrete. The effects of 0.5%, 4.0%, and 8.0% C-S-H/SRPCA on the shrinkage and strength of concrete were studied. Meanwhile, the internal mechanism was also explored through cement hydration, the physical aggregation morphology of hydration products, pore structure and classification, and the chemical properties of pore solution. The results suggest that C-S-H/SRPCA can shorten the setting time and accelerate cement hydration. Specifically, when the dosage of C-S-H/SRPCA is 4.0%, the initial setting time of concrete is shortened by 2.5 h and the final setting time is shortened by 6.2 h compared with the control group. As a result, the 1-day compressive strength is effectively increased by 29.5%, and the plastic shrinkage is reduced. In the stage of plastic shrinkage, the plastic shrinkage time of the concrete with 4.0% C-S-H/SRPCA is 4.1 h, which is 6.1 h shorter than that of the control group. In addition, C-S-H/SRPCA decreases the porosity. When the dosage is 4.0%, the porosity of the hardened cement paste at 28 days is reduced by 15% compared with the control group. It lessens the content of the capillary pores at 10–50 nm. At 24 h, the content of 10–50 nm capillary pores in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. It also reduces the surface tension of the pore solution. The surface tension of the simulated pore solution with 4.0% C-S-H/SRPCA is 34 mN/m, which is 53% of that of the control group, and it inhibits the volatilization of the pore solution. At 28 days, the evaporation rate of the pore solution in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. Thus, the drying shrinkage of concrete is inhibited. Given the above, at the optimum content of 4.0%, C-S-H/SRPCA improves the 1-day compressive strength of concrete by 29.5%, reduces the 28-day total shrinkage by 21.7%, and restrains the development of microcracks. Full article

The article investigates the characteristics of the phase composition and structure of supersulfated cement (SSC) during hardening using X-ray, electron microscopy, and ultrasonic analysis methods. The influence of different types of activators, hardening accelerators, and superplasticizers on the type and morphology of the newly formed phases during SSC hardening was studied. The effect of a polycarboxylate-type superplasticizer and calcium chloride on the standard consistency and setting times of SSC was experimentally determined. It was established that the introduction of the superplasticizer reduces the standard consistency by 10–16%. Experimental data showed higher effectiveness of phosphogypsum as a sulfate activator compared to gypsum stone. The strength increase of SSC at 7 days reached up to 35%, and at 28 days, up to 15%. Based on the kinetics of ultrasonic wave propagation during SSC hardening, the main stages of structure formation and the influence of cement composition on these stages were determined. The experimental results demonstrate the effect of SSC composition on its standard consistency, setting time, and mechanical properties. The impact of the type of activator and admixtures on the change in SSC strength during storage was investigated. It was found that the addition of a polycarboxylate-type superplasticizer significantly reduces the strength loss of SSC during long-term storage. Using mathematical modeling, experimentally obtained statistical models of strength were developed, which allow for the quantitative evaluation of individual and combined effects, as well as the determination of optimal SSC compositions. Full article

Cemented paste backfill with mine tailings provides a desirable solution for maximally utilizing mine tailings. Ordinary Portland cement (OPC) is the most widely used binder for cemented tailings backfills; however, the serious environmental problems resulting from OPC production and the drawbacks of OPC in cementing fine tailings motivate the investigation of novel binders characterized by environmental friendliness, cost-effectiveness, and efficiency. We previously synthesized solid-waste-based cementitious materials (SWCMs) for cementing fine tailings. In this study, CaCl₂ was added as an accelerator to further enhance the cementing performance of SWCMs for fine tailings. Adding a small amount of CaCl₂ accelerated the hydration of raw materials and prompted the formation of larger amounts of hydration products. As a result, the cementing performance of SWCMs for fine tailings was significantly enhanced through the combined effect of C-S-H gel and ettringite. The cemented fine tailings backfill can be hardened only after curing for ~36 h, with a 50% decrease in hardening duration compared to the control sample without CaCl₂. The optimal amount of CaCl₂ was controlled at 1.5 wt.%, and the sample strength reached 0.21 MPa at 36 h, even at a low binder-to-tailings ratio of 1:8, meeting the requirement of early strength of common cemented tailings backfills. The rapid hardening of cemented fine tailings backfills has significant implications for accelerating ore mining speed, improving mining production capacity, ensuring the safe environment of underground mining sites, and preventing the movement of surface masses in the terrain where mining production takes place. Full article

Corrosion is one of the causes of failure in reinforced concrete structures, and forming a passive film on the steel is essential for protection. Although several studies have looked at passive film formation in concrete pore solutions, few have considered its formation in hardened concrete and the influence of silica fume (SF) in the binder composition. This study aims to evaluate the influence of the SF content on passive film formation time in concrete. Periodic measurements assessed the electrical resistivity and corrosion current density of concrete samples containing 5%, 10%, 15%, and 20% SF. The alkalinity of the mixtures and the kinetics of the pozzolanic reaction were also monitored by XRD and titration tests. The control mixtures exhibited susceptibility to corrosion, regardless of the curing age evaluated. In contrast, the partial replacement of cement with SF accelerated the formation of the passive film on the steel surface, suggesting a delayed onset of corrosion due to modifications in the physical properties of the concrete. Also, the portlandite content and pH can predict passive film formation, with SF significantly accelerating this process. Full article

Properties of high early-strength concretes (HESCs) containing Type V, Type III, and rapid hardening calcium sulfoaluminate (CSA) cements were investigated at curing ages of opening time, 24 h, and 28 days. Investigated properties included the fresh (workability, setting time, air content, unit weight, and released heat of hydration), mechanical (compressive and flexural strengths), transport (absorption, volume of permeable voids, water penetration, rapid chloride permeability, and accelerated corrosion resistance), dimensional stability (drying shrinkage), and durability (de-icing salt and abrasion resistance) properties. Test results revealed that the HESC containing Rapid-Set cement achieved the shortest opening time to attain the required minimum strength, followed by Type III and Type V cement HESCs. For the most part, Type V cement HESC produced the best transport and de-icing salt resistance, whereas Rapid-Set cement HESC displayed the best dimensional stability and wear resistance. Full article

This study examines the impact of sodium citrate and a plasticizing additive, along with their sequential introduction into a cement slurry or concrete mix, on the heat evolution of the cement slurry, the microstructure, phase composition of the cement paste, and the compressive strength of fine-grained concrete. The binder used in this research was a blended binder consisting of 90% Portland cement and 10% calcium aluminate cement. This type of binder is characterized by an increased heat evolution and accelerated setting time. The addition of sodium citrate at 5% of the binder mass alters the phase composition of newly formed compounds by increasing the quantity of AFt and AFm phases. The presence of sodium citrate significantly delays the hydration process of tricalcium silicate by a factor of 3.3. Initially, it accelerates belite hydration by 31.6%, but subsequently slows it down, with a retardation of 43.4% observed at 28 days. During the hardening process, the hydration of tricalcium aluminate and tetracalcium aluminoferrite is accelerated throughout the hardening process, with the maximum acceleration occurring within the first 24 h. During the first 24 h of hydration, the dissolution rates of tricalcium aluminate and tetracalcium aluminoferrite were 40.7% and 75% faster, respectively. Sodium citrate enhances heat evolution during the initial 24 h by up to 4.3 times and reduces the induction period by up to 5 times. Furthermore, sodium citrate promotes early strength development during the initial curing period, enhancing compressive strength by up to 6.4 times compared to the reference composition. Full article

Shotcrete is one of the most important types of concrete used in engineering construction, and its properties are significantly influenced by accelerators. This study investigates the effects of aluminum sulfate series alkali-free accelerator (AKF) and alkali accelerator (ALK) on the strength, hydration process, characteristic hydration products, and microstructure properties of shotcrete. Techniques such as setting time measurement, isothermal calorimetry, simultaneous thermal analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS), and mercury intrusion porosimetry (MIP) were utilized. The results indicate that both ALK and AKF significantly accelerate and increase the early hydration heat release rate and cumulative hydration heat of Portland cement, producing the characteristic hydration products hexagonal plate AFm and rod AFt, respectively. This acceleration notably speeds up the setting process of Portland cement. ALK negatively impacts the later-stage microstructural development and pore structure filling of hardened cement paste, leading to average reductions of 15.3% and 19.9% in flexural and compressive strengths at 28 days, respectively. Specifically, compared to ALK, AKF shows a faster hydration heat release rate during the induction period and a more significant increase in cumulative hydration heat during the hydration process; the cumulative hydration heat is on average 18.2% higher than AKF. Furthermore, AKF does not hinder the subsequent C₃S hydration and C-S-H gel densification process. After 28 days of curing, EDS analysis indicates an average Ca/Si ratio of 1.171 for the AKF-treated shotcrete; the average Ca/Si ratio shows minimal variation from the reference group and is classified as the same type of C-S-H gel as the reference group. Therefore, the strength of hardened cement paste with AKF continues to increase steadily in the later stages. At 28 days, the average flexural strength increased by 10.2%, while the compressive strength decreased by only 3.0%. These findings suggest that AKF enhances the microstructural development and strength of shotcrete, making it a more effective accelerator for engineering applications. Full article

Shrinkage of concrete during hardening is a serious problem in attempts to maintain the integrity of concrete structures. One of the methods of combating shrinkage is compensating for it using an expansive agent. The purpose of this work is to develop and study an expanding agent to concrete, including high-calcium fly ash and calcium nitrate as an expansion activator. The content of Ca(NO₃)₂ can be used to regulate the degree of expansion of the additive itself during hydration and, accordingly, to control shrinkage, thus obtaining shrinkage-free or expanding concrete. Shrinkage–expansion deformations of concrete can also be regulated by the amount of expanding additive replacing part of the cement. With the Ca(NO₃)₂ content of fly ash being 10% or more, concrete experiences expansion in the initial stages of hardening. The transition of deformation through 0 to the shrinkage region occurs depending on the composition and content of the additive after 8–15 days of hardening. It has been established that replacing cement with pure fly ash with a curing period of 90 days or more has virtually no effect on the strength of concrete, either in bending or in compression. The use of an expanding additive containing 5–15% Ca(NO₃)₂ reduces the strength of concrete by an average of 9%, despite the fact that calcium nitrate is a hardening accelerator. Full article

A water-soluble ternary copolymer bearing carboxyl, sulfonic, and amide functional groups was synthesized using ammonium persulfate-catalyzed free radical polymerization in water, resulting in high monomer conversion. This copolymer was then complexed with aluminum sulfate, forming an admixture containing Al(SO₄)(OH)·5H₂O, which was subsequently combined with silica gel. Characterization revealed that the synthesized copolymer formed a large, thin membrane that covered both the aluminum compounds and the silica gel blocks. The introduction of this complex admixture, combining the copolymer and aluminum sulfate, not only reduced the setting times of the cement paste but also enhanced the mechanical strengths of the mortar compared to using aluminum sulfate alone. The complex admixture led to the formation of katoite, metajennite, and C₃A (tricalcium aluminate) in the mortar, demonstrating significant linking effects, whereas pure aluminum sulfate could not completely transform C₃S within 24 h. Further addition of silica gel to the complex admixture further shortened the setting times of the paste, slightly reduced compressive strength, but improved flexural strength compared to the initial complex admixture. The silicon components appeared to fill the micropores and mesopores of the mortar, accelerating cement setting and enhancing flexural strength, while slightly decreasing compressive strength. This study contributed to the development of new cementing accelerators with improved hardening properties. Full article