Search Results (60)

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 preparation of construction waste into eco-friendly recycled powder (RP), partially replacing cement to produce foam concrete with thermal insulation properties, provides a new approach for the resource utilization of RP. In this study, different components of construction waste were used to prepare recycled paste powder (RPP), recycled brick powder (RBP), and recycled concrete powder (RCP). The effects of RP on the microstructural and macroscopic properties of foam concrete were investigated at replacement rates ranging from 0% to 30%. The research results indicate that the microstructure of all three types of RP exhibits irregular shapes, and their chemical compositions show significant differences. Partial replacement of cement with these RP leads to the deterioration of the matrix microstructure, which negatively affects the workability and mechanical properties of the foam concrete. However, the addition of RP effectively mitigates the drying shrinkage of the foam concrete, with RBP showing particularly outstanding performance in this regard. Specifically, the maximum drying shrinkage rate of F-30RBP is 9.33% and 11.31% lower than that of F-30RPP and F-30RCP, respectively. Furthermore, the incorporation of RP has a minimal effect on the thermal conductivity of the foam concrete, indicating that RP is well-suited for use in foam concrete. Full article

Construction and demolition waste (CDW) was successfully utilized as an aggregate with 100% replacement of natural aggregates and mineral admixtures, with up to 60% replacement of ordinary Portland cement (OPC) in the production of recycled concrete. The effects of ratios of concrete-based CDW (concrete-CDW)/brick-based CDW (brick-CDW) in both aggregates and CDW mineral admixture contents in the binder on recycled concrete were investigated in terms of their workability and compressive strength, microstructure, and sustainability. The results showed that with an increase in the ratios of brick-CDW/concrete-CDW aggregates, the concrete workability continuously deteriorated, while the compressive strength firstly increased and then decreased. Compared to the 100% dosage of concrete-CDW aggregates, the 28-day compressive strength of the recycled concrete was 37.4 MPa; the optimized relative proportions of brick-CDW and concrete-CDW aggregates were 20% and 80%, respectively; and the 28-day compressive strength was the highest, reaching to 46.7 MPa, and increasing by 24.9%. In a binder study, the microstructure of the paste was found to be improved, with the dosage of brick-CDW and concrete-CDW admixtures at up to 20%. In this range, the workability changed slightly when the relative proportion of brick-CDW admixture increased, the 28-day compressive strength of the recycled concrete increased, and the pore structure was refined. Furthermore, the utilization of a large amount of CDW as a mineral admixture and aggregate in concrete significantly reduced costs and CO₂ emissions in different regions. Full article

Cementitious materials are multiscale and multiphase composites whose frost resistance at the macroscale is closely governed by microstructural characteristics. However, the interfacial transition zone (ITZ) between clinker and hydrates, recognized as the weakest solid phase, plays a decisive role in the initiation and propagation of microcracks under freezing conditions. Understanding the frost damage mechanism of ITZ is therefore essential for improving the durability of concrete in cold regions. The motivation of this study lies in revealing how freezing affects the mechanical integrity and microstructure of ITZ in its early ages, which remains insufficiently understood in existing research. To address this, a nanoscratch technique was employed for its ability to quantify local fracture properties and interfacial adhesion at the submicronscale, providing a direct and high-resolution assessment of ITZ behavior under freeze–thaw action. The ITZ thickness and fracture properties were characterized in unfrozen cement paste and in cement paste frozen at 1 and 7 days of age to elucidate the microscale frost damage mechanism. Moreover, the enhancement effect of nano-silica modification on frozen ITZ was investigated through the combined use of nanoscratch and mercury intrusion porosimetry (MIP). The correlations among clinker particle size, ITZ thickness, and ITZ fracture properties were further established using nanoscratch coupled with scanning electron microscopy (SEM). This study provides a novel micromechanical insight into the frost deterioration of ITZ and demonstrates the innovative application of nanoscratch technology in characterizing freeze-induced damage in cementitious materials, offering theoretical guidance for designing durable concrete for cold environments. Full article

This study quantifies the effects of freeze–thaw (FT) cycling and dynamic water scouring, and establishes links between mesoscale pore evolution and macroscale strength degradation in cement-stabilized macadam (CSM) bases. The objective is to provide quantitative indicators for durability design and non-destructive evaluation of CSM bases. First, laboratory tests were conducted to simulate alpine service conditions: CSM cylindrical specimens (Ø150 × 150 mm) with 4.5% cement content, cured for 28 days, were exposed to 0, 5, or 20 FT cycles (−18 °C for 16 h ↔ +25 °C for 8 h), followed by dynamic water scouring (0.5 MPa, 10 Hz) for 15, 30, or 60 min. Second, the resulting damage was tracked at two scales. Acoustic emission (AE) sensors monitored internal damage during subsequent splitting tests, while industrial computed tomography (CT) was used to scan selected specimens and quantify porosity, pore number, and average pore diameter. Third, gray relational analysis correlated pore structure parameters with strength loss. The results indicate that under 30 min of scouring, increasing FT cycles from 0 to 20 increased mass loss from 0.33% to 1.27% and reduced splitting strength by 28.8%. AE cumulative ringing count and energy decreased by 97.9% and 98.4%, respectively, indicating severe internal degradation. CT scans revealed porosity and pore count increased monotonically with FT cycles, while average pore diameter decreased (dominated by microcrack formation). Frost-heave pressure and cyclic suction enlarged edge pores and interconnected internal voids, accelerating erosion of cement paste. FT cycles compromise the cement–aggregate interfacial bond, thereby predisposing the matrix to accelerated deterioration under dynamic scouring; the ensuing evolution of pore structure emerges as the pivotal mechanism governing strength degradation. Average pore diameter exhibited the strongest correlation with splitting strength (r = 0.763), and its change was the primary driver of strength loss (r = 0.774). These findings facilitate optimizing cement dosage, validating non-destructive evaluation models for in-service base courses, and erosion durability of road base materials in permafrost regions. Full article

To enhance the support capacity of cemented paste backfill (CPB) in goaf areas and its ability to sequester CO₂, steel slag and ethylenediaminetetraacetic acid (EDTA) were incorporated into gangue-based cemented backfill materials. A stress–carbonation-coupled reaction system was employed to carbonate the CPB, and the effects of steel slag and EDTA on compressive strength, CO₂ uptake, and microstructure were studied. The findings indicate that steel slag remarkably enhanced the performance of the CPB, with both strength and CO₂ uptake initially increasing before declining as steel slag content increased. The optimum performance was achieved at a steel slag content of 10%. The incorporation of EDTA further enhanced the compressive strength and CO₂ uptake, with the best results at 0.5 g/L. Microstructural analyses demonstrated that steel slag increased the availability of Ca²⁺ and Mg²⁺ in the cement paste system, while EDTA accelerated their leaching, promoted hydration products, and catalyzed carbonation via chelation. However, excessive steel slag or EDTA reduced hydration products and deteriorated material performance. This work may provide a reference for enhancing the properties of CPB and promoting the efficient utilization of coal-based solid wastes. Full article

Foamed concrete has been widely applied in construction engineering; however, the performance requirements vary across different structural components. Its production typically involves a substantial consumption of cement, which imposes both environmental and economic burdens. Therefore, this study examined foamed concrete with dry apparent densities of 500–1000 kg/m³, in which cement was partially replaced (0–30%) by recycled powder from construction and demolition waste. Macroscopic performance was evaluated through drying shrinkage, compressive strength, softening coefficient, carbonation coefficient, and thermal conductivity, while microstructural analysis was conducted to clarify the underlying mechanisms. The results indicate that the internal composition of the recycled powder primarily consists of SiO₂, CaCO₃, and C-S-H gel. When recycled powder is used to replace cement, the microstructure of the resulting paste gradually deteriorates compared to that of the control group without recycled powder, and a significant amount of inert SiO₂ is introduced. As the replacement ratio of recycled powder increases, the compressive strength of foamed concrete across various density grades exhibits a gradual decline. Notably, when the replacement ratio reaches 30%, the reduction in mechanical performance becomes more substantial. However, the incorporation of recycled powder can effectively mitigate the drying shrinkage of foamed concrete. Moreover, the incorporation of recycled powder exerts minimal influence on the thermal conductivity and porosity of foamed concrete, demonstrating its favorable compatibility and potential for application in foamed concrete systems. Full article

Chloride-based deicing salt solutions have been contacted with concrete pastes containing slag cement at different conditions, such as slag replacement (20–80%), type (CaCl₂, MgCl₂, NaCl), and concentration (1 M–5 M) of the deicing salt, as well as temperature (ambient & −18 °C), and the extent of their reactions have been studied using XRD and ICP-OES. Also, solubility of Friedel salt (FS) has been measured in different types and concentrations of deicing salt solutions. It has been observed that the chemical deterioration arising from the formation and then dissolution of FS is more significant than the damage caused by the formation and expansion of oxychlorides in the pastes containing slag. While calcium oxychloride in its dried form can linger inside the paste for a long time, FS undergoes incongruent dissolution in CaCl₂ and MgCl₂ solutions and leaves the system. Presence of higher levels of AFm phases in pastes containing slag will further underscore this phenomenon. The extent of this chemical deterioration is relatively lower in NaCl solutions. Also, it was found that the nature of the chemical interaction changes with the concentration of the salt, as some disappeared phases might reappear and then disappear again. Using XRD and ICP-OES, this study provides a mechanistic understanding of salt-induced chemical deterioration in slag cement pastes by identifying phase-specific vulnerabilities and tracking the formation, transformation, and dissolution of key phases, such as Friedel’s salt and calcium oxychloride; additionally, the influence of various parameters have been studied, and chemical mechanisms have been proposed. Full article

This investigation examines the impact of machine-made sand methylene blue (MB) values on mortar properties and microstructure through controlled clay type and content testing, encompassing macro-performances, microstructures, and mechanisms measuring compressive strength, flexural strength, drying shrinkage, frost resistance, impermeability, pore structure, microstructure, interfacial transition zones (ITZs), and hydration products. MB testing demonstrates that montmorillonite and illite exhibit a significant sensitivity divergence, where 1% montmorillonite achieves an MB value of 1.42, exceeding 1.40, while illite requires a 5% content to attain an MB of 1.50, complying with SL/T 352-2020 specifications. Increasing MB values induce an initial rise followed by a decline in 7d compressive strength yet a persistent increase in flexural strength for montmorillonite mortars, with both strength parameters decreasing at 28d and 90d. Illite mortars exhibit progressive declines in compressive and flexural strength across all curing ages (7d, 28d, and 90d) with rising MB values. SEM-EDS analyses reveal a deteriorating mortar microstructure, reduced paste compactness, and thickened ITZ under identical clay types as MB values increase. Combined XRD and TG-DTA analyses demonstrate a diminishing hydration degree and decreased hydration products in mortars with ascending MB values. Given a constant clay mineralogy, elevated MB values inhibit hydration-product formation, causing incomplete cement hydration reactions and deteriorated ITZ microstructures, consequently impairing mortar macro-performances. Full article

In marine environments, chloride salt ingress into concrete having a porous structure can lead to physical salt attack (PSA) deterioration. However, the slow deterioration of cement-based materials caused by PSA under current laboratory conditions limits the understanding of the mechanisms of PSA. To improve the efficiency of accelerated testing for PSA in a laboratory, this study investigates the accelerated methods for PSA deterioration of cement-based specimens exposed to NaCl solution by adjusting curing ages and water-to-cement ratios. The results indicate that specimens with shorter curing age exhibit accelerated damage due to insufficient hydration, while specimens with higher water-to-cement ratios experience expedited surface scaling due to increased porosity. Reducing curing age from 28 to 7 days shortened the deterioration time of specimens by 50%. For the 28-day-cured specimens, increasing the w/c ratio from 0.4 to 0.5 accelerated the initial damage by 25%. Despite the variations in the curing age and water-to-cement ratio, the fundamental deterioration mechanism remained consistent across specimens. Notably, deterioration predominantly occurred in regions with relatively reduced external salt crystallization, which can serve as an indicator for predicting potential deterioration locations. The findings provide a theoretical basis for enhancing the efficiency of accelerated PSA testing protocols in a laboratory environment. Full article

This study examines freeze–thaw deterioration patterns and predicts the service life of wet-sprayed concrete with composite cementitious materials in cold-region tunnels. The microstructure and particle size distribution of four materials (cement, fly ash, silica fume, and mineral powder) were analyzed. Subsequent tests evaluated the rebound rate, mechanical properties, and durability of wet-sprayed concrete with various compositions and proportions of cementitious materials, emphasizing freeze–thaw resistance under cyclic freezing and thawing. A freeze–thaw deterioration equation was developed using damage mechanics theory to predict the service life of early-stage wet-sprayed concrete in tunnels. The results indicate that proportionally combining cementitious materials with different particle sizes and gradations can enhance concrete compactness. Adding mineral admixtures increases concrete viscosity, effectively reducing rebound rates and dust generation during wet spraying. Concrete incorporating binary and ternary mineral admixtures shows reduced early-age strength but significantly enhanced later-age strength. Its frost resistance is also improved to varying degrees. The ternary composite binder fills voids between cement particles and at the interface between paste and aggregate, resulting in a dense microstructure due to a ‘composite superposition effect.’ This significantly enhances the frost resistance of wet-mixed shotcrete, enabling it to withstand up to 200 freeze–thaw cycles, compared to failure after 75 cycles in plain cement concrete. The relative dynamic modulus of elasticity of wet-shotcrete follows a parabolic deterioration trend with increasing freeze–thaw cycles. Except for specimen P5 (R² = 0.89), the correlation coefficients of deterioration models exceed 0.94, supporting their use in durability prediction. Simulation results indicate that, across all regions of China, the service life of wet-shotcrete with ternary admixtures can exceed 100 years, while that of plain cement concrete remains below 41 years. Full article

Previous research on cement aging mainly focuses on construction cement, exploring the mechanisms through which aging conditions affect cement properties. However, the impact of aging on oil well cement remains understudied. Aging of cement under high-humidity conditions leads to significant alterations in its properties, indicating that the cement formulation needs to be adjusted to reduce the negative effects during cementing operations. The effect of aging on particle size, mineral composition, and early hydration behavior of oil well cement after 0, 7, 14, and 28 d at 90% relative humidity (±3%RH) and 25 °C (±2 °C) was investigated. The results showed that, during the aging process, the uptake of H₂O and CO₂ from the surrounding atmosphere by cement leads to slight hydration. This process was associated with a reduction in the specific surface area and surface energy. The contents of hydration products ettringite (AFt) and calcium hydroxide (CH) increased, whereas the amounts of C₃S and C₃A decreased. Consequently, the early hydration rate of cement decreased along with a reduction in the cumulative heat release. As the aging time increased, the compressive strength and thickening time of the cement pastes decreased, and the rheological properties deteriorated. Under the experimental temperature and humidity conditions, the permissible aging time without significant deterioration should not exceed 7 d, with a maximum permissible aging time of 14 d. Full article

This study investigated the sulfate resistance of modified recycled aggregate concrete (RAC) by applying carbonation and nano-silica soaking methodologies. Recycled concrete aggregates (RCA) derived from concretes of C30 and C60 strength grades were subjected to these modification techniques and subsequently utilized in the fabrication of RAC specimens. The results show notable porosity and crack density within the interfacial transition zone (ITZ) interfacing recycled aggregate and cement paste in recycled aggregate concrete (RAC). Specifically, the porosity within the ITZ of RAC is observed to be up to 30% higher than that of virgin aggregate concrete. These pathways facilitate the penetration of sulfate ions, subsequently inducing deterioration and resulting in a compression strength reduction of up to 40%. While carbonation treatment exhibits a moderate enhancement in sulfate resistance, decreasing the sulfate penetration depth by 15%, the incorporation of 2% nano-silica by weight of cement proves significantly more effective. This addition reduces the sulfate penetration depth by over 30% and lowers the sulfate concentration by 25%. Furthermore, the compressive strength of RAC modified with nano-silica increases by 15% following 28 days of sulfate exposure. Additionally, a 30% reduction in the sulfate ion mass equilibrium depth is observed in nano-silica-modified RAC, accompanied by a markedly lower sulfate concentration in the pore solution. After 56 days of sulfate attack, the compressive strength of nano-silica-modified RAC retains 85% of its initial value, whereas unmodified RAC decreases to 70%. Notably, the quality of recycled aggregate significantly impacts sulfate resistance, with high-strength RCA (exceeding 40 MPa) demonstrating superior resistance compared to low-strength RCA (below 20 MPa). Consequently, RAC produced with high-strength RCA experiences only a 20% loss in compressive strength under sulfate attack, whereas RAC containing low-strength RCA suffers a 40% loss. The novelty of this study is the effective use of nano-silica soaking and carbonation to enhance the sulfate resistance and compressive strength of recycled aggregate concrete originated from both normal and high-strength reference concrete. Full article

The incorporation of circulating fluidized bed (CFB) fly ash into self-leveling cement-based (SLC) paste production presents significant environmental advantages. However, its addition deteriorates the fresh properties of the paste, posing challenges for practical implementation. This research examined the fresh properties of SLC paste blended with CFB fly ash, emphasizing fluidity, rheological characteristics, and bleeding rate. To enhance flowability, polycarboxylate superplasticizer (PCE) was incorporated, with particular emphasis on its interaction with CFB fly ash. The findings reveal that adding CFB fly ash to cement-based paste significantly decreased fluidity while increasing yield stress and plastic viscosity. Incorporating 20 wt.% CFB fly ash reduced paste fluidity by 51.4%, while plastic viscosity and yield stress increased by factors of 2.3 and 73, respectively. While PCE enhanced the fluidity of the blended paste, its water-reducing efficiency diminished, and the bleeding rate of the paste increased with higher CFB fly ash dosage. The water-reducing capability of PCE in the CFB fly ash-blended cement paste with 20 wt.% CFB fly ash decreased by 40.0%, and the bleeding rate of the paste increased from 0.6% to 6.7%. This effect was primarily attributed to the poor compatibility between PCE and CFB fly ash. The decline in PCE efficiency with higher CFB fly ash content, along with its lower adsorption capacity on CFB fly ash compared to cement particles, further confirmed this incompatibility. Full article

The expansion, cracking and deterioration of properties during utilisation and solidification of municipal solid waste incineration bottom ash are key problems that are caused by the reaction of metallic aluminium in the bottom ash in the highly alkaline environment of hardened Portland cement. In this study, polyaluminium sulphate (PAS) was introduced into ordinary Portland cement (OPC) to inhibit the corrosion of aluminium alloy. The results indicate that PAS successfully inhibited the corrosion of Al in hardened OPC paste, prevented the expansion and cracking, reduced the amount of hydrogen gas release and formed a thinner and dense corrosion layer on the Al plate surface. The mechanism of corrosion inhibition of Al by PAS was the increase of initial Al(OH)₄⁻ concentration by hydrolysis, which expanded the pH range of passivation and transformed the porous loose bayerite layer to a dense homogeneous one around the Al plate without modification of the corrosion product (bayerite). The corrosion rate of the Al alloy in hardened OPC paste was reduced by 213 times by the addition of PAS, from 288.30 mm a⁻¹ without PAS addition to 1.35 mm a⁻¹ with PAS addition. This study casts light on the effective inhibition of corrosion of the Al alloy in OPC. Full article