Cement and Concrete Composites(2024 - 2024)
Evaluation of the thermal stability of metakaolin-based geopolymers according to Si/Al ratio and sodium activator
Kim G.; Cho S.; Im S.; Yoon J.; Suh H.; Kanematsu M.; Machida A.; Shobu T.; Bae S.
Cement and Concrete Composites, Elsevier Ltd., Vol.150, 2024, JUL.
(https://doi.org/10.1016/j.cemconcomp.2024.105562)
Abstract
The thermal stability of geopolymer pastes with different Si/Al ratios (1.5?3.0) and sodium activators (Na2SiO3 and/or NaOH) were evaluated in this study. The inclusion of Na2SiO3 induced an active geopolymer reaction, enhancing pre-heating compressive strength, but gradually reduced from 300 ‹C due to vapor pressure and microstructural deterioration caused by entrapped physically bound water. Conversely, the strength of geopolymers containing only NaOH remained stable or increased up to 600 ‹C, owing to additional geopolymerization from unreacted raw materials and compact matrix induced by thermal shrinkage. The aluminosilicate networks of all geopolymers remained relatively stable until crystalline nepheline formed above 800 ‹C; larger crystalline particles were formed in geopolymers with Na2SiO3 owing to its denser geopolymer matrix, whereas smaller crystalline particles were formed in geopolymers with smaller Si/Al and only with NaOH, resulting in fewer voids and stable mechanical strength. Swelling of silica was notable in Na2SiO3-activated geopolymer, resulting in greater strength loss upon heating, indicating that Na2SiO3 hinders the thermal stability of geopolymers at extreme temperatures. c 2024
Meso-scale simulation of moisture transport in concrete during wet-dry cycles using 3D RBSM conduit model with variable diffusion model
Ren D.; Jiradilok P.; Waghmare D.; Nagai K.
Cement and Concrete Composites, Elsevier Ltd., Vol.149, 2024, MAY.
(https://doi.org/10.1016/j.cemconcomp.2024.105497)
Abstract
This paper introduces the 3D RBSM Conduit model, an improved simulation program based on the Rigid Body Spring Model (RBSM), designed to accurately depict the intricate phenomenon of moisture transfer within concrete. This is achieved by incorporating an expanded diffusion model into the simulation framework. The diffusion coefficient is dynamically adjusted at each location and at each time step to accurately represent the precise moisture transfer processes occurring in the concrete. This dynamic adjustment allows the program to efficiently simulate the rate of moisture transport in concrete based on the moisture degree and the dry or wet state of the material. By adapting the diffusion coefficient to the changing conditions in this way, the program achieves simulated moisture transfer rates that align closely with the real-world behavior of moisture in concrete. It offers a dynamic model of the changes in concrete transmission during the switch from drying and wetting conditions. Further, the program exhibits inherent advantages in addressing water transfer problems involving nonlinear boundary conditions and transfer equations. In this study, the program is used to simulate various processes including drying, wetting, local wetting, and combined wet-dry cycles. The results reveal the lag in wet-dry transitions between the interior and exposed surface during the wet-dry conversion process due to retained interior moisture. The simulation captures the moisture variation in the transitional zone. c 2024 Elsevier Ltd
Investigation into the changes in the splitting tensile strength of concrete subjected to long-term drying using a three-phase mesoscale RBSM
Sasano H.; Maruyama I.
Cement and Concrete Composites, Elsevier Ltd., Vol.148, 2024, APR.
(https://doi.org/10.1016/j.cemconcomp.2024.105462)
Abstract
The fracture process of dried concrete under splitting tensile loading was investigated using a three-phase mesoscale rigid-body spring model (RBSM). Although many studies have reported changes in the splitting tensile strength (ft) owing to drying, the mechanism remains unclear. The ft values under different drying conditions were predicted using a mesoscale concrete model that accounted for the drying-induced mechanical property change (DMPC) of mortar and microcracking. This result supports the assumption that DMPC and microcracking predominantly affect the strength under long-term drying. An investigation of the fracture behavior showed that drying-induced microcracking degraded ft by uniformly reducing the load-bearing capacity of the mortar. However, the cracks also had a slightly positive effect on ft by spreading the fracture area near the peak load, but this benefit was outweighed by the negative effect of cracking at 80 % RH or more intense drying conditions. c 2024 Elsevier Ltd
Effect of accelerated carbonation of fully recycled aggregates on fracture behaviour of concrete
Tang Y.; Xiao J.; Zhang H.; Wang D.; Zhang M.; Zhang J.
Cement and Concrete Composites, Elsevier Ltd., Vol.148, 2024, APR.
(https://doi.org/10.1016/j.cemconcomp.2024.105442)
Abstract
Accelerated carbonation of recycled aggregates (RAs) enables fast carbon sequestration while improving engineering properties of prepared concrete. This paper presents a systematic experimental study on the fracture behaviour of concrete with carbonated recycled coarse and fine aggregates (RCA and RFA), in terms of fracture parameters and fracture process zone (FPZ). The relevant influencing mechanism was investigated by relating the macroscopic fracture responses to microstructural features. Results indicate that carbonated RFA mainly enhanced the initial fracture toughness of concrete, while carbonated RCA primarily increased its unstable fracture toughness. The enhancement of fracture energy was closely associated with carbonated RCA and RFA. The utilization of RFA resulted in the looseness and higher porosity of mortar matrix, which reduced the crack initiation resistance of concrete, mainly affecting its initial cracking stage. The incorporation of RCA reduced crack bifurcation and deflection of concrete, leading to a smaller width of FPZ, which made the initial descending segment of the tension softening curve (TSC) steeper and its tail segment shorter. The carbonation treatment of RAs improved the densification of the adhered old mortar and reduced the porosity of prepared concrete, rising the cracking resistance and inducing a wider FPZ. As a result, the initial descending segment of TSC became flatter and its tail segment tended to be longer. c 2024 Elsevier Ltd
Microstructural analysis of cement paste blended with blast furnace slag using 1H NMR relaxometry
Joseph S.; Mutti M.; Ohkubo T.; Maruyama I.; Cizer O.
Cement and Concrete Composites, Elsevier Ltd., Vol.146, 2024, FEB.
(https://doi.org/10.1016/j.cemconcomp.2023.105377)
Abstract
1H NMR is a powerful technique for characterizing the microstructure of cement pastes; however the analysis of the data poses significant challenges. This work presents a comparative analysis of the microstructure evolution in cement pastes using 1H NMR relaxometry, MIP and thermoporometry. We developed and implemented a cumulative T2 distribution to improve the reproducibility and robustness of the results obtained from the CPMG raw data. Although NMR and thermoporometry measured similar pore sizes, MIP measured considerable deviations at pores>10 nm for matured white cement pastes, but blends with slag were more comparable. Nevertheless, deviations were observed in the blends when the degree of slag hydration was low. These discrepancies are attributed to the C/(A + S) ratio of C?S?H which decreases with slag hydration. The findings suggest that the pore refinements typically measured with MIP for blended cement compared to reference Portland cement may be an artefact of sample preconditioning. c 2023 The Author(s)
Development and verification of an integrated hydration, geochemical and transport model for the hydrated cement paste exposed to an aggressive chemical environment
Krishnya S.; Kopitha K.; Yoda Y.; Kitagaki R.; Elakneswaran Y.
Cement and Concrete Composites, Elsevier Ltd., Vol.146, 2024, FEB.
(https://doi.org/10.1016/j.cemconcomp.2023.105374)
Abstract
This study established a new transport model by using the COMSOL-IPHREEQC interface to simulate the changes in the morphology of hydrated cement paste due to the diffusion of carbon dioxide and chloride ion. A series of constitutive models such as the cement hydration model (to compute the dissolution rate of each clinker mineral), thermodynamic model (to perform the hydration reaction, reaction due to transport of ions, chemical and physical adsorption of chloride ion during the chloride ion ingression and dissolution rate of calcium-silica-hydrate (C?S?H) simultaneously with portlandite for the carbonation), porosity determination, and COMSOL Multiphysics (for the calculation of transport problems) were integrated using MATLAB language to determine the pore solution chemistry, hydrates assemblage, and porosity of the cement paste exposed to aggressive environments. During the diffusion of carbon dioxide gas, the decalcification of C?S?H was realistically considered by assuming that the Ca/Si ratio of C?S?H decreased from 1.67 (Jennite type C?S?H) to 0.67 (Tobermorite C?S?H), and then from 0.67 to 0 (silica gel). The proposed integrated platform was well verified with different sets of reported and raw experimental results and existing models, indicating a realistic predictability for chloride ion ingression and carbonation. The developed model discloses the effect of coupling the progression of hydration with reaction due to the transport of ions by using the free chloride ion profile and phase assemblages during the chloride ion ingression. c 2023 The Authors
Natural carbonation process in cement paste particles in different relative humidities
Saeki N.; Cheng L.; Kurihara R.; Ohkubo T.; Teramoto A.; Suda Y.; Kitagaki R.; Maruyama I.
Cement and Concrete Composites, Elsevier Ltd., Vol.146, 2024, FEB.
(https://doi.org/10.1016/j.cemconcomp.2023.105400)
Abstract
Finely ground hardened cement pastes (hcp) under 75 ƒÊm were carbonated at varying relative humidities (23?95 % RH) to determine the carbonation rate that minimized CO2 diffusion in the bulk paste. The amounts of portlandite (CH) and calcium carbonate (CC) polymorphs were quantified, and the decalcification and decomposition of C-A-S-H were analyzed using TGA, XRD, mass balance calculation, FTIR, and 29Si MAS NMR spectroscopy. A higher RH accelerated the reaction rate, and CH dissolution stopped midway at 58 % RH or below because of the loss of connectivity with the pore solution in the CC shell around CH. The precipitated CC phase included calcite at high RH, whereas all three polymorphs formed at 58 % or lower RH; amorphous CC were detected at low RH. The decalcification and decomposition of C-A-S-H were observed at 45 % or higher RH; it was rarely carbonated at 33 % or lower RH. c 2023 Elsevier Ltd
Mechanisms on the inhibition of alkali-silica reaction in supersulfated cement
Ban J.; Fan D.; Li K.; Yao J.; Lu J.-X.; Wang Z.; Poon C.-S.
Cement and Concrete Composites, Elsevier Ltd., Vol.145, 2024, JAN.
(https://doi.org/10.1016/j.cemconcomp.2023.105320)
Abstract
The supersulfated cementitious system gains increasing popularity for lowering carbon emissions of the construction industry. However, the alkali-silica reaction (ASR) behavior of this sulfate-rich cement is still unclear. This study first reports the characteristics and suppressing mechanisms of ASR in supersulfated cement (SSC). The results show that nearly no ASR-induced expansion is found in the SSC even if 100% glass aggregates are used, while the expansion ratio of the traditional OPC is twenty times higher than the SSC. In the SSC, the further hydration of unreacted slag during the ASR test refines the pore size and enhances the compressive strength of the matrix. Meanwhile, the consumption of the alkali ions in the pore solution during the formation of N-A-S-H gel reduces the concentration of the alkali ions and the contact possibility with the aggregates. The high bulk electrical resistance of the SSC can mitigate the diffusion of alkali ions from the external solution to the mortar. Moreover, the higher Al/Si ratio of C-A-S-H in the SSC than C?S?H in the OPC system results in a higher adsorption capacity of Na+ due to the more negatively charged surface, reducing the concentration of alkali ions in the pore solution. Furthermore, the high concentration of SO42? in the pore solution of SSC can inhibit the diffusion of OH?. As a result, the low-carbon SSC system would be a promising cementitious material to mitigate the ASR risk. c 2023 Elsevier Ltd
Assessment and mitigation of early-age cracking for on-site concrete structures by a combined scheme using Temperature Stress Testing Machine (TSTM) and long-span restraining frame
Ou G.; Lin Z.; Kishi T.; Liu L.
Cement and Concrete Composites, Elsevier Ltd., Vol.145, 2024, JAN.
(https://doi.org/10.1016/j.cemconcomp.2023.105321)
Abstract
From the field survey, thermal cracking frequently occurs in substructural concrete walls, although a high dosage of Calcium Sulfoaluminate (CSA) expansive additive is widely utilized. Conventional assessment methods fail to characterize the failure of CSA expansion under coupled realistic conditions. Therefore, an assessment scheme is proposed in this study: Firstly, mix proportions, temperature, restraint and curing conditions are used as input for a developed Temperature Stress Testing Machine (TSTM). By testing, limited expansions of CSA additive under low water-to-binder ratio, high temperature or drying condition, is unfolded. Corresponding countermeasures are also proposed. Secondly, long-span restraining frames are constructed to validate, and upscale findings from TSTM. Eventually, normal and optimized mixtures are applied and compared in real substructural walls. This combined scheme newly reveals that the lack of free water is the mechanism for CSA's stagnant expansion and a combination of CSA and lightweight aggregates shows promising potential as a countermeasure. c 2023 Elsevier Ltd