Defects in the internal structure of cement-based materials can be identified through the electrical resistivity of cement-based materials. This letter focuses on detecting defects using an electrode array with optimized distribution. The defects were imaged and analyzed by demonstrating the effectiveness of this detection method. Based on the results of resistivity measurements with different main electrode spacing and auxiliary electrode positions, the optimal electrode distribution was determined to be 150 mm for the main electrode spacing and 5 mm for the auxiliary electrode from the edge of the test block. Defects were successfully detected using the electrode array with optimized distribution; the imaging results can accurately show the location of the defect and can initially reflect the shape of the defect. Therefore, this optimized electrode array makes it possible to detect defects through resistivity measurements of the cement-based materials, which provides us a new solution to the application of the optimized electrode array for the detection of cement paste or concrete defects in the practical engineering project.

The utilization of tailings for producing solid waste-based concrete can effectively mitigate the environmental consequences stemming from tailings accumulation. However, the presence of high concentrations of heavy metal ions in certain tailings can deleteriously affect the microstructure of the cement matrix. This study investigated the in-situ immobilization performance of Pb2+ and Mn2+ by calcium-aluminum layered double hydroxides (CaAl-LDH) and LDH@biochar blended in cement-based materials. The equilibrium adsorption capacity of CaAl-LDH/LDH@biochar for heavy metals in aqueous solution was assessed by adsorption kinetics, while the effect of pH value on the removal rate of heavy metals was also studied. The workability and mechanical properties of cement mortar are significantly improved with the addition of 4% LDH@biochar. LDHs@biochar can effectively reduce the deteriorating effects of heavy metal ions on the fundamental properties of cement materials. Compared with CaAl-LDH, the leaching concentrations for Pb2+ and Mn2+ in LDH@biochar at 28 d is decreased by 59% and 39%, respectively. The stable adsorption of Pb2+ and Mn2+ by LDH@biochar is attributed to multiple mechanisms, including interlayer anion exchange effect, chemical adsorption of surface functional groups and electrostatic attraction. This study provide valuable insights for enhancing the efficiency of heavy metal immobilization in cement-based materials.

The agglomeration of reclaimed asphalt pavements (RAP) affects the low-temperature performance and water stability of hot-recycled asphalt mixtures (HAM). To address this problem, a surface-active regeneration agent which named polyacrylamide (PAM) was employed. The effects of the coarse-grained RAP agglomeration rate, the alterations in the aged asphalt before and after the addition of PAM, and the degree of diffusion and fusion between old and new asphalt on HAM performance were investigated using macroscopic experiments, molecular dynamics simulations, Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The findings revealed that the water stability of HAM first decreased and then slightly increased as the RAP agglomeration rate increased. Notably, at a 60% coarse-grained RAP agglomeration rate, the water stability of the mixture was at its poorest, accompanied by a declining low-temperature stability. Utilizing Fick's diffusion principle, in conjunction with molecular dynamics simulations, FTIR, and SEM analyses, it was observed that PAM effectively diffused and fused with the aged asphalt. This resulted in enhanced diffusion coefficients for both old and new asphalt, consequently mitigating aging effects on the aged asphalt. Furthermore, the regeneration agent facilitated the diffusion and fusion process between old and new asphalt, thereby improving HAM performance and reducing the adverse impact of coarse-grained RAP agglomeration on the road performance of thermally recycled mixtures.

The control of air void structures introduced by air-entraining agents (AEAs) has long been a crucial problem. However, the mechanism through which the air void structure is refined by the extrusion of the slurry and the cutting of the aggregate remains insufficiently studied. This study aims to investigate the screening effect of coarse aggregate on the air void structure. The gradations of coarse aggregates were controlled by the fractal dimension (D). For casting air-entrained concrete, multiple continuously graded aggregates with D of 2.1, 2.3, 2.5 and 2.7 as well as two single graded aggregates with D=1.8 and 3.0 were used. Various parameters such as air content, air void structure, freeze-thaw durability, chloride ion permeability and water absorption were measured. Various air void structure parameters were compared to macroscopic properties. To evaluate the screening effect of coarse aggregate, the bubble rising and splitting behavior when passing through a single simulated aggregate (SA) and SA layer were observed. The findings indicate that finer and continuously graded coarse aggregates lead to higher air content and promoted air bubble trapped rate, which suggests a stronger screening effect. This results in a smaller bubble spacing coefficient and stronger frost resistance.

Machine learning techniques can predict the compressive strength of cement-based materials with good accuracy and learning capacity. Traditional compressive strength prediction according to machine learning techniques such as the support vector machine (SVM), decision tree, and Gaussian regression are normally based on the mix proportion of concrete compositions. Resistivity can realize the long-term, real-time and in-situ monitoring of compressive strength of the concrete structures. Therefore, electrical resistivity is regarded as a key nondestructive testing parameter to improve the accuracy of the compressive strength prediction model according to machine learning techniques in this study. When the resistivity was taken into consideration as an input variable accounting for 0.166, the fitting degree of the compressive strength in the decision trees model is increased from 0.77 to 0.79. In the SVM model, the fitting degree remains 0.79, the RMSE decreases from 8.490 to 8.335, which indicates the reliability is improved. The fitting degree in the Gaussian model model is increased from 0.81 to 0.82. As a new parameter variable, the accuracy of the compressive strength prediction model modified with electrical resistivity can be significantly increased. Therefore, the nondestructive testing method can be combined with machine learning techniques to promote the development of civil engineering building structure monitoring, diagnosis and facilitate the development of intelligent buildings through data-driven approaches.

Rheological properties and the interaction between air bubbles and solid particles are important factors influencing the air bubble system in air-entrained concrete. The former may affect the bubble motion and the latter may influence the strength of bubbles. However, none of them were properly discussed by direct observation. In this study, we observe the rising behavior of various-sized bubbles in air-entrained cement paste. By a modified drag equation, the bubble-particle interaction is determined and represented by the interaction coefficient Ki. By Ki the bubble-particle interaction can be quantified and correlated with air entrainment properties. In combination with rheological properties and surface tension, the influence of inorganic salts on air entrainment is systematically studied. It is found that the content of bubbles <200 μm and the specific surface area of bubbles in the fresh mortar decrease with the increase of Ki, and the content of bubbles >1000 μm and the spacing factor increase with increasing Ki.

Cementitious materials are well acknowledged as one of the most adaptable materials for immobilizing heavy metals. Belite calcium sulfoaluminate cement (BCSA), one of the low-carbon alternative binders to cement with superior properties regarding chemical resistance and mechanical properties, is found with a desirable capability for waste immobilization. In this study, BCSA was used for Co(II) immobilization with a dosage of up to 2.5% by weight of BCSA. The results showed that Co(II) could promote the hydration of BCSA pastes, specifically accelerated the hydration of ye’elimite. More hydration products could be generated in the Co(II)-doped BCSA pastes, leading to the construction of a denser microstructure. The compressive strength of BCSA pastes would be slightly improved when BCSA was used for Co(II) immobilization, and the electrical resistivity would decrease. In terms of Co(II) immobilization, BCSA cement exhibited a desirable capacity for Co(II) immobilization. The majority of the Co(II) could be immobilized within the first 100 min of mixing BCSA with Co(II) solutions. The immobilization degrees of Co(II) in hardened BCSA pastes could approach about 99.99% after 7d. The acquired results indicated that BCSA cement is effective for Co(II) immobilization. Therefore, BCSA has a low-carbon advantage with superior strength development over time and prospective capacity of heavy metals immobilization.

Coal gangue is a solid waste with low carbon content discharged during the course of the coal mining process. The resource utilization of coal gangue could solve environmental problems caused by its excessive production, such as soil contamination and land occupation. This study proposed to produce high-strength thermal insulation bricks using coal gangue as the primary material and three other mineral powders as auxiliary materials, including K-feldspar, CaCO3 and fly ash. A systematic analysis was conducted to explore the optimum raw material addition ratio and optimum sintering temperature; then, the intrinsic structure of thermal insulation bricks and their sintering formation mechanisms were revealed. The results showed that the optimal ratios of coal gangue, K-feldspar, CaCO3 and fly ash were 65 wt%, 15 wt%, 10 wt% and 10 wt%, respectively; the compressive strength of the thermal insulation brick produced under this ratio was 22.5 MPa; thermal conductivity was 0.39 W m−1 k−1. During sintering processes, mineral powders sufficiently fused to form a skeleton, and the CO2 derived from CaCO3 formed pores. The optimum sintering temperature was 1150 °C, because at this temperature, K-feldspar had the best effect in promoting the conversion of CaCO3 to Ca-feldspar. The high level of the relative crystallinity of Ca-feldspar (about 76.0%) helped raise the Si–O network’s polymerization degree (NBO/T = 1.24), finally raising the compressive strength of thermal insulation bricks. The innovative method of using coal gangue to make thermal insulation bricks not only solved the environmental pollution caused by coal gangue but also provided excellent construction materials with high practical application value.

The electrochemical parameters of cement-based materials with different water–cement ratios in carbon curing and water curing were measured with electrochemical impedance spectroscopy (EIS). The optimized circuit model and corresponding electrical parameters were obtained to illustrate the variation of the microstructure of cementitious materials after carbon capturing. The results show that, to a large extent, the semicircle diameter in the high frequency area gradually increased along with carbon curing and water curing. However, carbon curing showed a difference that the semicircle diameter in the high frequency appeared at the minimal value at 3 days, which was higher than that at 1 day and 7 days. This should be the result of the joint influence of water content and porosity in the cement matrix. It was also found that the mass increase rates of carbonation with water–cement ratios of 0.4, 0.5, and 0.6 were basically stable at 3.4%, 5.0%, and 5.5%, respectively. The electrochemical parameters ρct2 of cement mortar corresponding to carbon curing were around three times that of water curing specimens, mainly due to the reduction of soluble materials and refinement of connecting pores in the microstructure of cementitious materials. A quadratic function correlation between the mass increase rate and ρct2 in the carbonation process of cement mortar was built, which proved that EIS analysis could be applied to monitor the carbon capturing of cement-based materials, either for newly mixed concrete or for recycled concrete aggregates.

The mechanical strength of cement mortar is normally decreased in a low-temperature environment, which is mainly due to the slow cement hydration reaction rate and an increase in the total pore volume, especially the harmful pore volume. With the modification of nano-materials, the early mechanical strength of cement-based materials can be effectively compensated. In this study, the effect of multilayer graphene oxide (MGO) on the early mechanical properties and microstructure of cement mortar at low temperatures of 0 degrees C, 5 degrees C, and 10 degrees C is studied by measuring the fluidity, setting time, mechanical property, scanning electron microscope (SEM), thermogravimetric (TG), and nitrogen adsorption/desorption test. Compared with the mechanical strength of control mortar with curing temperature of 20 degrees C, the mortars with the addition of MGO curing at low temperature present higher mechanical strength and denser microstructure. And the mechanism of the interaction between cement particles and MGO nano particles at low temperatures (0 degrees C, 5 degrees C, and 10 degrees C) is further discussed. The initial hydration reaction of MGO mortar is accelerated, which is mainly due to the nucleation effect of MGO and chemical reactions between -COOH on the edge of MGO nanosheets and the Ca2} of Ca(OH)2. The promotion of the initial hydration refined the pore diameter of samples by reducing the pore size and total porosity, thus making the microstructure of samples denser and enhancing the strength of specimens.

As an industrial by-product containing pozzolanic components, recycled ferronickel slag (FNS) has the potential to be supplementary cementitious materials (SCMs) to reduce the massive carbon footprint of the cement industry, however, the main limitation of ferronickel slag as SCMs is the low hydration rate at an early age. In this study, the pozzolanic activity property results indicate that if the proportion is more than 10 %, FSN can hardly participate in the cement hydration reaction during the early stage, even the mechanical strength of FNS-mortar decreases obviously with the higher proportion of ferronickel slag. Therefore, mechanical grinding and steam curing at an early age are applied to promote the reaction activity of the recycled ferronickel slag tailing in this study. Compared with standard curing, the compressive strength of hardened FNS-cement paste with steam curing at 60 °C or 80 °C increased by 8.2 % or 33.8 %, and the connected porosity decreased by 18.9 % or 17.3 %. And MgO in the ferronickel slag exists as Mg2SiO4 in raw materials and enters the C-S-H gel with the formation of M-S-H gel during the secondary hydration stage. This study provides a theoretical basis for solid waste-based concrete and promotes the recycling, conservation, and resources of solid waste in building materials.

Pretreated coconut fibers can be applied as renewable damping components in fiber-reinforced cement mortars to reduce the adverse effects of vibration and promote the recycling of waste fibers. In this study, FT-IR, TG, XRD, and SEM measurements were performed to analyze the morphologies and physicochemical properties of the coconut fibers before and after pretreatment. The compressive strength, porosity, and damping properties were tested to study the effects of fiber pre-treatment on the mechanical properties of cement mortar. In contrast to original fibers, pretreated coconut fibers induced a moderate air-entraining effect in cement mortars. Therefore, the mortars with pretreated fibers exhibited significantly lower strength loss, indicating consistency with the porosity results. Furthermore, by adding 0.75 vol% coconut fibers pretreated by a mixed solution of NaOH and H2O2, the loss tangent of the cement mortar can be increased by 25%. The enhanced damping properties were attributed to the weakened interface between the fibers and cement mortar matrix, which facilitated dissipation of the vibration energy. Moreover, a simplified shear-lag model was proposed to describe the relationship between the weakened interface frictional sliding and the damping properties of the cement mortars.

In this study, a double p-n heterojunction based on g-C3N4@NiO/Ni@MIL-101 ternary composite was successfully prepared by a feasible method. Results suggested that the rational growth of NiO and MIL-101 on the surface of g-C3N4 can improve the absorption capacity for visible light. Importantly, the formation of double p-n heterojunction can effectively inhibit the recombination of photogenerated electrons and holes. Due to the significant Fermi level differences among NiO, MIL-101 and g-C3N4, the internal electric fields were established on the interface after constructing the composite, consequently reducing the electrical resistance and accelerating the transfer of photoinduced charge carriers. As expected, the g-C3N4@NiO/Ni@MIL-101 ternary composite with good stability had excellent photocatalytic performance in the degradation of emerging pollutants, and the removal efficiency of ibuprofen was as high as 95.6%, wherein •O2−, •OH and 1O2 were the major active species in the reaction system. Furthermore, developed photocatalytic oxidation system can also achieve the rapid inactivation of bacteria. Based on the synergistic effects of energy band position and double internal electric fields, the separation and transfer of photogenerated charge carriers followed a traditional type-II route. In short, current work not only constructed a promising photocatalyst, but also shared an effective strategy for boosting the photocatalytic activity.

Non-destructive testing methods such as electrochemical impedance spectroscopy are promising in assessing the long-term properties of cement-based materials. However, the inherent correlation between electrical properties and cement hydration parameters is still unclear, which hinders the application of this method in the replacement of the traditional hydration test method. The hydration of cement with Na2CO3 was investigated using electrochemical impedance spectroscopy combined with the conventional characterization methods. It is found that Na2CO3 facilitates the early-age hydration but retards the later stage hydration with the same trend in the development of compressive strength. The mechanism of Na2CO3 on cement hydration is revealed through microstructure characterization results which are related to the diminishing of AFt and excess formation of CH. The resistivity evolution of cement pastes well-captured the hydration process altered by Na2CO3. And the correlation between microstructure, the hydration products and the electrical resistivity of cement paste is proposed to follow the linear or exponential functions. All these results indicate that electrochemical impedance spectroscopy is a promising method for characterizing both the microstructure and macroscopic properties of cement-based materials.

Alternative current impedance spectroscopy (ACIS) is a promising non-destructive testing method to monitor long-term change and assess the durability of concrete. This study investigates the influences of Ethylene Diamine Tetraacetic Acid (EDTA) on the hydration of hardening cement by ACIS. It is found that EDTA retards the early-age hydration of cement but can facilitate the later age reaction. Pastes with EDTA show comparable or higher compressive strength than Control at 28 d, especially when the dosage is higher than 0.4%. Microstructural characterization results reveal the working mechanism of EDTA originating from its complexing effect on free ions. The resistivity evolution of the pastes detected by ACIS can well reflect the effects of EDTA on the cement hydration in different ages. Proportional relations are identified between the resistivity and other hydration parameters, such as reaction degree, chemical shrinkage, compressive strength. The results of this study indicate a wider prospect of ACIS in monitoring the microstructure evolution and macro-properties of cementitious materials.

To reduce the threat of dynamic loads, the anti-vibratory performance of the building structure is improved by using materials possessing excellent damping properties. Inspired by the microstructure of shells, the bionic inorganic-organic laminated structures which consisted of RHA, thin layer polymer and cement-hydrates matrix are built and used to enhance the damping properties of cement mortar. A nanometer-sized bionic function interface is prepared by immobilizing methyl methacrylate (MMA) and styrene (ST) onto rice husk ash (RHA), which is firstly modified by vinyltriethoxysilane (VTES). Detailed characterizations including FT-IR, TG, XRD, BET, contact angle, and SEM are conducted to investigate the surface characteristic of RHA. The loss tangent, storage modulus, loss modulus, porosity, compressive strength, and flexural strength are tested to investigate the effect of bionic inorganic-organic laminated structures on cement mortar physical properties, and hydration products are interpreted by the adoption of XRD and FT-IR. The test results of cement mortar show that compared with the loss tangent value of plain cement mortar (0.02219), the loss tangent value of the cement mortar containing 12% wt.% RHA (0.02575) shows an obvious increase of 16.05%. Moreover, compared with the cement mortar containing 12% wt.% RHA, the loss tangent of the cement mortar containing 12% wt.% modified RHA (0.0363) is significantly increased by 40.97%. This indicates that the built and use of bionic inorganic-organic laminated structures can effectively enhance the damping properties of cement mortar by multi-phase viscoelasticity interface interaction.

In this study, the impact of water-to-cement (w/c) ratios of belite calcium sulfoaluminate cement (BCSA) on the hydration kinetics and the electrochemical impedance spectroscopy (EIS) parameters is studied. According to the analysis of classic hydration measurements, such as calorimetry tests, chemical shrinkage content, and chemically bound water content, it can be concluded that a higher w/c ratio clearly accelerates the hydration of BCSA cement paste. The electrical resistivity of BCSA0.35 cement paste is more than 4.5 times that of BCSA0.45 and BCSA0.5, due to the gradually densified micropore structure blocking the electrical signal transmission rather than the free charged-ion content. The porosity of BCSA0.5 is 27.5% higher than that of BCSA0.35 and 7.8% higher than that of BCSA0.45, which proves the resistivity is clearly related to the variation in microstructure, especially for the porosity and pore size distribution. The novelty of this study is the linear regression with logarithm terms of electrical resistivity and classic hydration parameters such as chemical shrinkage, cumulative hydration heat, and chemically bound water is established to extend the classical expression of cement hydration degree. It indicates that the electrochemical impedance spectroscopy can be taken as a nondestructive testing measurement to real-time monitor the cement hydration process of cement-based materials.

This study reports on the effects of multilayer graphene oxide (MGO) on compressive strength, flexural strength, and microstructure of cement mortar. The cement mortar was prepared with type P. II. 52.5 Portland cement, standard sand, and MGO. Four mixes were prepared with inclusion of MGO (0%, 0.02%, 0.04%, and 0.06% by weight of cement). The testing result shows that the compressive of GO-cement mortar increased by 4.84%–13.42%, and the flexural strength increased by 4.37%–8.28% at 3 d. GO-cement mortar’s compressive strength and flexural strength at 7 d increased by 3.84%–12.08% and 2.54%–13.43%, respectively. MGO made little contribution to the increases of compressive strength and flexural strength of cement mortar at 28 d. The results of X-ray diffraction (XRD), scanning electron microscope (SEM), and nitrogen (N2) adsorption/desorption tests show that the types of hydration products and crystal grain size did not change after adding MGO. Still, it can help to improve the microstructure of the cement mortar via regulating hydration products and can provide more condensed cores to accelerate hydration. Furthermore, the regulating action of MGO for the microstructure of cement mortar at an early age was better than that at 28 d.

In this paper, the interconnectedness among the electrical resistivity, the calorimetric heat and the chem-ical shrinkage of high Belite calcium sulfoaluminate (BCSA) cement are discussed. Microstructure analyses by using XRD, NMR, ICP-MS and SEM are applied to further demonstrate the relationship among the mi-crostructure, the compositional variation and the electrical resistivity of the hydrated cement paste. Two distinctive hydration stages can be differentiated from the BCSA cementitious materials. First stage is mainly controlled by hydration process of both the ye'elimite and the gypsum, and the second stage is mostly controlled by the formation of AFm and amorphous phases as a secondary hydration process. Ac-cording to the classic equation depicted the hydration degree of cementitious materials, a specific mathe-matical equation combining not only traditional parameters but also the electrical resistivity is proposed for the BCSA. (c) 2021 Elsevier Ltd. All rights reserved.

This paper investigates the prediction of carbonation depth for recycled aggregate concrete (RAC) with machine learning models. Nine parameters including RAC intrinsic properties and environmental conditions were considered as input variables. A dataset comprising 593 test data was used to train, validate, and test machine learning models. Results show that the Random forest model shows superior performance than the Gaussian progress regression model and standalone artificial neural network (ANN) model. All ANN models hybridized with swarm intelligence algorithms outperform the standalone ANN model, especially for the ANN model hybridized with the whale optimization algorithm. All machine learning models show higher accuracy than the existing code models and statistical models. The variable importance analysis shows that the carbonation resistance for RAC was determined by both internal and external factors. Based on the parametric analysis, the robustness of the proposed machine learning models was further confirmed.