Improving Sag Resistance in Geopolymer Coatings Using Diatomite Filler: Effects on Rheological Properties and Early Hydration.

The reduction in the rheological parameters and dissolution rate of precursors in geopolymer coatings during early hydration significantly contributes to sagging. This study aims to improve the sag resistance of these coatings by incorporating diatomite filler. Rheological testing was conducted to assess the impact of diatomite and its concentration on the yield stress, plastic viscosity, and thixotropy of the geopolymer coatings. The results indicated that diatomite's large specific surface area and high reactivity have a significant influence on the rheological parameters and early dissolution rate of precursors. With a diatomite concentration of 1.1%, the coating exhibited a yield stress of 2.749 Pa and a plastic viscosity of 0.921 Pa·s, maintaining stability, homogeneity, and no sagging at a thickness of 600 µm. Furthermore, the highly active SiO2 in diatomite participates in the secondary hydration reaction of the geopolymer materials led to the formation of substantial C-(A)-S-H gel. This gel enhances internal interconnectivity within the coating, thereby improving its rheological and mechanical properties.

Experimental and Numerical Study on Chloride Transport in Unsaturated Concrete: Highlighting Temperature, Humidity, and Mineral Admixtures.

Chloride transport within concrete is critical for the durability of reinforced concrete structures; however, its diffusion under the coupling action of temperature and humidity has not been fully comprehended. Therefore, in this work, the coupling effects of temperature, relative humidity, and mineral admixtures on chloride transport in concrete were investigated through experimental and numerical simulation work. The results show that the chloride diffusion coefficient decreases with the decreased temperature and growth of relative humidity; however, the chloride concentration on the concrete surface is increased with the growth of temperature and relative humidity. Moreover, compounding about 15% fly ash (FA) and 30% granulated ground blast furnace slag (GGBS) to replace the cement is the most beneficial for improving the antichloride capacity of concrete, considering also the strength. In addition, the numerical simulation considering the coupled effect of temperature and relative humidity of chloride transport in concrete has good agreement with that of experimental results.

Chloride Transport and Related Influencing Factors of Alkali-Activated Materials: A Review.

Chloride transport is a vital issue in the research on the durability of alkali-activated materials (AAMs). Nevertheless, due to its miscellaneous types, complex mix proportions, and limitations in testing methods, the reports of different studies are numerous and vary greatly. Therefore, in order to promote the application and development of AAMs in chloride environments, this work systematically reviews the chloride transport behavior and mechanism, solidification of chloride, influencing factors, and test method of chloride transport of AAMs, along with conclusions regarding instructive insights to the chloride transport problem of AAMs in future work.

The Effect of Temperatures on the Passivation Behavior of Q235 Steel in the Simulated Concrete Pore Solution.

Concrete, especially mass concrete, releases a large amount of heat during the hydration process, resulting in the passivation of reinforcement at high temperatures. However, the passivation study of reinforced concrete is mostly conducted at room temperature, and the influence of temperature on passive film behavior is not clear at present. The passivation film of reinforcing steel directly determines the corrosion resistance of reinforcing steel and affects the service life of reinforced concrete. Herein, the passivation of Q235 steel soaking in simulated concrete pore (SCP) solution at 20 °C, 40 °C, and 60 °C is explored. It is found that the passivation process is divided into two stages, with 24 h as the boundary; within 24 h the passivation was carried out rapidly, and the passive film is in a relatively stable state after 24 h. In addition, the higher the temperature, the faster the passivation. Moreover, under the condition of higher temperatures, more Fe3+ compounds are produced, and the semiconductor properties of passivated films are more stable. Based on experiments, the passivation mechanism affected by temperature was analyzed in detail.

New Self-Repairing System for Brittle Matrix Composites Using Corrosion-Induced Intelligent Fiber.

Brittle matrix composites such as concrete are susceptible to damage in the form of cracks. Most of the current self-repair and self-healing techniques have repair limits on crack widths or high costs of an external stimulator, or have an unfavorable effect on the composite's strength. This paper proposes a new concept of corrosion-induced intelligent fiber (CIF) and a new self-repairing system that uses the CIFs to close cracks in brittle matrix composites within a corrosive environment without external help, and without compromising the strength. The CIF comprises an inner core fiber and an outer corrodible coating that are in equilibrium, with the core fiber in tension and the corrodible coating in compression. The preparation steps and shape recovery mechanism of the CIF and the self-repair mechanism of the CIF composites are explained. Based on these concepts, this paper also describes several mechanical models built to predict the magnitude of pre-stress stored in the core fiber, and the maximum pre-stress released to the matrix composites, and the minimum length of the reliable anchor ends of CIF. The sample calculation results show that the recovery strain was 0.5% for the CIF with the steel core fiber and 12.7% for the CIF with the nylon core fiber; the maximum crack closing force provided by the CIF to concrete can be increased by increasing the amount of the CIFs in concrete and the initial tensile stress of the core fiber. This paper provides some suggestions for enhancing the self-repair capability of brittle composites in complex working environments.

Non-uniform model of relationship between surface strain and rust expansion force of reinforced concrete.

When operating within the environments rich with sodium chloride, steel bars of reinforced concrete structures are often subject to corrosion caused by surrounding erosive materials, and the associated rust expansion force due to corrosion takes a critical role in determining the durability of relevant reinforced concrete structures. By investigating the corrosion course of steel reinforcement with theory of elasticity, a numerical rust expansion model is established for the moment of concrete surface rupture based on non-uniform sin function. Cuboid reinforced concrete specimen with squared cross sections is tested to analyze the rust expansion when concrete cracks due to corrosive forces. The utility of the established expansion model is validated by numerical simulation with Abaqus through the comparison between the associated outcomes. The impacts of steel bar diameter and concrete cover thickness on the magnitude of rust expansion force are discussed.

Relationship model between surface strain of concrete and expansion force of reinforcement rust.

Concrete cracking caused by corrosion of reinforcement could significantly shorten the durability of reinforced concrete structure. It remains critical to investigate the process and mechanism of the corrosion occurring to concrete reinforcement and establish the theoretical prediction model of concrete expansion force for the whole process of corrosion cracking of reinforcement. Under the premise of uniform corrosion of reinforcing steel bars, the elastic mechanics analysis method is adopted to analyze the entire process starting from the corrosion of steel bars to the cracking of concrete due to corrosion. A relationship model between the expansion force of corrosion of steel bars and the surface strain of concrete is established. On the cuboid reinforced concrete specimens with square cross-sections, accelerated corrosion tests are carried out to calibrate and verify the established model. The model can be able to estimate the real-time expansion force of reinforced concrete at any time of the whole process from the initiation of steel corrosion to the end of concrete cracking by measuring the surface strain of concrete. It could be useful for quantitative real-time monitoring of steel corrosion in concrete structures.

Durability and Aesthetics of Architectural Concrete under Chloride Attack or Carbonation.

Architectural concrete has been wildly used nowadays, and those served in an offshore environment often suffer from chloride penetration and carbonation. To assess the protection and decoration performances of architectural concrete, this study exposed architectural concrete to actual marine environments and accelerated carbonation conditions. The chloride and carbonation resistance of architectural concrete was determined to evaluate the protection performance, and the corresponding surface-color-consistency was adopted to characterize its decoration performance. The results show that the total and free chloride of concrete in the marine atmosphere zone and the tidal zone generally decreases with depth; chloride content arguments significantly with exposure time, with a chloride maximum peak near the surface. Moreover, the chloride diffusion coefficient is small throughout the measurements, indicating the superior chloride resistance of architectural concrete. Furthermore, architectural concrete also possesses excellent carbonation resistance based on the carbonation depth data obtained from the carbonation experiment. Therefore, architecture concrete served as protection covers can withstand both the chloride attack and carbonation tested in this paper. In addition, carbonation was found to have a profound influence on the aesthetics of architectural concrete. Therefore, carbonation should be carefully handled for better maintaining the aesthetic appearance of architectural concrete in long-term service.

Perfect Combination of LBL with Sol-Gel Film to Enhance the Anticorrosion Performance on Al Alloy under Simulated and Accelerated Corrosive Environment.

Given their outstanding versatile properties, multilayered anticorrosion coatings have drawn great interest from researchers in the academic and engineering fields. However, the application of multilayered coatings is restricted by some limitations such as low interlayer compatibilities, the harsh preparation process, etc. This work introduced a composite film fabricated on a 2A12 aluminum alloy surface, including an anodic oxide film, a sol-gel film, and a layer-by-layer (LBL) self-assembling film from bottom to top. The microstructure and elemental characterization indicated that the finish of the coating with the LBL film resulted in a closely connected multilayered coating with a smoother surface. The anticorrosion performance was systematically evaluated in the simulated corrosive medium and neutral salt spray environment. The integrated coating with the LBL film presented an excellent anticorrosion ability with system impedance over 108 Ω·cm2 and a self-corrosion current density two orders of magnitude lower than that of the other coatings. After the acceleration test in a salt spray environment, the multilayered coatings could still show a good protective performance with almost no cracks and no penetration of chloride ions. It is believed that the as-constructed multilayered coating with high corrosive properties and a fine surface state will have promising applications in the field of anticorrosion engineering.

Feasibility of Utilizing Recycled Aggregate Concrete for Revetment Construction of the Lower Yellow River.

To explore the feasibility of utilizing recycled aggregate concrete (RAC) in revetment construction of the lower Yellow River, a series of mix proportions with local recycled aggregates (RA) were designed to evaluate its mechanical properties and durability. The morphology and micro-hardness of the interface transition zone (ITZ) were also characterized to explain the performance of RAC. Based on the compressive strength data of 13 groups of mixtures, which is larger than 30 MPa, and with the RA substitution rate not less than 50%, the RAC containing 50% recycled fine aggregate (RFA) (HDX50), 70% RFA (HDX70), and 50% recycled coarse aggregate (RCA) (HDC50) were selected. The experiment results suggest that the mechanical performance, frost resistance, and carbonation resistance of the selected RAC is generally poorer than that of natural aggregate concrete (NAC), but can meet the performance requirement of concrete for the revetment construction of the lower Yellow River. The comprehensive performance of these three mixtures ranks as: HDX50 > HDX70 > HDC50. When considering the RA substitution ratio as a priority, HDX70 would be the best choice and can be applied in the revetment engineering. A number of defects are observed on the surface of RA with old pastes attached. Furthermore, the ITZs formed around RA are loose and with low micro-hardness, which is deemed to be the dominating reasons leading to the weaker performance of RAC than that of NAC.

Effect of Oxidization Temperatures and Aging on Performance of Carbonate Melt Oxidized Iridium Oxide pH Electrode.

Iridium oxide pH electrodes employing the carbonate melt oxidation method were fabricated with oxidation temperatures of 750 °C, 800 °C and 850 °C, respectively. Scanning electron microscope (SEM) and atomic force microscope (AFM) images showed that the oxide film regularized with the increase in oxidation temperatures. The pH response, response time and long-term stability of the electrodes indicated that the electrodes made at 850 °C had the best performance. X-ray photoelectron spectra (XPS) surveys investigated the change in the electrodes' chemical composition and element oxidation states at 850 °C, and the results showed that the relative content of Ir3+ had increased by 23.9%, and the Ir4+ and Ir6+ had decreased by 10.9% and 13%, respectively, in the surface oxide layer after one month of aging. However, the relative contents of Ir3+, Ir4+ and Ir6+ were almost constant for the inner oxide layer. Meanwhile, the XPS result also indicated that the outer oxide layer of the electrode had a higher hydration degree than the inner oxide layer.

Influence of Moisture Content on Electromagnetic Response of Concrete Studied Using a Homemade Apparatus.

In this study, we examined the influence of moisture content on the electromagnetic response of concrete. A novel homemade electromagnetic monitoring apparatus was developed and used to evaluate the Hall effect voltage at both ends of concrete based on our previous study of the Hall effect. We used four different concrete mix water/binder ratios: 0.30, 0.28, 0.26, and 0.24, and three conditions (relative humidity, carbonation, and water absorption) were examined in this experiment. The results show that the moisture content inside concrete influences the relative permeability of concrete. The variation in the Hall effect voltage is more influenced by carbonation than changes in relative humidity; water absorption increases the Hall effect voltage the least amongst the other examined factors. According to the experiment, a calibration system was established, and the relevant correction factors are provided.

Correction: Hydrophobic silane coating films for the inhibition of water ingress into the nanometer pore of calcium silicate hydrate gels.

Correction for 'Hydrophobic silane coating films for the inhibition of water ingress into the nanometer pore of calcium silicate hydrate gels' by Jiao Yu et al., Phys. Chem. Chem. Phys., 2019, DOI: .

Effects of Transverse Crack on Chloride Ions Diffusion and Steel Bars Corrosion Behavior in Concrete under Electric Acceleration.

Cracks greatly impact the durability of concrete structures due to their influence on the migration of chloride ions and the corrosion process of steel bars. This study investigates the effects of transverse cracks on chloride diffusion and the corrosion behavior of two types of steel bars (low carbon steel and corrosion resistant steel) in fly ash concrete with 1 kg/m3 solution-polymerized super absorbent polymer. Electrochemical impedance spectroscopy was used to monitor the chloride-induced corrosion behavior of steel bars in concrete. The chloride profile around cracks was tested via chemical titration. The corrosion products diffusion area was photographed and measured to evaluate the influences of cracks on the corrosion degree of steel bars. Transverse cracks greatly influence the chloride ion transport. When their width is less than 0.15 mm, cracks exert little influence on both chloride diffusion and steel corrosion. When the crack width exceeds 0.15 mm, the chloride ion transmission coefficient is significantly improved and steel corrosion is accelerated. However, when the crack width exceeds 0.20 mm, this effect is gradually weakened. Based on the experimental data, a quantitative relationship between the crack width and the chloride ion transmission coefficient in electric acceleration was established.

Hydrophobic silane coating films for the inhibition of water ingress into the nanometer pore of calcium silicate hydrate gels.

The super-hydrophobic nature of surfaces is greatly dependent on the interfacial molecular structure of coating materials. In this study, to understand the structure, dynamics and interfacial behavior of hydrophobic coating, molecular dynamics is utilized to study the capillary transport of water molecules through the nanometer channel of calcium silicate hydrate (C-S-H) with the interior surface impregnated with silane. The C-S-H surface is coated by connecting the bridging silicate tetrahedron with an oxygen-containing group in isobutyl-triethoxysilane (C10H24O3Si) with a silane molecule coverage rate ranging from 25% to 100%. We demonstrated that the silane coating with a coverage exceeding 25% can effectively inhibit the water molecule and detrimental ion invasion in the gel pore. The grafted silane groups reduce the number of non-bridging oxygen atoms in the surface silicate chains that provide plenty of sites to accept the H bonds from the surface water molecules. This results in the reduction of the dipole moment of the surface water molecules and transforms the hydrophilic C-S-H substrate to hydrophobic. The silane molecules, immobilized by the Si-O-Si bond on the C-S-H substrate, are protruded to the gel pore with the hydrophobic tail of branch-like isobutyl groups. It transforms the smooth surface to a lotus-leaf-like rough surface with distributed nanoscale papillae. The isobutyl groups, freely vibrating and rotating, further blocks the connectivity of the transport channel and weakens the interaction between penetrated ions and C-S-H substrates. Furthermore, spatial correlation analysis demonstrates that the immobilized silane molecules disturb the tetrahedron distribution of water molecules in the gel pore and break the hydration shell of the counter Ca ions that associate with less water molecules. The dramatic degradation of the time correlation function for the surface solution species in the presence of isobutyl-triethoxysilane exhibits that the coated C-S-H surface can repel the surface water molecules and calcium ions by weakening the H bond and the Ca-O ionic bond strength. These nanostructure results provide guidance for the construction of artificial super-hydrophobic surfaces and the design of cementing materials with controllable wettability.

Preparation and Physical Properties of High-Belite Sulphoaluminate Cement-Based Foam Concrete Using an Orthogonal Test.

Prefabricated building development increasingly requires foam concrete (FC) insulation panels with low dry density (ρd), low thermal conductivity coefficient (kc), and a certain compressive strength (fcu). Here, the foam properties of a composite foaming agent with different dilution ratios were studied first, high-belite sulphoaluminate cement (HBSC)-based FCs (HBFCs) with 16 groups of orthogonal mix proportions were subsequently fabricated by a pre-foaming method, and physical properties (ρd, fcu, and kc) of the cured HBFC were characterized in tandem with microstructures. The optimum mix ratios for ρd, fcu, and kc properties were obtained by the range analysis and variance analysis, and the final optimization verification and economic cost of HBFC was also carried out. Orthogonal results show that foam produced by the foaming agent at a dilution ratio of 1:30 can meet the requirements of foam properties for HBFC, with the 1 h bleeding volume, 1 h settling distance, foamability, and foam density being 65.1 ± 3.5 mL, 8.0 ± 0.4 mm, 27.9 ± 0.9 times, and 45.0 ± 1.4 kg/m³, respectively. The increase of fly ash (FA) and foam dosage can effectively reduce the kc of the cured HBFC, but also leads to the decrease of fcu due to the increase in mean pore size and the connected pore amount, and the decline of pore uniformity and pore wall strength. When the dosage of FA, water, foam, and the naphthalene-based superplasticizer of the binder is 20 wt%, 0.50, 16.5 wt%, and 0.6 wt%, the cured HBFC with ρd of 293.5 ± 4.9 kg/m³, fcu of 0.58 ± 0.02 MPa and kc of 0.09234 ± 0.00142 W/m·k is achieved. In addition, the cost of HBFC is only 39.5 $/m³, which is 5.2 $ lower than that of ordinary Portland cement (OPC)-based FC. If the surface of the optimized HBFC is further treated with water repellent, it will completely meet the requirements for a prefabricated ultra-light insulation panel.

Molecular dynamics study on calcium silicate hydrate subjected to tension loading and water attack: structural evolution, dynamics degradation and reactivity mechanism.

The coupled effects of mechanical loading and chemical attack can dramatically weaken the durability of a material. In this study, reactive force field molecular dynamics and the GCMC method were utilized to investigate the degradation mechanism for a deformed C-S-H gel subjected to water attack. In the elastic region of the stress-strain relation, no water molecules invaded the deformed C-S-H gel tensioned along the y-direction. On the other hand, in the failure stage, the tension loading stretched/broke the Si-O-Si bond, resulting in the distortion of the "dreierketten" silicate chain distribution and ordered zigzag sheets built by the calcium oxygen octahedron. More water molecules penetrated into the defective silicate sheets and dissociated into the Si-OH and Ca-OH surrounding the highly reactive non-bridging oxygen sites induced by the silicate chain breakage. The water invasion and hydrolytic reaction reduced the cohesive stress of the tensioned C-S-H structure. Furthermore, the cracks in the calcium silicate sheet connected with the interlayer region, enhancing the channel connectivity for the water transport. This resulted in the water dynamic transformation from the cage stage to the diffusive stage. The high mobility of confined water molecules further weakened the stability of the hydrogen bonds in the calcium silicate skeleton. Moreover, the tensile loading and water attack contributed to the silicate morphological rearrangement. The long silicate chains were first destroyed to form shorter chains and then re-polymerized to form a branch and ring structure to strengthen the weak interlayer regions.