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The purpose of this study is to evaluate the influence of cation types on the rust resistance performance of new nucleic acid corrosion inhibitors in simulated concrete pore solution. Four common chlorinated ice salts (sodium chloride (NaCl), potassium choride (KCl), calcium chloride (CaCl2) and magnesium chloride (MgCl2)) were selected to study the corrosion behaviour of chloride ions by linear polarisation resistance method, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The results show that the cationic type will affect the thickness of the passivated film on the steel surface and the compactness of the passivated film formed by iron oxide and nucleic acid rust inhibitor. The type of cations affects the critical chloride concentration of nucleic acid inhibitor, following Na+ ≈ K+ > Ca2+ > Mg2+.

The effects of limestone powder (LP) of different fineness on the packing density, strength and hydration behaviour of cement blended with a high volume of LP were studied using Vicat setting time tests, strength tests, isothermal calorimetry and scanning electron microscopy. Three different LP fineness values were examined and it was found that cement replacement by LP with the smallest median particle size produced the highest density. The compressive strength of the blended cement mortar increased with increasing LP fineness. The packing densities of the blended cement were used to infer the evolution of mechanical strength of the blended cement mortar. The addition of LP with a median particle size smaller than that of cement accelerated the hydration of cement in the acceleration period. Compared with LP with a median particle size smaller than that of cement, the production rate of nuclei of cement blended LP with a median particle size larger than that of cement decreased with increasing amounts of LP replacement, while the growth rate of nuclei increased with increasing amounts of LP replacement.

High-belite cement (HBC) is increasingly used in hydraulic concrete, of which the frost resistance is an important index in cold climatic areas. In the present study, the mass loss, dynamic modulus, cube compressive strength, micro-morphology and pore characteristic parameters of HBC concrete and reference specimens were investigated under freezing and thawing, and their internal frost damages were quantified by micro-Vickers hardness and ultrasonic theory. Test results showed that the frost resistance of HBC concrete was similar to that of ordinary Portland cement (OPC) concrete under the same proportioning, and that their freeze–thaw failure was a continuous and asynchronous process of both surface-to-interior damage and overall deterioration caused by the test mechanism and specimen size. Service life of concrete could be calculated through the thickness of the damaged layer obtained by ultrasonic theory, and models of Vickers hardness distribution and compressive strength prediction based on equivalent Vickers hardness were proposed to evaluate the performance of frozen concrete with high accuracy. This potentially provides a better understanding of freeze–thaw damage and a reference for the popularisation and application of HBC concrete.

High-performance and ultra-high-performance concrete have been increasingly used as building materials in modern construction, and their superior properties derived from a tailored microstructure. In this study, Rietveld quantitative X-ray diffraction analysis was adopted to investigate phase evolutions of four cementitious pastes with varied addition of fly ash or silica fume at very low water/binder ratios. The results indicate that the phase changes concerning the contents of the major clinker phases mainly happen within the first 7 d. The content of portlandite (CH) in pure cement paste increases monotonically while, due to pozzolanic reaction, it decreases in blended pastes between 7 and 28 d. At early age, the addition of silica fume accelerates the hydration of cement, while the introduction of fly ash shows significant retardation especially at 1 d. The degree of hydration (DOH) of cement is mainly determined by the effective water/cement ratio of the paste, and the incorporation of fly ash, while initially producing little reaction, results in a higher DOH of cement. With steam curing, the content of CH significantly decreases in the blended pastes, which indicates a promoted pozzolanic reaction.

Limestone powder has been widely used in concrete structures because of its easy availability and positive effect on reducing the heat of hydration. The addition of limestone powder has a significant influence on the hydration and microstructure of cement paste and therefore affects the strength and durability of concrete. This paper examines the hydration and microstructure of cement paste blended with limestone powder using multiple techniques. The results reveal that when the content of limestone powder is up to 50 wt%, the compressive strength, cumulative heat and portlandite show a significant reduction. However, the addition of limestone powder has no effect on the species of hydration products and pore solution composition in cement paste specimens. The porosity and relative threshold diameter of cement paste samples increase while the pore volume (

The pore structure is of great concern to the mechanical properties and durability of concrete. This paper investigated the pore structures of slag-blended cement paste under steam curing using two non-destructive testing (NDT) tools: electrochemical impedance spectroscopy and ultrasonic methods. Four different pastes containing 0, 20, 50 and 70% slag were prepared. For the electrochemical impedance spectroscopy method, parameters R1 and n fitted from an equivalent circuit model, Re(CR1(QR2)), were used to evaluate the porosity and the fractal dimension, respectively. For the ultrasonic method, ultrasonic pulse velocity was used to determine the porosity. Moreover, X-ray diffraction and mercury intrusion porosimetry were employed to characterise the properties of the cement paste. The results showed a linear relationship between R1 and ultrasonic pulse velocity. It was found that the porosity of the steam-cured paste was lower than that of the standard-cured paste at 3 d. However, an opposite trend was observed at 28 d. In addition, the fractal dimension of the pore structure dropped as the porosity increased during the hydration process. Although the slag-blended cement paste had a lower porosity than the pure cement paste, the fractal dimension increased with the increasing slag replacement ratio due to the complicated slag hydration products.

Graphene can be used in cement-based materials to enhance their electrical properties. In this work, electrical conductivity measurements on graphene/cement nanocomposites were performed using the four-probe method. The influences of graphene content, water/cement ratio, curing age and water content on the electrical conductivity of the system were studied. The micro-morphology of the composites was observed by scanning electron microscopy (SEM). The results showed that electrical conductivity increases with increasing graphene content, which can be explained by percolation theory. A logistic function is introduced to describe the relationship between the electrical conductivity of the system and graphene content. SEM observation revealed that the conductivity of system depends on the distribution and connection of graphene in the cement matrix. When the graphene content exceeded a threshold concentration, curing age and water content had little influence on the electrical conductivity of system. The findings provide an important guide for research on the electrical properties of graphene/cement composites.

Compressive fatigue generally causes permanent internal damage to concrete structures. This paper presents an experimental study to evaluate the influences of steam curing and slag replacement on the compressive fatigue performance of concrete. After 10 000 cycles loading at stresses of 0·5, 0·6 and 0·7, the residual compressive strength of concrete was measured to investigate the mechanical property after fatigue. The fatigue damage was determined using two non-destructive techniques: the ultrasonic pulse velocity and electrochemical impedance methods. In the electrochemical impedance method, the R1 and n parameters fitted from the Re(CR1(QR2)) circuit model were used to characterise the pore structure. The results showed that the residual compressive strength of standard-cured concrete without slag replacement decreased with increasing stress. The compressive fatigue damage of specimens after steam curing was greater than that after standard curing. In steam curing conditions, the fatigue damage of concrete increased with increasing slag replacement. The results indicated that electrochemical impedance is an appropriate method to evaluate compressive fatigue damage.

An electrochemical technique was applied to improve mortar durability. Silicate ions were injected into mortar pores to react with calcium hydroxide by applying an electric field, resulting in densification of the mortar. The calcium ions in the pores of the mortar were electro-transported to the sodium silicate solution, which led to the formation of a surface coating. The durability of mortars after electrochemical treatment was evaluated by examining water absorption, carbonation depth, sulfate resistance and chloride diffusion. In addition, microscopic observations were made using scanning electron microscopy, mercury intrusion porosimetry, thermogravimetric analysis and differential scanning calorimetry. The results indicated that the resistances to water absorption, carbonation, sulfate and chloride diffusion were improved by use of the electrochemical technique. The improvement in mortar durability was attributed to the combined actions of densification and the coating layer produced on the mortar surface during the electrochemical treatment. However, the coating layer, being thin and of high porosity, played only a minor role.

A novel electrodeposition method for both surface coating treatment and densification of mortar is presented. Mortar specimens of dimensions 2 cm × 4 cm × 16 cm were used to evaluate the effectiveness of the method: the produced coating layer thickness and flexural strength, compressive strength and resistivity of the electrochemically treated mortar specimens were measured. The results indicate that the coating layers were 0·2–1·1 mm thick, and thickness strongly depended on technological parameters such as applied voltage, sodium silicate concentration, conduction time and temperature. With suitable technological parameters, the electrodeposition method was found to produce moderate increases in flexural and compressive strengths. Resistivity was significantly increased, indicating great potential for upgrading durability. The microstructures of the electrochemically treated mortar specimens were observed using a mercury intrusion porosimeter and a scanning electron microscope equipped with an energy dispersive spectrometer.

Expanded polystyrene (EPS) lightweight concrete is composed of a plain concrete matrix and EPS beads. The aim of the study was to characterise the mechanical properties of EPS lightweight concretes containing various volumes of EPS (0%, 5%, 10%, 15%, 20%, 25%, 30%, 35% and 40%) and of two water/cement ratios (0·45 and 0·55). The test results indicate that the apparent density, compressive strength, splitting tensile strength and elastic modulus of EPS concrete decrease when the content of EPS particles increases. Based on the experimental results and composite approach, a new simplified mathematical model is proposed for evaluating the compressive strength and elastic modulus of EPS concrete as a function of the mechanical properties of the plain concrete, the mechanical properties of the EPS and the percentage of EPS particles.

The objective of this study is to examine the influence of chloride salt type on the critical chloride content for the corrosion of steel reinforcement in concrete. The experiment was carried out in Portland cement concrete and concrete incorporating fly ash and ground granulated blast-furnace slag. Sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl2) and magnesium chloride (MgCl2) were chosen to corrode the steel reinforcement. The initiation of pitting corrosion was detected by half-cell potential, linear polarisation and electrochemical impedance spectra methods. The critical chloride content was expressed as free chloride or total chloride by mass of binder, as well as [Cl−]/[OH−]. In addition, X-ray diffraction and differential scanning calorimetry–thermogravimetry analyses were employed to study the products in concrete. Test results show that the chloride salt type has a marked influence on the critical chloride content. The critical chloride contents expressed as free Cl− for the specimens contaminated by external magnesium chloride and calcium chloride are lower than those for the specimens contaminated by external sodium chloride and potassium chloride. Contrary to the trend for free Cl−, the critical chloride contents expressed as total Cl− and [Cl−]/[OH−] for the specimens contaminated by external magnesium chloride and calcium chloride are higher than those for the specimens contaminated by external sodium chloride and potassium chloride. The concrete specimens contaminated by external sodium chloride have the same critical chloride content as those contaminated by external potassium chloride, regardless of the water–binder ratio, mineral admixtures, or the form of critical chloride content.

The objective of this study is to evaluate the influences of silica fume and pulverised fly ash on the chloride threshold value for the corrosion of steel reinforcement in concrete. The chloride ions were introduced into concrete specimens made with ordinary Portland cement alone and a mixture of ordinary Portland cement and the partial replacements of cement with silica fume and pulverised fly ash at three levels by an immersing–drying cycle procedure. Half-cell potential and linear polarisation methods were applied to detect the initiation of pitting corrosion for the steel reinforcement. The threshold values were expressed as free chloride or total chloride by mass of binder, as well as [Cl−]/[OH−] and [Cl−]/[H+]. It has been found that the changes of the chloride threshold value with the additions of pulverised fly ash and silica fume intensively depend on the representations. Especially, the critical [Cl−]/[H+], regardless of the type and content of mineral admixtures, approximately keeps constant at 0·02. Accordingly, [Cl−]/[H+] is suggested as a more suitable form to represent the chloride threshold value.