Search

The durability of concrete under combined acidic exposure and flexural loading was studied. Concrete specimens were made and subjected to up to 168 days of exposure to flexural loading (stress levels of 0, 0.35 and 0.50 of the 28 day flexural strength) and an acidic environment. Concrete properties such as neutralisation depth, mass change, compressive strength and flexural strength were measured. The compositions of different samples were determined by means of X-ray diffraction and chemical analysis and the microstructure of samples was examined using scanning electron microscopy. The results showed that, with an increase in stress level from 0 to 0.50, the performance of the concrete deteriorated gradually and the deterioration in the tension zone was greater than that in the compression zone. The concrete performance improved within 28 days but deteriorated dramatically afterwards. The acidic solution reacted with the cement paste, leading to the formation of harmful gypsum and the decomposition and dissolution of cement hydration products. An increase in stress level increased the porosity of the concrete, accelerating the rate of acid erosion, which led to a deterioration of concrete performance. [ABSTRACT FROM AUTHOR]

The effect of sulfate attack (SA) on the strength of ultrahigh-performance concrete (UHPC) made with different replacements (0%, 50% and 100% by weight) of natural fine aggregate with recycled fine aggregate (RFA) was investigated. The UHPC samples were soaked in 10% sodium sulfate solution for 0–180 days (SA duration). The sulfate ion concentration, flexural strength and compressive strength of the UHPCs were investigated. The microstructure of the UHPCs before and after SA was analysed using a multi-technique approach (analysis of interfacial transition zones (ITZs), X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy and mercury intrusion porosimetry). The results showed that, with an increase in SA duration, the compressive strength of the UHPC made with RFA increased. The generated gypsum and ettringite refined the microstructure by converting harmful pores (>20 nm) into harmless pores (<20 nm) in the UHPC matrix. For the same SA duration, the sulfate ion concentration of UHPC increased with an increase in RFA content due to more ITZs, pores and microcracks introduced by the RFA. Strength development models for UHPC with different RFA contents under SA were developed. [ABSTRACT FROM AUTHOR]

Geopolymer-based materials have attracted increasing interest in construction and infrastructure engineering due to their high mechanical properties but low carbon dioxide emissions. However, it remains unclear whether this type of material can work in a stable way in harsh environments. This study investigated the chemical–physical–mechanical stability of metakaolin-based geopolymer (MKG) mortars under the immersing actions of sodium sulfate and magnesium sulfate solutions up to 180 days by testing their visual appearance, mass, dimensions, strength, microstructure and chemical components. Results show that the immersion actions do not cause obvious changes in mass, dimensions or compressive and flexural strengths of the MKG mortars. A white deposit, consisting of magnesium hydroxide, magnesium sulfate, magnesium carbonate and magnesium silicate aluminate hydrate, appeared on the surfaces of the high-alkali MKG specimens immersed in magnesium sulfate solution for 30 days, and disappeared at later ages due to the continual consumption of magnesium hydroxide. Mg–Na exchanges occurred for the MKG mortars immersed in magnesium sulfate solution. Overall, the chemical–physical–mechanical stability of geopolymer-based materials depends on the material components and interactions with the environment. Caution should be used before any engineering applications, and relevant issues deserve further rigorous investigation in the future. [ABSTRACT FROM AUTHOR]

The erosion behaviour of cement mortar subjected to sand particle blasting was investigated using the gas-blast method with different impact angles and sand particle fluxes. The corrosion regime under the action of freeze–thaw cycles was also investigated. The erosion rate of the cement mortar was measured and analysed under freeze–thaw cycles using two concentrations of corrosive solutions (8% and 10% mass fractions of salt solutions of sodium chloride and sodium sulfate). Moreover, with the support of scanning electron microscopy images, X-ray diffraction patterns and laser scanning confocal microscopy, the erosion mechanism at different impact angles and freeze–thaw cycles was investigated. The results showed that the erosion rate increased with increasing impact angle and decreased with an increase in sand particle flux. The erosion rate of the cement mortar was also found to first decrease and then increase with an increasing number of freeze–thaw cycles. The erosion damage of the cement mortar surface with significant mass loss was found to be due to two mechanisms – horizontal cutting and vertical pressing. [ABSTRACT FROM AUTHOR]

A repair material for use in the marine environment is developed. The ternary cementitious system (TCS), which contains ordinary Portland cement, calcium aluminate cement and gypsum, causes rapid hardening and displays high strength, and hence can be used as a building repair material. However, the hydration products of the TCS contain a large amount of ettringite, calcium hydroxide and calcium aluminate hydrate, which can be easily corroded by the inorganic ions in seawater. Owing to its pozzolanic activity and ability to bind chloride, slag is added to the TCS. The content of slag in the TCS is a critical factor; hence, the effects of 0–35 wt% slag on the compressive strength, mass change, apparent porosity, and free and total chloride content of TCS immersed in seawater are investigated. The systems with slag show a higher compressive strength than the TCS in seawater, and the reduction in compressive strength of the system with 25 wt% slag is only 14·14% after 180 d. Also, the addition of slag significantly reduces the free and total chloride content of the TCS immersed in seawater. The optimum amount of slag in the TCS is determined to be about 25 wt%. [ABSTRACT FROM AUTHOR]

Cementitious materials are highly vulnerable to rapid and serious deterioration in aggressive acidic environments. The aim of this paper is to understand the microstructure deterioration of cement paste due to acid attack using the non-destructive technique called X-ray computed micro-tomography (micro-CT). To investigate this deterioration, an accelerated static immersion based leaching test with periodic abrasive action was performed on cement paste specimens of size 10×10×60 mm, prepared using ordinary Portland cement with a water-to-cement ratio of 0·40. After 28 d of initial curing in saturated lime water, specimens were exposed to 1% sulfuric, 1% hydrochloric, 0·5 M acetic and 0·5 M citric acid solutions for a maximum period of 6 weeks. X-ray micro-CT was used for imaging the deteriorated specimens to investigate the changes in microstructure. Various parameters related to degradation were also deduced based on the image analysis performed on the CT images in order to evaluate the performance of the cement paste in various acid solutions. Parameters such as altered depth/area could be considered as primary indicators of attack, which can be best captured and analysed using X-ray CT. [ABSTRACT FROM AUTHOR]

Reinforced concrete structures are normally exposed to various aggressive environmental conditions; thus it is essential to protect the steel reinforcement from corrosion and the concrete from degradation. This experimental study is aimed at optimising the physico-mechanical properties of reinforced concrete by using iron mill scale as a replacement for cement. Reinforced cement mortars and concrete specimens with and without the addition of iron mill scale were partially immersed in 3·5% w/w sodium chloride solution for 17 months and 130 d, respectively. The mill scale was added to the aforementioned cementitious composites in proportions of 5% and 10% by weight of cement. Its effect on chloride penetration resistance was evaluated using various standardised and non-standardised experimental techniques. The results showed that the additive increased the resistance of reinforced cementitious composites to chloride penetration, reduced the corrosion of steel reinforcement and improved the compressive strength of concrete. Furthermore, iron mill scale decreased the concentration of chloride ions in cement mortars which were partially immersed in sodium chloride solution. [ABSTRACT FROM AUTHOR]