Search

Fibers are essential in reinforcing the mechanical properties of ultra-high performance concrete (UHPC), particularly the tensile and flexural strength. This paper conducts a systematic review on the effect of fibers with different textures and geometry on the properties of UHPC from the following aspects: (1) the bond mechanism of steel fibers with the UHPC matrix; (2) the effect of fiber shape, fiber orientation and hybridization of steel fibers on the microstructure, failure mode, mechanical properties, autogenous shrinkage and durability of UHPC; (3) the reinforcing mechanism of synthetic fibers (polyvinyl alcohol fibers (PVA), polypropylene fiber (PP), polyethylene fibers (PE)), mineral fibers (basalt fibers, wollastonite fibers) and carbon fibers in UHPC and their effect on the performance of UHPC; (4) the use of hybrid fibers and their synergetic effect on the mechanical performance and shrinkage of UHPC. Finally, this paper discussed the further research trends of fibers in UHPC. [ABSTRACT FROM AUTHOR]

The accurate measurement of chloride content in concrete is of great importance, because chloride-initiated corrosion of the reinforcement significantly influences the durability of reinforced concrete. This paper summarizes the research status of Ag/AgCl ion-selective electrode (ISE) used for non-destructive chloride detection in concrete. In general, four main topics are reviewed, including preparation of Ag/AgCl-ISE, main factors influencing the potential of Ag/AgCl-ISE, detection limit/potential response of Ag/AgCl-ISE, and novel composites/electrochemical methods based on Ag/AgCl-ISE for chloride detection in concrete. Further, the existing problems and research strategies are also discussed in detail in this present review. [ABSTRACT FROM AUTHOR]

The chemical composition and surface properties of reaction products in alkali activated slag (AAS) are different from ordinary Portland cement, thus significantly affect their chloride binding behavior. The objective of this study is to investigate the relationship between physical/chemical characteristics and chloride binding capacity of the reaction products in AAS, i.e. C-S-H gel, C-A-S-H gel and LDHs. In this study, typical reaction products with different chemical compositions in AAS were chemically synthesized and characterized by XRD, NMR, BET and zeta potential. The chloride binding capacity of synthesized reaction products was evaluated in neutral and alkaline NaCl solution. The results indicate that C-S-H and C-A-S-H gels bind chlorides more physically. The increase of Ca/Si ratio and Al/Si ratio results in more positive zeta potential, which is beneficial for chloride binding of C-(A)-S-H gels. Further, the chloride binding capacity of C-A-S-H gels is higher than C-S-H gel, due to more positive zeta potential and larger layer spacing in the presence of Al. LDHs possess higher chloride binding capacity than C-(A)-S-H gels, due to its higher specific surface area (120.3–125.5 m2/g) and positive zeta potential (22.5–28.5 mV); the increase of Mg/Al ratio results in increased layer spacing and subsequently higher chloride binding capacity. The physical binding contributed most chloride binding of LDHs; due to the difficulty of exchange between Cl− and intercalated CO 3 2− in Mg-Al LDH, its chemical binding is weak, which is resulted from the entering of chlorides into the interlayer from the edge of LDHs. [ABSTRACT FROM AUTHOR]

Alkali-activated materials (AAMs) are usually recognized having better acid resistance compared to ordinary Portland cement (OPC), however, the detailed mechanism has not been well studied due to the complexity of raw materials and hardened matrix. In this study, two typical binding gels in AAMs, sodium aluminum silicate hydrate (N-A-S-H) and calcium (sodium) aluminum silicate hydrate (C-(N)-A-S-H), were chemically prepared by laboratory synthesized aluminosilicate powders. The microstructure and composition evolution of the two gels exposed to sulfuric acid were investigated. The results showed that the C-(N)-A-S-H gel had higher degree of structural order than the N-A-S-H gel, while the N-A-S-H gel showed higher polymerization. The AlIV in N-A-S-H gel transferred completely to AlV during sulfuric acid attack, while trace amount of AlIV was still detected in the exposed C-(N)-A-S-H gels. Both gels had increased silica polymerization degree after exposure to sulfuric acid solution. Due to rapider dealumination of N-A-S-H gel than C-(N)-A-S-H gel, the Si/Al ratio increased much higher in the former. The molecular framework changes of N-A-S-H gel caused by dealumination was found to have less effect on the integrity though than that of C-(N)-A-S-H gel, in which coarse gypsum crystalline grains formed and led to destructive stress in hardened matrix. This paper provides an insight at microstructure level of the two typical gels, which is essential for the manufacturing and application of alkali-activated materials. [ABSTRACT FROM AUTHOR]

Corrosion performance of the reinforcement was dramatically affected by chemical compositions of pore solution in alkali-activated fly ash. In this study, the electrochemical behavior and surface properties of reinforcement were characterized in chloride-contaminated simulated alkali-activated fly ash pore (SAFP) solution. The results indicated that due to the high alkalinity and formation of zeolite-like adsorption layer (about 20 nm), the reinforcement was still in passive state and exhibited high corrosion resistance in chloride-contaminated SAFP solution. However, the protective effect of passivation film (consisted of mainly FeOOH outer film and FeO inner film) and corrosion resistance of the reinforcement were reduced by chlorides. Further, the chloride threshold in SAFP solution was 10 times larger than simulated cement pore solution, due to the competitive adsorption between OH−/silicates and chlorides. [ABSTRACT FROM AUTHOR]

The present paper explores the methods of transforming low grade kaolinitic clay, sandstone clay, into supplementary cementitious material (SCM). The sandstone clay is the waste product in the sand manufacturing from sandstone mineral. Thermal calcination at 600–900 °C was applied to activate the initial sandstone clay. Microstructure characterization was then carried out on the obtained activated sandstone clays (ASCs) to study the microstructure and chemical changes during the thermal activation. The pozzolanic reactivity of ASCs were assessed by direct methods (isothermal calorimetry test and lime consumption test) and indirect method (strength measurement of ASC blended mortar). The compressive strength of the ASCs blended cement based mortar showed that ASCs possessed high pozzolanic reactivity through the dehydroxylation and the alternation of Al and Si coordination environments during thermal activation. Agglomeration occurred after the sandstone clay calcination, however the particle size was reduced. Even at a short time calcination for 1 h, the pozzolanic reactivity of ASCs improved as the calcination temperature increased from 600 to 900 °C. Mortar strength test results confirmed that the ASCs with a fineness similar to OPC, can be used as high quality SCMs. Further isothermal calorimetry results indicated that ASCs not only contributed to pozzolanic reaction but also had a good filler effect in the early hydration stage due to the existence of micro-quartz. [ABSTRACT FROM AUTHOR]

The development of a green alternative that can partially or completely substitute the Ordinary Portland cement (OPC) as a practical construction material with low CO 2 footprint is an important subject to researchers for decades. The use of artificial pozzolanic materials, e.g. fly ash, slag, silica fume, etc. cannot meet the huge-scale demand for cement replacement. Kaolinitic clay, with its high pozzolanic reactivity after activation and abundant resources worldwide, is regarded as a promising supplementary cementitious material (SCM) for cement. Recent studies have addressed that the utilization of calcined kaolinitic clay as SCM can effectively improve the properties of cement products. This paper presents a comprehensive review of the research progress on activated kaolinitic clay as SCM in the last decade. It systematically introduces the essential properties, activation mechanism and pozzolanic reactivity assessment of kaolinitic clay. Application of calcined clay in two different cementitious systems (calcined clay blended cement and limestone calcined clay cement) is reviewed from the aspects of workability, mechanical properties, and long-term durability properties with the mechanism discussion. Finally, the environmental and economic impacts of the application of calcined clay in different cementitious systems are discussed. [ABSTRACT FROM AUTHOR]