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Developing a suitable activation method to extend the application of molybdenum tailings (MT) in construction materials is crucial for maximizing MT utilization and reducing carbon emissions in the construction industry. In this study, MT was substituted for different levels of cement to prepare cement-MT composite paste, and direct electric curing (DEC) was used attempting to improve the reactivity of MT. The results indicated that MT doping decreased the compressive strength of the specimens, increased the pore content, and reduced the generation of hydration products. On the other hand, DEC effectively improved the early strength of the cement-MT composite and the hydration activity of MT. At 28 d, the cementitious coefficient of MT using DEC increased by 6.25 %-675 % compared with standard curing (SC) prepared specimens. Meanwhile, XRD and TG test results showed that DEC effectively promoted the hydration process through the ohmic heating effect. However, it did not change the types of hydration products or the morphology of the specimens. DEC reduced the pore content and refined the pore size distribution in the early stage of the specimens. Moreover, the carbon emission and cost per unit of strength in the early stage of DEC specimens were significantly reduced compared with those of SC. This paper proved that DEC was a feasible way to activate MT activity, and DEC could effectively promote the application of MT in construction materials. more...

The concrete hydration heat and different construction methods directly affects the early crack resistance of massive concrete structures and may lead to concrete cracks. In this work, the hydration heat test and finite element simulation were conducted to investigate the early cracking resistance and the main influencing factors of the massive concrete bridge pier. The size of the bridge pier was 8 m (length) × 4 m (width) × 22.5 m (height), and the pier was poured in two layers (8.5 m/14 m) with an interval of 37 h. The internal temperature of the pier was tested 16 days after the concrete pouring using pre-buried temperature sensors, and the crack expansion on the surface of the pier was continuously observed during the test period. The test obtained a maximum concrete temperature of 64.25 °C after 100 h after the concrete pouring, and 3 cracks were found on the surface of the pier. The pier simulation model is established according to the actual situation in the test, The maximum temperature inside the bridge pier obtained from the simulation is 61.73 ℃, and the temperature field development pattern and the crack expansion on the pier surface were basically consistent with the test, which confirmed the reliability of the simulation method. Besides, the simulation process found that the maximum main tensile stress on the surface of the bridge pier was 2.77 MPa, which was mainly distributed near the interface of the 2 layers, indicating that the layered pouring scheme adopted in the test was improper. The effect of layered casting schemes on the early cracking resistance of massive concrete has rarely been discussed, and further simulations revealed that: the maximum main tensile stress can be reduced by gradually improving the number of layers, layer height and interval between each layer. By changing the original layered pouring scheme to 3 layers with an interval of 3 days between each layer, the maximum principal tensile stress is reduced to 1.50 MPa, and early cracking of the bridge pier can be avoided. This work provides a general numerical analysis method with high accuracy, parameterization and quantification for early crack control of massive concrete structures in a multi-factor complex environment. more...

In order to explore the applicability of different cementitious paste systems used for massive concrete structures, the hydration characteristics and mechanism of the binary and ternary cementitious paste systems in Portland cement pastes containing fly ash and slag were investigated in this work. The hydration heat release rate and cumulative heat release of each cementitious paste system at 20 ℃ and 60 ℃ were measured by an isothermal calorimeter. The characteristic parameters of each cementitious paste system hydration process were calculated and analyzed using the Knudsen equation and Krstulović-Dabić hydration kinetic model. Besides, the hydration mechanism was analyzed. It was obtained that the cement-fly ash (C-F) binary system is not effective in reducing the early reaction rate but its overall heat release is lower; while, the cement-slag (C-S) binary system is more effective in reducing the reaction rate but its overall heat release is higher, and the cement-fly ash-slag (C-F-S) ternary system has the advantages of both above binary systems. Probably, the C-F-S ternary system is more consistent with the heating characteristics of massive concrete structures, and it is more applicable. There are few studies on the hydration characteristics of C-F-S ternary systems from the perspective of hydration kinetic, and this work provides theoretical basis for the selection of cementitious paste systems for massive concrete structures. more...

Along with the construction industrialization degree, precast concrete is being utilized more and more frequently in construction to speed up the process and guarantee the quality of structural concrete. In order to make the precast concrete reach the required strength quickly and speed up the turnover of formwork, various rapid concrete curing methods emerged. Currently steam curing, direct electric curing, and microwave curing are the widely studied rapid curing methods. Rapid curing methods could promote early strength growth by changing the internal and external temperature and humidity of concrete, but may lead to late strength shrinkage, increased internal micro-cracks, coarsening of pore structure and other problems. Based on the comprehensive analysis of the curing principles of the three rapid curing methods, this paper reviews the effects of the three rapid curing methods on the hydration process, properties, and microscopic pore structure of concrete. The shortcomings of different rapid curing methods, and the topics of future researchs are also included. more...

The hot-dry environment in high geothermal tunnels negatively affects the development of properties of cementitious materials. In order to improve the mechanical properties of cementitious materials in such environments, a proper curing system was developed, the material compositions were taken into account, and the related mechanism was analyzed by microscopic tests. The results showed that the mechanical properties and microstructure of cementitious materials can be improved effectively by adopting film curing and an adequate dosage of fly ash in a hot-dry environment. Compared with standard curing, a hot-dry environment accelerates the evaporation of water in fresh cementitious materials and the early hydration rate of cementitious materials, resulting in lots of unhydrated cement particles and an uneven distribution of hydration products, seriously decreasing the properties of the cementitious materials. Film curing keeps the water from evaporating and ensures enough water is available for hydration at an early age. The addition of FA reduces the amount of cement clinker and decreases hydration reaction rate in high temperature environments at an early stage, avoiding the unevenness of hydration products. High temperatures stimulate the pozzolanic activity of fly ash and promote the degree of secondary hydration reaction, producing more hydration products, as well as contributing to the improvement of the mechanical properties and microstructure of cementitious materials. In this study, in a hot-dry environment, the mechanical properties of mortar with 25% fly ash and 2–3 d film curing are better than those of other experimental groups, including those of standard curing. more...

Deterioration mechanisms of some premature damaged steam-cured concrete track slabs (CTS) in Chinese railway less than 4 years were investigated. Field investigation, raw materials test and suspicious products analysis were carried out. Results show that steam-cured heat damage (SCHD) of concrete takes place in steam-cured process. Expansion products are ettringite in hydrated products and alkali-silica gels between the interface of hydrated products and coarse aggregate. SCHD makes CTS surface layer loose, porous and more micro-cracks. Long-term fatigue load from high-speed train acting on CTS enlarges concrete microcracks, leading to water penetrating into concrete easily in moist and rainy environment. In the process of water ingression, alkali-silica reaction (ASR) and delayed ettringite formation (DEF) take place, hence resulting in CTS cracking and premature deterioration. more...

The use of recycled aggregates as raw materials in mortar and concrete can not only achieve the effective use of renewable resources but also achieve environmental friendliness. In this paper, the influence of recycled fine aggregate (RFA) content and additives on fluidity, fluidity loss, strength, drying shrinkage, and hardened surface roughness of self-leveling mortar (SLM) was investigated. The surface roughness of SLM was characterized by fractal dimension, and the main influence factors of the surface roughness were discussed. Results showed that the strength of SLM decreased with the addition of RFA content. When the RFA content was 100 %, the 1-day and 28-day compressive strength of SLM decreased by 19.2 % and 12.4 % respectively, and the 1-day flexural strength decreased by 16.7 %, but the 28-day flexural strength changed little. With the increase of RFA content in SLM, more polycarboxylic acid superplasticizer was required in order to achieve the appropriate workability. Meanwhile, a high content of RFA led to an increased fluidity loss of SLM within 30 min. The addition of silica fume enhanced the early strength of SLM, fly ash decreased the drying shrinkage, and slag improved fluidity. The size and gradation of RFA had a significant effect on the surface roughness. The addition of RFA reduced the surface roughness of SLM. The surface fractal dimension of SLM mixed with 25 % RFA was the lowest. The addition of silica fume further reduced the surface roughness of the SLM. The mortar with different surface roughness can be prepared by adjusting the content and gradation of RFA and the fluidity of mortar. [ABSTRACT FROM AUTHOR] more...

In rheological testing, different test regimes can result in large differences in the rheological curves, and the pre-shear procedure is one of the most important parts of the test. In order to investigate the effect of the pre-shear regime on the rheological properties of cement pastes, an Anton Paar 102 rotary rheometer was used for the test. Two different pre-shear regimes (RF and CP pre-shear), three different pre-shear duration times (50 s, 100 s, 150 s), and four different pre-shear rates (25 s-1, 50 s-1, 100 s-1) are carried out for the test. The results showed that with the increase of pre-shear duration or rate, the four rheological parameters (plastic viscosity, yield stress, rheological index, and thixotropy) of the pastes decrease in different degrees. RF pre-shear was more influenced by shear time, while CP pre-shear was influenced by shear time and rate. The pre-shear regimes led to different changes in the flocculation structure of the cement paste, which had a great influence on rheological parameters ultimately. Based on the improved Krieger-Dougherty model, the effective solid volume fraction of cement paste was calculated. Analysis shows that the phenomenon of "deflocculation" and "re-flocculation" occurs when the cement paste is subjected to shear action, thus, the pre-shear regimes destroy the structure of the pastes. With the increase of pre-shear duration or rate, the effective volume fraction decreased, which indicated that the initial flocculation structurewas broken. The greater the damage degree of pre-shear, the slower the establishment rate of the internal structure of the pastes in dynamic shear, resulting in lower rheological parameters. [ABSTRACT FROM AUTHOR] more...

Natural gas hydrate (GH) is a significant potential energy source due to its large reserves, wide distribution, high energy density and low pollution. However, the gas production rate of past gas hydrate production tests is much lower than the requirement of commercial gas production. Reservoir stimulation technologies like hydraulic fracture provide one potential approach to enhance gas production from GH. The reservoir reformation behaviour of the hydrate-bearing sediments (HBS), particularly sediments with a high clay content, is a complex process during a hydraulic fracturing operation, which has been poorly understood and thus hardly predictable. This paper presents an experimental facility that was developed to analyze the hydraulic fracture mechanism in synthesized HBS. This facility can be used to form GH in sediments, conduct visual observation of hydraulic fracturing experiments, and measure the permeability of HBS under high pressure (up to 30 MPa) and low-temperature conditions (from 253.15 K to 323.15K). It is mainly composed of a pressure control and injection unit, a low temperature and cooling unit, a cavitation unit, a visual sapphire reactor, and a data acquisition and measurement unit. The hydraulic fracture module is consisting of a gas cylinder, fracturing pump, hopper, proppants warehouse and valves. The sapphire reservoir chamber is applied to observe and measure the fracture of HBS during hydraulic fracturing. The permeability test module is composed of a constant-flux pump and pressure sensors, which can evaluate the permeability performance before and after hydraulic fracture in HBS. The fundamental principles of this apparatus are discussed. Some tests were performed to verify hydraulic fracture tests and permeability tests could be practically applied in the HBS exploitation. [ABSTRACT FROM AUTHOR] more...