Direct electric curing (EC) is a new green curing method for cement-based materials that improves the early mechanical properties via the uniform high temperature produced by Joule heating. To ...understand the effects of EC and steam curing (SC) on the mechanical properties and microstructure of cement-based materials, the mortar was cured at different temperature-controlled curing regimes (40 °C, 60 °C, and 80 °C). Meanwhile, the mechanical properties, hydrates and pore structures of the specimens were investigated. The energy consumption of the curing methods was compared. The results showed that the EC specimens had higher and more stable growth of mechanical strength. The hydration degree and products of EC samples were similar to that of SC samples. However, the pore structure of EC specimens was finer than that of SC specimens at different curing ages. Moreover, the energy consumption of EC was much lower than that of SC. This study provides an important technical support for the EC in the production of energy-saving and high early-strength concrete precast components.
Addressing the scarcity of research on the residual expansion deformation of steam-cured concrete and its underlying factors, this study introduces a testing apparatus to characterize the internal ...stress in fresh concrete during the thermal treatment. It explores the impact of water-to-cement ratio (w/c) and paste-to-aggregate ratio (p/a) on internal stress and elucidates the developmental patterns of internal stress in concrete throughout the thermal treatment phase. The key findings are as follows: (i) During the thermal treatment, the internal stress in concrete under constraint is predominantly compressive, exhibiting a rapid increase in the heating stage, followed by a gradual decline in the constant temperature treatment stage, and a continued decrease with a decelerating trend during the cooling stage. (ii) Both w/c and p/c significantly influence internal stress in concrete, and this stress can be enhanced by reducing the initial free water content and augmenting the initial structural strength of the concrete. This paper serves as a valuable foundation and reference for further investigations into the residual expansion deformation of steam-cured concrete.
Ultra-high performance concrete (UHPC) is one of the most promising materials to reduce the detrimental effects of building materials on the environment because UHPC requires fewer materials and ...post-maintenance in comparison with normal concrete. However, the high cost and CO2 emission of UHPC are the major factors influencing its large-scale application in China. This study presents a series of comprehensive methods, including the optimization design of the binder system, utilization of nano-particles and chemical activators as well as heat curing, to prepare UHPC with relatively lower environmental impact. The workability, compressive strength, flexural strength, hydration heat and environmental impact of UHPC were investigated. The results indicate that each of the above methods has a significant effect on the strength and carbon emission of UHPC. UHPC with cement content lower than 200 kg/m3 can be prepared by optimizing its binder system and curing conditions, and its embodied CO2 index is lower than 3 kg/MPa·m3, while the embodied CO2 index of conventional UHPC ranges from 5.5 to 7 kg/MPa·m3. The designed UHPC has a low hydration release heat which may decrease the cracking risk caused by hydration temperature rise. For improving the mechanical strength and the eco-friendliness of UHPC, chemical activating method and heat curing have a synergistic effect.
•The low carbon emission UHPC has been successfully prepared in this study.•The effect of compositions and curing regime on environmental impact of UHPC were studied.•The effect of nano-silica and chemical activators on environmental impact of UHPC were studied.•The environmental impact of the optimized and normal UHPC is compared.
•The influences of FA and SL on the nucleation and growth of cement paste were studied.•FA retards the early hydration of cement at 20℃ due to its hindering nucleation.•SL has stronger seeding effect ...than FA because it produces more additional C-S-H gel.
In this study, isothermal calorimetry and the boundary nucleation and growth model (BNG model) were adopted to investigate the influence of fly ash (FA) and slag (SL) on the nucleation and growth of cement hydrates at early-stage. Afterwards, the mechanism was discussed by analyses of instantaneous activation energy and microtopography. The results show that the hinderance of FA on the early hydration at 20℃ is possibly contributed by the inhibition of nucleation, while SL promotes nucleation at both 20℃ and 60℃. Moreover, it is found that the paste containing FA has the lowest activation energy at early-stage, while the paste containing SL has the highest activation energy, which further indicate that the nucleation of the paste containing FA is more difficult. Besides, the surface microtopography analysis of particles confirm that SL has higher reactivity than FA in paste to generates more additional C-S-H gel to provide stronger seeding effect for producing C-S-H.
Steam-curing at elevated temperature involves a complex hygrothermal coupling action, which greatly affects the hydration process, microstructure buildup and thus the mechanical and long-term ...properties of cement-based materials. This paper investigated the multi-scale pore structure characteristics of concrete, paste and paste-aggregate interfacial zone during steam-curing process by a combined method of microscopic image analysis, scanning electron microscopy and mercury intrusion porosimetry. The evolution model of pore structure of steam-cured concrete during steam-curing stage and corresponding mechanism are also discussed. The results show that the multi-scale pore structure evolution of concrete is remarkably influenced by the steam-curing process, which can be divided into three stages. The initial free water content is one of the critical factors affecting the pore structure evolution of steam-cured concrete, especially in the interfacial transition zone. The morphology of hydrated cement pastes in interfacial transition zone is looser than that of matrix. The expansion pressure of vapor in the heating period of steam-curing process when water interior expands more severely in a higher initial free water content hinders the precipitation and diffusion of hydrates, bringing a relative weaker interfacial transition zone for steam-cured concrete. A hydration degree-dependent and water to cement ratio-related theoretical model is established to characterize the pore structure evolution during steam-curing process, which is proved to be well consistent with the experimentally measured results.
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•The pore structure of concrete undergoes a three-stage decrease during steam-curing process.•The initial free water content is critical for the pore structure evolution of steam-cured concrete, especially in the ITZ.•A theoretical model is established to characterize the pore structure evolution during steam-curing process.
•A numerical method was proposed to analyze the hydration of steam curing cement.•The effect of steam curing regimes on the hydration of cement was investigated quantitatively.•The strength evolution ...of concrete under steam curing was analyzed.•An optimized steam curing regime was put forward to mitigate the heat damage.
The hydration process under unsteady steam curing condition is difficult to monitor in real time by experimental method. This study developed a numerical method to describe the hydration heat release characteristics of Portland cement under atmospheric steam curing condition. Instantaneous activation energy and an iterative algorithm were employed in this method to improve the accuracy. It was found that prolonging the precuring time to over 7 h or heating time to over 3 h would both decrease the peak value of hydration heat release rate, while a treatment time over 8 h would slightly affect the total heat release during steam curing. The validity of the method was confirmed by the experimental results. This proposed method can rapidly analyze the effect of steam curing regimes on hydration heat release behavior of Portland cement and thus optimize the hydration process of Portland cement under steam curing process in order to mitigate or avoid heat damage of steam curing concrete.
•Micro-silica has slight influence on hydration kinetics of cement at 20 °C and 40 °C.•The effects of micro- and nano-silica on hydration kinetics are very similar under 50 °C and 60 °C.•Nano-silica ...improves the phase boundary reaction at 60 °C.
Exothermic characteristics of Portland cement incorporating 1%wt micro- and nano-silica were researched by measuring the heat releases at different testing conditions. The presence of nano-SiO2 reduces the induction period, and this effect changes with temperature. The rising temperature makes silica fume generate more significant influence on the induction period. The maximal heat release rate is influenced slightly at normal temperature and decreased to some extent at elevated temperatures through replacing 1%wt cement by micro- and nano-silica. The effects of nano-SiO2 and silica fume at 60 °C are almost completely consistent with each other within 7 h of hydration including induction state, acceleration state and deceleration state. Silica fume presents the property of nano-SiO2 at high temperatures, which can also be observed by analyzing their cementitious indexes. The reaction kinetics of cement with nano-SiO2 and silica fume was analyzed by the classical Krstulović-Dabić model. The analysis results indicate that nano-SiO2 and silica fume can improve the hydration degrees (α1 and α2) of the transformation from nucleation and crystal growth (NG) process to phase boundary reaction (I) process or I process to diffusion (D) process only at 60 °C. Nano-SiO2 can improve the phase boundary reaction at 50 °C and 60 °C judging by the difference between α1 and α2. The possible mechanism of nano-SiO2 and silica fume influencing the hydration kinetics was also proposed.
•Electric curing is proposed for the rapid curing of cement at room temperature.•Suitable electrical curing regime can effectively improve the early strength of cement.•Joule heat is the key to ...accelerate the hydration of cement.•Electric curing consumes much less energy than steam curing.
To understand the effects of direct electric curing condition on the mechanical strength and microstructure of cement paste, a series of experiments were carried out to investigate the effects of alternative current voltages and frequency on compressive strength of samples. And the corresponding microstructure of samples were also analyzed by several measurements including X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and mercury intrusion porometer. The results show that alternative current electric voltage and frequency can greatly influence the early compressive strength of a cement paste. And a suitable electric curing condition can make hardened cement paste keep a steady growth of compressive strength and excellent microstructure. The Joule heat of direct electric curing on cement paste is the main reason that accelerates the hydration and improve the early strength development of sample. This study will provide an important technical support for exploring low carbon rapid electric curing method to produce green high early strength cement concrete prefabricated element.
•C-S-H seeds and sulphoaluminate cement were used to prepared RH-UHPC.•The flexural load–deflection response of RH-UHPC was studied by AE technology.•The flexural failure pattern, strength, energy ...absorption capacity, and toughness index of RH-UHPC were investigated.•Micromorphology, phase composition, and pore structure of RH-UHPC were studied.
With the increasing requirements for emergency repair projects, there is a pressing need for high-quality repair materials. In this work, a series of rapid hardening ultra-high performance concrete (RH-UHPC) were prepared by using C-S-H seeds and SAC as early-strength agents. The effects of these early-strength agents on RH-UHPC’s flexural behaviors, including failure pattern, strength, energy absorption capacity, and toughness index, were investigated. Also, the variations of acoustic emission signals and microstructure were studied to provide a thorough insight into the influence of early-strength agents on RH-UHPC. The results show that C-S-H seeds and SAC exhibited a good effect on improving the 1-day flexural strength of RH-UHPC, but increased the risk of shear failure at 28 days. The flexural strength, energy absorption capacity, and toughness of RH-UHPC at 28 days were reduced by adding C-S-H seeds. However, combining C-S-H seeds with SAC alleviated this reduction. Moreover, incorporating early-strength agents enhanced the ringing counts, AE energy, damage rate, and the proportion of shear cracks. The noticeable increase in r value (i.e., RA/AF) judged the damage stage of RH-UHPC more reliably. Micro-analysis revealed that introducing C-S-H seeds increased the internal cracks and the Ca/Si ratio of C-S-H gel, but the hybrid of C-S-H seeds and SAC partially mitigated above problem. The incorporation of SAC notably increased the AFt content and decreased the CH content. C-S-H seeds presented little effect on the hydration degree of RH-UHPC, but roughened the pore size and enlarged the average pore size.
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