One-part geopolymer similar to conventional Portland cement by just adding water is a new type of green cementitious material. Na2SiO3-anhydrous is an excellent powder activator for preparing ...one-part geopolymer, but its CO2-e emission and alkalinity are very high. In this study, the Na2CO3 was used to replace a portion of Na2SiO3-anhydrous, and the properties of one-part geopolymer with composite activators were investigated systematically. The possible influencing mechanism of the complex use of activators in one-part geopolymer was proposed. Test results indicate that the replacement level of Na2CO3 has slight impact on the fluidity and increases the final setting time significantly. This is directly related to the initial heat release rate and the type of network structures formed. The compressive strength decreases with increasing the Na2CO3 content. On the one hand, the quantity of gelatinous products reduces and calcite is produced as the new phase after replacing Na2SiO3-anhydrous by Na2CO3. On the other hand, the gathering of calcite and low-crystalline CaCO3 in some regions leads to the generation of many large micro-cracks, which explains the change in the porosity of the unfavorable pores. Judged from the cost per MPa and total CO2-e emission for a cubic meter of cementitious materials, one-part geopolymer activated by composite activators is cleaner than that activated by single Na2SiO3-anhydrous or Na2CO3. This study provides a guidance for preparing one-part geopolymer with lower alkalinity and potential environmental impact.
•One-part geopolymer was prepared by using the compound activator of Na2SiO3-anhydrous and Na2CO3.•The properties of one-part geopolymer and influencing mechanism of Na2CO3 were investigated.•Na2CO3 makes one-part geopolymer cleaner without sacrificing the performance and economic benefit.
•The surface permeability of steam-cured concrete is higher than that of standard-cured concrete.•The subsequent curing condition has great influence on the surface permeability of concrete.•The ...subsequent water-soaking and elevated temperature curing decrease the performance of concrete.
Steam curing is commonly used to produce the prefabricated elements to meet the requirement of the high early strength. However, this curing regime was proved to lead to the thermal damage on the microstructure and properties of concrete. And the subsequent curing after initial steam curing is critical to the development of performances of steam-cured concrete. In this study, four typical subsequent curing conditions simulating the extreme service environments were employed on the curing of steam-cured concrete. And the surface permeability indexes involving water and air permeations and compressive strength were measured to analyze the effects of subsequent curing condition on performance of steam-cured concrete. The results indicate that the surface permeability and strength development of steam-cured concrete are remarkably influenced by the subsequent curing condition. The subsequent curing such as long-term water-soaking curing at 20 °C or oven-dry curing at 60 °C deteriorates the performances of steam-cured concrete. Their surface permeability indexes are 75.1% and 88.0% higher than that of standard-cured concrete at 90 days, respectively. The continuous dissolution of calcium hydroxide is considered the main reason for deterioration of impermeability and strength of steam-cured concrete.
•Full scale experiment of uncovering plate on-site.•28-day continuous detecting of on-site full scale experiment plate.•Bonding interface defects change the response spectrum.•Correlation between age ...and the good bonding rate of bonding interface.
The bonding interface between the self-compacting concrete (SCC) filling layer and the steam-cured concrete precast plate plays an important role in the durability of the China Rail Track System III (CRTS III) slab ballastless track structure. It is of great practical value to accurately detect the quality of the bonding interface. This paper investigates the influence of age on the detection of bonding interface defects via the impact-echo method. For different ages of the SCC filling layer, the stress nephograms produced under the same impact load are investigated by the finite element method. Via full-size field experiments, the results of the bonding interface quality detected by the impact-echo method are compared with the actual quality state determined by the field test. It is found that there is a good correlation between the age of the SCC filling layer and the bonding rate of the bonding interface. The research also shows that the existence of a bonding interface cavity changes the vibration mode. This work provides important technical support for the detection of the quality state of the bonding interface.
Display omitted
•The temperature and humidity gradient during steam curing process is closely related to its long-term performance.•Surface covering can significantly reduce surface porosity and ...damage of steam-cured concrete.•The effects of steam-curing regime and surface covering method on the permeability of concrete were evaluated.•The high temperature-humidity gradient during steam curing is detrimental to the development of concrete microstructure.
Steam curing has an adverse effect on the long-term performance of concrete, which is inseparable from the early hygro-thermal coupling action. The temperature-humidity field and performance of steam-cured concrete with different surface covering methods were characterized under the treatment temperature ranging from 45 to 75 ℃ by water permeability along with hydration characteristics, porosity, microhardness, and microstructure. The results show that steam-curing regimes directly affect the long-term performance of steam-cured concrete by changing internal temperature-humidity field. During the steam curing process, a higher treatment temperature often corresponds to a more complex temperature-humidity gradient, so that the steam-cured concrete exhibits low compactness and high permeability. In addition, the surface covering methods can reduce the surface porosity and interface transition zone width of steam-cured concrete. Meanwhile, with the increase of treatment temperature, the improvement effect of the surface covering methods on the impermeability of steam-cured concrete is more obvious.
•The fineness modulus index was used to evaluate the crushing degree of concrete under impact load.•The steam-cured concrete shows the stronger strain rate sensitivity at higher curing ...temperature.•The influence of temperature on key parameters of damage constitutive models was analyzed.
In order to clarify the influence of curing temperature on the dynamic properties of steam-cured concrete, series of SHPB tests were conducted to investigate the impact mechanical characteristics of a typical steam-cured concrete applied in prefabricated components of high-speed railway in China. The index of fineness modulus was proposed to evaluate the degree of concrete breakage under different strain rates. The results showed that the peak stress of steam cured concrete increased with increasing the strain rate, while the peak strain showed a decreasing trend, the fineness modulus of broken concrete particles decreased, and the degree of fracture increased obviously. As curing temperature increases, the peak stress of steam-cured concrete decreased significantly, and the concrete specimens showed a stronger strain rate sensitivity. A statistical damage constitutive model considering the strain rate was established and agreed well with the experimental stress–strain curves. Higher temperatures lead to an increase in internal defects of concrete, which makes the impact mechanical properties different from that of normal temperature cured concrete.
•Full-scale size field experiments of CRTS III type ballastless track structure.•Factors influencing the experimental results of the impact-echo method.•Apparent state of the bonding interface ...corresponds to the spectra of impact-echo method.•Flaw identification function F(y) based on the spectra.
The bonding interface is a part of China Rail Track System III (CRTS III) type ballastless track structure that is prone to concealed flaws. The bonding interface is between the self-compacting concrete (SCC) filling layer and the steam-cured concrete precast plate. It is very important to detect the quality of the bonding interface. This paper investigates the feasibility of using the impact-echo method to detect the bonding interface flaws. The testing identification results of the bonding interface via the impact-echo method is compared with the actual quality state obtained by on-site uncovering plate. The main factors affecting the corresponding results are analyzed by field experiments. And the spectra of the impact-echo method under different flaw conditions are discussed. The relationship between the spectra and the actual flaws is analyzed in detail. A flaw identification function based on spectra is established to effectively identify bonding interface flaw types. This work will provide important technical supports for developing a reliable non-destructive testing method to inspect the bonding interface quality state.
Rheological curves of cement–fly ash (C–FA) paste incorporating nanomaterials including nano-SiO2 (NS), nano-CaCO3 (NC) and nano-Al2O3 (NA) at different resting times (hydration time of 5 min, 60 ...min, and 120 min) were tested with a rheometer. The rheological behaviors were described by the Herschel–Bulkley (H–B) model, and the influences of these nanomaterials on rheological properties of C–FA paste were compared. Results show that the types, content of nanomaterials and resting time have great influences on the rheological properties of C–FA paste. Incorporating NS and NA increases yield stress and plastic viscosity, and decreases the rheological index of C–FA paste. When the content of NS and NA were 2 wt%, the rheological index of C–FA paste was less than 1, indicating rheological behavior changes from shear thickening to shear thinning. Meanwhile, with rising resting time, yield stress and plastic viscosity increased significantly, but the rheological index decreased evidently, showing paste takes on shear thinning due to the rise of resting time. However, incorporating 3 wt% NC and the rising of resting time did not change the rheological properties of C–FA paste. These differences are mainly that the specific surface area (SSA) of NS (150 m2/g) and NA (120 m2/g) are much larger than that of NC (40 m2/g). The huge SSA of NS and NA consume lots of free water and these tiny particles accelerate the hydration process during resting time.
In order to improve the stability of air bubbles in fresh concrete, it is of great significance to have a better understanding of the mechanisms and main influencing factors of bubble stability. In ...the present review, the formation and collapse process of air bubbles in fresh concrete are essentially detailed; and the advances of major influencing factors of bubble stability are summarized. The results show that the surface tension of air-liquid interface exerts a huge impact on bubble stability by reducing surface free energy and Plateau drainage, as well as increasing the Gibbs surface elasticity. However, surface tension may not be the only determinant of bubble stability. Both the strength of bubble film and the diffusion rate of air through the membrane may also dominate bubble stability. The application of nano-silica is a current trend and plays a key role in ameliorating bubble stability. The foam stability could be increased by 6 times when the mass fraction of nano-particle reached 1.5%.
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.
PDF
