The Performance and Mechanism Analysis of Cement Pastes Added to Aluminum Sulfate-Based Low-Alkali Setting Accelerator

We proposed a type of low-alkali liquid state setting accelerator, named HLSA; it was environmentally friendly product. To investigate the temperature adaptation and cement flexibilities of HLSA, the setting time and strength development properties of cement with HLSA were discussed in this paper. The effects of HLSA on hydration process, hydration products, and microstructure were studied by means of X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP). The results show that four typical 42.5-grade ordinary Portland cement types with 6–8% HLSA could satisfy the first-grade requirements according to JC477-2005 even at a lower temperature (e.g., 10°C). Further, the percentage ratio of 28 d compressive strength of cement with 6–8% HLSA was over 90%; the XRD diffraction peak of AFt integrated area of cement with 7% HLSA was 3818 at 5 min of hydration; SEM observation revealed that AFt crystals were filled in the pore of cement at 28 d of hydration; the temperature adaptation and cement flexibilities of HLSA were excellent; the cement with HLSA coagulating in a short time attributed to promoting the formation of abundant AFt and the hydration of C3S

Analysis of Cubic Boron Nitride Single Crystal Defects Growth under High Temperature and High Pressure

Cubic boron nitride (cBN) single crystals are synthesized under high temperature and high pressure in the Li-based system. The growth defects on hexagonal and triangular (111) surfaces of cBN single crystals after rapid cooling are discussed systemically for the first time using the atomic force microscope. Some impurity particles, triangle cone hole defects, lamellar-fault structures, and big steps are obvious on the surfaces of cBN single crystals. The formation mechanism of these defects is analyzed briefly at the synthetic process of cBN single crystals, and the growth mechanism of cBN single crystals transform from the two-dimensional growth to dislocation growth mechanism under high temperature and high pressure

Effect of Atmospheric Pressure on Performance of AEA and Air Entraining Concrete

The effect of atmospheric pressure on performance of air entraining agent (AEA) and air entraining concrete was studied in Tibet and Hubei, China. The result shows that the reduced atmospheric pressure increased surface tension and lowered foaming property of solution. The AEA with excellent foaming ability and stability is preferred in low atmospheric pressure. The freeze-thaw deterioration process of hardened pastes is relatively faster under low atmospheric pressure. The effect of air pressure on concrete frost resistance performance is more sensitive than the mechanical property. The bigger pores and poor uniformity of internal pore size distribution led to the deterioration of concrete macroscopic properties. Therefore, the AEA varieties should be preferred, the dosage of AEA should be increased, and pore structure of pastes should be optimized to ensure the durability of concrete frost resistance for construction in low-pressure areas

Investigation and Application of Fractal Theory in Cement-Based Materials: A Review

Cement-based materials, including cement and concrete, are the most widely used construction materials in the world. In recent years, the investigation and application of fractal theory in cement-based materials have attracted a large amount of attention worldwide. The microstructures of cement-based materials, such as the pore structures, the mesostructures, such as air voids, and the morphological features of powders, as well as the fracture surfaces and cracks, commonly present extremely complex and irregular characteristics that are difficult to describe in terms of geometry but that can be studied by fractal theory. This paper summarizes the latest progress in the investigation and application of fractal theory in cement-based materials. Firstly, this paper summarizes the principles and classification of the seven fractal dimensions commonly used in cement-based materials. These fractal dimensions have different physical meanings since they are obtained from various testing techniques and fractal models. Then, the testing techniques and fractal models for testing and calculating these fractal dimensions are introduced and analyzed individually, such as the mercury intrusion porosimeter (MIP), nitrogen adsorption/desorption (NAD), and Zhang’s model, Neimark’s model, etc. Finally, the applications of these fractal dimensions in investigating the macroproperties of cement-based materials are summarized and discussed. These properties mainly include the mechanical properties, volumetric stability, durability (e.g., permeability, frost and corrosion resistance), fracture mechanics, as well as the evaluation of the pozzolanic reactivity of the mineral materials and the dispersion state of the powders

Investigation and Application of Fractal Theory in Cement-Based Materials: A Review

Cement-based materials, including cement and concrete, are the most widely used construction materials in the world. In recent years, the investigation and application of fractal theory in cement-based materials have attracted a large amount of attention worldwide. The microstructures of cement-based materials, such as the pore structures, the mesostructures, such as air voids, and the morphological features of powders, as well as the fracture surfaces and cracks, commonly present extremely complex and irregular characteristics that are difficult to describe in terms of geometry but that can be studied by fractal theory. This paper summarizes the latest progress in the investigation and application of fractal theory in cement-based materials. Firstly, this paper summarizes the principles and classification of the seven fractal dimensions commonly used in cement-based materials. These fractal dimensions have different physical meanings since they are obtained from various testing techniques and fractal models. Then, the testing techniques and fractal models for testing and calculating these fractal dimensions are introduced and analyzed individually, such as the mercury intrusion porosimeter (MIP), nitrogen adsorption/desorption (NAD), and Zhang’s model, Neimark’s model, etc. Finally, the applications of these fractal dimensions in investigating the macroproperties of cement-based materials are summarized and discussed. These properties mainly include the mechanical properties, volumetric stability, durability (e.g., permeability, frost and corrosion resistance), fracture mechanics, as well as the evaluation of the pozzolanic reactivity of the mineral materials and the dispersion state of the powders

The Effect of Sodium Gluconate on Pastes’ Performance and Hydration Behavior of Ordinary Portland Cement

The goal of this paper provides better understanding of the effect of sodium gluconate (SG) on ordinary Portland cement (OPC) hydration behavior. Pastes’ performances of ordinary Portland cement, including setting time at 20°C and 35°C curing temperature, mechanical strength, fluidity, and zeta potential are studied. Furthermore, the effects of SG on cement hydration behaviors are investigated by the means of isothermal calorimetry measurements, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The results show that SG is difficult to maintain significant retarding effect at the temperature of 35°C compared to that at the temperature of 20°C. SG is able to reduce the cement cumulative hydration heat and delay the occurrence time of heat evolution peak in a certain extent, but it has little impact on reducing the cement evolution rate peak. The effects of SG on mechanical properties and dispersion properties of cement depend on its dosages. Specifically, the positive effect occurs when the addition dosage is less than 0.15% (i.e., by cement weight), but the negative effect emerges if the addition dosages exceed this limitation. Similarly, SG plays different roles on cement hydration at different hydration periods. It inhibits the hydration of C3S and the formation of portlandite (CH) at the early hydration period. On the contrary, it promotes the C3S hydration when hydration time is beyond 1 d. Meanwhile, SG also plays different roles on cement hydration at different dosage additions. Specifically, SG promotes ettringite (AFt) formation at the dosage less than 0.06%, but it inhibits AFt formation at the dosage more than 0.06%