Evolution of multi-scale pore structure of concrete during steam-curing process

Evolution of mechanical properties and permeability of concrete during steam curing process

Multiscale pore structure determination of cement paste via simulation and experiment: The case of alkali-activated metakaolin

In this article, micronized sand is selected as filler to partially replace Portland cement in order to reduce the global warming potential, i.e. CO2 emission. It is necessary to figure out the effects of micronized sand on the properties of cement paste. Microstructure, especially the pore structure, influences the transport properties, which are widely recognized as the most important factors to determine the durability of cementitious materials. In this study, it focused on the microstructure of cement paste blended with micronized sand in comparison with the reference ordinary Portland cement paste. Non-evaporable water test was carried out to calculate the degree of hydration of cement. By means of mercury intrusion porosimetry measurements and electronic scanning electron microscope, the pore structure of cement pastes blended with micronized sand as well as interfacial transition zone between micronized sand and hydration products have been investigated. Parameters, including the Portland cement replacement percentages: 10, 20 and 30 % and fineness of micronized sand: M6, M300 and M600, have been taken into account. The results show that although micronized sand is considered as inert filler, it can indirectly modify the hydration kinetics by increasing the degree of hydration of cement and influence the porosity and pore size distribution of the paste. There is a clear interfacial zone between micronized sand and hydration products. This interfacial zone caused by the micronized sand is considered as the decisive factor which influences the transport properties of the material.

Experiments on influence of species of aggregate and mixing method on interfacial zone in recycled aggregate concrete were investigated. SEM observations revealed that a recycle normal-strength concrete aggregate consist of loose and porous interfacial structure, whereas a recycled high performance concrete (HPC) aggregate and a triple mixing (TM) consist mainly of dense hydrates. Various admixtures on ITZ was produced that consumed CH in the pore, modified attached cement mortar. Strength of recycled concrete was explained by interaction between cements paste and recycled aggregate. The result verified that the relatively dense pore structure of the recycled concrete benefit to development of mechanical properties.

How to treat the iron tailings of mining solid waste with high value is an urgent problem on a global scale. In recent years, the application of iron tailings in the building materials industry has attracted the attention of many scholars. The conversion of iron tailings into green building materials helps achieve carbon neutrality and high-value utilization of solid waste, and promotes sustainable development. Although iron tailings have been extensively studied as supplementary cementitious materials, the performance of concrete is not ideal due to its low activity. In this study, the hybrid supplementary cementitious materials system was prepared by iron tailings, phosphorus slag, and steel slag, and the effects of supplementary cementitious materials type, iron tailings content, iron tailings grinding time, and supplementary cementitious materials content on concrete performance were studied. The compressive properties, iron tailings properties, pore structure, interfacial transition zone, and element distribution of hydration products of concrete were tested by compressive strength tests, X-ray Diffractometer (XRD), X-ray Photoelectron Spectroscopy (XPS), Mercury Intrusion Porosimetry (MIP), Backscattering Electron Tests (BSE), and Energy Dispersive Spectrometer (EDS). The results show that further grinding improves the iron tailings activity. There is a synergistic mechanism between steel slag and phosphorus slag in the composite supplementary cementitious materials, which overcomes the low activity defect of iron tailings and produces concrete with a compressive strength exceeding 40 MPa. The composite supplementary cementitious materials can optimize the interfacial transition zone of the concrete interface and reduce the calcium–silicon ratio of the hydration products. However, it will deteriorate the pore structure of the concrete matrix, cause part of the concrete matrix to be damaged and lead to a loss of compressive strength, and the loss is acceptable. This work broadens the methods of comprehensive utilization of iron tailings and also provides a reference for a more detailed understanding of the properties of iron tailings-based concrete.

Lightweight modeling is one of the most important research subjects in modern fabrication manufacturing, and it provides not only a low-cost solution but also functional applications, especially for the fabrication using 3D printing. This approach presents a multi-scale porous structure-based lightweight framework to reduce the weight of 3D printed objects while meeting the specified requirements. Specifically, the triply periodic minimal surface (TPMS) is exploited to design a multi-scale porous structure, which can achieve high mechanical behaviors with lightweight. The multi-scale porous structure is constructed using compactly supported radial basis functions, and it inherits the good properties of TPMS, such as smoothness, full connectivity (no closed hollows) and quasi-self-supporting (free of extra supports in most cases). Then, the lightweight problem utilizing the porous structures is formulated into a constrained optimization. Finally, a strength-to-weight optimization method is proposed to obtain the lightweight models. It is also worth noting that the proposed porous structures can be perfectly fabricated by common 3D printing technologies on account of the leftover material, such as the liquid in SLA, which can be removed through the fully connected void channel.

A novel multiscale porous composite structure for sound absorption enhancement

This work reports the results of structural analysis in novel dextran-acrylate (dexT70-VA) hydrogels generated chemoenzymatically. Porous structure as well as hydrogel surface and interior morphologies were evaluated by mercury intrusion porosimetry (MIP), nitrogen adsorption (NA), and scanning electron microscopy (SEM) analyses, as a function of the degree of substitution (DS), and initial water content used in the preparation of the hydrogel. MIP analysis showed that the overall networks were clearly macroporous with pore sizes ranging from 0.065 to 10 microm. As expected, the average pore size decreased as DS increased and as initial water content decreased. Moreover, the porosity values ranged from 75 up 90%, which shows that these hydrogels present an interconnected pore structure. Nitrogen adsorption analyses showed that the specific surface area of dexT70-VA hydrogels increased either by increasing the DS or by decreasing the initial water content of the hydrogel. SEM results revealed that the surface of hydrogels with lower DS presented either a porous structure or a polymeric "skin" covering the pores, whereas hydrogels with higher DS were totally porous. Furthermore, the interior morphology varied according to the DS and the initial water content of the hydrogels. Finally, the average pore size was also determined from the swelling of hydrogel using a theoretical model developed by Flory-Rehner. The comparison of the SEM and MIP results with the ones obtained by the equilibrium swelling theory of Flory-Rehner shows that this approach highly underestimates the average pore size.

Evolution of temperature stress and tensile properties of concrete during steam-curing process

Investigation of the factors that control the development of pore structure in lacustrine shale: A case study of block X in the Ordos Basin, China

This paper presents observed soil moisture redistribution within unsaturated soil in response to imposed boundary temperatures. Both salinized and solute‐free soil conditions are studied. Two different uniform initial soil water contents and solute concentrations are used for the salinized soil columns. Likewise, two different uniform initial soil water contents are used for solute‐free soil columns. High and low boundary temperatures are similar for all of the soil columns. Thus, the experiments are designed to directly observe the impacts of thermal, soil matric, and osmotic gradients on redistribution of soil water. In all cases, appreciable amounts of water move in the direction of decreasing temperature within the soil columns. This is not a general rule of soil water movement, but a result of the uniform initial water content, the presence of solute, and the imposed boundary temperatures under salinized soil, or the initial water content and the imposed boundary temperatures under solute‐free soil. For both solute‐free and salinized soils, the net amount of transported water to achieve steady‐state conditions is greater for low initial water content than for high initial water content. Therefore, the steady‐state variations in the soil moisture content increased by decreasing the initial moisture content. When the same initial water content is used for both salinized and solute‐free soils, the net water transported to achieve steady‐state conditions is greater for the solute‐free soil than for salinized soil. These observations indicate clearly that solute concentration affects soil water transport with unsaturated, nonisothermal conditions.

A pore-network-based upscaling framework for the nanoconfined phase behavior in shale rocks

Coal rock pores are the space in which coalbed gas is stored and flows. Accurately characterizing the pore structure of coalbed gas is the foundation of coalbed gas reserve assessment and production forecasting. Traditional experimental methods are unable to characterize the multi-scale pore structure characteristics of coal rock. In this paper, a multi-scale pore structure characterization method is proposed by coupling various experimental methods, including low-pressure nitrogen gas adsorption experiments, X-ray computed tomography (XCT) imaging technology, and scanning electron microscopy (SEM). Using Zhengzhuang coalbed gas as an example, the micro-pore structure of coalbed gas reservoirs is characterized and depicted from a multi-scale perspective. The results indicate that a single experimental approach can only partially reveal the microstructure of coal rock pores. The combined use of multiple methods can accurately reveal the full-scale microstructure of coal rock pores. The pore structure of the experimental coal rock samples exhibits multi-scale characteristics, with a complex variety of pore types, including inorganic pores, organic pores, and fractures. Organic pores are predominant, with a small number of inorganic pores, and their sizes range from 2 nm to 50 μm. Mineral particles and fractures are observed at both the nanoscale and microscale, exhibiting typical multi-scale characteristics, with quartz being the predominant mineral.

Influence of low vacuum condition on mechanical performance and microstructure of hardened cement paste at early age

The composition and structure of the interfacial transition zone (ITZ) in concrete were investigated to analyze the mechanism of silica fume in suppressing alkali-silica reaction (ASR). Alkali active concrete specimens with varying addition of silica fume were prepared for the experiment. Changes of compositions and pore structures at ITZ and the elements distribution across ITZ were analyzed by SEM and SEM-BSE. The expansion of the corresponding mortar bar was also measured. The results show that the accumulation of element of alkali metal occurred at ITZ in alkali active concrete without silica fume, while the characteristics of the ITZ are basically similar to those in conventional concrete. By addition of silica fume, the ITZ in alkali active concrete are changed and have the following features: disappearance of the accumulation, occurrence of the C-S-H gel with a lower CaO/SiO2 ratio, decrease in crystal phases of Ca(OH)2 and ettringite, and in porosity. These changes account for the mechanism of Silica fume in suppressing ASR.