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Cementitious materials have potential for infrastructure development in low-temperature marine environments, including in seawater at high latitudes and in deep-sea environments (water depths of >1000 m). Although the marine deterioration of cementitious materials has been widely investigated, the influence of seawater temperature has not been elucidated. In this study, to determine the effects of low-temperature seawater on the durability of cementitious materials, cement paste specimens were immersed in a seawater tank at room temperature and 2 °C for 433 days. The specimen immersed in low-temperature seawater exhibited significant deterioration with a partially collapsed surface, whereas the specimen immersed in room-temperature seawater maintained its original shape. Following low-temperature immersion, Ca dissolution was more pronounced and dissolved portlandite, decalcified calcium (alumino)silicate hydrate (C–(A-)S–H), magnesium (alumino)silicate hydrate (M–(A-)S–H), and thaumasite were observed on the collapsed surface. Such significant deterioration can be attributed to the increased solubility of portlandite under low-temperature conditions, which could promote Ca dissolution and subsequently lead to C–(A-)S–H decalcification and the formation of M–(A-)S–H and thaumasite. These insights are expected to contribute to the successful construction and maintenance of cementitious structures in low-temperature seawater.

Sulfate corrosion is a significant durability issue for concrete used in sewage and hydraulic infrastructure. In sulfate-rich environments, the formation of expansive products (e.g., ettringite and thaumasite) leads to a progressive loss of performance. Unlike conventional protection methods, which rely on surface-applied coatings or impregnation, this study examines the use of water-dilutable epoxy resins as an internal, volume-wide admixture dispersed throughout the concrete matrix to provide whole-body protection. The experimental program evaluated the mechanical performance, microstructure, and sulfate ion ingress/penetration dynamics of resin-modified concretes. The results suggest that using the appropriate amount of resin can limit the penetration of aggressive ions and slow the harmful changes associated with sulfate attack while maintaining the material’s overall performance. Overall, these findings suggest that water-based epoxy admixtures are a promising strategy for improving the durability of concrete in sulfate-exposed environments. They also provide guidance for designing more resistant cementitious materials for modern infrastructure applications.

To reduce CO 2 emissions associated with cement production, the use of calcined clay and limestone has gained increasing attention as supplementary cementitious materials. However, the addition of limestone can potentially compromise the lifetime of a structure due to the risk of thaumasite formation. Thaumasite is stable only at low temperatures and forms when cement is exposed to sulphate-rich environments such as seawater. This study investigates the early signs of thaumasite formation and other hydration phase changes in calcined clay-limestone cement at 5 °C. Cement samples containing 35 % calcined clay or limestone were exposed to a 0.3 % Na 2 SO 4 solution simulating the SO 3 2- concentration in the Femern Belt, Denmark, with a solution-to-cement mass ratio of 1:100, to create conditions optimal for the formation of thaumasite. The samples were analysed using X-ray diffraction and thermogravimetric analysis. The study focuses on early changes in the phase assemblage to provide insights into the sulphate resistance of calcined clay-limestone cement. By identifying the early changes, the aim is to screen several binders and gain a better understanding of the durability of calcined clay-limestone cement when exposed to sulphate-rich environments. The screening focuses on processes that might be limited by the low exposure concentrations used in this study.

Thaumasite has been synthetized with and without ettringite in the reaction medium. The compositions of solids have been analyzed over time by means of X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM-EDX) in order to check the relation between ettringite and thaumasite. The results indicate that in the synthesis in absence of ettringite, the thaumasite appears at 6 months. By contrast, in the synthesis with presence of ettringite, at 7 days much of the ettringite has become thaumasite and at 2 months ettringite has disappeared and only thaumasite is detected. The results obtained at the end of two years experience, using the Rietveld method, show that thaumasite forms from ettringite by substitution of aluminum with silicon.

In this work, synthesis procedures to obtain thaumasite and different influences on its formation were revised. Theoretical background is compiled in this paper from published sources with focus on synthesis of pure thaumasite in laboratory environment.

This study aims to resolve the “secondary activation” challenge when erecting structures over goaf zones by employing a novel grouting and filling material. It delves into the performance degradation of the innovative ECS soil grouting filling material (ESGF material) within the goaf’s ionic erosion context. Erosion tests were performed on ESGF material specimens with varying mix designs to mimic the sulfate and chloride erosion scenarios commonly encountered in practical engineering. The macro-mechanical properties and microstructural changes of ESGF materials under ionic erosion environment were systematically investigated by various testing methods, such as unconfined compressive strength (UCS), SEM, XRD, TG, FTIR, and Raman. The findings indicate that both sulfate and chloride erosion lead to a reduction in the strength of the ESGF material. As erosion progresses, the specimens experience a mass increase followed by a decrease, with their strength exhibiting a consistent downward trend. In sulfate erosion conditions, the buildup of expansion product like ettringite (AFt) and thaumasite (TSA) inflicts substantial internal structural damage. Conversely, Friedel’s salt, the primary product of chloride erosion, exhibits relatively weaker expansiveness, and chloride concentration exerts a less pronounced effect on material degradation. Moreover, the cementitious material content and the proportion of quick-setting component play a significant role in determining the ESGF material’s resistance to erosion. By adjusting the quick-setting components ratio in response to changes in the water content of soft soil, the anti-ion erosion performance of solidified soil can be effectively enhanced. Notably, curing with a 5% sulfate maintenance could significantly improve the erosion resistance of ESGF material. This suggests that ESGF materials can be used without concern for curing issues in high-salinity environments during grouting. The research addresses the root cause of goaf subsidence while facilitating the recycling of solid waste, offering an environmentally friendly solution.

Ettringite, reported with ideal formula Ca Al (SO (OH) ·26H O, is recognized as a secondary-alteration mineral and as an important crystalline constituent of Portland cements, playing different roles at different time scales. It contains more than 40 wt% of H O. The crystal structure and crystal chemistry of ettringite were investigated by electron microprobe analysis in wavelength-dispersive mode, infrared spectroscopy, and single-crystal neutron diffraction at 20 K. The anisotropic neutron structure refinement allowed the location of (22+2) independent H sites, the description of their anisotropic vibrational regime and the complex hydrogen-bonding schemes. Analysis of the difference-Fourier maps of the nuclear density showed a disordered distribution of the inter-column (“free”) H O molecules of the ettringite structure, modeled (in the structure refinement) with two independent and mutually exclusive configurations. As the disorder is still preserved down to 20 K, we are inclined to consider that as a “static disorder.” The structure of ettringite is largely held together by hydrogen bonding: the building units [i.e., SO tetrahedra, Al(OH) octahedra, and Ca(OH) (H O) polyhedra] are interconnected through an extensive network of hydrogen bonds. The ettringite of this study has ideal composition Ca Al (SO (OH) ·27H O, with (Mn+Fe+Si+Ti+Na+Ba) < 0.04 atoms per formula unit. The effect of the low-temperature stability of ettringite and thaumasite on the pronounced “Sulfate Attack” of Portland cements, observed in cold regions, is discussed.

Over the last 20 years, flue gas desulfurization gypsum (FGD gypsum) has become a valuable and widely used substitute for a natural raw material to produce plasters, mortars, and many other construction products. The essential advantages of FGD gypsum include its high purity and stability, which allow for better technical parameters compared to natural gypsum, and, until recently, its low price and easy availability. This FGD gypsum is obtained in the process of desulfurization of flue gases and waste gases in power plants, thermal power plants, refineries, etc., using fossil fuels such as coal or oil. The gradual reduction in energy production from fossil raw materials implemented by European Union countries until its complete cessation in 2049 in favor of renewable energy sources significantly affects the availability of synthetic gypsum, and forces producers of mortars and other construction products to look for new solutions. The gypsum content in commonly used light plaster mortars is usually from 50 to 60% by mass. This work presents the results of tests on mortars wherein the authors reduced the amount of gypsum to 30%, and, to meet the strength requirements specified in the EN 13279-1:2008 standard, added Portland cement in the amount of 6–12% by mass. Such a significant reduction in the content of synthetic gypsum will reduce this raw material’s consumption, thus extending its availability and developing other solutions. The study presented the test results on strength, density, porosity, pore size distribution, and changes in the microstructure of mortars during up to 180 days of maturation in conditions of increased relative humidity. The results show that decreased porosity and increased mechanical strength occur due to the densification of the microstructure caused by the formation of hydration products, such as C-S-H, ettringite, and thaumasite.

A new type of cemented paste backfill (CPB) was prepared using sodium hydroxide (NaOH) as the activator, slag and silica fume (SF) as the binder, and tailings as the aggregate. The effects of proportion of replacement of 0%, 5%, 10%, 15%, and 20% silica fume on the properties of CPB were studied. The strength formation mechanism of CPB was explored through a combination of scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and Fourier transform infrared (FTIR) spectroscopy. The SEM images were analyzed by IMAGE J software, and the porosity of CPB with different silica fume contents was obtained. The results show that as the amount of silica fume increases, the unconfined compressive strength (UCS) increases first and then decreases. When the amount of silica fume was approximately 5%, CPB with a larger UCS can be obtained. When the silica fume content increased from 0% to 5%, because silica fume has good activity and small particles, more calcium silicate hydrate (C–S–H) gels and Mg-Al type layered double hydrotalcites (LDHs) were generated in CPB, which made it denser and improved its strength compared with the non-silica fume group. C–S–H gels were the main source of CPB strength. With a further increase in the amount of silica fume, thaumasite produced inside of CPB, reducing the content of C–S–H gels. Moreover, due to the expansion of thaumasite, it is easy to generate a large number of micro cracks in CPB, which weakens the strength of CPB.

The introduction of the European Union’s climate change legislation and the intended use of renewable energy sources instead of fossil fuels will significantly reduce the production of flue gas desulfurization (FGD) gypsum used as the raw material for gypsum mortar plasters’ production. This has forced mortar producers to look for alternative materials, including gypsum–cement composites. This work investigated the mechanical strength and linear extension of four gypsum–cement mortars with the gypsum content reduced to 30%. The authors showed that the cement admixture of 6 to 12% introduced into the prepared mortars resulted in the formation of gypsum–cement mortars, which fulfill the requirements of the EN 13279-1:2008 standard concerning mechanical strength. This publication took into account the use of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffractometry to characterize the chemical and phase composition of the mortars up to 180 days of dry air curing and increased relative humidity (RH) conditions. The formation of thaumasite, ettringite, and mixed ettringite–thaumasite phases was interesting because of their deleterious effect on the durability of plaster mortars.