This review represented various methods used to explore the electron transfer mechanism within S-scheme heterojunctions. Each method was introduced with a fundamental explanation of the mechanism, followed by practical application examples. Some challenges persisted in elucidating the electron transfer mechanism within S-scheme heterojunctions. Despite the progress in characterization techniques for electronic transfer mechanisms, understanding the S-type heterojunctions and optimizing the photocatalytic systems need some techniques with atomic-scale resolution to analyze the dynamics of photogenerated charge carriers from a microscopic perspective. Such techniques, like ISI-XPS, zeta potential measurements and fs-TAS, should be complemented by advanced methods for in-situ characterization, providing research avenues for understanding the photocatalytic mechanisms of S-scheme heterojunctions in solar energy conversion.

The high-temperature eutectic composites composed of high-entropy alloy and ceramic have superior eutectic structural stability and performance such as high-temperature strength, which are expected to be applied in the high-temperature fields. However, the corresponding research on these eutectic composites is still in infancy, a relationship among composition, structure and performance is relatively unclear. In the future, the development of these eutectic composites is mainly carried out from five aspects, i.e., theoretical design and computational simulation, synthesis and preparation of new materials, multi-scale structure design, high-temperature oxidation resistance of enhanced materials, and improvement of mechanical properties of materials.

During the long-term service of the tunnel in high-geothermal environment, one side of the lining structure is in contact with the high-temperature rock wall, and another side is the normal temperature opening area of the tunnel. The temperature field is formed inside the lining concrete. The durability deterioration of lining concrete is the result of the coupling effect of temperature field and sulfate attack. The existence of temperature field under the coupling effect accelerates the diffusion of SO₄^2- from the outside to the inside of concrete as well as the chemical reaction and damage degree of attack. The deterioration mechanism of sulfate attack in high-geothermal environment is different from that in normal temperature environment. However, an indoor simulation test in high-geothermal environment adopts a hot sulfate solution soaking method. Although this method can achieve the acceleration of high temperature, it cannot simulate the effect of temperature field. The concrete corner is subjected to two-way high temperature under the method of hot sulfate solution immersion, resulting in more serious corner deterioration. Therefore, the scientific basis and rationality of the hot sulfate solution soaking method are still debatable. In addition, the pore structure characteristics of concrete are a key to affecting sulfate attack. The existing research on the pore structure of sulfate attack concrete mostly adopts the index of pore size distribution and its corresponding pore content, which cannot fully reflect the mechanism of sulfate attack. Fractal theory is proved to quantitatively characterize the complexity and irregularity of the pore structure of cement-based materials, and can also link the pore structure characteristics of cement-based materials with macroscopic properties. Therefore, this paper proposed a durability simulation system for the coupling of one-dimensional temperature field and sulfate attack. A fractal dimension method was used to investigate the pore structure characteristics of lining concrete under the coupling of temperature field and sulfate attack. Based on the layered damage theory, the relationship between the pore structure characteristics and the average compressive strength of the damaged layer, and the overall concrete strength under the coupling effect were analyzed.

A Portland cement 42.5 (GB175—007 standard) and a fly ash with grade II (GB/T 1596—2017 standard) were used. The fineness modulus of the river sand was 2.7, and the coarse aggregate was gravel with a gradation of 5-25 mm. A water reducing agent was polycarboxylate as a superplasticizer, and the water reducing rate was 25%. According to the actual engineering test, a mix ratio is cement of 320 kg/m³, fly ash of 80 kg/m³, water-binder ratio of 0.38, as well as sand and gravel of 740 kg/m³ and 1 108 kg/m³. After de-moulding, the specimens were placed and cured in a curing box for 28 d at different curing temperatures (i.e., 40, 60 ℃ and 80 ℃, respectively). After reaching the curing age, the durability test of specimens was carried out in a self-designed high geothermal environment attack test device. The wet sand containing SO₄^2- solution was used to simulate the attack environment. The wet sand and heating system were placed on the top of the test block. The coupling effect of single-sided attack and one-dimensional temperature field from top to bottom was realized. Water was added every 12 h to keep the humidity of the wet sand constant. The wet sand changed every 7 d to keep the salt concentration constant. The upper part of the wet sand was sealed with a plastic film to prevent water evaporation.

The change of compressive strength of concrete under the coupling effect of temperature field at 40 ℃ and 60 ℃ and sulfate attack are divided into the early growth and the subsequent decline. The coupling effect is 30.0% and 23.6% lower than that of single factor. However, the compressive strength of the coupling effect of temperature field at 80 ℃ decreases with a maximum decrease of 22.5%. The temperature field at 40-80 ℃ accelerates the attack chemical reaction, but a greater initial damage of concrete under the temperature field at 80 ℃ occurs. The most probable pore size and pore size distribution of concrete increase with the increase of temperature field. The large pores and transition pores in concrete are dominant, which are 40.38%-46.72 % and 31.09%-38.51%. Under the coupling effect of temperature field at 60 ℃, the content of gel pores and capillary pores decreases, while the content of macropores increases with the increase of attack depth. The surface of transition pores and capillary pores of concrete under coupling action has fractal characteristics, while the surface of gel pores and macropores does not have fractal characteristics. The pore tortuosity also has fractal characteristics in different pore size ranges. The D_t of gel pores increases with the increase of temperature field. The D_t of macropores decreases with the increase of temperature field, but the D_t of transition pores, capillary pores and macropores does not change with the depth. The influence of the surface roughness of the transition pore and the large pore on the average compressive strength of the damaged layer concrete is dominant. The surface roughness of the gel pore has a less influence. On the contrary, the complexity of gel pore structure has a dominant effect on the average compressive strength of damaged layer concrete. The influence of fractal dimension of pore structure on the strength of integral concrete is not consistent. There is a negative linear correlation between D_s of macropores and the compressive strength of integral concrete. However, the influence of D_s of capillary pores, transition pores and gel pores on the compressive strength of integral concrete is dominant, which is different from the results of previous studies. However, the D_v of macropores, capillary pores and transition pores is negatively linearly correlated with the compressive strength of the whole concrete.

The overall compressive strength of concrete under the coupling effect of temperature field at 40-80 ℃ was 22.5%-30.0% lower than that of single factor. The proportion of gel pores in concrete was decreased by 43.17%, and the capillary pores were increased by 573.10% as the temperature field was increased from 40 ℃ to 80 ℃. The surface of transition pore became more rougher, and the spatial structure of gel pore and transition pore was more complex than that of capillary pore and macropore under the coupling action. The tortuosity of gel pore, transition pore, and capillary pore increased but the macropore decreased. The surface roughness of transition pores and macropores, and the structure complexity of gel pore had a dominant influence on the concrete damaged layer average compressive strength. There was a negative linear correlation between the fractal dimension of pore volume of macropores/capillary pores/transition pores and the overall concrete compressive strength.

"Lithium-sulfur catalysis" is considered as an important strategy to solve the bottleneck of practical application of Li-S batteries. Although a variety of catalytic material systems are developed, the catalytic mechanism still remains unclear, and advanced theories need to guide the rational design of high-efficiency catalysts. Therefore, it is extremely urgent to develop a novel lithium-sulfur battery system and explore an effective research paradigm of Li-S catalysis for the conversion reaction process of Li-S battery. Future research directions focus on three aspects: 1) The failure mechanism and stability of catalysts should be investigated; 2) The structure-activity relationship between the electronic structure and catalytic performance of the catalyst should be deeply explored by AI technology; and 3) The conversion mechanism of solid-state Li-S battery should be clarified, and a novel practical solid-state Li-S battery system should be constructed for some future extreme applications.

Phosphates are one of commonly used inorganic retarding agents in Portland cement, including monophosphates (i.e., sodium phosphate) and polyphosphate compounds (i.e., sodium pyrophosphate, sodium tripolyphosphate, and sodium hexametaphosphate). Different phosphate salts exhibit distinct retarding mechanisms. Among these, sodium phosphate is extensively investigated as an alkaline metal monophosphate-type retarder. However, there exist varying interpretations of the retarding action of sodium phosphate in different studies, potentially impacting its practical application, and necessitating a clear elucidation of its retarding mechanism. Ammonium dihydrogen phosphate as an acid phosphate can also serve as a retarder and exhibits a more enduring retarding effect rather than the commonly used sodium phosphate. Compared to PO₄^3-, the volatility of NH₄⁺ or its potential to mitigate the influence of alkaline metal ions on the cement hydration and the larger ionization constant of H₂PO₄^- can have a quicker reaction with alkaline cement particles, affecting the hydration process. Some work indicate that ammonium dihydrogen phosphate has a favorable retarding effect on cementitious slurries, but its specific retarding mechanism remains less explored. It is thus necessary for the analysis of their impact on the kinetics of Portland cement hydration to conduct a comparative study between ammonium dihydrogen phosphate and sodium phosphate.

Cement used was conformed to the GB 8076—2008 standard with a strength grade of 42.5. Ammonium dihydrogen phosphate and sodium phosphate were of analytical reagent grade (AR). The dosage of ammonium dihydrogen phosphate and sodium phosphate was 0.5% (in mass fraction), with a fixed water-to-cement ratio of 0.4. Cement samples for microstructural tests were prepared in 60 mL plastic bottles. The cement paste was prepared via dissolving ammonium dihydrogen phosphate and sodium phosphate, and mixed with cement. The well-mixed cement paste was placed in the plastic bottles and cured under natural conditions at (20±2) ℃ until the specified age.

Cement setting time and paste flowability were determined according to GB/T 1346—2011 and GB/T 8077—2000 standards, respectively. The hydration exothermic curve of the cement paste in the early 72 h at 20 ℃ was recorded by a model TAM isothermal calorimeter. The zeta potential within the first 10 minutes of cement paste hydration was measured by a model DT-1200 electroacoustic spectrometer. The element concentration of the pore solution was measured by a model PS-6 inductively coupled plasma optical emission spectrometer (Baird Co., USA). The thermogravimetric analysis was conducted by a model STA449C synchronous thermal analyzer (NETZSCH Co., Germany). The X-ray diffraction (XRD) patterns of the samples were obtained by a model Advance D8 fully automated powder diffractometer (Bruker Co., Ltd., Switzerland). The surface morphology and elemental identification of the samples were observed by a model Nova NanoSEM230 scanning electron microscope.

The incorporation of sodium phosphate and ammonium dihydrogen phosphate results in a corresponding extension of both initial and final setting time of the cement paste. Ammonium dihydrogen phosphate with an equal mass exhibits longer initial and final setting time, compared to sodium phosphate. Compared to the pure cement group, the addition of sodium phosphate and ammonium dihydrogen phosphate improves the flowability of the cement paste and delays the time-dependent loss of flowability, showing a more pronounced retarding effect.

Hydration heat, thermogravimetric and XRD results indicate that sodium phosphate and ammonium dihydrogen phosphate both can inhibit the dissolution rates of C₃S and C₃A, thus retarding the formation of hydration products. This leads to a reduction in the rate of heat evolution during Portland cement hydration, an extension of the induction period of hydration, a decrease in the total heat evolved during the first three days of hydration, and an overall inhibition of Portland cement hydration. However, an inhibitory effect of ammonium dihydrogen phosphate is dominant.

Based on the analysis of zeta potential and SEM results, after the addition of sodium phosphate and ammonium dihydrogen phosphate in cement, a coating layer is formed on the surface of the cement particles. Sodium phosphate precipitates as calcium phosphate on the surface of Portland cement particles, while ammonium dihydrogen phosphate precipitates as hydroxyapatite. Ammonium dihydrogen phosphate exhibits a greater retarding effect rather than sodium phosphate due to its higher adsorption capacity of hydroxyapatite for inorganic ions, compared to calcium phosphate.

The early pore solution analysis indicates that ammonium dihydrogen phosphate can be adsorbed more rapidly on Portland cement particles, leading to the rapid precipitation of solid-phase products. In addition, no nitrogen (N) element appears when analyzing the early pore solution of the cement paste with ammonium dihydrogen phosphate, indicating that NH₄⁺ rapidly escape into air as ammonia when adding to the Portland cement paste. This escape of NH₄⁺ effectively prevents the influence of alkali metal ions in sodium phosphate on the early hydration of Portland cement.

Ammonium dihydrogen phosphate and sodium phosphate both were capable of prolonging the setting time of Portland cement and retarding the time-dependent loss of paste flowability with ammonium dihydrogen phosphate exhibiting a more pronounced retarding effect. These agents could inhibit the dissolution rates of C₃S and C₃A, thereby reducing the rate of heat evolution during Portland cement hydration, extending the induction period of hydration, lowering the total heat evolved during the first three days of hydration, and overall suppressing Portland cement hydration. However, the inhibitory effect of ammonium dihydrogen phosphate was dominant. Compared to sodium phosphate, ammonium dihydrogen phosphate could be more rapidly adsorbed on Portland cement particles, leading to the rapid precipitation of solid-phase products, and thereby inhibiting cement hydration. Sodium phosphate precipitated as calcium phosphate on the surface of Portland cement particles, suppressing cement hydration, while ammonium dihydrogen phosphate precipitated as hydroxyapatite on the surface of Portland cement particles, similarly inhibiting cement hydration.

Although clinical and basic application studies have confirmed the potential of BBGs in wound healing applications, the structural composition of BBGs, especially the types and concentrations of therapeutic ions released, as well as the regularity and molecular biological mechanisms of pathological wound healing, still need a further exploration. The risk of soft tissue and organ toxicity, and even systemic toxicity caused by BBGs, is needed to be evaluated. Also, it is necessary to effectively evaluate and ensure the biosafety of BBGs dissolved ions, and to construct a more advanced in-vitro simulated wound dynamic microenvironment. In addition, combining BBGs as additives with other treatment strategies (such as stem cell therapy or growth factors or drugs) or technologies (such as negative pressure drainage) could be a possible synergistic strategy for promoting wound healing and repair. In summary, the existing researches indicate that the full potential of BGs in medicine is not fully developed, and the related market is expected to further grow in the future.

Transition metal doping can promote the radiative recombination of excitons, and the defect formation energies of V_Cs, V_Br and V_Pb are effectively improved, resulting in an improvement of the luminescence performance of CsPbX₃ NCs. However, some problems need to be solved in the future. The PL linewidth of nanocrystals for the application of bioluminescence imaging and LED fields needs to be reduced. The external quantum efficiency (EQE) and luminous efficacy (LE) of transition metal doped CsPbX₃ NCs still need to be improved, compared to that of Cd-based QDs. Finally, there are still potential risks associated with the reaction, and the PLQY still needs to be further improved. The investigation of transition metal ion doping all-inorganic CsPbX₃ NCs has a profound impact on the scientific research and commercial applications of fluorescent materials.

Application of silica nanoparticles in concrete has attracted much attention. Its unique filling characteristics and high volcanic ash characteristics can improve the pore structure of concrete and promote hydration reaction, having a positive effect on the mechanical properties of concrete. After concrete is subjected at a high temperature, the pore pressure inside the structure, the deterioration of the transition zone of the slurry aggregate interface and the change of phase are all reasons for the decline of the mechanical properties of concrete at a high temperature. Silica nanoparticles have a filling effect, increasing the compactness of the material, while silica nanoparticles can undergo a volcanic ash reaction to generate more hydrated calcium silicate (C-S-H) gel, improving the high-temperature resistance of concrete. The inside of alkali slag mortar is alkaline environment and its hydration reaction is completely different from concrete, and the influence of silica nanoparticles on the high-temperature resistance of alkali slag cementitious material is different from that of ordinary concrete. Therefore, silica nanoparticles can improve the high-temperature mechanical properties of cement-based materials, but the influence of silica nanoparticles on the high-temperature resistance of seawater sea sand alkali slag mortar is more complicated. This paper was to investigate the effect of silica nanoparticles on the high-temperature resistance of seawater sea sand alkali slag mortar.

Simulated seawater and sea sand with a fineness modulus of 2.3 were used as mixing water and fine aggregates. Liquid sodium silicate and solid sodium hydroxide were formulated as alkali initiators in a molar ratio of 1.7 for SiO₂ and Na₂O. Silica nanoparticles with the sizes of 20-50 nm and a specific surface area of 670 m²/g as an admixture were supplied by Taihong Shengda New Materials Co., Ltd., China. Four sets of alkali slag mortar with different concentrations of silica nanoparticles (i.e., 0%, 1%, 2% and 3%) were produced. After mold removal, it was placed in a curing room at (20±2) ℃ and a relative humidity of > 90% for 27 d, and then dried naturally for 1 d. The changes of mass and length in specimens were recorded by scales and vernier calipers before and after high-temperature heating. The flexural and compressive strengths were determined on seawater sea sand alkali slag mortar specimens (40 mm×40 mm×160 mm) at different high temperatures (i.e., 200, 400, 600 ℃ and 800 ℃).

At room temperature, silica nanoparticles have little effect on the compressive strength of seawater sea sand alkali slag mortar, and the compressive strength of the specimen is only increased by 1.5% and 2.3% at the concentrations of silica nanoparticles of 2% and 3%, respectively. Also, silica nanoparticles can reduce the pH value of alkali initiator, which is unstable in salt solutions where Mg²⁺ is > 0.1% (in mass fraction). Monomeric silicon ([SiO_n(OH)_4-n]_n^-) is formed when silica nanoparticles are added into the base initiator, improving the SiO₂/Na₂O ratio of the solution.

At 200 ℃, compared with ordinary concrete, the addition of silica nanoparticles can increase the compressive strength of seawater sea sand alkali slag mortar. The compressive strength of the specimen with silica nanoparticles content of 1%-3% is 103%-123% of the normal temperature state. This may be related to the superior dispersion of silica nanoparticles in alkaline environments. OH^- can convert the silicon hydroxyl group on the surface of silica nanoparticles to Si-O^- groups, so that the surface of silica nanoparticle generates a negative charge and increases the dispersion. The deformation results at a high temperature show that the residual deformation of the specimen can be effectively alleviated when the silica nanoparticles content is 3%. Combined with the analysis of the results of mass loss, the addition of silica nanoparticles can reduce the content of evaporable water in the specimen. This decreases the pressure generated by the evaporation of partially pore water, thereby reducing the partial crack generation.

The addition of silica nanoparticles improved the compressive strength of seawater sea sand alkali slag mortar at 200 ℃. This could be related to the dispersion of silica nanoparticles in alkaline environments rather than in neutral environments. Silica nanoparticles could reduce the evaporable water content inside seawater sea sand alkali slag mortar. At the silica nanoparticles content of 3%, the deformation and cracks of the specimen at a high temperature were effectively inhibited.

Alkali-activated fly ash (AAFA) can reduce the carbon emissions from the production of ordinary Portland cement (OPC) by >80%. The utilization of AAFA and the research on the electrochemical behavior of steel in AAFA have thus attracted much attention. Compared with OPC, a higher resistance to chloride-induced corrosion can be achieved in AAFA due to its high alkalinity and zeolite-like adsorption layer. However, the corrosion of steel in AAFA cannot be completely inhibited in harsh environments contaminated with chlorides.

Carboxylate groups-based corrosion inhibitors (such as sodium citrate) improve the pore structure and compressive strength of concrete, and have a high inhibition efficiency for steel. Especially at a high chloride concentration, the pitting potential (E_pit) of steel treated with carboxylate groups-based corrosion inhibitors is greater than that treated with amine salt and amino acid. Therefore, sodium citrate can be considered as a promising organic corrosion inhibitor to enhance the corrosion resistance of steel in AAFA. However, the inhibition mechanism of sodium citrate for steel in AAFA is still unclear. In this paper, the effect of sodium citrate on the corrosion behavior of reinforcing steel in extracted AAFA solution under chloride attack was investigated by electrochemical measurements and surface characterization techniques, and the inhibition mechanism of sodium citrate was revealed.

In this study, the preparation of AAFA solution was carried out at room temperature ((25±2) ℃). Firstly, a low calcium fly ash was mixed with an alkali-activated solution (80 g/L NaOH solution) at a mass ratio of 1∶1, then the mixture was stirred in a closed vessel for 24 h, and finally it was filtered to remove the residues. The concentration of sodium citrate added into the AAFA solution was 100 mmol/L.

Conventional ribbed carbon steel (HRB400E) with a diameter of 16 mm was used. The electrochemical measurements, namely electrochemical impedance spectroscopy (EIS) and the cyclic potentiodynamic polarization (CPP), were performed in a classic three-electrode electrochemical system, i.e., the working electrode (HRB400E steel), the reference electrode (saturated calomel electrode, SCE) and the counter electrode (platinum electrode). The surface morphology and chemical composition of steel passive films were characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. After chloride attack, the corrosion morphology and corrosion products of steel were analyzed by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) in a model FEI Quanta 3D FEG environmental scanning electron microscope.

In the absence of protective passive film, the capacitive loop of the non-passivated steel (the Blank specimen) decreases in AAFA solution containing chlorides, thereby gradually reducing the corrosion resistance of steel. However, the capacitive loop and impedance modulus (|Z|) of steel treated with citrate (the Cit specimen) increase. Citrate can effectively improve the corrosion resistance of the non-passivated steel in AAFA solution containing chlorides.

Moreover, the EIS results of pre-passivated steel in chlorides-free AAFA solution show that the capacitive loop of steel treated with citrate (the P-Cit specimen) is somehow smaller than that of the blank group (the P-Blank specimen), indicating that citrate is unfavorable to the formation of steel passive films in AAFA solution. However, in the presence of chlorides, an enhanced corrosion resistance can be obtained for the P-Cit specimen, which is the same as that of the non-passivated steel.

According to the results of AFM and XPS measurements, citrate can modify the surface morphology and chemical composition of steel passive film in AAFA solution. There are a large number of fine particles on the steel surface in AAFA solution, and the surface roughness of steel is less and more uniform due to the adsorption effect of citrate, compared with that without citrate. For steel with citrate, the intensities of C—C/C—H and C==O peaks increase and the oxidation of Fe may be suppressed to a great extent. Furthermore, O^2-/OH^- ratio of steel passive film treated with citrate slightly decreases, and the corresponding content of bound water in steel passive film decreases.

Clearly, citrate ions can combine with iron ions to form an adsorption film on the steel surface, therefore suppressing the formation of passive film to some extent. However, when steel is attacked by chlorides, the adsorption film of citrate can prevent chloride ions from reaching steel surface. In addition, the negatively charged carboxylic acid group is also dominant in the competition with chloride ions, effectively inhibiting the depolarization of chloride ions, and finally improving the corrosion resistance of steel in AAFA solution.

Although sodium citrate had a negative effect on the formation of a stable steel passive film, it was suitable for improving the corrosion resistance of steel in AAFA solution. Citrate ions could be adsorbed on the steel surface to form an adsorption film, thereby acting an effective physical barrier. Also, citrate ions played a dominant role in the competitive adsorption with chloride ions on the steel surface. The corrosion resistance of steel in AAFA solution contaminated with chlorides was improved by the addition of sodium citrate.

Calcium sulphoaluminate cement has a potential application in sub-zero temperature environments due to its rapid hydration, rapid setting, fast strength development, and high early strength. In cold construction sites, such as polar regions, the materials and the concrete both are exposed to sub-zero temperatures for extended periods. It is thus necessary to investigate the formation, condensation, hardening and microstructures of calcium sulphoaluminate cement at sub-zero temperatures. Cold concreting method widely recognized involves directly blending raw materials on-site at the same temperature as the ambient environment, even at sub-zero temperatures, follows via pouring and curing fresh concrete without supplemental heating. Choosing a suitable mixing solution is crucial for the effective cold concreting technology. The mixing solution must prevent freezing of both the solution and the cement pastes at sub-zero temperatures, initiating the cement hydration reaction and enabling continuous progression. Also, the properties of the mixing solution (i.e., pH value and ion concentration) affect the dissolution and hydration of clinker minerals, leading to consequential changes in cement characteristics. In this paper, the performance evolution laws and mechanisms of calcium sulphoaluminate cement mixed with different mixing solutions at sub-zero temperatures were investigated.

Deionized water was used as a solvent. Six inorganic analytical pure chemical reagents and four organic analytical pure chemical reagents were used as solutes. Each of these reagents was dissolved individually in deionized water at room temperature. The solution was stirred until in a homogeneous state and left standing for 24 h. To ensure the temperature of the materials as the environmental temperature, the calcium sulphoaluminate cement and the mixing solution both were placed in a sub-zero temperature experimental system for a storage period of at least 24 h. All processes, i.e., sample shaping and curing, were carried out in the sub-zero temperature experimental system.

At -10 ℃, the mixing solution and cement were poured sequentially into a mixing bowl. They were stirred at a low speed for 120 s, paused for 15 s, and then stirred at a high speed for 120 s. The resultant paste was poured into the mold. After pouring, the vibrating table was used for 60 compactions. The samples were then placed on a rack for cured without being covered in the sub-zero temperature environment. Once the samples were cured to the specified age, a portion of them was taken out from the sub-zero temperature experimental system for the measurement of compressive strength. Another portion was crushed and sampled on-site. These samples were placed in anhydrous ethanol to terminate hydration at room temperature for at least 7 d. Afterwards, they were dried in vacuum at 40 ℃ and -0.08 MPa for no less than 24 h. A portion of the samples dried was used for the microscopic observation, while another portion was ground and further analyzed for the phase composition.

CaCl₂/Ca(NO₃)₂ solution is capable of accelerating hydration, resulting in an increased hydration temperature and a greater early strength. The peak temperature reaches up to 23.6 ℃ and 20.4 ℃, and the compressive strength at 12 h reaches 21.5 MPa and 16.5 MPa, respectively. Furthermore, the compressive strengths at 28 d are 101.8 MPa and 19.6 MPa, respectively. K₂CO₃ solution generates an increased hydration temperature and results in a high early strength, with a peak temperature of 7.7 ℃ and a compressive strength of 13.9 MPa at 12 h. However, the compressive strength only increases to 19.3 MPa after a 28-d period. MgCl₂, NaCl and NaNO₂ solutions exhibit a limited effect on enhancing hydration, presenting a low heat evolution and an early strength at their peak temperature of <5 ℃ and the compressive strength of <5 MPa after 12 h. Nonetheless, after a 28-d period, their compressive strength increases to 74.0, 42.7 MPa and 39.2 MPa, respectively. After 12-h hydration, the main products formed from CaCl₂, Ca(NO₃)₂, MgCl₂, NaCl and NaNO₂ in calcium sulphoaluminate cement are AFt and AFm phases, having a needle-bar and flake morphology. Also, the main products formed from K₂CO₃ solution-mixed samples are CaCO₃ and K₂SO₄ phases, having a cube and ellipsoid morphology.

CH₃OH, CH₃CH₂OH, (CH₂OH)₂ and C₃H₈O₃ solutions have a slight promotion effect on the hydration, resulting in a negligible heat evolution (peak temperature of < -5 ℃), minimal strength development at 12 h, and extremely low compressive strength for the samples after 28 d. However, the compressive strength of C₃H₈O₃ solution-mixed samples reaches 23.5 MPa after 28 d. For the samples mixed with four organic solutions, no hydration products such as AFt appear after 12-h hydration.

CaCl₂ and Ca(NO₃)₂ solutions with the same ions (Ca²⁺) as the cement minerals promoted cement hydration, resulting in an increased temperature and a greater early strength. MgCl₂, NaCl and NaNO₂ solutions had negligible effects on the hydration, leading to a decreased temperature and a weaker early strength. CO₃^2- in K₂CO₃ solution reacted with Ca²⁺ produced due to the dissolution of anhydrite to create CaCO₃ precipitates. This reaction increased the freezing point of the solution and reduced its frost resistance, causing the sample to freeze and preventing the effective cement hydration reactions.

The hydroxyl group of alcohol molecule in CH₃OH, CH₃CH₂OH, (CH₂OH)₂ and C₃H₈O₃ solutions was adsorbed by Ca²⁺ on the surface of cement particles to form an adsorption film. This film delayed hydration. In addition, the interaction between water and cement particles also impeded when the alcohol molecule contained a methyl group, exacerbating a retarding effect.