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The usage of manufactured sand concrete is widespread in modern engineering, and it is important to study its performance to improve the overall engineering quality. This paper presents an experimental study on the working performance and durability of 12 groups of manufactured sand high-performance concrete (MSHPC) with varying mix ratios, in the context of the construction of the Dalian Bay undersea immersed tube tunnel. The study reveals that the stone powder content significantly affects the physical and mechanical properties, as well as the durability, of manufactured sand concrete. At an approximately 9% stone powder content, the concrete achieves the highest slump and best workability. However, excessive stone powder reduces early crack resistance. Furthermore, an optimal stone powder content (ranging from 5% to 13%) enhances the compressive strength, with the 28-day compressive strength reaching 60 MPa at a 13% stone powder content, while the effect on the splitting tensile strength is negligible. The stone powder content does not significantly impact impermeability and frost resistance, but at 7–9%, the RCM method shows the lowest chloride ion diffusion coefficient. Additionally, a lower water–binder ratio enhances resistance to chloride ion diffusion. High-performance RCM concrete with a 9% stone powder content was used in the construction of the Dalian Bay Cross-Harbor Tunnel, achieving 28-day and 56-day compressive strengths of C45 and C50, respectively, an impermeability grade of P14, a chloride ion diffusion coefficient of 1.9 × 10−12 m2/s, and a frost durability index of 92%, meeting the project's 100-year lifespan design requirements. [ABSTRACT FROM AUTHOR]

The natural environment in the high-altitude regions of Northwest China is extremely harsh, characterized by numerous salt lakes. The high concentrations of chloride salts, sulfates, and alkali metal ions in these areas can induce alkali–silica reactions (ASRs) in concrete. These reactions generate harmful gel within the concrete, causing expansion and cracking, which significantly impacts the durability of concrete structures. This study investigates the evolution of the mechanical properties in high-performance concrete (HPC) under long-term ASR by incorporating different admixtures and varying the equivalent alkali content. A three-dimensional random aggregate mesoscopic model was used to simulate static compression tests under various operational conditions. Non-destructive testing methods were utilized to determine the expansion rate, internal, and surface damage variables of the concrete. The experimental results indicate that the 10-year expansion rate differs from the 1-year rate by approximately 1%, and under long-term ASR mitigation measures, the internal damage in the HPC is minimal, though the surface damage is more severe. As the equivalent alkali content increases, the compressive strength of the concrete cubes decreases, initially rising before falling by 5–15% over time. The HPC with only air-entraining agent added exhibited better mechanical performance than the HPC with both air-entraining and corrosion inhibitors added, with the poorest performance observed in the HPC with only a corrosion inhibitor. A relationship was established between the surface and internal damage variables, with the surface damage initially increasing rapidly before stabilizing as the internal damage rose. Numerical simulations effectively describe the damage behavior of HPC under static uniaxial compression. Comparisons with actual failure morphologies revealed that, in the cube compression tests, crack propagation directly penetrated both coarse and fine aggregates rather than circumventing them. The simulations closely matched the experimental outcomes, demonstrating their accuracy in modeling experiments. This study discusses the compressive mechanical properties of concrete under prolonged ASR through a combination of experimental and simulation approaches. It also delves into the impact of surface damage on the overall mechanical performance and failure modes of concrete. The findings provide experimental and simulation support for the concrete structures in regions with high alkali contents. [ABSTRACT FROM AUTHOR]

In regions characterized by the challenging combination of brine corrosion in the salt lakes and river sand with alkali silica reaction (ASR) activity in areas of the Northwest, high-performance concrete (HPC) formulated with high-volume composite mineral admixtures as ASR suppression measures has been preferred for civil engineering structures in the region. This study investigates the splitting tensile strength, corrosion products, microscopic structure characteristics, and mesoscopic mechanical mechanisms of splitting failure of such HPC under 10-year corrosion from salt lake brine. The relationship between mechanical properties and corrosion damage, as well as the characteristics of internal crack propagation paths and failure mechanisms of HPC under splitting load, are explored. The findings reveal that as the alkali content within HPC rises, corrosion damage intensifies, resulting in a reduction in splitting tensile strength. Moreover, a linear association between mechanical properties and corrosion damage is observed. Microscopic structural analysis and numerical simulation of the splitting failure process of HPC elucidate that while the substantial presence of mineral admixtures effectively suppresses the ASR risk associated with alkali-reactive aggregates in concrete, uneven ASR gel products persist. These discontinuous micro-fine interface cracks induced by the gel products and the cracks induced by the gel products around the selective alkali-active aggregate particles distributed in the local area are the initiation sources of mortar cracks in HPC splitting failure. In terms of the overall failure state observed during the concrete splitting process, mortar cracks manifest two distinct extension paths: along the coarse aggregate interface and directly through the aggregates themselves. Notably, a greater proportion of coarse aggregates are directly penetrated by mortar cracks, as opposed to the number of interface failures bypassing coarse aggregates. More importantly, the above work establishes a theoretical reference in three dimensions: macroscopic, mesoscopic, and microscopic, for studying concrete corrosion damage in complex environments such as salt lake brine corrosion and ASR inhibition. [ABSTRACT FROM AUTHOR]

This paper comprehensively investigates the dynamic mechanical properties of concrete by employing a 75 mm diameter Split Hopkinson Pressure Bar (SHPB). To be detailed further, dynamic compression experiments are conducted on coral aggregate seawater concrete (CASC) to unveil the relationship between the toughness ratio, strain rate, and different strength grades. A three-dimensional random convex polyhedral aggregate mesoscopic model is also utilized to simulate the damage modes of concrete and its components under varying strain rates. Additionally, the impact of different aggregate volume rates on the damage modes of CASC is also studied. The results show that strain rate has a significant effect on CASC, and the strength grade influences both the damage mode and toughness index of the concrete. The growth rate of the toughness index exhibits a distinct change when the 28-day compressive strength of CASC ranges between 60 and 80 MPa, with three times an increment in the toughness index of high-strength CASC comparing to low-strength CASC undergoing high strain. The introduction of pre-peak and post-peak toughness highlights the lowest pre-to-post-peak toughness ratio at a strain rate of approximately 80 s−1, which indicates a shift in the concrete's damage mode. Various damage modes of CASC are under dynamic impact and are consequently defined based on these findings. The LS-DYNA finite element software is employed to analyze the damage morphology of CASC at different strain rates, and the numerical simulation results align with the experimental observations. By comparing the numerical simulation results of different models with varying aggregate volume rates, it is reported that CASC's failure mode is minimized at an aggregate volume rate of 20%. [ABSTRACT FROM AUTHOR]

This study presents a comprehensive investigation into the mechanical properties of Basic Magnesium Sulfate Cement Concrete (BMSC) in comparison to Ordinary Portland Cement Concrete (OPC) within reinforced concrete components. The main objective is to evaluate BMSC's applicability for practical engineering purposes, with a focus on its with early high strength, improved toughness, and superior crack resistance compared to conventional concrete. Experimental procedures involved fabricating beam specimens using OPC concrete with a C40 strength grade, alongside BMSC beams with varying strength grades (C30, C40, and C50). These specimens underwent bending resistance tests to analyze crack patterns and mechanical characteristics. The findings reveal that BMSC beams demonstrate enhanced bending and tensile properties at equivalent strength grades compared to OPC beams. Particularly, cracking mainly occurred at the mid-span region of BMSC beams, characterized by narrower cracks, indicating superior crack resistance. However, it was noted that the toughness of BMSC beams decreases as the strength grade increases. The maximum mid-span deflection of the BMSC test beam was smaller than that of the OPC test beam, which was 3.8 mm and 2.6 mm, respectively. The maximum crack width of the OPC beam was 4.7 times that of the BMSC beam. To facilitate practical implementation, the study developed calculation models for estimating the crack bending distance and ultimate bending distance in BMSC beams, offering valuable tools for engineering design and optimization. Overall, this research provides significant insights into the mechanical behavior of BMSC, presenting potential advantages for structural engineering applications. [ABSTRACT FROM AUTHOR]

This study first employs TurbSim and OpenFAST (Fatigue, Aerodynamics, Structures, Turbulence) programs for secondary development to comprehensively model the NREL-5MW semi-submersible wind turbine and OC4-DeepC wind floating platform with wind–wave interaction. Next, we investigate the dynamic response of floating wind turbines under the complex coupling of turbulent winds and irregular waves. Turbulent wind fields were simulated using the IEC Kaimal model with turbulence intensities of 5% and 20%. Additionally, two irregular waves were simulated with the Pierson–Moskowitz (P–M) spectrum. The results indicate that in turbulent wind conditions, the aerodynamic power of the wind turbine and the root bending moments of the blades are significantly influenced by turbulence, while the impact of waves is minimal. The coupled motion response of the floating platform demonstrates that turbulence intensity has the greatest impact on the platform's heave and pitch motions, underscoring the importance of turbulence in platform stability. This study provides essential insights for designing and optimizing floating wind turbines in complex wind–wave coupling offshore environments. [ABSTRACT FROM AUTHOR]

This study employed a rapid freezing method to investigate the impact of individual additions of expansion agent, steel fibers, and High Elasticity Module Polyethylene Fiber (HEMPF), as well as their combinations, on the freeze-thaw resistance of High-Strength Concrete (HSC). The findings reveal that the non-air-entrained HSC of C80 exhibits excellent freeze-thaw resistance. However, this resistance is sensitive to the curing environment's humidity. The expansion agent has a negative impact on the freeze-thaw resistance of HSC, while steel fibers and HEMPF fibers have positive effects. Combining HSC with an expansion agent and high elasticity modulus fibers ensures not only high freeze-thaw resistance but also a total alteration of humidity sensitivity, leading to an extended freeze-thaw life of HSC under dry curing conditions. [ABSTRACT FROM AUTHOR]

The durability of concrete is critical for the service life of concrete structures, and it is influenced by various factors. This paper investigates the impact of the relative humidity (RH) of the curing environment on the durability of five different concrete types. The aim is to determine a suitable approach for designing concrete that is well-suited for use in the salt lake region of Inner Mongolia. The concrete types comprise ordinary Portland cement (OPC), high-strength expansive concrete (HSEC), high-strength expansive concrete incorporating silica fume, fly ash, and blast furnace slag (HSEC-SFB), steel fiber-reinforced high-strength expansive concrete (SFRHSEC), and high elastic modulus polyethylene fiber-reinforced high-strength expansive concrete (HFRHSEC). All these concrete types underwent a 180-day curing process at three distinct relative humidities (RH = 30%, 50%, and 95%) before being subjected to freeze–thaw cycles in the Inner Mongolia salt lake brine. The curing environment with a 95% RH is referred to as the standard condition. The experimental results reveal that the durability of OPC and HSEC decreases significantly with increasing relative humidity. In comparison with the control sample cured in 95% RH, the maximum freeze–thaw cycles for concrete cured in lower RHs are only 31% to 76% for OPC and 66% to 77% for HSEC. However, the sensitivity of the durability of HSEC-SFB, SFRHSEC, and HFRHSEC to variations in RH in the curing environment diminishes. In comparison with the corresponding reference value, the maximum freeze–thaw cycles for samples cured in dry conditions increase by 14% to 17% for HSEC-SFB and 21% for SFRHSEC. Specifically, the service life of HFRHSEC cured in a low RH is 25% to 46% higher than the reference value. The durability of HSEC-SFB, SFRHSEC, and HFRHSEC has been proven to be appropriate for structures located in the salt lake region of Inner Mongolia. [ABSTRACT FROM AUTHOR]

This study explores a cost-effective and efficient method for snow melting and deicing using a concrete pavement structure with pre-mixed chloride-salt antifreeze material. The mixed design method for road cement concrete with chloride-type antifreeze material is investigated by encapsulating NaCl and CaCl2 in organic resin capsules. The snow melting and deicing efficacy of cement concrete with chloride antifreeze material is evaluated through an automatic tracking measurement system. The results indicate that NaCl-type antifreeze material is more effective in reducing the freezing point of concrete surface solution than CaCl2-type antifreeze material. A composite antifreeze material (4% NaCl + 1% CaCl2) provides the strongest snow melting and deicing ability. An outward migration and diffusion model of chloride ion in cement concrete with antifreeze material is established to calculate the chloride ion diffusion coefficient, surface chloride ion content, and decay rate of concrete. The effective age of snow melting and deicing is analyzed using this model. [ABSTRACT FROM AUTHOR]

To study the durability of magnesium oxychloride cement (MOC) in practical applications, samples from 6 to 80 years in different regions of China, including both north and south China, were collected. Phase composition and microstructure of MOC were analyzed, and phase transition patterns and microstructure characteristics of MOC were explored. The results show that, for MOC material, the main hydration product is 5Mg(OH)2·MgCl2·8H2O (phase 5·1·8), and the carbonation products are Mg(OH)2·MgCl2·2MgCO3·6H2O (phase 1·1·2·6) and 4MgCO3·Mg(OH)2·4H2O (phase 4·1·4). For MOC samples with ages less than 20 years, the content of phase 5·1·8 and the total content of carbonized phases (phase 1·1·2·6 and phase 4·1·4) increases and decreases in turn. The change of the total content of the carbonized phase is opposite to that of the phase 5·1·8 and is affected by phase 5·1·8. A calculation formula for the carbonation degree of MOC material system is proposed. The calculation results show that when the age is less than 14 years, the carbonation rate of MOC samples in the dry environment from north China is slow, but obviously increases when the age exceeds 14 years. Carbonation degree of the MOC samples at 6 years in the high humid environment of south China is close to that of the samples at about 20 years in north China. The crystal morphologies of different phases in MOC are varied. There are needle rod-like and flocculent crystals of phase 5·1·8 , short rod-like crystals of phase 1·1·2·6 , flaky crystals of phase 4·1·4 , and nubbly crystals of MgCO3 in MOC. Phase 5·1·8 ensures the integrity of the glass fiber, while sawdust shows disadvantages on the long-term service of MOC. [ABSTRACT FROM AUTHOR]

High strength concrete (HSC) with superior strength advantage has been gaining widespread attention and engineering endorsement in recent years. For a better understanding of the mechanical responses and failure mechanisms of HSC, a novel three‐dimensional (3D) mesoscale model considering the meso‐structural features of concrete was proposed in this paper. First, a series of uniaxial compressive tests were conducted to investigate the compressive properties of HSC specimens with a strength of 80–90 MPa. After that, a 3D three‐phase mesoscale model consisting of mortar, coarse aggregate, and the interfacial transitional zone (ITZ) between them, was developed to perform the mesoscopic simulations of HSC. Then, systematic analysis and discussions on the compressive properties of HSC were presented in terms of the stress–strain curves, compressive strengths/toughness, failure patterns, cracking process, etc. Results indicate that HSC exhibited obvious brittle failure characteristics, and its strength was significantly related to the water‐to‐cement ratio and mineral admixture. At meso‐level, the cracking behaviors of mortar and ITZ phases were well modeled using the developed mesoscale model. Additionally, it was found that the cracking behavior of HSC was significantly associated with the ITZ characteristics. The developed mesoscale modeling approach has been proven to simulate and investigate concrete's compressive properties in a reliable manner, and can be applied further to the property analysis of concrete materials and structures. [ABSTRACT FROM AUTHOR]

Basic magnesium sulfate cement coral aggregate concrete (MCAC) is a new type of concrete consisting of basic magnesium sulfate cement, coarse coral aggregate, coral reef sand and seawater. The rebound hammer (RH), the ultrasonic pulse velocity (UPV) and the compressive strength (fcu) tests of 14 sets of cube specimens of the MCAC after 28 d of aging were conducted. The impact of the content and length of sisal fiber on the relationship between the fcu - RH and the fcu - UPV was determined. A mathematical model was established to predict the strength of the MCAC using the UPV, RH, and comprehensive UPV/RH methods and to obtain the curves of test strength. The applicability of the test strength curves of ordinary portland concrete (OPC), light-weight aggregate concrete (LAC), and coral aggregate concrete (CAC) to MCAC was assessed. The results showed that the test strength curves of OPC, LAC and CAC were inappropriate to determine the strength of MCAC using non-destructive method. The relative standard error of the curves of test strength of the RH method and the comprehensive method met the specifications, whereas that of the UPV method did not. [ABSTRACT FROM AUTHOR]

Split Hopkinson pressure bar (SHPB) was used to investigate the dynamic compressive properties of sisal fiber reinforced coral aggregate concrete (SFCAC). The results showed that, with the increase of strain rate, the dynamic compressive strength, peak strain and toughness index of SFCAC are all greater than its static properties, indicating that SFCAC is a kind of rate-sensitive material. When the sisal fiber was blended, the failure mode showed obvious ductility. At high strain rates, the SFCAC without sisal fiber specimen was comminuted, and the SFCAC showed a "cracked without breaking" state. The results indicated that the sisal fiber played a significant role in reinforcing and strengthening the properties of concrete. The finite element software LS-DYNA was used to simulate two working conditions with strain rates of 78 and 101 s−1. The stressstrain curves and failure patterns obtained were in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

The penetration resistance of the new material Basic Magnesium Sulfate Cement (BMSC) is studied through comprehensive application of an experimental and numerical simulation method. This paper consists of three parts. The first part introduces the preparation of Basic Magnesium Sulfate Cement Concrete (BMSCC) and the study of its dynamic mechanical properties. In the second part, on-site testing was carried out on both BMSCC and an ordinary Portland cement concrete (OPCC) target, and the anti-penetration performance of the two materials was analyzed and compared from three aspects: penetration depth, crater diameter and volume, and failure mode. In the last part, the numerical simulation analysis was carried out based on LS-DYNA, and the effects of factors, such as material strength and penetration velocity on the penetration depth, are analyzed. According to the results, the BMSCC targets have better penetration resistance performance than OPCC under the same conditions, mainly manifested in smaller penetration depth, smaller crater diameter and volume, as well as fewer cracks. [ABSTRACT FROM AUTHOR]

Basic magnesium sulfate cement (BMS) is a new type of magnesium cementitious material, which has similar advantages with magnesium oxychloride cement, such as lightweight, fast setting, high strength, wear resistance, and is not easy to absorb the moisture. It can widely replace magnesium oxychloride cement in the field of building insulation materials. In addition, the BMS has 2.5 times the flexural strength of Portland cement of the same strength grade, and its durability and reinforcement protection performance is similar to Portland cement, which also has a good application prospect in the field of civil engineering. The preparation and hydration process of BMS is reviewed in this paper. The microstructure and properties of BMS, magnesium oxychloride cement and magnesium oxysulfate cement are comparatively analyzed. The effects of content and activity of α‐MgO in the magnesium materials, the type of blending solution, the α‐MgO/MgSO4 ratio, the type and dosage of chemical admixture on the microstructure and properties of BMS were clarified. This paper is helpful for the application of BMS in civil engineering. [ABSTRACT FROM AUTHOR]

Split Hopkinson pressure bar (SHPB) with 75-mm diameter is used to conduct dynamic impact compression tests on different strength grades of basic magnesium sulfate cement-coral aggregate concrete (BMSC-CAC). The failure process of BMSC-CAC under dynamic impact compression is simulated using the finite element software LS-DYNA and a three-dimensional random aggregate mesoscopic model. The results show that BMSC-CAC has a significant strain rate effect, and the dynamic impact strength increases as the strain rate increases. Because of the high toughness of BMSC, BMSC-CAC has higher energy absorption than coral aggregate seawater concrete (CASC) and sisal fiber coral aggregate seawater concrete (SFCASC). The LS-DYNA software is used for the numerical simulation of the mesoscale dynamic impact compression. The relative error of the peak stress and strain between the test and simulation results is small. Analysis of the failure process indicates that the damage extent of the aggregate increases, and the failure time decreases with an increase in the strain rate. [ABSTRACT FROM AUTHOR]

The frost resistance of concrete is an essential evaluation index in cold areas; however, quantitative analyses of macroscopic mechanical properties and interfacial strength of concrete under freeze–thaw (F–T) action are scarce. Based on rapid F–T cycle tests of concrete, mortar, and interface specimens, with limestone, quartz, and basalt as variables, the internal damage and basic mechanical properties of specimens under F–T action were obtained. Furthermore, the quantitative relationship between the mechanical properties of the specimens, and between the internal damage and the basic mechanical properties of specimens under F–T action was established. The results showed that the properties of a coarse aggregate–mortar interface affected the mechanical properties and frost resistance of concrete specimens to a certain extent. After F–T action, the internal structure of the specimen tended to be loose, and the specimen was in a frozen state. The degree of damage on concrete specimens was more substantial than that on mortar ones. Quantitative analysis also indicated that the F–T erosion of concrete was mainly caused by the damage in the interface transition zone between the coarse aggregate and mortar. [ABSTRACT FROM AUTHOR]

Adding mineral admixtures is a green, economic and effective way of improving the properties of magnesium oxysulfate cement (MOSC). Based on MOSC modified with low-calcium fly ash (L-FA), high-calcium fly ash (H-FA) and granulated blast-furnace slag (G-BS), the effects of L-FA, H-FA and G-BS and silica fume (SF) double mixing on the properties of MOSC were studied by characterising the compressive strength, water resistance, hydration heat, hydration products and micropore structure characteristics. The modification mechanism of the mineral admixtures was also investigated. The results showed that L-FA, G-BS and SF double mixing optimised the hydration product composition and the micropore structure of the MOSC, thus improving its compressive strength and water resistance. The hydration reaction rate of MOSC and 5.1.7 phase content in the hydration products was increased by H-FA and SF double mixing, but the micropore structure was not further optimised, which is why the water resistance of MOSC with H-FA and SF was not improved. The research results enrich the basic theory of mineral admixtures for improving the properties of MOSC and provide theoretical guidance for practical applications. [ABSTRACT FROM AUTHOR]

This paper is based on the basic mechanical properties and volume deformation test, as well as the characterization of hydration products and microstructure properties such as x-ray diffraction (XRD), scanning electron microscope (SEM), and mercury injection method (MIP). The effects of CO2 curing on the mechanical properties, hydration product composition, and microstructure of magnesium oxysulfate (MOS) cement were studied. The results show that the carbonation properties of MOS cement are closely related to the standard air curing age after demolding. The CO2 curing immediately after demolding is not conducive to the development of compressive strength of MOS cement, while the compressive strength of MOS cement in standard air curing for 14 or 28 days after demolding is increased during CO2 curing. The CO2 can prevent the continuous hydration of residual MgO to form Mg(OH)2 or 5·1·7 phase, which affects the development of the compressive strength of MOS cement. The appropriate addition of low-calcium fly ash (L-FA) can compensate the effect of CO2 on the mechanical properties of MOS cement. During CO2 curing, some Mg(OH)2 is transformed into an amorphous phase to increase the compactness of its microstructure and a small amount of Mg(OH)2 reacts with CO2 to form MgCO3·3H2O. The results provide an important theoretical and experimental foundation for the further study of the effects of CO2 on the properties of MOS cement and the use of CO2 curing modification. [ABSTRACT FROM AUTHOR]

Basic magnesium sulfate cement (BMS) is a new kind of cement, which has many advantages. In order to study the corrosion of steel in basic magnesium sulfate cement concrete (BMSC), nine steel bars were tested by dry and wet cycling. The simulated solution of BMS was used in this test. In addition, 24 corroded steel bars in BMSC beams and columns that exposed for two years in the natural environment were intercepted for the tensile test. It can be found that the maximum section loss rate, the yield strength, the ultimate strength, the modulus of elasticity and the elongation at break of the corroded steel bars are linearly related to the corrosion rate. Through the research, the calculation formula of the relationship between the yield strength, ultimate strength, elongation at break and elastic modulus of the steel bar and the maximum corrosion rate of section of the steel bar were established. It provides a reference and basis for the durability study of BMSC beams and columns in the future. [ABSTRACT FROM AUTHOR]

In order to investigate the influence of different naphthalene series water-reducing agent content, cement content and coral sand rate on cube compressive strength (fcu) of basic magnesium sulfate cement coral aggregate concrete (BMSC-CAC), the C30 strength grade concrete as the research object, the experiment was carried out. The results show that the fcu of BMSC-CAC increases firstly and then decreases with the increase of cement content, naphthalene series water-reducing agent content and coral sand rate. When the cement content is 600 kg/m³, the naphthalene series water-reducing agent content is 3% and the coral sand rate is 40%, the fcu reaches the maximum value of 35 MPa. [ABSTRACT FROM AUTHOR]

Based on seawater immersion, drying-wetting cycles, carbonation and drying-wetting cycles for coral aggregate sea-water concrete (CASC) with different strength grades, the effect of carbonation and drying-wetting cycles on chloride diffusion behavior of CASC is studied. The results show that the free surface chloride concentration (Cs), free chloride diffusion coefficient (Df) and time-dependent index (m) of CASC in the drying-wetting cycles is obviously higher than that in seawater immersion. The Df and m of CASC of carbonation and drying-wetting cycles is higher than that in the drying-wetting cycles. Carbonation increases the Df and m of CASC, which is against CASC to resist chloride corrosion. The corrosion possibility of CASC structures in different exposed areas is as follows: splash zone (carbonation and drying-wetting cycles)>tidal zone (drying-wetting cycles) >underwater zone (seawater immersion). Besides, the chloride diffusion rate of C65-CASC is 17.8%–63.4% higher than that of C65-ordinary aggregate concrete (OAC) in seawater immersion (underwater zone). Therefore, anti-corrosion measures should be adopted to improve the service life of CASC structure in the oceanic environment. [ABSTRACT FROM AUTHOR]

Magnesium oxysulfate (MOS) cement is often seen as a 'green engineering material in the twenty-first century'. However, low mechanical strength is one of the major drawbacks for large-scale applications of MOS cement. In this paper, MOS cement with desirable strength was prepared by adding phosphoric acid. The effects of phosphoric acid on the hydration process, hydration products and microstructure of MOS cement were studied in detail using isothermal calorimetry, X-ray diffraction, Fourier transform infrared spectrometry, mercury intrusion porosimetry and scanning electron microscopy. It was found that the incorporation of phosphoric acid could retard the hydration process and increase the setting time of MOS cement. Furthermore, the incorporation of phosphoric acid was shown to increase the compressive strength through its effect in promoting the formation of 5Mg(OH)2·MgSO4·7H2O. The space-filling properties and directional growth characteristics of 5Mg(OH)2·MgSO4·7H2O contributed to a more compact and uniform microstructure of the MOS cement formed. [ABSTRACT FROM AUTHOR]

To study the behavior of coral aggregate concrete (CAC) column under axial and eccentric compression, the compression behavior of CAC column with different types of steel and initial eccentricity (ei) were tested, and the deformation behavior and ultimate bearing capacity (Nu) were studied. The results showed that as the ei increases, the Nu of CAC column decreases nonlinearly. Besides, the steel corrosion in CAC column is severe, which reduces the steel section and steel strength, and decreases the Nu of CAC column. The durability of CAC structures can be improved by using new organic coated steel. Considering the influence of steel corrosion and interfacial bond deterioration, the calculation models of Nu under axial and eccentric compression were presented. [ABSTRACT FROM AUTHOR]

In this paper, the hydration reaction and properties of magnesium oxysulfate (MOS) cement modified with ammonium citrate tribasic are discussed and compared with MOS cement without chemical additives. The hydration reaction characteristics, hydration products, mechanical properties, and microstructures of MOS cement prepared with and without chemical additives were characterized by isothermal calorimetry, quantitative X-ray diffraction (QXRD), mercury intrusion porosimetry (MIP), thermogravimetry (TG), and scanning electron microscopy (SEM), and the mechanism of action of the chemical additives–was investigated. The results show that when the molar ratio of α-MgO:MgSO4·7 H2O:H2O is 8∶1∶11 , the basic salt hydrate 5 Mg(OH)2·MgSO4·7H2O phase (5·1·7 phase) can be formed in MOS cement with and without chemical additives. However, ammonium citrate tribasic is an effective chemical additive that can (1) adjust the concentration of H+ and OH− ions in the liquid phase through buffering to meet the requirements of the H+ or OH− ion concentration (or pH value) required for the neutralization reaction of MgO particles, (2) strengthen the neutralization reaction of MgO particles, and (3) reduce the hydration reaction of MgO particles to form Mg(OH)2 through the complexation of the complex ions. Therefore, a large amount of the 5·1·7 phase and small amounts of Mg(OH)2 and the amorphous phase are formed in MOS cement with ammonium citrate tribasic, which optimizes the micropore structure and improves its mechanical strength and water resistance. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of coral sand powder (CSP) and corrosion inhibitors (SBT®‐ZX(V), SBT®‐RMA(II)) on reinforcement corrosion in coral aggregate seawater concrete (CASC) in a marine environment. A reasonable time‐variation model for reinforcement corrosion in CASC is proposed and verified. Based on the time‐variation model and chloride diffusion model, the cracking time and service life of CASC are predicted. The results show that the replacement of cementitious materials by 5% CSP can improve the corrosion resistance of steel bar in CASC. Both SBT®‐ZX(V) and SBT®‐RMA(II) can reduce the corrosion rate of steel bar in CASC, and this effect of SBT®‐RMA(II) is more effective than that of SBT®‐ZX(V). Among all CASC, CASC containing 5% CSP and SBT®‐RMA(II) possesses the best corrosion resistance. The results also reveal that the corrosion current density of steel bar in CASC increases firstly and then decreases over time. The ascending segment of the fitting curves for reinforcement corrosion in CASC conforms to the logarithmic‐type time‐variation model, and the descending segment of the fitting curves for reinforcement corrosion in CASC conforms to the exponential‐type time‐variation model. The cracking time and service life of CASC containing 5% CSP and SBT®‐RMA(II) will reach more than 10 years when the thickness of protective layer is 30 mm. [ABSTRACT FROM AUTHOR]

The effects of silica powder (SP) with different fineness values on the properties of magnesium oxysulfate cement (MOSC) were compared with the effects of fly ash (FA). X-ray diffraction, mercury porosimetry and scanning electron microscopy were used to analyse the effects of the particle morphology and fineness of FA and SP on the mechanical properties, hydration products and microstructure of MOSC. The results showed that when the α-MgO/MgSO4.7H2O/H2O molar ratio was constant, the concentration of magnesium sulfate and the uniformity of magnesium oxide particles in the slurry were the main factors determining the composition of the hydration products and the microstructure of MOSC. Spherical FA was found to improve the uniformity of magnesium oxide particles in the slurry and exert a nucleation effect, thereby increasing the formation of the 5.1.7 phase of the hydration products. Polyhedral SP with a certain fineness increased the concentration of magnesium sulfate in the slurry and had a filling effect, thus promoting formation of the 5.1.7 phase and optimising the microstructure. Therefore, with a certain magnesium sulfate concentration and a homogeneous slurry, both active FA and inert SP can improve the water resistance of MOSC by optimising the composition of the hydration products and microstructure. [ABSTRACT FROM AUTHOR]

To investigate the dynamic responses and comprehending the damage mechanism of coral aggregate concrete (CAC) in compression, both the experimental and numerical investigations were implemented in the present work. Firstly, the dynamic mechanical properties of CAC at different strain rates were tested through the Split-Hopkinson pressure bar (SHPB) tests. Moreover, the effects of concrete strength grade and strain rate on CAC were discussed and analyzed. Subsequently, we developed the three-dimensional (3D) random mesoscale model considering the randomness of aggregate shape, size and distribution at meso-level, which was validated and employed in the numerical simulation of CAC in compression. The results indicate that the splitting failure passing through the coral aggregate is CAC'S primary failure mode. It has been found that the failure pattern, deformation process, and dynamic increasing factor of CAC are associated with both the strain rate and concrete strength grade. Furthermore, by comparing the experimental and mesoscopic results, it has been proven to be reliable to employ the developed 3D mesoscale modelling method to simulate CAC's dynamic performances, which has enormous potential in future research of CAC under intense dynamic loadings. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to compare the effect of rust inhibitors and surface strengthening materials on the service life of RC structures in tropical marine environments and ultimately to provide basis and recommendations for the durability design of reinforced concrete (RC) structures. Design/methodology/approach: Slag concrete specimens mixed with four kinds of rust inhibitors and coated with four kinds of surface strengthening materials were corroded by seawater exposure for 365 days, and the key parameters of chloride ion diffusion were obtained by testing. Then a new service life prediction model, based on the modified model for chloride ion diffusion and reliability theory, was applied to analyze the effect of rust inhibitors and surface strengthening materials on the service life of RC structures in tropical marine environments. Findings: Rust inhibitors and surface strengthening materials can effectively extend the service life of RC structures through different effects on chloride ion diffusion behavior. The effects of rust inhibitors and surface strengthening materials on the service life extension of RC structures adhered to the following trend: silane material > cement-based permeable crystalline waterproof material > hydrophobic plug compound > spray polyurea elastomer > water-based permeable crystalline waterproof material > calcium nitrite > preservative > amino-alcohol composite. Originality/value: Using a new method for predicting the service life of RC structures, the attenuation law of the service life of RC structures under the action of rust inhibitors and surface strengthening materials in tropical marine environments is obtained. [ABSTRACT FROM AUTHOR]

Based on the drying-wetting cycles experiment and the carbonation-drying-wetting cycles experiment for coral aggregate seawater concrete (CASC) with different strength grades, the effects of carbonation-drying-wetting on the durability of CASC are studied with the surface state, mass loss rate, relative dynamic elastic modulus, ultrasonic wave velocity and cube compressive strength as indices. Results show that the mass loss rate of CASC increases gradually with the increase in cycle times in the drying-wetting and carbonation-drying-wetting cycles. The mass loss rate increases relatively slowly at the initial stage but it increases remarkably after 10 cycles. The relative dynamic elastic modulus and ultrasonic wave velocity decrease gradually with the increase in cycle times. After 6 cycles, the decrease rate of the relative dynamic elastic modulus and ultrasonic wave velocity of CASC tends to be flat and the surface is slightly damaged. Compared with the initial 28 d cube compressive strength, the cube compressive strength of CASC decreases by 8.8% to 11.0%. Drying-wetting cycles and carbonation can accelerate seawater erosion on CASC, and drying-wetting cycles result in salting-out and accelerate the destruction of concrete. Therefore, the carbonation-drying-wetting accelerates the destruction of CASC. [ABSTRACT FROM AUTHOR]

Sodium dihydrogen phosphate (NaH2PO4) and potassium dihydrogen phosphate (KH2PO4) were selected as additives for magnesium oxysulfate (MOS) cement. The phase composition and the microstructure of MOS cement were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TG-DSC), Flourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). It is found that both NaH2PO4 and KH2PO4 lead to an increase in the compressive strength and an improvement in the volume stability of MOS cement. The XRD, MIP and SEM results show that the addition of NaH2PO4 or KH2PO4 does not change the phase composition of MOS cement but promotes the formation of strength phase of 5Mg(OH)2·MgSO4·7H2O (5·1·7 phase). This phase brings a considerable improvement in the microstructure of MOS cement, which has a positive effect on the properties of MOS cement. [ABSTRACT FROM AUTHOR]

Reinforced coral aggregate concrete beam and reinforced ordinary aggregate concrete beam with different concrete type, concrete strength, and steel type were designed in this study to investigate their shear behavior. The shear behavior of reinforced coral aggregate concrete beam was tested, the failure mode and deflection behavior were studied, and the calculating model for the ultimate shear capacity (V cs) of reinforced coral aggregate concrete beam was proposed. Results showed that the failure mode of reinforced ordinary aggregate concrete beam and reinforced coral aggregate concrete beam were basically the same. As the concrete strength increases, the normal section cracking load (V cr), inclined section V cr, and V cs of reinforced coral aggregate concrete beam increased gradually. Furthermore, V cr and V cs of reinforced coral aggregate concrete beam were as follows: 316 stainless steel > common steel > zinc-chromium coated steel > new organic coated steel. For the coral aggregate concrete structure in ocean engineering, in order to prolong its service life, the use of new organic coated steel was suggested. At the same time, the influence of high-strength coral aggregate concrete and stirrup corrosion was comprehensively considered and the calculation model for the V cs of reinforced coral aggregate concrete beam was presented and was then verified. [ABSTRACT FROM AUTHOR]

The brine-freeze-thaw durability (defined as the durability under freeze-thaw cycles in Qinghai salt lake brine) of concrete (ordinary Portland cement concrete (OPC), high performance concrete (HPC-a), high performance concrete with steel fiber (HPC-b), and high performance concrete with high Young's modulus polyethylene fiber (HPC-c)) was systematically investigated by the relative dynamic elastic modulus, the relative mass, the appearance, the scanning electron microscopy, and the X-ray diffraction. In addition, the low-temperature physical and chemical corrosion mechanism and a crack density model after the modified relative dynamic elastic modulus being taken into consideration were proposed. The results show that the deterioration of OPC is the severest, followed by HPC-a, HPC-c and HPC-b. The admixture or the fiber is mixed into concrete, which can improve the brine-freeze-thaw durability of concrete. The critical mass growth of the failure of concrete is 3.7%. The cause of the deterioration of concrete under the brine-freeze-thaw cycles is physical and chemical corrosion, not freezing and thawing. The crack density model can effectively describe the deterioration evolution of concrete. [ABSTRACT FROM AUTHOR]

To improve the workability of magnesium oxysulfate (MOS) cement with a low water/cement ratio, polycarboxylate (PC) and naphthalene (NF) superplasticisers were used as modifiers and their effects on the physical properties of MOS cement were studied. Various approaches were used to analyse and explain the action mechanisms of the superplasticisers, including zeta potential measurements, scanning electron microscopy, X-ray diffraction, hydration analysis and mercury intrusion porosimetry. The results showed that the addition of superplasticisers decreased the compressive strength and water demand of the MOS cement and also retarded the hydration process, prolonged the setting time, changed the porosity and pore distribution, and decreased the relative content of the main hydration products in the cement. In the magnesium oxide–magnesium sulfate–water system studied, the hydrophobic interaction of the NF superplasticiser in the MOS cement was stronger than that of the PC superplasticiser. NF superplasticiser thus appears to be more suitable for MOS cement, improving its fluidity without significantly reducing its compressive strength. A dosage of 0·5% NF superplasticiser was found to give the best performance. [ABSTRACT FROM AUTHOR]

There is no systematic research on the basic physical and mechanical properties of the basic magnesium sulfate cement (BMS) concrete, which limited the application and development of BMS concrete. This paper offers a review of the new experimental results on structural performance of BMS concrete in recent five years. The flexural performance and shear behavior of BMS concrete beams, the compressive behavior of BMS columns as well as the seismic behavior of the column-beam joints are introduced. The structural components of BMS concrete show the higher value of the cracking moment and excellent flexural performance. The existing empirical equations for calculating the bearing capacity of beams and eccentric compressive columns are modified. The seismic behavior of the column-beam joints of BMS concrete is obviously influenced by the axial compression ratio of the joints. This paper would assist engineers and researchers in better understanding the performance of the new kind of structural concrete. [ABSTRACT FROM AUTHOR]

An investigation that combines both experimental tests and mesoscopic modelling is conducted to characterize the dynamic splitting-tensile behavior of coral aggregate concrete (CAC). Static and dynamic splitting-tensile strength and failure patterns of CAC with different uniaxial compressive strength (30–70 MPa) are tested by means of MTS machine and Split-Hopkinson pressure bar device, respectively. A three-dimensional (3D) randomly mesoscopic model for the simulation of the splitting-tensile strength and failure of CAC under different strain rates (1–200 s−1) is developed and validated by contrasting tested and numerical results. The experimental and numerical results indicate that the splitting-tensile strength and failure pattern are significantly affected by concrete strength and strain rate. The dynamic splitting failure mechanism that the damage outside the specimen is more serious than the inside, and the fracture in the center of the specimen is more severe than the edge, has been explained from the localized failure patterns of concrete and aggregates. Furthermore, it can be learned from the tensile dynamic increase factor of CAC is sensitive to strain rate significantly, which has a profound significance in the further investigation of reef CAC structures. [ABSTRACT FROM AUTHOR]

Purpose: Coral aggregate seawater concrete (CASC) is a new type of lightweight aggregate concrete that is becoming widely used in reef engineering. To investigate the corrosion behavior of different kinds of rebar in CASC exposed to simulated seawater for 0-270 d, the electrochemical techniques, including linear polarization resistance (LPR) technique and the electrochemical impedance spectroscopy (EIS), were used in the present work. Design/methodology/approach: The electrochemical techniques, including LPR technique and the EIS, were used in the present work. Findings: Based on the time-varying law of linear polarization curves, self-corrosion potential (Ecorr), polarization resistance (Rp), corrosion current density (Icorr), corrosion rate (i), and the characteristics of EIS diagrams for different types of rebar in CASC, it can be found that the anti-corrosion property of them can be ranked as epoxy resin coated steel > 2205 duplex stainless steel (2205S) > 316 L stainless steel (316 L) > organic coated steel > ordinary steel. Additionally, the linear regression equation between Rp and charge transfer resistance (Rct) was established. Finally, the EIS corrosion standard of rebar was established from the LPR corrosion standard, which provides a direct standard for the EIS technique to determine the condition of rebar in CASC. Originality/value: The linear regression equation between polarization resistance and charge transfer resistance was established. And the EIS corrosion standard of rebar was established from the LPR corrosion standard, which provides a direct standard for the EIS technique to determine the condition of rebar in CASC. [ABSTRACT FROM AUTHOR]

In this review, investigations on the product of hydration, mechanical behavior, durability and application of basic magnesium sulfate cement (BMS) have been carried out in the past seven years in China. Structural characterization of 5·1·7 phase and hydration product of BMS are introduced in the first part of this paper. The second part gives the research on the basic properties of BMS: strength, setting time, and microstructure of BMS. The third part show the durability of the cement: water resistance, chloride penetration, ability of protecting reinforcing bars and carbonization. The last part is devoted to the production and application of BMS, which indicates that basic magnesium sulfate cement has wide application potential in the construction industry. [ABSTRACT FROM AUTHOR]

Supplementary materials Such As Slag (SG), Fly Ash (FA) and Silica Fume (SF) are necessary for reducing the drying shrinkage of High-performance Concrete (HPC) in arid environments and extremely arid area. This paper investigates the drying shrinkage of HPC with the composites of 10% Silica Fume (SF) and 20% Fly Ash (FA) additions and another HPC incorporated with 10% SF?20% FA and 20% slag (SG), which were exposed to the conditions of 30% Relative Humidity (RH) and 50% RH. The inhibitive mechanism of Aluminate Expansion Agent (AEA) on the drying shrinkage of HPC reveals that the dry shrinkage can be inhibited by the expansion agent as long as the high density of concrete structure and the internal "surplus water" can be ensured. Furthermore, the additions of the composites of SF, FA and SG, super-plasticizer, high modulus fiber (PF) and AEA, can reduce the drying shrinkage and improve the moisture sensitivity of shrinkage of the concrete. The drying shrinkage rate of HPC in extremely harsh environment decreased to about 300 × 10−6 and no shrinkage cracks appeared on the surface of the HPC incorporated with SF, FA and SG. The perfect combination of high volume stability and high durability of HPC was realized. [ABSTRACT FROM AUTHOR]

Basic magnesium sulfate cement (BMS) has many advantages, such as early strength, high strength, resistance to water and corrosion resistance, and so on. In order to explore the difference in performance between BMS and conventional reinforced concrete (RC) large‐eccentric columns, the experiments of eight BMS concrete columns, including their deflection, cracking, yielding, ultimate moments, and failure modes are studied. The results show that large‐eccentric compression column of BMS concrete has 20% higher values of cracking load and the ultimate capacity compared with the conventional RC column. The curing relative humidity has no obvious effect on the eccentric compressive performance of BMS concrete columns. There is no obvious difference in the failure modes of BMS concrete columns and conventional columns under large‐eccentric compression. In addition, with the increase of the strength of BMS concrete, the cracking moment and the eccentric compressive capacity for BMS concrete columns can be improved. The general eccentric compressive theory and current code provisions for eccentric compressive column design are modified in order to be applicable to eccentric BMS concrete columns. [ABSTRACT FROM AUTHOR]

Magnesium oxysulfate cement (MOS) prepared by light-burned dolomite has excellent performance in terms of light weight, high strength and low carbon dioxide emission. In this research, dolomite ores are used to produce MOS cement at different calcination temperatures and calcination times; the parameters investigated are the rate of loss on ignition, free lime (f-CaO), dihydrate gypsum (CaSO4×2H2O) and impact of active magnesium oxide (α-MgO) content exerted on the setting time and compressive strength of the MOS cement. X-ray diffraction analysis, scanning electron microscopy, the time of colour change for citric acid and hydration-heat release rate were adopted to assess the effects of calcination temperature, calcination time and the molar ratio of α-magnesium oxide/magnesium sulfate on the MOS cement. The experimental results showed that the MOS cement prepared by dolomite ores at the calcination temperature of 850°C for 30 min with the α-magnesium oxide/magnesium sulfate molar ratio of 4 demonstrates better mechanical properties than those made in other circumstances. In addition, the main strength phase of MOS cement with tri-sodium citrate dihydrate (Na3C6H5O7·2H2O) generated by light-burned dolomite ores is the needle-like 5Mg(OH)2·MgSO4·7H2O crystal phase. [ABSTRACT FROM AUTHOR]

The properties of air-entrained concrete (AEC) subjected to freeze–thaw cycles in various salt lake brines were investigated. The relative dynamic elastic modulus (RDEM) and weight loss of the AEC were measured after different numbers of freeze–thaw cycles. In addition, the microstructure and composition of the AEC under the combined action of freeze–thaw cycles and salt lake brine attack were analysed using X-ray diffraction and scanning electron microscopy. The results showed that the RDEM and weight losses of AEC subjected to freeze–thaw cycles in salt lake brines from Tibet and Inner Mongolia decreased sharply. For AEC immersed in salt lake brines from Qinghai and Sinkiang, the RDEM was slightly reduced and the weight loss was slightly increased. The coupling effect of freeze–thaw cycles and salt lake brine thus had only a slight influence on concrete damage in Sinkiang and Qinghai regions. The X-ray diffraction and scanning electron microscopy results demonstrated that only chemical corrosion occurred in the AEC under freeze–thaw action in the Qinghai and Sinkiang salt lake brines. However, the coupled effects of water frost-heaving force, salt crystallisation and chemical corrosion rapidly led to damage to the concrete under freeze–thaw action in the Tibet and Inner Mongolia salt lake brines. [ABSTRACT FROM AUTHOR]

Purpose This paper aims to reduce the cost, limit the time and increase raw material source availability, coral aggregate seawater concrete (CASC) composed of coral, coral sand, seawater and cement can be widely used for the construction of ports, levees, airports and roads to achieve practical engineering values. However, the naturally porous coral structure and abundant Cl− in the seawater and coral lead to extremely severe reinforcement corrosion for CASC. It is well known that Cl− is the main cause of reinforcement corrosion in the marine environment. Therefore, it is necessary to research the reinforcement corrosion of CASC in the marine environment.Design/methodology/approach In this study, linear polarization resistance was adopted to test the linear polarization curves of reinforcement in CASC with different exposure times. Ecorr, Rp, Icorr and Vcorr were calculated according to the weak electrochemical polarization theory and Stern–Geary formula. The effects of concrete cover thickness, exposure time, reinforcement types and inhibitor on reinforcement corrosion in CASC were analysed. The reinforcement corrosion degradation rule was determined, which provided theoretical support for the durability improvement, security assessment, service life prediction and service quality control of CASC structures in marine islands and reef engineering.Findings The corrosion resistance was enhanced with increased concrete cover thickness, and the concrete cover thickness for organic new coated steel should be at least 5.5 cm to reduce the reinforcement corrosion risks in CASC structures. The corrosion resistance of different types of reinforcements followed the rule: 2205 duplex stainless steel > 316 stainless steel > organic new coated steel > zinc-chromium coated steel > common steel. In the early exposure stage, the anti-corrosion effectiveness of the calcium nitrate inhibitor (CN) was superior to that for the amino alcohol inhibitor (AA). With the extension of exposure time, the decreasing rate of anticorrosion effectiveness of CN was higher than that of AA.Originality/value Reinforcement corrosion of CASC in a marine environment was studied. Concrete cover thickness, exposure time, reinforcement type and inhibitor influenced the reinforcement corrosion were investigated. New technique of reinforcement anti-corrosion in marine engineering was proposed. Possible applications of CASC in marine engineering structures were suggested. [ABSTRACT FROM AUTHOR]

A new magnesium phosphate bone cement (MPBC) was prepared as a byproduct of boroncontaining magnesium oxide (B-MgO) after extracting LiCO from salt lakes. We analyzed the elementary composition of the B-MgO raw materials and the effects of calcination temperature on the performance of MPBC. The phase composition and microstructure of the B-MgO raw materials and the hydration products (KMgPO·6HO) of MPBC were analyzed by X-ray diffraction and scanning electron microscopy. The results showed that ionic impurities and the levels of toxic elements were sufficiently low in B-MgO raw materials to meet the medical requirements for MgO (Chinese Pharmacopeia, 2010 Edition) and for hydroxyapatite surgical implants (GB23101.1-2008). The temperature of B-MgO calcination had a marked influence on the hydration and hardening of MPBC pastes. Increasing calcination temperature prolonged the time required for the MPBC slurry to set, significantly decreased the hydration temperature, and prolonged the time required to reach the highest hydration temperature. However, the compressive strength of hardened MPBC did not increase with higher calcination temperatures. In the 900-1 000 °C temperature range, the hardened MPBC had a higher compressive strength. Imaging analysis suggested that the setting time and the highest hydration temperature of MPBC pastes were dependent on the size and crystal morphology of the B-MgO materials. The production and microstructure compactness of KMgPO·6HO, the main hydration product, determined the compressive strength. [ABSTRACT FROM AUTHOR]

Magnesium oxysulfate (MOS) cement is a promising material for fire-proofing and insulation applications. Here, we have studied the effects of phosphoric acid and phosphates (HPO, KHPO, KPO and KHPO) on the setting time, mechanical strength and water resistance of MOS cement. X-ray diffraction was used to examine phase composition, and analytical reagents were used to prepare samples of the new phase found so that it would be examined by chemical and thermogravimetric analyses. Adding phosphoric acid and phosphates can extend the setting time and improve the compressive strength and water resistance of MOS cement by changing the hydration process of MgO and the phase composition. A new subsulfate phase 5Mg(OH)·MgSO·7HO (517 phase) is formed, which is needle-like and insoluble in water. Phosphoric acid or phosphates ionize in solution to form HPO, HPO and/or PO, and these anions adsorb onto [Mg(OH)(HO)] to inhibit the formation of Mg(OH) and further promote the generation of a new magnesium subsulfate phase, leading to the compact structure, high mechanical strength and good water resistance of MOS cement. The improvement produced by adding phosphoric acid or phosphates to MOS cement follows the order of HPO = KHPO ≫ KHPO > KPO. [ABSTRACT FROM AUTHOR]

In order to make full use of salt lake magnesium resources and improve the strength of the thermal decomposed magnesium oxychloride cement (TDMOC), the effects of citric acid on the hydration process and mechanical properties of TDMOC was studied. The hydration heat release at initial 24 h and strengths at 3, 7, and 28 days of TDMOC specimens were conducted. The hydration products and paste microstructure were analyzed by XRD, FT-IR and SEM, respectively. The results showed that citric acid can not only reduce the 24 h hydration heat release and delay the occurring time of second peak of TDMOC, but also produce more 5Mg(OH)·MgCl·8HO and less Mg(OH) in hydration process of TDMOC. More perfect and slender crystals were observed in the microstructure of the TDMOC pastes with citric acid. The results demonstrated that citric acid as an additive of TDMOC can decrease the hydration heat release and increase the compressive strength and flexural strength of TDMOC. The possible mechanism for the strength enhancement was discussed. [ABSTRACT FROM AUTHOR]

A new magnesium oxysulfate hydrate phase, 5 Mg( OH)2· MgSO4·7 H2 O was synthesized and structurally characterized. The new phase crystallizes in the form of microsized needles and its crystal structure was solved using X-ray powder diffraction. The crystal packing is made of infinite triple MgO6 chains, intercalated with sulfate groups, water molecules, and hydroxide anions. Using vibrational spectroscopy [infrared (IR) and Raman] and Rietveld refinement, the arrangement of water molecules and hydroxide anions were observed to be statistically distributed and dynamically disordered. The high-temperature behavior and decomposition were studied by thermal analyses and in situ X-ray powder diffraction. The high-temperature measurements reveal the presence of five decomposition stages from the new phase to the production of MgO. After the reactions with atmospheric carbon dioxide were analyzed using micro-Raman spectroscopy, two new carbonate phases were detected on the surface of the material. [ABSTRACT FROM AUTHOR]

Effects of alumina-leached coal fly ash residue (ACFAR) on magnesium oxychloride (MOC) cement are comprehensively investigated. The results show that the addition of ACFAR delayed the setting time of MOC paste. The compressive strength, the flexural strength and the water resistance of MOC are improved by the incorporation of ACFAR. Furthermore, the improvement effects of ACFAR are compared with those of raw coal fly ash (RFA) on MOC cement. Scanning electron microscopy analysis of MOC cement reveals that ACFAR decreases the interspaces in MOC cement and facilitates the radial growth of 5Mg(OH)·MgCl·8HO. X-ray diffraction analysis of MOC cement with ACFAR indicates that the addition of ACFAR promotes the production of magnesium silicate, which can improve the compactness of 5Mg(OH)·MgCl·8HO and the water resistance of MOC cement. [ABSTRACT FROM AUTHOR]

By incorporation of fly ash or silica fume into magnesium oxychloride (MOC) cement, a high water resistance material can be formed for successful industrial applications. The influences of fly ash and silica fume on water-resistant property were investigated by SEM and EDS. It is found that the incorporation of fly ash or silica fume can improve the water-resistance of the MOC. The improvement of the water resistance of the MOC incorporated with fly ash or silica fume may be attributed to the alumino-silicate 5·1·8 gel or silicate 5·1·8 gel. [ABSTRACT FROM AUTHOR]