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Molybdenum tailings are accumulated in large quantities, occupying the land and polluting the environment. Therefore, the resource utilization of molybdenum tailings is imperative. In this paper, the influence of molybdenum tailings powder (MTs) as admixture on hydration characteristics, including hydration heat, hydration products, phase composition, and mechanical properties evolution of cement paste were investigated. Results showed that the early hydration of cement paste can be promoted when the content of MTs is not more than 15%. The compressive strength of cement paste with 15% MTs at 3, 28, and 120 days was 89.37%, 96.14%, and 91.68% of that of the portland cement (PC) group, respectively. When the content of MTs does not exceed 15%, the second exothermic peak of hydration heat of cement paste appears ahead of time and the peak value increases. With the increase of MTs content, the cumulative heat release of 72 h gradually decreased. The addition of MTs does not produce new hydration products, but introduces the mineral phase of MTs itself, and leads to the early transformation of ettringite (AFt) into monosulfoaluminate (AFm) in paste. There is still a close linear relationship between the compressive strength and cumulative heat release and chemically bound water content after addition of MTs into cement paste. It was also discovered that the pozzolanic activity of MTs in cement paste mainly occurred from 3 to 28 days. After 28 days, the particle-filling effect of MTs was dominant mainly. The research results are expected to provide basic research for the resource utilization of MTs. [ABSTRACT FROM AUTHOR]

To compensate for the defects of silica fume (SF), which aggravates drying shrinkage, and increase the strength and durability of cement mortar, it was modified using 1 wt% nanosilica (NS) or calcium sulfate whisker (CSW) compounded with 2 wt% SF. The strength, volume stability, and durability of the cement mortar were characterized using the compressive strength, drying shrinkage, and capillary water absorption. The physical composition and microstructure of the samples are discussed in detail based on thermogravimetry, mercury intrusion porosimetry, and scanning electron microscopy measurements. The combination of SF and NS increased the generation of C–S–H gels and reduced the total porosity, thus increasing the early strength of the cement mortar and decreasing the capillary water absorption. Notably, on combining SF and CSW, additional calcium aluminate hydrates (AFts) were generated, and the mesoporosity (10–50 nm) was reduced. The fibrous AFt phase increased the later strength of the cement mortar, and the presence of additional AFt increased the solid phase volume, which compensated for the drying shrinkage. [ABSTRACT FROM AUTHOR]

As an inorganic–organic composite material, cement and emulsified asphalt (CA) mortar has been used widely in ballastless slab track structures, and its performance is influenced significantly by environmental conditions. In this study, we tested the long-term creep of CA mortar at different temperatures using a self-designed loading device, and explored the mechanisms by which high temperatures influence CA mortar creep based on several microscopic testing methods. The results indicated that high temperatures cause the creep degree and rate of CA mortar to increase significantly. Subsequent to creep at high temperatures, the mechanical properties and microstructure of CA mortar changed remarkably, and asphalt migration may be an important cause of changes in CA mortar at high temperatures. This will be useful for future studies of the evolution of CA mortar performance under long-term service conditions. [ABSTRACT FROM AUTHOR]

The calculation of the rheological parameters of fresh cement pastes plays a key role in understanding the rheology of cement‐based mixes. Because cement paste is not a simple Bingham fluid, a suitable nonlinear model must be found for characterizing its flow. A test system in which the rotational speed or shear rate can be changed in multiple steps is regarded as a suitable rheological test protocol because the paste reaches a steady state. Furthermore, theoretical derivations show that the solution of the Couette inverse problem corresponding to the modified Bingham model and the Herschel–Bulkley (H‐B) model is complex. However, a comparative analysis revealed that the yield stress of fresh paste could easily be obtained through a calculation process based on a Parabolic model. This study presents the complete calculation procedure for this model. The influence of the plug flow is considered, and test points with low minimum shear stress (τmin) are excluded. Finally, the accuracy of the proposed method is verified through comparisons with the results obtained using mini‐cone slump tests. These results show that the dynamic yield stress calculated using the expression of the Couette inverse problem based on the Parabolic model in consideration of the plug flow is very close to the yield stress obtained using the mini‐cone slump flow test. This proves that the proposed method could precisely characterize the dynamic yield stress of cement pastes. [ABSTRACT FROM AUTHOR]

To investigate the reasons for the mitigation of drying shrinkage but earlier cracking in limestone powder cement paste, various admixtures such as fly ash, metakaolin, super-absorbent polymer, and expansion agent were selected for the above-mentioned experiments. The effects of mixing two admixtures on the drying shrinkage deformation and crack resistance performance of the limestone powder-cement paste was studied. Additionally, hydration exotherm, hydration products, microscopic morphology of samples were analyzed to understand the underlying mechanisms. Based on the hydration process, the addition of fly ash and metakaolin can promote the formation of carbon aluminate in the reaction system, thereby reducing the porosity. Fly ash is more beneficial for the hydration heat peak value of cement paste, effectively eliminating temperature strain. The results indicate that effects of mixing two admixtures on the improvement of drying shrinkage and crack resistance of limestone powder cement paste are much better than using a single admixture. Among them, the combination of metakaolin and expansion agent shows the best overall mitigating effect on the drying shrinkage and crack resistance of limestone powder cement paste. This compound admixtures reduces the drying shrinkage of the paste by approximately 30% and delays the cracking time of the specimens by 34.4 h. • The utilization of limestone powder in limestone powder cement paste can be effectively enhanced by mixing two admixtures. • Using a smaller ring size for the ring test to monitor the cracking resistance of cement paste. • The application of linear fitting revealed that the moisture loss characteristics of cement paste manifest in two stages. • Analyzing the reasons for the mitigation of drying shrinkage but earlier cracking in limestone powder cement paste. [ABSTRACT FROM AUTHOR]

Steam curing is a crucial stage for the strength development of steam-cured concrete. This study comprehensively investigated the hydration kinetics, mechanical properties, and autogenous shrinkage (AS) of cement pastes during steam curing. A hydration-dependent analytical method was proposed to analyze the mechanisms governing the development of mechanical properties and AS of steam-cured cement pastes (SCPs). Results revealed that in this investigated curing regimes, the peak hydration rate of SCPs was approximately 7 times higher than that of standard-cured pastes (NCPs). SCPs exhibited faster time-dependent mechanical property development but slower hydration-dependent mechanical property development than NCPs. This difference was primarily attributed to variations in initial structure establishment and free water content. Moreover, an elevated curing temperature reduced the growth rate of hydration-dependent AS due to large capillary pores and low surface tension, which suggested that AS should not be considered as a cause of heat damage to SCPs. [ABSTRACT FROM AUTHOR]

The nanometer materials and technology are becoming new ways for cementitious composite innovation due to the significant improvement of microstructure and mechanical performance of cement-based materials. In this study, the isothermal calorimetry was employed to measure the heat release rate and total heat release of multi-scale cement system incorporated with different nanoparticles at elevated temperatures. The nucleation and growth processes of hydration products were simulated through a kinetics model. It is discovered that the effects of nanoparticles on cement hydration depend on itself chemical reactivity and physical properties as well as ambient temperature. Both nano-SiO 2 and nano-C-S-H can obviously shorten the induction period of cement hydration and have acceleration effects obviously. Addition of 1% nano-CaCO 3 seems to have no obvious effect on cement hydration process at elevated temperatures. The acceleration effects of nano-particles mainly refer to the improvement of the nucleation rate of hydrates. Generally, the effect of nanoparticles on nucleation process is more significant than that on growth process. Nano-SiO 2 and nano-CaCO 3 have slight influence on the growth rate, but the nano-C-S-H can increase the growth rates to some extent at different temperatures. [ABSTRACT FROM AUTHOR]

• The drying shrinkage behavior of mortars under low vacuum conditions was studied. • The low vacuum increased mass loss and drying shrinkage. • The mechanisms of shrinkage under low vacuum conditions were analyzed. • Incorporation of FA and SRA reduced the drying shrinkage under low vacuum conditions. In this work, the time-dependent drying shrinkage behaviors of cement mortar under low vacuum and standard drying conditions were tested and compared, in which the specimens were prepared based on various mixtures. Also, a series of microscopic tests were conducted to investigate the hydration and microstructure of the prepared specimens, and thus, to explain the drying shrinkage behavior of different specimens. The results showed that compared with standard drying condition, the specimens under the low vacuum condition exhibited higher drying shrinkage and mass loss, but lower sensitivity of drying shrinkage to mass loss. Under both conditions, the incorporation of fly ash and silica fume or shrinkage-reducing agent could greatly decrease the drying shrinkage, with the better performance identified in the case of fly ash and shrinkage-reducing agent; while the addition of sulphoaluminate cement had no inhibiting effect on the drying shrinkage deformation. Although the specimen solely based on sulphoaluminate cement showed the lowest drying shrinkage under standard drying condition, it exhibited greater drying shrinkage than the Portland cement specimens under low vacuum conditions. The microscopic test results suggested that low vacuum treatment could decrease the hydration degree while the decomposition of ettringite in the sulphoaluminate cement could be observed under low vacuum conditions. Besides, the gel pore volume fractions decreased while the cumulative total pore volume increased after low vacuum treatment, which resulted in a loose and porous microstructure with high vulnerability to drying shrinkage. [ABSTRACT FROM AUTHOR]

• The physical effect of intense UV radiation accelerates the mass loss of the cement mortar. • The chemical effect of the intense UV radiation exacerbates the carbonation of the cement mortar. • FA and GGBS have the effect of reducing drying shrinkage under conditions of intense UV radiation. • The incorporation of FA and GGBS weakens the carbonation resistance of the cement mortar. The intense UV irradiation factor in the plateau area is easy to ignore, but it may have adverse effects on the microstructure and properties of concrete. Theredore, to clarify the evolution characteristics and the corresponding mechanism of the drying shrinkage of cement-based materials under intense UV irradiation environments at high altitudes cement mortars incorporating three types of mineral admixture were prepared to investigate the drying shrinkage, mass loss and microstructure, respectively. The hydration process and microstructure of samples were discussed in detail by microscopic experiments including TG-DTG, MIP, XRD, and IR patterns analysis. The results show that the mass loss and drying shrinkage of the cement-mineral admixture mortar exposed to intense UV irradiation were higher than those under the standard environment. Under intense UV radiation environment, the total porosity, mesoporosity (<50 nm), and carbonization degree of each group of samples increased. However, UV irradiation did not change the phase composition, chemical bond type, and polymerization degree of the C S H gel of the cement-based material. The incorporation of fly ash and slag resulted in the reduction of drying shrinkage of mortars, an increase in carbonation degree, and decreasing the hydration degree. [ABSTRACT FROM AUTHOR]

• Higher calcination temperature reduced the dissolved degree of Al in sludge ash. • S600 ash promoted the formation of Al-bearing products but inhibited the C 3 S hydration significantly. • The reaction of S800 ash with cement contributed to more hydration heat and hydration products. Sewage sludge ash has potential to be used as supplementary cementitious material in the production of building materials. The chemical effect of sewage sludge ash on early-age hydration of cement was investigated in this study. Two types of sludge ash calcined at 600 °C (S600) and 800 °C (S800) were blended with cement for analysis of hydration heat evolution, solid phase assemblage and aqueous composition. The aluminate dissolution of S600 ash promoted the formation of ettringite and consumption of gypsum, resulting in a high initial hydration heat. However, high concentrations of Al and Si caused by continuous dissolution of aluminate and silicate in S600 ash inhibited significantly the C 3 S dissolution. Interestingly, S800 ash had slight effect on early cement hydration since higher calcination temperature decreased the activity of aluminate. As compared with the reference, cumulative hydration heat of blended paste with 30% S800 ash at 7 days was increased by 18.72% indicating the occurrence of reaction between sludge ash and cement. Further study is recommended to focus on the long-term performance of cement-based materials blended with sludge ash. [ABSTRACT FROM AUTHOR]

Degradation in grade of manganese ore aggravates the complexity of electrolytic manganese residue (EMR). Calcination is one of the most practical pretreatment methods to improve EMR activity and dispose the hazardous elements. In this paper, the evolution of mineral phase, pozzolanic activity, pore structure and harmful components induced by calcining EMR was investigated. The results show that EMR calcined at 800 °C has the strength activity index (SAI) of 84.79 at 28 d, which is attributed to the decomposition of dihydrate gypsum and the formation of activated calcium, silicon and aluminum oxide. The formation of β-type hemihydrate gypsum increases the pozzolan activity, while the latter is limited by the formation of stable Mn-spinel (Mn 3 O 4) and Mn-hercynite (MnFe 2 O 4). In the EMR-doped mortar matrix, the production of a large amount of ettringite due to the existence of gypsum, as well as common C-S-H, portlandite and AFm, which strongly verify the pozzolanic activity of EMR. Leaching results show that Mn2+ and NH 4 +-N could not be eliminated completely at low temperature (<600 °C) , but could be completely stabilized in the alkaline environment provided by the cement. The Mn2+ and NH 4 +-N levels in mortar are fully below the regulatory standards when calcinated above 800 °C. All heavy metals are fixed in the cement and calcination process, ensuring the cleaner utilization of EMR in building materials. [Display omitted] • EMR calcined at 800 °C has the optimum strength activity index of 84.79 at 28-d. • Pozzolan activity of EMR is boosted in β-CaSO 4 ·0.5H 2 O and weakened in Mn 3 O 4 and MnFe 2 O 4. • Presence of gypsum in EMR retards the cement hydration and forms ettringite. • Mn2+ and NH 4 +-N below national standards at calcination above 800 °C. [ABSTRACT FROM AUTHOR]

• Physicochemical properties and pozzolanic activity of sludge ashes were investigated. • The sludge ash calcined at 800 ℃ presented high pozzolanic activity. • Sludge ash addition enhanced the formation of Al-bearing hydrates. • No leaching risk was detected for the use of sludge ash in building material. The application of aluminum-based flocculant in wastewater treatment results in a large amount of aluminum-rich sewage sludge. This work investigated the influence of calcination process on physicochemical characteristics and pozzolanic activity of aluminum (Al)-rich sludge ash and studied the effect of sludge ash on cement hydration. The results showed that higher calcination temperature from 600 ℃ to 900 ℃ increased the amorphous content in sludge ash. The pozzolanic activity of sludge ash calcined at 800 ℃ and 900 ℃ was confirmed by Frattini test. In view of strength activity index of blended mortar and energy conservation, the optimal calcination condition of sewage sludge ash was calcined at 800 ℃ with air-cooling. The addition of sludge ash promoted the transformation of ettringite to monosulfate phase in cement paste. However, the high Al concentration dissolved from S6 and S7 ash inhibited significantly the cement hydration and resulted in low compressive strength values of the blended mortars. The pozzolanic reaction of S8 and S9 ash produced more hydration heat and additional Al-bearing products such as katoite and monosulfate which contributed to the strength development of mortars. Furthermore, the heavy metals in sewage sludge can be immobilized in ash structure during calcination process and the structure of hydration products, which ensures the environmental security of sludge ash utilization in construction materials. [ABSTRACT FROM AUTHOR]

• Calcium laurate is synthesized and proves the presence of carboxylate in LA-samples. • The addition of LA and PEG4000 promotes the formation of carboxylate in specimens. • The carboxylate can be decomposed into carbonate phase at high temperature. • PCMs have almost no influence on the phase compositions of cement pastes. The application of phase change materials (PCMs) in steam-cured cement-based materials is very interesting, and a good improvement effect has been achieved in the water resistance of specimens. However, there are few studies on the interaction between PCMs and steam-cured cement-based materials. The influence of PCM, including lauric acid (LA), polyethylene glycol (PEG4000) and paraffin (P-58) on the hydration products of steam-cured cement paste were explored in this study. Calcium laurate is synthesized from lauric acid and calcium hydroxide in atmospheric water medium without additives to facilitate analysis the impact of LA on hydration products. The physical and chemical interactions occurring in the PCMs modified cement pastes were investigated using thermal gravimetric analyzer (TG) and Fourier transform infrared spectrometer (FTIR). The type and morphology of hydration products were studied using X-ray diffraction (XRD), Scanning electron microscope (SEM) and Energy Dispersive Spectroscopy (EDS). The results show that pure calcium laurate is synthesized, and calcium laurate has only low calcium content and free acid. The P-58 evaporates during the thermal decomposition of samples. The addition of P-58 delayed the decomposition of calcium hydroxide. There is no new product formed in the steam-cured cement pastes containing P-58. The addition of LA and PEG4000 into the steam-cured cement pastes promotes the formation of organic salt-carboxylate. The carboxylate can be decomposed into carbonate phase at high temperature. In the current investigated area, the addition of LA, PEG4000 or P-58 has almost no influence on the quantity of cement hydration products and phase compositions of steam-cured cement pastes. [ABSTRACT FROM AUTHOR]

• Green UHPC based on optimization of the physical and chemical effects of binders is proposed. • Nanomaterials and chemical activator proven to improve the properties of green UHPC. • The designed green UHPC shows a better performance and environmental impact. The development of Green Ultra-High Performance Concrete (GUHPC) has become a hotspot of concern in the construction industry. GUHPC with very low cement content can be prepared based on the combined optimization of the physical and chemical effects of the binder materials. On this basis, this paper proposes to prepare a new GUHPC by further enhancing chemical effects of the binder materials through the addition of nanomaterials and chemical activator. Series of experiments are designed to investigate the mechanical properties and environmental impact in respect to embodied CO 2 -emissions of GUHPC. The hydration characteristics and the pore structure of GUHPC are evaluated by employing thermogravimetry analysis and mercury intrusion porosimetry. The results show that GUHPC with good mechanical performance and very low environmental impact can be prepared with very low cement content (no more than 370 kg/m3). The high initial packing density of GUHPC can be achieved by application of multi-scale particles system, especially in the presence of nano-scale particles. Chemical activator can effectively improve the reactivity of binder materials and promote the hydration of GUHPC. The GUHPC has lower porosity and finer pore size than UHPC. [ABSTRACT FROM AUTHOR]

• The influences of FA and SL on the nucleation and growth of cement paste were studied. • FA retards the early hydration of cement at 20℃ due to its hindering nucleation. • SL has stronger seeding effect than FA because it produces more additional C-S-H gel. In this study, isothermal calorimetry and the boundary nucleation and growth model (BNG model) were adopted to investigate the influence of fly ash (FA) and slag (SL) on the nucleation and growth of cement hydrates at early-stage. Afterwards, the mechanism was discussed by analyses of instantaneous activation energy and microtopography. The results show that the hinderance of FA on the early hydration at 20℃ is possibly contributed by the inhibition of nucleation, while SL promotes nucleation at both 20℃ and 60℃. Moreover, it is found that the paste containing FA has the lowest activation energy at early-stage, while the paste containing SL has the highest activation energy, which further indicate that the nucleation of the paste containing FA is more difficult. Besides, the surface microtopography analysis of particles confirm that SL has higher reactivity than FA in paste to generates more additional C-S-H gel to provide stronger seeding effect for producing C-S-H. [ABSTRACT FROM AUTHOR]

[Display omitted] • The steam-cured cement paste has a higher porosity and pore connectivity. • The pore structure of steam-cured cement paste has a less fractal characteristic. • Steam curing process brings a microstructural damage to hydrated cement paste. Cement pastes subjected to initial steam-curing cycle have a poor later strength and durability. This adverse effect is originated from the steam-curing process. In order to investigate the microstructure changes of cement paste during steam-curing process, the XCTs with different spatial resolutions were combined used to detect the pore structure and the hydration development of cement pastes subjected to different curing regimes. The hydration heterogeneity and spatial distribution of pore network of cement pastes at various steam-curing ages were obtained on both mesoscale and microscale. The results indicate that the XCT is competent for figuring out the hydration development and of which the steam-cured cement paste is much higher than the standard-cured one at early age but has slight difference at later age. Besides, the steam-cured cement paste has a higher porosity and pore connectivity in the studied scales than the standard-cured one with the same hydration volume. Moreover, the pore structure of steam-cured cement paste has a less fractal characteristic, resulting in a microstructural damage and thus a lower fracture toughness and higher brittleness under elevated temperature steam-curing condition. [ABSTRACT FROM AUTHOR]

• Small amount of CPCM can accelerate the early hydration of steam-cured cement paste. • Both paraffin and carriers in CPCMs jointly promote the hydration of cement. • The combination of two carriers promotes cement hydration better than single carrier. • The phase compositions of steam-cured cement paste are almost not affected. The application of phase change materials (PCMs) in building materials has attracted more and more attention. However, there is little information on the interactions between PCMs and cementitious material during hydration process. This study investigates the influence of PCMs on cement hydration behavior under steam curing condition. Three kinds of composite phase change material (CPCM) are synthesized, denoted by D/P, EG/P and D/EG/P, respectively. The thermal stability and crystalline phase of PCMs are characterized. The effects of above PCMs on hydration process of cement are evaluated by several testing methods, including isothermal calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. Results indicate that CPCMs exhibit good thermal stability compared with single paraffin when the operating range is less than 200 °C. The time to reach the heat release rate peak of binders containing paraffin, D/P, EG/P and D/EG/P is 20 min, 20 min, 20 min and 30 min earlier than the cement pastes without PCM at 60 °C isothermal condition, respectively. The addition of a small amount of CPCM can accelerate the early hydration of steam-cured cement paste due to the combined action of paraffin and carriers with high surface area that can provide nucleation sites. Moreover, the combination of two carriers promotes cement hydration better than the single carrier. However, the number of hydration products and phase compositions of steam-cured cement paste are almost not affected by the addition of CPCM. [ABSTRACT FROM AUTHOR]

• The free water in HCP undergoes a three-stage decrease during steam-curing. • Most of the free water in HCP are distributed in pores less than 500 nm. • The hydration is intense within the first 4 h of steam-curing process. The presented study focuses on the water evolution and hydration characteristics of Portland cement paste under steam-curing condition. The detailed information concerning water consumption and micro-pore network as well as the corresponding hydration kinetics of hydrating cement paste (HCP) during steam-curing process were discussed based on the low-field nuclear magnetic resonance measurement. The results indicate that, within steam-curing period, most of the free water in HCP are distributed in smaller pores less than 500 nm. The water evolution rate of paste system can be divided into three stages, i.e. acceleration stage within heating period and the first 2 h of constant-temperature period, rapid low-down stages in later constant-temperature period and diffusion stage in rest period. The curing temperature predominantly controls the water evolution rate. Finally, the hydration kinetics model of paste within steam-curing period is established and verified by experiment. [ABSTRACT FROM AUTHOR]

In order to solve the problems of the sudden loss of fluidity and low expansion rate of CAM I (cement asphalt mortar type I) in a construction site with high environmental temperature, this paper studies the effect of temperature on the fluidity, expansion ratio and pH value of CAM I. The mechanism of action was analyzed by IR (infrared spectrometry), SEM (scanning electron microscopy) and other test methods. The results showed that a high temperature accelerates aluminate formation in cement paste. Aluminate adsorbs emulsifiers leading to demulsification of emulsified asphalt, and wrapped on the surface of cement particles, this causes CAM I to lose its fluidity rapidly. The aluminum powder gasification reaction is inhibited, resulting in an abnormal change in the expansion ratio. Based on findings, the application of an appropriate amount of superplasticizers can effectively improve the workability and expansion characteristics of CAM I at a high temperature. [ABSTRACT FROM AUTHOR]

Low vacuum is one of the most challenging service environments for cementitious materials, which would affect their hydration and microstructure, and thereby impair strength and durability. In this study, the hydration phases and microstructural evolution of pastes based on Portland cement (PC), sulphoaluminate cement (SAC), and tricalcium silicate (C 3 S, the clinker of PC) were investigated under low vacuum conditions. The results show that rapid mass loss could be observed at the initial stage of low vacuum treatment, which was mainly attributed to the dissipation of free water. After that, the mass loss rate was greatly slowed down and maintained at a relatively low level, which was primarily due to the loss of chemical bound water in hydration products. As a result, C 3 S samples, with the slowest early hydration procedure, exhibited the highest mass loss, which was followed by PC and SAC samples. The dissipation effect by low vacuum treatment resulted in the inhabitation of cement hydration, and correspondingly the contents of hydration products remained almost constant, or even decreased especially for ettringite (AFt). Moreover, the examination of the hydration product morphology demonstrates that under low vacuum conditions, the needle-like morphology of the hydration product C-S-H transferred gradually from divergence to convergence or underwent collapse, while the AFt crystals experienced degradation and then transformed into an amorphous state. This consequently led to the sample after low vacuum treatment exhibiting increased total volume of pores less than 100 nm and elevated percentage of pores with sizes from 50 nm to 100 nm. Overall, this research holds great scientific significance in guiding the engineering application of concrete in low vacuum environments. • Hydration and microstructure evolution of PC, SAC and C 3 S pastes were investigated. • Low vacuum inhibits hydration and even affects the chemical bound water in hydration products. • Low vacuum induces pore coarsening and increased porosity. • The crystallization of ettringite gradually degrades under low vacuum. [ABSTRACT FROM AUTHOR]

• The width of ITZ of steam-cured concrete was analyzed by microhardness. • A novel analysis method was used to obtain the microhardness distribution. • The change in ITZ observed by microhardness indicates the heat damage in steam-cured concrete. In order to understand the influencing mechanisms of curing temperature on the performances of steam cured concrete further, the microhardness characteristics of the interfacial transition zone between cement paste and coarse aggregate under different curing temperatures were studied through microhardness measurements, backscattered electronic microscopy (BSEM) and mathematical statistical analysis. The experimental results indicate that the microhardness of the hardened cement paste presents a significant statistical distribution characteristic. That is to say, there are two microhardness characteristic values which reflect the hydrated products and unhydrated cement particles, respectively. The curing temperature has a significant effect on the microhardness of interfacial transition zone. In other words, the microhardness decreases, and the thickness range of interfacial transition zone increases with increasing the curing temperature. The relationship of the microhardness value of the interfacial transition zone to the distance from the aggregate surface conforms to the Peal function. [ABSTRACT FROM AUTHOR]

Highlights • A novel porous material with high early strength is prepared at standard curing condition. • Metakaolin can improve the strength and water resistance of aerated MPC significantly. • Aerated MPC with metakaolin has a more homogeneous pore structure with more finer pores. Abstract Magnesium phosphate cement (MPC) was proved to be a cementitious material for preparing porous materials with high early-stage strength under standard curing condition. However, the poor water resistance of MPC-based porous materials limits the further application. This study employs metakaolin to improve the performance of aerated MPC. Testing results indicate that the presence of metakaolin produces more finer pores which makes the pore structure more homogeneous. For this reason, metakaolin improves both the compressive strength and water resistance with little negative impact on the thermal insulation property of aerated MPC. The metakaolin modified aerated MPC with a density of about 650 kg/m3 has 3-h strength of about 3 MPa and thermal conductivity of about 0.08 W/mK. [ABSTRACT FROM AUTHOR]

Dynamic mechanical thermo-analysis (DMTA) is an effective approach to evaluate the viscoelastic behaviour of materials. In this paper, DMTA of cement and asphalt mortar (CA mortar) with different asphalt to cement ratios (A/C) and water to cement ratios (W/C) was performed within a temperature range of − 40 to 60 °C. The results demonstrate that the storage modulus E ′ gradually decreases with increasing temperature. At the same temperature, E ′ decreases with the increase of A/C and W/C, with the decreasing trend becoming gradually more pronounced. The loss modulus E ″ of CA mortar increases initially but subsequently decreases with an increase in temperature. E ″ gradually increases with an increase in A/C and W/C at the same temperature, though the increasing trend with W/C is less pronounced. The variation of the loss factor tan δ is similar to that of E ″, and the temperature corresponding to the peak points of tan δ is slightly higher than that of E ″. [ABSTRACT FROM AUTHOR]

• A modified isothermal calorimeter was used to measure the hydration heat of DEC cement paste. • The influence of DEC excluding OH effect on fresh cement paste is small. • A theoretical model of simulating electric power per unit mass was established and is well consistent with the measured results. A modified apparatus based on isothermal calorimetry was employed to determine the influence of direct electric curing (DEC) on the hydration of cement paste other than ohmic heating effect. In addition, the effects of DEC on the microstructure of cured cement pastes were investigated by TG, XRD, NMR, SEM and MIP. The results show that the heat release of DEC samples at early-age is increased significantly due to the electrical power. However, the hydration and microstructure of DEC samples in the isothermal environment are familiar with the normal curing sample. This indicates that, other than OH effect, the impact of DEC on the hydration process and microstructure of cement is negligible. Moreover, a theoretical model for predicting the electric power evolution related to porosity, free water content, hydration degree and initial electric power per unit mass was established which is matched well with the actual measurement results. [ABSTRACT FROM AUTHOR]

• Electric curing is proposed for the rapid curing of cement at room temperature. • Suitable electrical curing regime can effectively improve the early strength of cement. • Joule heat is the key to accelerate the hydration of cement. • Electric curing consumes much less energy than steam curing. To understand the effects of direct electric curing condition on the mechanical strength and microstructure of cement paste, a series of experiments were carried out to investigate the effects of alternative current voltages and frequency on compressive strength of samples. And the corresponding microstructure of samples were also analyzed by several measurements including X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and mercury intrusion porometer. The results show that alternative current electric voltage and frequency can greatly influence the early compressive strength of a cement paste. And a suitable electric curing condition can make hardened cement paste keep a steady growth of compressive strength and excellent microstructure. The Joule heat of direct electric curing on cement paste is the main reason that accelerates the hydration and improve the early strength development of sample. This study will provide an important technical support for exploring low carbon rapid electric curing method to produce green high early strength cement concrete prefabricated element. [ABSTRACT FROM AUTHOR]

The introduction of metakaolin into magnesium phosphate cement (MPC) can improve the compressive strength, volume stability, water resistance and freeze-thaw resistance significantly. Two possible reasons were proposed in this study. Firstly, metakaolin decreases the total porosity of MPC and improves the pore structure which is directly related to the mechanical properties. Secondly, part of metakaolin can dissolve in MPC and take part in hydration, even though no new hydration products can be observed in XRD patterns. This study not only gives a compressive evaluation on the application of metakaolin in magnesium phosphate cement, but also explains the possible reason for the improvement of the properties of magnesium phosphate cement. • Strength and durability of MPC incorporating metakaolin were systematically studied. • Possible mechanism of metakaolin influencing MPC was proposed by multi-analysis. • The more homogeneous microstructure in metakaolin modified MPC can explain the property improvement. Metakaolin has been generally used as a filler in Portland cement-based materials, and it was proved to be able to enhance the performance of magnesium phosphate cement composites. In this research, metakaolin was introduced into MPC, and compressive strength, tensile bonding strength, porosity and durability of MPC were measured. The microstructure of MPC before and after water immersion was evaluated. The experimental results indicate that the presence of metakaolin can lead to higher plastic viscosity of fresh MPC mortar, 28-day compressive strength, higher tensile bonding strength, better freeze-thaw resistance, better water resistance, lower drying shrinkage and better optimized microstructure. Physical effects of metakaolin including filling effect and inducing effect play the leading role in MPC. Metakaolin particles can fill in the pores or cracks, resulting in the decrease in porosity. In addition, metakaolin particles can provide lots of sites for crystallization of hydration products, inducing in the generation of more homogeneous microstructure. Finally, the dissolution and chemical reaction of metakaolin particles were confirmed by BSE analysis. [ABSTRACT FROM AUTHOR]

• Metakaolin improves the mechanical properties and durability of MPC. • Metakaolin decreases the early-term hydration heat of MPC and the total porosity. • Mechanisms of metakaolin in MPC include providing nucleation cites, filling effect and pozzolanic effect. Different particle sizes of metakaolin were employed to replace magnesia partially, and it goes MK1, MK2 and MK3 in order of the specific surface area. Experimental results indicate that the presence of metakaolin indeed improves the compressive strength, tensile bonding strength and water resistance of MPC mortar through sacrificing the workability to some extent, and 30% MK2 blended MPC mortar has the best performances except fluidity. Metakaolin decreases the porosity and improves the pore structure of MPC mortar, especially for specimens after water immersion. The dissolution and reaction of metakaolin in MPC are confirmed by BSE analysis. Nevertheless, no new hydration products can be detected in MPC paste with metakaolin through XRD and TG-DTG analysis. The condensation of intermediate products, crystallization and crystal growth occurring on the surface of metakaolin promote the formation of the more compacted and homogeneous microstructure, which is the physical mechanism of metakaolin improving the properties of MPC. [ABSTRACT FROM AUTHOR]