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The mechanical properties of concrete are the important parameters in a design code. The amount of laboratory trial batches and experiments required to produce useful design data can be decreased by using robust prediction models for the mechanical properties of concrete, which can save time and money. Portland cement is frequently substituted with metakaolin (MK) because of its technical and environmental advantages. In this study, three mechanical properties of concrete with MK, i.e., compressive strength ( f ′ c ), splitting tensile strength ( f st ), and flexural strength (FS) were modelled by using four machine learning (ML) techniques: gene expression programming (GEP), artificial neural network (ANN), M5P model tree algorithm, and random forest (RF). For this purpose, a comprehensive database containing detail of concrete mixture proportions and values of f ′ c , f st , and FS at different ages was gathered from peer-reviewed published documents. Various statistical metrics were used to compare the predictive and generalization capability of the ML techniques. The comparative study of ML techniques revealed that RF has better predictive and generalization capability as compared with GEP, ANN, and M5P model tree algorithm. Moreover, the sensitivity and parametric analysis (PA) was carried out. The PA showed that the most suitable proportions of MK as partial cement replacement were 10% for FS and 15% for both f ′ c and f st . [ABSTRACT FROM AUTHOR]

It has been long recognized that low air pressure (AP) affects the moisture transfer in concrete, and thus its microstructure and bulk property, but less is known about the thermodynamic mechanism. This research investigated the microstructure evolution of young white Portland cement paste during wetting or drying at different APs (i.e. 101 kPa, 60 kPa, 20 kPa), the experimental and numerical analyses reveal the moisture transfer changes at low APs, which matters for the microstructural features of hardened cement paste. It was found that low APs increase the water vapor diffusion and convective mass transfer coefficients, and the intrinsic permeability of cement paste sample, which results in higher water contents within cement paste during wetting at 98 % RH but more rapid water losses during drying at 43 % RH. Owing to the distinct moisture transfer behaviors, the C-S-H structures were linked with water contents within confined pores. [ABSTRACT FROM AUTHOR]

Air entrainment in cement-based materials is believed to be affected by low air pressure (AP) which is the atmospheric characteristic in high plateaus, and thus the rheology may be affected. This paper attempts to study rheology and air entrainment of fresh Portland cement mortar in simulated low APs. The results indicate that the rheological responses with the varying AP gradients are relevant to the air entrainments in fresh mortar mixtures. The volume changes of air voids in fresh mortars are linearly related to the varying APs, which is owing to not only the AP difference between the inside and outside air voids but also the additional pressure on the liquid films of air voids stemming from the surface tension changes. Moreover, the Bingham capillary number is calculated to account for the air void deformability. Then rheological parameters are correlated to the content and deformability of air voids in fresh mortars. [ABSTRACT FROM AUTHOR]

This paper elucidated the temperature dependence of cement setting by comparing the storage modulus (Gʹ) evolutions under different temperatures ranging from 10 to 55 °C. The temperature-induced changes in cement hydration and colloidal interaction were also explored to reveal the underlying physio-chemical effects. It was found that the increase in temperature significantly accelerates the growth of Gʹ in all the stages of setting process, which mainly arises from the promotion of forming "calcium silicate hydrates (C–S–H) bridges" and compressing the electrical double layer of particles. For the same hydration degree, the rise of temperature leads to a greater Gʹ of cement paste. It was attributed to the diminution of the electrostatic repulsive force between particles caused by not only the direct effect of rising temperature but also the higher SO 4 2-/Ca2+ concentration ratio of interstitial solution in the paste cured at the higher temperature. [ABSTRACT FROM AUTHOR]

AC impedance spectroscopy (ACIS) is a promising technique for monitoring the microstructure evolution of fresh cement paste in real-time. This paper compared the change of bulk electric resistivity (ρ bulk) obtained from ACIS with the developments of storage modulus (G′) and the mean transverse relaxation times (T 2) of fresh Portland cement pastes within 5 h. It was found that the three different phases (Phase I, II, and III) on the microstructural build-up process of fresh paste can be accurately distinguished by analyzing ρ bulk , as well as G′ and T 2. The use of ρ bulk fails to characterize the microstructural changes in Phase I due to the great sensitivity on the electrical conductivity of the interstitial solution. However, it can successfully reflect the developing features of microstructure in Phases II and III, and reliably evaluate the impacts of water to cement ratio, superplasticizer, and supplementary cementitious materials on the microstructural development. [ABSTRACT FROM AUTHOR]

• The rheological properties of LHC pastes with MEA and SRA were studied. • The variations of dynamic yield stress and plastic viscosity were estimated by response surface methodology. • The roles of MEA and SRA on the static yield stress development of LHC pastes were investigated. The combined use of low heat Portland cement (LHC), MgO-based expansive agent (MEA) and shrinkage reducing admixture (SRA) is beneficial to reduce the cracking risk of concrete. In this study, the effects of MEA and SRA on the rheological behaviour of LHC paste were investigated using dynamic and static shearing tests. The response surface methodology was used to estimate the effects of MEA, SRA, and superplasticizer on dynamic rheological parameters, while the zeta potential, calorimetric, and solid phases tests were conducted to explore the mechanisms of time-dependent rheological behaviour. Results indicate that MEA contributes to higher dynamic yield stress and plastic viscosity, while the effect of SRA is dependent on its dosage. MEA promotes the static yield stress development for accelerating the hydration of blends and the formation of Mg(OH) 2. SRA retards the hydration of LHC and blended paste and reduces the number of main hydration products. However, the static yield stress is further increased by SRA, showing a consistent changing trend with the surface area of hydrated particles. [ABSTRACT FROM AUTHOR]

To investigate the synergistic effects of steel slag (SS) and granulated blast furnace slag (GBFS) on cementitious systems during carbon dioxide (CO 2) mineralization curing. Ternary paste specimens containing SS, GBFS, and Portland cement (PC) were prepared to analyze the influence of drying pre-treatment time and SS–GBFS mixing ratios on mechanical properties, CO 2 fixation rate, and microstructure evolution. Results emphasize the significance of appropriate drying pre-treatment in enhancing mineralization degree and compressive strength. At 60 °C drying pre-treatment temperature, extending drying pre-treatment time up to 12 h enhances both compressive strength and CO 2 fixation rate. However, beyond 12 h, a decline is observed in both parameters. Moreover, under the optimal drying pre-treatment time, the compressive strength of ternary paste specimens post-mineralization increases with higher SS content, with SS exerting a more pronounced positive effect on CO 2 fixation rate and compressive strength compared to GBFS. After 4 h of mineralization curing, specimens with 50% SS and 10% GBFS demonstrate a 13.16% increase in compressive strength and a 9.16% increase in CO 2 fixation rate compared to specimens with 10% SS and 50% GBFS. Microscopic test results reveal that the primary crystalline product in ternary paste specimens after mineralization is calcite, with traces of aragonite. The SS–GBFS mixing ratio has minimal effect on the type of reaction products but has a predominant influence on the total content and crystallinity of calcarea carbonica. • The increase of steel slag content can improve the degree of mineralization curing. • Increasing blast furnace slag reduces calcite crystallinity. • Steel slag boosts carbon fixation more than blast furnace slag. [ABSTRACT FROM AUTHOR]

Aging of Portland cement, due to its reaction with moisture and atmospheric CO 2 prior to regular use, has the potential to severely impair its hydration reactivity. The effectiveness of using metakaolin to improve the properties of aged Portland cement exposed to a laboratory conditions (RH=65 %, 20 °C) as well as the associated mechanism are studied. The incorporation of 5–10 % metakaolin changes the early-stage hydration kinetics of aged cement obviously by enhancing reactions of both C 3 S and C 3 A through the filler effect at the early hydration stage, thereby mitigating detrimental influences of aging on setting time and early-age strength development (up to 7 days). Metakaolin exhibits significant effectiveness on enhancing 28-day strength of aged cement, and the enhancement arises from the synergic reaction between metakaolin and CaCO 3 , which converts CaCO 3 resulting from aging into carboaluminate phases and produces secondary C-S-H. The incorporation of 5 % metakaolin increases the 28-day compressive strength of the sample with cement aged for one month to a comparable level as that of fresh cement. • Aging leads to relaxation and reconstruction of the cement surface can hinder the activity of the cement. • The filler effect caused by metakaolin promotes the reaction of C 3 S and C 3 A in aged cement. • The synergistic reaction between metakaolin and CaCO 3 formed in aged cement improves the mechanical property. [ABSTRACT FROM AUTHOR]

This study aims to formulate solid and aqueous compositions of hydrated Portland cement incorporated with reactive alumina (RA) via thermodynamic modeling, with the goal to prevent alkali–silica reaction (ASR). Based on SEM/EDS point analysis, the uptake of Al into C–S–H was corrected in the thermodynamic modeling to improve the accuracy. The predicted solid and aqueous compositions were validated by means of XRD, TGA, and ICP. An accelerated mortar bar test was also carried out to confirm the ASR mitigating efficiency, and a preliminary relationship among the ASR expansion, solid assemblage, and aqueous species was established. The critical content for RA is around 5.75% in the studied systems, above which katoite precipitates, resulting in increased Al concentration in the pore solution (∼0.8 mmol/L); hence, ASR can be effectively inhibited. [ABSTRACT FROM AUTHOR]

• The effect of roughness on bonding performance between old OPC concrete and new MPC concrete was studied. • Fractal dimension was used to define the roughness of the bonding surface. • Roughness has significant influence on the bonding strength and microstructure of interface between new and old concretes. This paper investigated the bonding performance between magnesium phosphate cement concrete (MPC) and ordinary portland cement concrete (OPC) with different roughness. The bonding interface was characterized using fractal dimension, backscattered electronic microscopy, and microhardness. The optimal mixture proportion of MPC is selected with the mass ratio of MgO-KH 2 PO 4 (M/P) by 3:1 and Na 2 B 4 O 7 ·10H 2 O-MgO (B/M) by 40%, which has the compressive strength of 52.8 MPa and the initial setting time of 24 min 20 s. It is found that the roughness indicated by fractal dimension has a positive correlation to rough depth and the bonding performance. The bonding interface can be categorized into two forms that either MPC paste bonds to OPC paste or bonds to the aggregate. Micro-analysis shows that the former has denser hydration products and lower porosity, providing better chemical bonding and the latter contributes to the mechanical interlocking which is the major form of the bonding force. MPC is a potential material for the field application of wet joints of the floating slab tracks. [ABSTRACT FROM AUTHOR]

• Carbonation of steel slag results in the formation of CaCO 3 barrier which blocks the reaction of silica gel. • The incorporation of metakaolin enhances the reactivity of steel slag by promoting the reaction of CaCO 3 and formed silica gel. • The coupled substitution of carbonated steel slag and metakaolin leads to significant improvement of strength and water absorption. This study aims to promote the reactivity of basic oxygen furnace slag (BOFS) through accelerated carbonation and the combined use of metakaolin (MK). Influences of the incorporation of the combination of carbonated BOFS and MK at various cement substitution levels on strength development, water absorption, phase composition, and microstructure are investigated. It is revealed that the coupled substitution with carbonated BOFS and MK improved the formation of carboaluminate phases and stabilized ettringite owing to the synergic reaction between metakaolin and CaCO 3 derived from the carbonation of BOFS; and the consumption of CaCO 3 facilitates the pozzolanic reaction of the amorphous SiO 2 gel formed during carbonation, generating more hydration products. All these reactions contribute to improved impermeability and enhanced strength development; 45% of ordinary Portland cement (OPC) can be replaced while achieving a comparable strength to sample consisting of 100% OPC. In addition, compared to the sample incorporated with uncarbonated BOFS and MK, the compressive strength of the sample blended with 30% carbonated BOFS and 15% MK was improved by 32.5% at the age of 56d; meanwhile, the water absorption during 72 h was lowered by 67.7%. In terms of the environmental impact, the CO 2 emission and intensity for sample incorporated with 30% carbonated BOFS and 15% MK only account for 59% and 62% of those in sample comprised of 100% OPC. The study proposes an approach to improve the reactivity of BOFS efficiently and sequestrate a significant amount of CO 2 simultaneously. [ABSTRACT FROM AUTHOR]

[Display omitted] • The role of CGS on the workability of cement paste with and without PCE was investigated. • The decrease of interparticle force leads to the improvement of fluidity when added CGS. • The aggregation of PCE in pore solution leads to its efficiency decrease in the cement pastes blended with CGS. Coal gasification slag (CGS) is the main by-product of coal gasification technology, and the utilization of CGS as mineral admixture is environmentally sound. In this paper, the effect and mechanisms of CGS on the workability (i.e., fluidity and rheological behaviors) of ordinary Portland cement (OPC) paste with and without polycarboxylate superplasticizer (PCE) were studied. The zeta potential and surface free energies of OPC and CGS were tested to calculate the electrostatic force and Van der Waals force variations due to the addition of CGS particles. The results revealed that CGS mainly decreases the van der Waals attraction force between binder particles, enhancing the fluidity and reducing cement pastes' yield stress and plastic viscosity. When blended with PCE, the dissolution of CGS mainly increased the ionic concentrations of Si(OH) x (4-x), which promoted the aggregation of PCE and decreased its adsorption, thus decreasing the fluidity and increasing the rheological parameters. [ABSTRACT FROM AUTHOR]

• Application of steel slag in cementitious materials based on dense packing can achieve high utilization efficiency of both steel slag and cement. • Early age hydration kinetics, fluidity of blends, and properties including strength and water absorption are investigated on paste specimens. • The appropriate amount of gypsum addition can enhance long-term strength through improving the early-age hydration kinetics SS-SPC cementitious systems. • The findings of this research help to valorize steel slag utilization in cement-based materials without sacrificing mechanical property and durability. Steel slag is an industrial solid waste with latent hydration nature, possessing a high potential use in cement and concrete but has not been fully utilized yet. This study reveals an implication approach based on dense packing to achieve high utilization efficiency of both steel slag and cement. Steel slag-based reverse filling approach utilizes higher volume of relatively coarser steel slag particles as micro fillers and a smaller quantity of fine cement grains as the major binding component. Thus, giving an initial compact packing structure and an enhanced strength of the resulting materials. Based on the packing density of various mixes, steel slag-based reverse filling cementitious materials are formulated. Early age hydration kinetics, fluidity of fresh pastes, and properties including mechanical strength and water absorption are further investigated on paste specimens. Results indicated that a maximum packing density of 0.6956 was attained for the blend containing 85 % steel slag with 15 % superfine Portland cement. The compressive strength of formulated mixes reached 96.4 MPa, 105.6 MPa and 123.8 MPa at 180 days for pastes containing 15 %, 25 % and 35 % of superfine Portland cement, respectively. The addition of appropriate amount of gypsum can enhance long-term strength by 8 ∼ 18 MPa through improving the early-age hydration kinetics of the steel slag based cementitious systems. The findings of this research help to valorize steel slag utilization in cement-based materials without sacrificing mechanical property and durability. [ABSTRACT FROM AUTHOR]

To understand the effect of elevated Al concentration on early hydration in Portland cement, hydration heat, the evolution of solid and aqueous composition in paste blended with reactive alumina were investigated in this study. The increase of Al concentration depletes gypsum rapidly with massive precipitation of ettringite during the pre-induction period, resulting in decreased Ca and S concentration in pore solution. The elevated Al concentration passivated the active sites on C 3 S surface, and the decreased Ca concentration inhibited the nucleation of C–S–H and precipitation of portlandite, hence prolonging the induction period. Meanwhile, the reaction of C 3 A has not been accelerated even after the complete depletion of gypsum, which is associated with the lowered undersaturation degree caused by the increased Al concentration. Additionally, the precipitation of large amounts of ettringite and foil-shaped C–S–H reduced available nucleation sites, thus hindering the reaction of both C 3 S and C 3 A during the acceleration period. [ABSTRACT FROM AUTHOR]

The incorporation of steel slag usually leads to retardation on the early-age hydration of cement, which limits its use as supplementary cementitious material in cement or concrete. Aiming to a better understanding of the retarding mechanism, the influences of two representative basic oxygen furnace steel slags on setting behaviour, hydration kinetics, aqueous and solid composition during early age were investigated. Different from the existing mechanisms, this study highlights the role of aluminate phase contained in steel slags in the retardation of hydration. The incorporation of steel slag with high aluminate phases content reduces C 3 S/aluminate phase ratio significantly. This reduction in C 3 S/aluminate phase ratio leads to undersulfation in the system, where aluminate phases react prior to C 3 S, resulting in flash setting, reduced C 3 S reaction, and thus the hindered early-age strength gain. This study also proposes an efficient way to alleviate the retarding effect of steel slag on early-age hydration by increasing the sulfate supply, which allows the system to react in a properly sulfated manner. [ABSTRACT FROM AUTHOR]

• SCE method is a convenient, reliable and environmental friendly method. • SCE method can determine sulfate ion content in CSA cement based materials. • SCE method is improper for PO cement due to sulfate ion could be trapped by C-S-H. The penetration of sulfate ions into the concrete often causes durability problems such as spalling, cracking, and strength loss. Traditional methods for sulfates content test in cement-based materials, a large number of toxic and high-risk chemical reagents such as hydrochloric acid, barium chloride, barium chromate are used. In this research, the sodium carbonate extraction (SCE) method was used to determine the sulfates content in Ordinary Portland cement (OPC) and Calcium sulphoaluminate (CSA) cement while the gravimetric method was used as standard method to evaluate the accuracy of SCE method. The optimum SCE method operation conditions were determined as 0.30 mol/L sodium carbonate solution, liquid–solid ratio at 100:1 and particle size smaller than 500 μm. The results indicated that the replacement of gravimetric method with SCE method for sulfates content test is acceptable in CSA cement but improper for Portland cement system. Through the FTIR, XRD, and TG-DSC analysis, it was found that the absorption of sulfate ions by C-S-H gels was responsible for the failure of SCE method in Portland cement system. [ABSTRACT FROM AUTHOR]

To improve the environmental efficiency of the reverse filling system, three strategies aim to optimize the packing density, and the mechanical property were adopted in this study. Based on the compressive packing model (CPM), the relationship between the D50 ratio and maximum theoretical packing density for a reverse filling system with 25% and 30% superfine Portland cement was established. For comparison, silica fume and steel fiber were also added to the reverse filling system, respectively. The improvement of packing density by adjusting the D50 ratio was verified through the minimum water demand method, CPM, and modified Andreasen and Andersen (MAA) model. Compared to the reverse filling system added with 3 wt % silica fume, which possesses a comparable mechanical property with the optimized group (adjusted D50 ratio), the incorporation of steel fiber shows a more significant increase. The environmental efficiency of all the samples was quantified into five aspects through the calculation based on the mix proportion, compressive strength, and hydration degree. The comprehensive evaluation demonstrated that the optimized reverse filling system exerts a lower environmental impact and possesses a much higher cement use efficiency compared to the majority of ultra-high performance concrete (UHPC)/ ultra-high performance fiber-reinforced concrete (UHPFRC) reported in published papers. [ABSTRACT FROM AUTHOR]

In this study, the sulfate attack on uncarbonated cement paste partially exposed to Na2SO4 solution was experimentally investigated and compared with that on carbonated specimens with the same exposure regime and uncarbonated specimens without exposure. N2 was used to protect specimens from carbonation throughout the sulfate exposure period. The effects of the water-to-cement (w/c) ratio and the fly ash as cement replacement on the sulfate attack were evaluated. Portland cement paste specimens with different w/c ratios of 0.35, 0.45, and 0.55 or fly ash replacement rates of 10%, 20%, and 30% were prepared. These specimens were partially immersed in 5% Na2SO4 solution for 50 d and 100 d exposure periods. The micro-analysis was conducted to evaluate the effect of the partial sulfate attack on the uncarbonated cement paste using X-ray diffraction (XRD) and thermo-gravimetric (TG) techniques. The results confirmed that, for uncarbonated cement paste, the chemical attack rather than the physical attack is the deterioration mechanism and is responsible for more severe damage in the evaporation zone (dry part) compared with the immersed zone (immersed part). When the effect of carbonation is well excluded, there is an optimal w/c ratio of 0.45 for minimizing the sulfate attack, while incorporating fly ash tends to reduce the sulfate attack resistance. [ABSTRACT FROM AUTHOR]