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• 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]

• 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]

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]

• 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]

• 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]