Viscoelasticity of reactive magnesia cement (RMC) pastes containing 3 different hydration agents (HCl, Mg(CH3COO)2 and MgCl2) were investigated. Amplitude sweep, frequency sweep and time sweep of RMC pastes were examined within 3 h of hydration. Time-dependent evolution of storage modulus, loss modulus, phase angle, and shear stress were recorded. Measurements of pH, isothermal calorimetry, XRD, TG-DTG and FTIR were used to analyze hydration reaction and products. Addition of hydration agents (HAs) accelerated the growth rate of storage modulus/loss modulus over time. MgCl2 demonstrated the greatest acceleration influence, also reflected in non-destructive structural build-up and buildability related to 3D printing applications. Addition of MgCl2 and HCl advanced the initial setting time of RMC pastes to 100–110 min, during which yield stress reached maximum, and decreased afterwards. Within 3 h of hydration, pastes containing MgCl2 revealed lowest pH, highest heat release and brucite concentration. HAs inclusion precipitated brucite away from MgO particles in the bulk solution, creating a bridge between MgO particles and enabling denser microscopic network structure.

This study investigates the microstructural changes of cement paste due to the inclusion of polymeric microfiber at different water-to-cement (w/c) ratios. A procedure to quantify the porosity of epoxy impregnated interfacial transition zone (ITZ) is also presented. Results show that the microstructures of the ITZ beneath and above a microfiber, with respect to the gravity direction, are largely different. Though the ITZ at both sides of the fiber are more porous than the bulk matrix, the porosity of the lower ITZ (i.e., the ITZ beneath a fiber) is significantly higher than the upper side (i.e., the ITZ above a fiber). This difference can be attributed to the combined effects of fiber on the initial packing of surrounding cement grains and on the settlement of the fresh mixture. The porosity gradients of the upper ITZs are found to be nearly identical for all the tested w/c ratios, while the porosity gradients of the lower ITZs become steeper when the w/c is higher. The lower side is also found to be the preferred location for the precipitation of calcium hydroxide crystals. Results of energy-dispersive X-ray spectroscopy (EDS) and nano-indentation analyses confirm that the chemical and mechanical properties of the ITZ are also asymmetric.

In the present work, the synergetic effect of fly ash (FA) and ground granulated blast furnace slag (GGBS) on the early compressive strength and microstructure development of CO2-cured mortars was investigated. A rim of several micrometers was found around carbonated cement particles, which contained not only silica-rich gel but also crystal calcium carbonate. The calcium carbonate formed around the cement particles were surrounded by an amorphous layer of around 3 nm, while no layers were observed around the calcite formed on FA or GGBS particles. The calcite formed on FA particles were hexagonal plate shaped, while the one on GGBS particles were rhombohedral shaped. The use of FA resulted in the increase of crystal size and crystallinity of calcite, while GGBS decreased the crystal size of calcite. The incorporation of FA and GGBS increased the calcite content and polymerization of silica-rich gel. However, this didn't result in a higher compressive strength. This was due to the looser microstructure and nanopores within the carbonation products compared to the pure ordinary Portland cement sample. The compressive strength of the ternary binder system showed linear relationship with capillary pores and the crystal size of calcite. Moreover, the effect of GGBS on the compressive strength reduction was more obvious than that of FA.

This study aims to investigate the effects of ferronickel slag (FNS) on the hydration heat, mechanical strength and microstructure of alkali-activated slag (AAS). Sodium hydroxide (SH) and sodium silicate (SS) were used as alkali activators to prepare various alkali-activated slag-FNS cements (AASF). The incorporation of FNS has no significant effect on the reaction process of the SH-activated AASF system while delaying the reaction process of the SS-activated AASF system. The influence mechanism of FNS on the reaction process of AASF is unaffected by variations in alkalinity and modulus of the alkali activators. The delay effect of FNS on the SS-activated AASF system is diminished at 60 °C. The reaction degree of FNS at a later age is significantly higher than that at an early age. Mg in FNS participates in the formation of Mg4Al2(OH)14·3H2O and gel products. The reaction degree of FNS increases with the rise of FNS content in the SH-activated AASF system but has no relationship with the FNS content in the SS-activated AASF system.

Cemented paste backfill (CPB) is an efficient and sustainable technology for the disposal of tailings in mining industry. However, the flowability of CPB is not completely clear due to the lack of robust characterization methods and sufficient studies. In this study, the effects of pre-shear rate and time, resting time, and shear protocol on the rheological measurements of fresh CPB were examined. Additionally, a yielding model was built, and the 2-h structural evolution was investigated. The structural evolution of CPB was observed using the oscillatory shear rheology method. The underlying mechanisms were investigated using zeta potential, thermogravimetry, ionic concentration, focused beam reflectance measurement, and 1H nuclear magnetic resonance. The pre-shear rate of 100 s−1, pre-shear time of 100 s, pre-shear resting period of 150 s and the stress ramp-up protocol were all recommended by the results. The dual yielding properties of CPB are adequately represented by the proposed model. It is possible to categorize the structural evolution of CPB at rest into three stages, each of which can be attributed to flocculation brought on by particle interactions, packing density enhancement from the massive formation of rigid links, and an increase in the volume fraction of solid, respectively.

Dynamic cyclic loading commonly encountered in engineering practice can pose great threat to the integrity and durability of concrete infrastructures. Ductile materials with enhanced self-healing performance could provide a promising solution under such conditions. This study proposes and investigates, for the first time, as self-healing SHCC (SH2CC) for dynamic reversed-cyclic (DRC) applications through incorporating a mineral substitute (MS) comprising of reactive MgO and quicklime and triethanolamine (TEA) additive in a high-volume-fly-ash matrix. Different DRC preloads were considered. Results indicated that applying 5% MS combined with 1.5% TEA additive showed optimal crack width control below 12 μm under up to 5 cycles of DRC preloading and the highest potential for mechanical strengthening of modulus of rupture, first cracking strength and flexural stiffness reaching up to 160%, 137% and 210% respectively after healing. Moreover the highest crack sealing efficiency was observed up to 88% under 14d water immersion. Microstructural analysis revealed that MS produced additional hydrates and promoted more carbonates to form in cracks during healing, while TEA effectively paused hydration under air curing but resumed the process upon water contact for massively enhancing crack healing (especially mechanical strengthening). Combining MS and TEA in SH2CC produced complementary healing products both in cracks and fibre-matrix interfaces indicating better filling and bridging of the cracks and demonstrating great potential for enhancing healing performance under aggressive loading conditions.

Micro-defects in UHPFRC, inevitably generated from the manufacturing to engineering service stage, impact its durability under extreme service environments. However, relevant understanding is still insufficient. This work assesses the corrosion risk and corrosion-induced deterioration in UHPFRC containing initial micro-defects, simulated by a combination of mechanical pre-loading and thermal treatment. Analytical analyses include electrochemical tests (OCP, Tafel, EIS), SEM, MIP, compressive strength measurements, etc. Results show that initial defect degree and steel fiber contents have significant effects on the corrosion resistance and mechanical performance of UHPFRC. Micro-cracks and pores are the major channels to deepen fiber corrosion risk, degrading mechanical performance up to 52%-56% in the most severely damaged UHPFRC. The porosity is increased by the corrosion/increased defects and fiber contents up to a growth rate of 35%, 56% and 78%, respectively, as corrosion triggers the occurrence of new defects (e.g., fiber splitting, newborn micro-cracks, pores). The present results provide a reference for predicting the corrosion potential of the defective UHPFRC.

This work investigates the effect of alternating current (AC) curing and ambient temperature curing on the hardening process of metakaolin-based geopolymer (MKG). Two processes were related to the conductive behavior in the hardening process: the slow process (the linear tail near the low frequency end) and the fast process (the typical hemi-circle near the high frequency). A thermo-electric model based on Arrhenius equation was presented and the experimental results were in good agreement with the calculated values. In addition, this article presented a new explanation of the coagulation mechanisms underlying the hardening behavior of electro-curing geopolymers: fast-coagulation under high voltages and slow-coagulation under low voltages exhibit significantly different effects on pore structure.

Spent fluidized catalytic cracking catalysts (SC) are considered toxic solid waste due to it is contained heavy metal ions, which can be polluting for the environment and harmful to humans. Since conventional disposal methods can be harmful to both the environment and people, therefore, new disposal methods need to be developed. In this study, 80 kg/m3 and 150 kg/m3 of SC are incorporated into the design of high-density cement-based materials (HDCM). The mechanical properties, hydration kinetics, durability, and environmental characteristics of HDCM containing SC are evaluated. The results shown that the HDCM has good mechanical properties, with a 6.62% increase in compressive strength at 28 days for HDCM containing 80 kg/m3 of SC. Furthermore, the HDCM containing SC has better long-term volumetric stability, with a 44.55% reduction in dry shrinkage compared to the reference. Further, the HDCM has a good immobilizing ability for heavy metal ions. The leaching concentrations of Ni and V from C1 are only 0.203 mg/L and 0.282 mg/L, and the immobilizing rate is 99.08% and 98.26%, respectively. For the C2, the leaching concentrations of Ni and V are 0.198 mg/L and 0.402 mg/L, the immobilizing rate is 99.31% and 98.25%.

This paper presents a systematic experimental study on the fracture behavior of low-carbon mortars with various contents of carbonated recycled sand and recycled powder (CRS and CRP). Three-point bending tests on pre-notched beams were performed to explore the load-induced fracture processes. Based on the strain and displacement fields measured using the digital image correlation technique, the evolution of the fracture process zone (FPZ) was studied qualitatively and quantitatively. Results indicated that the initial cracking toughness (KIcini), unstable fracture toughness (KIcun) and fracture energy (Gf) of mortar reduced gradually with the increasing recycled sand (RS) content but kept relatively steady with the rising recycled powder (RP) dosage except for KIcini. Carbonation treatment could effectively improve the KIcini of mortar with low RP content and Gf with a high proportion of RS but had limited effect on KIcun. A combination of the FPZ evolution characteristics and crack growth resistance curves suggested that the use of RS would increase microcracking in the stable crack growth stage and thus facilitate the connectivity of microcracks in the unstable fracture stage, while the use of RP mainly affected the crack initiation stage of cement paste in the mortar. The improvement of mortar fracture by CO2 treatment of RS or RP also corresponded to these stages. Regarding fracture characteristics and carbon footprint, mortars with CRS and CRP could achieve a synergy of safety and low carbon and thus can be considered as low-carbon cementitious materials compared to normal mortar.

Dynamic shear properties of concrete could influence the blast resistance and penetration properties of concrete structures. Compared with the dynamic compressive and tensile characteristics, the dynamic shear properties of concrete are far from being well studied. In this study, dynamic push-off tests under six loading velocities were carried out on concrete specimens with two strength grades to qualify their dynamic shear properties. The process of concrete failure was recorded by a high-speed camera, and the deformation of the specimen was obtained by the digital image correlation (DIC) technique. The test results show that the shear properties of concrete have remarkable loading rate sensitivity, and the dynamic shear strength can be increased even 10 times at high loading rates. The rate sensitivity of concrete under shear is higher than that under compression, but lower than tension if the stress rate is below about 2 × 105 MPa/s. The mechanical background of the rate effect on concrete shear strength is revealed through testing results and theoretical analysis according to the different failure modes of the specimen under different loading rates. Based on experimental results, empirical equations of the dynamic increase factors (DIFs) for the shear strength, the shear slip, and the shear slip stiffness are proposed to predict the dynamic shear properties of concrete.

The energy-saving concrete was developed by incorporating n-octadecane and diatomite composite shape-stabilized phase change materials (OC/D SSPCMs) into cement concrete. OC/D SSPCMs were prepared by vacuum impregnation of barrel diatomite. The results show that the melting point and enthalpy of OC/D SSPCMs are 25.02 °C and 60.26 J/g, and OC/D SSPCMs have good dispersion and compatibility in cement paste. As a structure-function integrated material, energy-saving concrete with compressive strength of 33.5 MPa has a broad application prospect. Self-designed room test results reveal that energy-saving concrete can reduce the indoor temperature by 4.5 °C. Long-term thermal/mechanical properties, volume deformation and microstructure after thermal cycling treatment were investigated. The results indicate that phase transformation behavior has a huge negative effect on the compressive strength and deformation tendency of energy-saving concrete, being mainly attributed to the significant volume evolution with the temperature of PCM and PCM encapsulation leakage.

To alleviate the storage of red mud (RM) and the resulting pollution, this study presented an encapsulation-based strategy for the safe management of RM, and adopted ultra-high performance concrete (UHPC) as an encapsulating material due to its superior impermeability and chemisorption properties. It is noteworthy to find that encapsulated material suffered from a selective chemisorption and density functional theory (DFT) calculations were innovatively employed to investigate the selective chemisorption mechanism. The simulation results suggested that distinct bonding forms lead to the selective chemisorption of heavy metals, with Pb being adsorbed by Si–O–Pb chemical bonds and a strong Si–O–Pb–O–Si long chain, while As and Cr only form relatively weak As–O and Cr–O bonds through their attraction to O atoms. With the thickness of the encapsulating material increases, the synergistic effect of chemical solidification and physical package is provided to achieve a stronger stabilization effect. The leaching results indicated that the encapsulated RM exhibited exceptionally low concentrations of hazardous components (i.e., Na at 1.580 mg/L, As at 0.003 mg/L, Cr at 0.027 mg/L, and undetected levels of Pb in encapsulated RM with a thickness of 1.5 mm), far below the limits set by the United States Environmental Protection Agency (EPA). This study opening a new window for the sustainable and efficient management of hazardous solid waste.

Due to the low density and the distinct surface hydrophobicity of EPS, EPS are prone to float in the mixing and forming process of EPS concrete and to distribute unevenly in the process of concrete hardening, which finally results in the poor mechanical properties of EPS concrete. To improve EPS hydrophilicity, a new modification method was proposed in this paper to graft Pluronic (PEO) and ammonium chloride (NH4Cl) with hydrophilic groups on EPS surface, thereby converting EPS from hydrophobicity to hydrophilicity. The effects of modification on both the EPS hydrophilicity and the working performance and mechanical properties of EPS concrete were studied. The results suggest that both the modified EPS hydrophilicity and the EPS concrete fluidity and uniformity were significantly improved, with the minimum hydrophilic angle of EPS of 29°, and the minimum quality difference between the upper and lower concrete parts of 4.7%. While at the final failure, unmodified EPS concrete presented the most severe damage, however, modified EPS concrete, remained relatively intact. Compared with unmodified EPS concrete, modified EPS concrete cube increased its compressive strength by 26.64%, and its flexural strength by 16.12%. The ascending section of the stress-strain curve shows a regular linear distribution, whereas its descending curve jittered down with slight fluctuations in load, owning to the resilience of EPS. In addition, each stress-strain curve was fitted by a two-stage method, which indicates a high overall fitting degree.

This research aligns with green and sustainable development principles, aiming to augment concrete's mechanical properties and durability. The low-carbon reactive powder concrete was developed by substituting ordinary Portland cement with sulfoaluminate cement. Additionally, an effective method was proposed to enhance the interfacial transition zone (ITZ), involving the pre-saturation of sand particles in a dispersion of carbon nanotubes (CNTs). The role of CNTs on the concrete's mechanical property, hydration process, and microstructure was extensively explored using experiments and MD (molecular dynamics) simulations. The results indicated a 27.8% and 10.1% increase in 28-day compressive strength, along with a 16.7% and 6.3% rise in 28-day flexural strength for the sample with pre-saturated sand dispersed in CNTs (CS2), in comparison to samples without CNTs (C0) and those with dispersed CNTs (C2). The primary enhancement mechanism is ascribed to the CNTs' ability to improve the ITZ through adsorption and nucleation. The CNTs facilitated the generation of AFt (ettringite) and AH3 (gibbsite), enhancing the hydration degree. This mechanism was corroborated by MD simulations that analyzed the effect of CNTs' on the mobility of calcium ions (Ca2+). When dispersed at the ITZ, CNTs had a seeding effect, leading to the generation of AFt and AH3 around the CNTs. This, in turn, reduced the ITZ's width and dramatically improved the bonding between the CNTs and the ITZ, thus effectively enhancing the ITZ's microscopic structure.

Diffusion of CO2 strongly influenced the carbonation efficiency of steel slag. A novel approach for preparing steel slag-based artificial lightweight aggregates (SALAs) was proposed, in which biochar was employed to provide internal CO2 curing and reduce the bulk density. Owing to the additional diffusion channels and temporary storage tank for CO2 provided by the biochar, the carbonation degree of steel slag was effectively improved, leading to improved performance of SALAs as well as concrete. For instance, with the addition of 3 wt% biochar, the CO2 uptake was increased by 24.3%, the crushing resistance was 5.7 MPa, while the bulk density was reduced to 1070 kg/m3. In addition, the SALAs improved the compressive strengths and volume stability of concrete.

With increasing interest in utilizing metallurgical slags for mineral carbonation, there is a need to find applications for carbonated slags. The scope of this study is to explore the utilization potential of carbonated steel furnace slags (SFS) as supplementary cementitious materials (SCM) to produce low-CO2 cement-based materials. Two different types of SFS are studied with comparable amounts of major oxides but significantly different mineral phase compositions. In the first phase of the study, several parameters that affect the CO2 uptake during mineral carbonation are considered (temperature, CO2 pressure, particle size, and duration) for the two slags namely, basic oxygen furnace slag (BOFS) and desulfurized slag (DeSS). Among the two slags, the DeSS exhibited significantly higher degrees of carbonation than BOFS, and the higher carbonation of DeSS is attributed to the presence of Ca as Ca(OH)2 rather than Ca–Si in BOFS. For both the slags, increase in temperature and CO2 pressure generally led to increase in the degree of carbonation. In the second phase of the study, 30 wt% of white Portland cement (WPC) was replaced by carbonated slags (used as SCM) in the cement mixture. The cement mixture with the BOFS carbonated at 60 °C and 1 bar CO2 pressure, as SCM, exhibited compressive strength comparable with that of WPC. Higher degrees and rates of carbonation under different conditions appears to have reduced their reactivities as SCM.

The early hydration of UHPC can be retarded by the low water/binder ratio, the high polycarboxylate superplasticizers dosage, and the large supplementary cementitious materials (SCM) content. In this study, the effects of the prepared nanoscale C–S–H seeds and nano-silica (NS) on the early performance of UHPC were compared. The mechanism of nanomaterials influencing the early hydration of UHPC was further characterized by calorimetry, thermal analysis, nuclear magnetic resonance (NMR), and mercury intrusion porosimetry. The results showed that the 1-day compressive strength of UHPC samples with 0.3 wt% C–S–H seeds and NS increased by 457% and 25% respectively compared to the control sample. Compared to NS, C–S–H seeds were more effective in accelerating the early (before 3 d) hydration to produce hydrates and refine the pore structure of UHPC. The NMR results further implied that compared with the NS, the C–S–H seeds was 12% and 343% more efficient in accelerating the early hydration of cement and SCMs, respectively. However, the performance enhancement of nanomaterials was effective at 1 d and had little effect thereafter.

The crack-bridging effect of fibers in strain-hardening cementitious composites (SHCCs) restricts the crack width to a very small value, which promotes autogenous self-healing. The healing should lead to a noticeable recovery of mechanical properties. To ensure healing-induced recovery of mechanical properties (HIRMP) is relied upon in the built environment, a novel strategy for in-situ assessment of HIRMP is developed. HIRMP causes the stress-strain response of healed specimen to be ‘close’ to that of the pre-damaged state. It was derived that the ratio of areas(ROA) under the stress-strain response between ‘healed’ and ‘undamaged’ state appropriately measures the ‘closeness’ between these stress-strain curves. Exploiting the fact, acoustic emission (AE) signals are associated with fracture energy (thus also with the area under stress-strain curve). We design novel damage parameters to statistically represent random-stochastic fracture processes in self-healed SHCCs. The ratio of these parameters was then utilized to estimate HIRMP. This strategy was then studied experimentally on self-healed SHCCs with a different healing environment, fiber content, and pre-damage level. The results computed from this strategy are more consistent and comprehensive as compared to the conventional technique. The predicted HIRMP matches well with the expectation over a long range of recoveries.

The continuous rise in global temperatures is an evidence of climate change. CO2 emissions have caused major problems owing to its contribution to climate change. In particular, the construction industry has a considerable carbon footprint. Therefore, investigations into climate change mitigation are indeed a priority. All steps in the construction process, from raw materials preparation to cement production, contribute to CO2 emissions. This can be mitigated to a certain extent by incorporating bio-based constituents into construction materials. However, bio-based materials may negatively affect cement reaction and structural performance, despite their positive environmental impacts. Biochar, a carbon-rich product of biomass pyrolysis, is considered a potential substitute for cement replacement that can enhance structural properties if used in appropriate amounts. Although biochar has conventionally been used as a soil amendment in the agricultural industry, researchers have recently investigated its applicability in concrete. Importantly, the results thus far have reported its contribution to the enhancement of the mechanical, thermal, and physical properties of cement. This review provides a comprehensive overview of the physicochemical properties of biochar added cementitious materials, including the fresh and hardened properties of biochar-cement mixtures considering both environmental and economic aspects.