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Application of 3D printing in building projects can enhance construction efficiency and productivity. Rheological properties play a key role in defining the suitability of different cement-based mixes for digital construction. In this study, the rheological behavior of the MgO-SiO 2 system involving metakaolin (MK) as the silica source was thoroughly investigated. Mixes with two different MgO/MK ratios (1 and 1.5) were prepared and evaluated for their hydration process, solid phase analysis, and dispersion stability of suspension particles by utilizing isothermal calorimetry, zeta potential, XRD, and TG-DTG analyses. Results revealed that the median differential viscosity (μ diff), dynamic yield stress (τ d), and static yield stress (τ s) of M50-MK50 were considerably greater than those of M60-MK40. The increase of τ s over time adhered to the linear structuration model within 60 min of hydration. The small amplitude oscillatory shear (SAOS) test findings indicated that M50-MK50 paste had a larger storage modulus. Although the printed structure of M50-MK50 closely approximated the intended dimensions of the design, notable instability was evident in the first and third layers from bottom to top. Microscopic analysis revealed that the formation of a modest number of hydration products in the early stage, the tendency of the system to agglomerate due to the smaller particle size of MK, and the irregular morphology of MK are significant variables influencing the rheological behavior of the MgO-MK mixes. [ABSTRACT FROM AUTHOR]

Various innovative research in the cement industry is looking into improving its environmental sustainability. Sodium carbonate-activated slag/fly ash (NC-SF) binders has recently evolved as a potentially more sustainable binding materials than both Portland cement and conventional alkali activated materials such as sodium silicate and sodium hydroxide activated materials. The reaction mechanism and some microstructural properties of NC-SF cements have been a major area of research recently. However, very few studies have scaled up the investigation of these binders into concrete specimens. This paper, therefore, provides new insight into the strength development and the durability performance of NC-SF concrete and MgO-modified NC-SF concrete. Concrete testing included measurements of compressive strength, split tensile strength, water absorption, depth of carbonation, sulphate exposure, acid exposure and elevated-temperature exposure. Microstructure studies were conducted using Powder X-Ray diffraction (PXRD), Thermogravimetric analysis (TGA) and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR). It is concluded that NC-SF concrete mixes develop acceptable mechanical strength and demonstrate high resistance to sulphate attack. They also showed higher resistance to acid attack than the control mix based on sodium silicate-activated slag concrete. Here, emphasis is placed on the potential of developing NC-SF concrete with excellent performance and less complicated production methods as well as a low carbon footprint. It is also found that the use of reactive MgO enhanced the strength development of NC-SF concrete as well as its resistance to acid and carbonation. [ABSTRACT FROM AUTHOR]

Engineering properties of soft soils can gain great improvement through the addition of reactive magnesia (MgO) and further carbonation of substantial gaseous CO 2 absorbed. The paper studies the influence of MgO activity index on engineering properties of the carbonated silt with different water-MgO ratio. The engineering properties are investigated mainly through unconfined compression tests, and then the strength development are explained by the scanning electron microscopy (SEM). The results demonstrate that the mechanical properties of carbonated MgO-stabilized soils were greatly influenced by MgO activity index and water-MgO ratio, and the unconfined compressive strength of reactive MgO-stabilized soil has increased obviously after CO 2 carbonation. With increasing MgO activity index and reducing water-MgO ratio, the unconfined compressive strength increased, and the failure mode of carbonated specimens approximately changes from elastic-plastic to brittleness as well as their failure strain mainly ranges between 0.5% and 2.3%. The deformation modulus of carbonated silt generally increases with increasing unconfined compressive strength, and the ratio of the deformation modulus to unconfined compressive strength is about 35 to 150. A simplified equation with combining MgO activity index and water-MgO ratio is proposed for accurately predicting the unconfined compressive strength of carbonated MgO-stabilized soils. Moreover, the microstructural characteristics explain the strength gain of carbonated MgO-stabilized soils. [ABSTRACT FROM AUTHOR]

A system of alkali-activated fly ash (FA)/slag (AAFS) mixtures as a clinkerless cement was investigated with different dosages of Na 2 CO 3 , as a sustainable activator. The effect of incorporating various proportions of reactive magnesia (MgO) was also examined. Mechanical, mineralogical, and microstructural characterisation of the cement pastes was carried out using the unconfined compressive strength, X-ray diffraction, thermogravimetric analysis, infrared spectroscopy and scanning electron microscopy. It was found that the strength of Na 2 CO 3 activated FA/slag mixtures generally increased with time and the Na 2 CO 3 dosage. The hydration products were mainly C–(N)–A–S–H gel of low-crystallinity, which is rich in Al and may have included Na in its structure, and hydrotalcite-like phases. Adding reactive MgO in the mixes showed an accelerating effect on the hydration rate as suggested by the isothermal calorimetry data. Additionally, findings revealed variations on the strength of the pastes and the chemical compositions of the hydration products by introducing reactive MgO into the mixtures. [ABSTRACT FROM AUTHOR]

The utilisation of reactive magnesia or quicklime as novel activators for slag offers a range of technical and environmental benefits over conventional caustic alkali activators and showed great potential in soil stabilisation. This paper investigates the mechanical and microstructural properties of two model soils, i.e., a clayey soil and a slightly silty clayey sand, stabilised by ground granulated blastfurnace slag (GGBS) using various techniques including unconfined compressive strength (UCS) test, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). A number of MgO and CaO mixtures with different MgO/CaO ratios were adopted for slag activation. The activator to GGBS ratio was 1:3 and the dosage of the binder (including MgO, CaO and GGBS) was 12% by weight of the dry soil. The result demonstrated that the increasing MgO/CaO ratio in the binder led to an increase in the UCS of the stabilised clayey soil up to 90 days, due to the increased homogeneity of C–S–H gel structure, the decreased Ca/Si ratio of C–S–H gel and the increased amount of voluminous hydrotalcite-like phases. On the other hand, slag activated with MgO–CaO mixtures showed poorer mechanical performance than slag activated with either MgO or CaO alone for sand stabilisation. In addition, strength enhancement was observed for the stabilised clayey soil upon different soaking conditions up to 7 days. After 28 days, although binders with higher MgO/CaO ratios showed slight strength degradation upon soaking, they still exhibited higher strength than those with lower MgO/CaO ratios. [ABSTRACT FROM AUTHOR]

In this study, reactive magnesia (MgO)- and carbide slag (CS)-activated ground granulated blastfurnace slag (GGBS) were used to stabilise a natural soil in comparison to Portland cement (PC). X-ray diffraction (XRD), scanning electron microscopy (SEM), and unconfined compressive strength (UCS) test were employed to investigate the microstructural and mechanical properties of stabilised soils. The results indicated that the main hydration products of CS-GGBS stabilised soil included calcium silicate hydrates (CSH), calcium aluminate hydrates (CAH), and ettringite. For MgO-GGBS stabilised soils, CSH was the only hydration product detected. These hydration products had different microstructure and binding ability, affecting the strength of stabilised soils. There was an optimum MgO or CS content, in a range of 10–20%, for yielding the highest UCS of MG-GGBS or CS-GGBS stabilised soil at the same age. The 90-day UCS of the optimum MgO-GGBS and CS-GGBS stabilised soils was 3.0–3.2 and 2.4–3.2 times that of the PC stabilised soil, respectively. [ABSTRACT FROM AUTHOR]

Ground granulated blastfurnace slag (GGBS) is widely used as a partial replacement for Portland cement or as the major component in the alkali-activated cement to give a clinker-free binder. In this study, reactive MgO is investigated as a potentially more practical and greener alternative as a GGBS activator. This paper focuses on of the hydration of GGBS, activated by two commercial reactive MgOs, with contents ranging from 2.5% to 20% up to 90 days. The hydration kinetics and products of MgO–GGBS blends were investigated by selective dissolution, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy techniques. It was found that reactive MgO was more effective than hydrated lime in activating the GGBS based on unconfined compressive strength and the efficiency increased with the reactivity and the content of the MgO. It is hence proposed that reactive MgO has the potential to serve as an effective and economical activator for GGBS. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydration and hardening properties of RMPC are evaluated comprehensively. • RM content obviously affects the hydration and hardening properties of RMPC. • Mg(OH) 2 crystal size can be bigger than 2 μm causing expansion and self-healing. Hydration and hardening properties of reactive magnesia and Portland cement composite were evaluated with macro and micro property tests, in order to better understand systematically hydration and hardening mechanisms of reactive magnesia and Portland cement composite. Reactive magnesia content (10 wt%, 35 wt%, 50 wt%, 65 wt% and 90 wt%) obviously affects the hydration and hardening properties of reactive magnesia and Portland cement composite. As the main hydration product of reactive magnesia, Mg(OH) 2 crystal shape affected by inner alkaline environment and Mg(OH) 2 crystal amount influence the macro volume expansion performance. The Mg(OH) 2 crystal size in hardened reactive magnesia and Portland cement composite pastes can be bigger than 2 μm and is big enough to cause micro crack in hardened RMPC pastes, and at the same time to heal micro crack in moist curing condition (20 ± 2 °C and relative humidity ≥ 95%). The loose cluster structure of Mg(OH) 2 crystals leads to low binding property of RMPC pastes and causes the increases of the amount of pores in range of 0.1–1 μm in hardened RMPC pastes. The findings will be beneficial for mixture proportion design, manufacture control and properties optimization of reactive magnesia and Portland cement composite concrete. [ABSTRACT FROM AUTHOR]

Stabilization/solidification (S/S) is a low-cost and effective remedial technique for dredged contaminated sediments. Quick lime (CaO)-activated and reactive magnesia (MgO)-activated ground granulated blast furnace slag (GGBS) are effective and low-carbon S/S binders. However, the existence of metals, especially Zn, in contaminated sediments, may hinder the hydration of GGBS. This study compared the performance and mechanisms of CaO-GGBS, MgO-GGBS and ordinary Portland cement (OPC) for the treatment of Zn-contaminated clay slurry using unconfined compressive strength (UCS) test, one-stage batch leaching test, and mineralogical and thermal analyses. The results showed that the application of the MgO-GGBS (GGBS dosage of 10 % and MgO of 0 %–3 % (of dry clay by mass)) had positive effects on the mechanical strength and Zn immobilization of the contaminated clay slurry while the CaO-GGBS affected negatively and the situation became even worse at a higher CaO dosage (0 %–1.5 % of dry clay by mass). In comparison with OPC, the application of MgO-GGBS produced higher mechanical strength and that for CaO-GGBS was the lowest. The Zn leaching difference depends on initial Zn concentrations. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) results showed that a retarder, calcium zinc hydroxide, formed in the immobilization process when adding the CaO-GGBS binder, hindering the GGBS hydration and further leading to inferior strength and higher Zn leachability. The clay slurry treated by the MgO-GGBS binder was found to have a higher calcium silicate hydrate content which explained its high strength and low leachability. • The S/S mechanisms of MgO- and CaO-GGBS were systematically compared and clarified. • A retarder calcium zinc hydroxide was formed when using CaO-GGBS binder. • Zn immobilization in contaminated clay slurry is largely pH-dependent. • Compared with GGBS, CaO-GGBS reduced strength of S/S sample while MgO-GGBS increased. [ABSTRACT FROM AUTHOR]

Nesquehonite is an attractive magnesium precipitation product as it sequesters CO 2 and exhibits good filterability. Upon calcination, high-quality reactive magnesia is produced. As part of the hydrometallurgical processing of nickel, magnesia is used as a precipitating agent to recover nickel and cobalt in the form of a Mixed Hydroxide Precipitate (MHP). However, a waste solution with elevated concentration of magnesium is formed which can be an environmental liability. Chemical thermodynamic equilibrium modeling of magnesium carbonate precipitation from magnesium sulfate liquor predicted a high reaction extent of 99% could be achieved. Magnesium carbonate precipitation experiments were carried out by gradually mixing sodium carbonate solution and synthetic industrial liquor in a batch reactor at 25 °C, while monitoring solution concentrations and pH, used to estimate the supersaturation index. Calcination of the resulting solid at temperatures ranging from 450 to 650 °C produced magnesia with acetic acid activities ranging from 14 s to 41 s. The most reactive magnesia was obtained at the lower temperature of 450 °C with an activity of 14.7 ± 0.6 s, which meets industrial product specifications. Periclase was identified as the dominant magnesia mineral phase along with thenardite as a minor component. [Display omitted] • Nesquehonite is recovered from nickel laterite process solution. • Nesquehonite precipitation was modeled using PHREEQC. • Precipitation was studied with respect to pH, liquor composition and the supersaturation index. • The MgO synthesized was suitable for reuse in the nickel process and consistent with the principles of a circular economy. [ABSTRACT FROM AUTHOR]

• Several additives and curing regimes were used in clay solidification. • Addition of CaO and MgO provided Ca2+, Mg2+ and OH− ions for the formation of varying hydration phases. • Morphology of hydration phases may be more important than their amounts and sample porosity. • Delayed expansion of MgO reduced the formation of micro-cracks of samples under oven curing. • MgO is a more sustainable additive than CaO and PC in clay solidification. Kaolin production requires large amounts of excavation waste to be removed, with each tonne of kaolin recovered typically producing up to 9 tonnes of waste. This study introduced quicklime (i.e., CaO), reactive magnesia (i.e., MgO) and sodium carbonate, together with an early-age oven curing regime as a means of re-using kaolin excavation waste to produce more sustainable cementitious materials. The strength development of solidified clay samples was measured and later interpreted by pH tests, porosity measurement, Fourier-Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy-Energy-dispersive X-ray spectroscopy (SEM-EDX). Besides, the environmental impacts with respect to the production of additives for clay solidification were also calculated. The results indicated that the incorporation of CaO and MgO effectively solidified the clay with acceptable compressive strength (CaO group: 12.2 MPa; MgO group: 20.3 MPa after 28 days). Meanwhile, given the strength development of samples investigated in this study, the morphology of hydration phases could be possibly more important than their contents and sample porosity. Furthermore, with the introduction of 3 days of oven curing, MgO took advantage over CaO in clay solidification, which was attributed to the formation of fibrous crystals (i.e., nesquehonite) and fewer micro-cracks. Finally, from a perspective of 'greenness' and sustainability, MgO is a more favorable additive than CaO and PC in soil solidification. [ABSTRACT FROM AUTHOR]

3D concrete printing technology greatly facilitates automation in construction which enhances efficiency, productivity and sustainability. This study develops a slag-based mixture as a cementless material for sustainable spray-based 3D printing. Effects of MgO and fly ash cenosphere (FAC) addition on setting, hydration and rheological properties of fresh mixtures are investigated to obtain the optimal mixture. Results show that inclusion of MgO effectively reduces initial setting time of the fresh mixtures. With 40 wt% of GGBS replaced by MgO, initial setting time is greatly reduced from 305 min to 67 min (78% reduction). Fourier-Transformed Infrared (FTIR) spectra suggest the acceleration is plausibly due to the physical aspects. Furthermore, the FTIR spectra show that MgO can effectively activate the slag and also improve water retention. Rheological tests reveal that FAC addition generally reduces dynamic yield stress and plastic viscosity while increases static yield stress of the fresh mixtures, resulting in lower pumping pressures and higher critical ratios. The mixture with 20 wt%/40 wt% FAC addition has 29%/31% lower pumping pressure and 78%/68% higher critical ratio compared with plain MgO-activated slag material, respectively. Hence, the material with tailored rheology leads to better delivery and deposition performance of the mixture and overall spray-printing quality. An optimal mixture was finally selected based on setting, hydration, rheological properties and spray performance. The developed cementless mixture was successfully applied in the vertical spray-based 3D printing of filament and profile, which confirmed its feasibility in engineering applications. Image 102134 • A cementless mixture has been designed with MgO-activated slag. • Fly ash cenosphere was introduced for rheological tailoring. • Setting and hydration behaviors of MgO/slag-based mixtures were analyzed. • Feasibility of developed mixture for spray-based printing was confirmed. [ABSTRACT FROM AUTHOR]

Elevated soil lead (Pb) concentrations are a global concern owing to the toxic effects of this heavy metal. Solidification/stabilization (S/S) of soils using reagents like Portland cement (PC) is a common approach for the remediation of Pb contaminated sites. However, it has been reported that under long-term field conditions, the performance of PC treatments can diminish significantly. Therefore, novel reagents that provide longer-term stabilization performance are needed. In this study, four magnesium oxide (MgO) products of different reactivity values were applied (5 wt%) to a Pb contaminated clayey soil. The short-term (1–49 days) and long-term (25–100 years) temporal stabilization effects were investigated by laboratory incubation and accelerated ageing methods, respectively. The concentration of Pb in Toxicity Characterization Leaching Procure (TCLP) leachate was ~14 mg/L for the untreated soil; ~1.8 times higher than the TCLP regulatory level (5 mg/L). Only one day after treatment with MgO, the leachate concentration was reduced to below the regulatory level (a reduction of 69.4%–83.2%), regardless of the MgO type applied. However, in the long-term accelerated ageing experiments, only treatments using the most reactive MgO type could provide leachate concentrations that were consistently below the TCLP threshold throughout the 100 years of simulated ageing. The soil treated with the MgO of lowest reactivity was the first to exceed the regulatory level, at simulated year 75. It is thus demonstrated that MgO reactivity has a significant effect on its long-term effectiveness for contaminated soil stabilization. This is attributed to differences in their specific surface area and readiness to carbonate, which may facilitate the immobilization of Pb in the long term. It is also noteworthy that compared to PC, reactive MgO is more environmentally friendly owing to lower energy consumption and reduced CO 2 emissions during its manufacture. • MgO products with different reactivity were applied to a Pb contaminated clay soil. • Leaching concentrations were below the regulatory criteria after only 1 day. • Short-term (1–49 days) and long-term (25–100 years) ageing effects were studied. • Only treatment with most reactive MgO was below criteria for 100 simulated years. • Good performance was ascribed to higher surface area and readiness to carbonate. [ABSTRACT FROM AUTHOR]