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The characteristics of reactive magnesia (MgO)-carbonated silt in respect to long-term stability have not been well understood in severely cold climate despite the usage of reactive MgO in enhancing the engineering performances. Under the binder content of 15% and initial water content of 25%, MgO-admixed silt specimens were carbonized for 3 h and 6 h and then subjected to different numbers of freezing-thawing (F-T) cycles. After different F-T cycles, the physico-mechanical properties of MgO- carbonated silt were analyzed in comparison with Portland cement (PC)-stabilized silt through physical and unconfined compression tests. Besides, a series of micro tests on MgO-carbonated specimens was performed including X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests. The results demonstrate that both mass change ratio and moisture content of carbonated/stabilized silt decrease, and these values of MgO-carbonated silt are significantly lower while the density is higher compared to PC-stabilized silt. The strengths and moduli of MgO-carbonated silt are still two times higher than those of PC-stabilized specimens and the strength change ratio of keeps above 0.8 after F-T cycles. There is no visible transformation between nesquehonite and dypingite/hydromagnesite, although the XRD peaks of nesquehonite decrease and the bonding and filling effects weaken slightly. After 6 and 10 F-T cycles, the pore-size characteristics changed from a unimodal distribution to a three-peak and bimodal distribution, respectively. The total, macro and large pore volumes increase obviously while the medium and small pore volumes decrease except for intra-aggregate pore. The findings show better F-T durability of MgO-carbonated silt, which would be helpful for facilitating the application of MgO carbonation in the soil treatment.

A new method for curing electrolytic manganese residue (EMR) using the biocarbonization of reactive MgO (r-MgO) is presented in this paper. This method uses microbial technology to provide CO2 to r...

The innovative carbonation technique based on reactive MgO and CO2 has been identified as an environmentally friendly and efficient method in the improvement of weak soils. Previous laboratory studies have indicated that carbonated MgO-admixed soils had significant improvement in mechanical properties. However, there are to date limited investigations on the soft-soil field application of this technique. In this study, a field trial was conducted to ascertain the feasibility of the MgO mass carbonation technique in improving shallow soft-soil subgrades. A series of field tests, including temperature, dynamic cone penetrometer, and light-weight deflectometer tests, were undertaken. The results indicated that compared with uncarbonated soil layers, there was two to three times increase in dynamic resilient moduli and soil resistances of carbonated MgO-admixed soils. The outcomes of this field investigation will contribute to the utilization of the combined stabilizer of MgO and CO2 and the mass carbonation technology in subgrade improvement.

Carbonation of reactive magnesia (MgO) has recently received increasing attention in the area of soil stabilization and ground improvement. However, as a critical parameter in terms of long-term seepage behavior in the geotechnical analysis, the hydraulic conductivity of carbonated reactive MgO-stabilized silt has not been fully studied. In this context, the effect of water-MgO ratio (ratio of initial water content to MgO content, w0/c) and carbonation time on hydraulic conductivity (or permeability) characteristics was systematically investigated. Serial microstructural tests including mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) analyses were applied to elucidate the intrinsic mechanisms. The obtained results indicate that as the initial water-MgO ratio decreases, the void ratio gradually decreases and the reduction of hydraulic conductivity becomes less prominent because of the little presence of flow paths. The hydraulic conductivity of carbonated MgO-admixed silt similar to that of PC-treated silt is mainly governed by the porosity, and its correlation with void ratio is proposed in the article. The variations of permeability with void ratio are consistent with those of the cumulative pore volume from MIP results in general, and the medium pores (3–30 μm) are substantiated to be the primary contributor in controlling the permeability. SEM and DSC analyses reveal that the cementation of soil particles and filling of hydrated magnesium carbonates marginally reduce the voids and permeability. The reasons for changes of permeability behaviors have been confirmed by the pore-size distribution and microstructure characteristics.

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 strength and hydraulic conductivity of vertical cutoff walls consisting of reactive magnesia-activated ground granulated blast furnace slag (GGBS), bentonite, and soil (MSB) have been investigated in previous studies. However, there has been little comprehensive study of the influence of wet-dry cycles on the mechanical and microstructural properties of MSB backfills. In this paper, the durability of MSB backfills when exposed to wet-dry cycles is investigated. The variations in mass change, dry density, pH value, pore size distribution, and mineralogy are discussed. The results show that the mass change of ordinary Portland cement (OPC)-based and MSB backfills increases with respect to wet-dry cycles. The MSB backfills exhibit up to 8.2% higher mass change than OPC-based ones after 10 wet-dry cycles. The dry density, pH value, and unconfined compressive strength of MSB backfill decrease with the increasing number of wet-dry cycles. Increasing the GGBS-MgO content from 5% to 10% in MSB backfills results in 2.1–2.3 times higher strength, corresponding to a reduction of 2%–12% in cumulative pore volume; while increasing the bentonite content slightly reduces the strength of MSB mixtures, corresponding to an increase of cumulative pore volume by 4.6%–7.9%. The hydrotalcite-like phases and calcium silicate hydrate (C-S-H) are the primary hydration products in MSB backfills. Moreover, the continuous wet-dry cycles result in the precipitation of calcite and nesquehonite.

A new loess solidification method using biocarbonization of reactive MgO is presented in this paper. The proposed method was employed to solidify loess, and the study focused mainly on exploring the influence of the reactive MgO content on the moisture content, dry density, unconfined compressive strength (UCS) with deformation, and microstructure of the solidified loess. The results indicated that with the increase in reactive MgO content, the moisture content of the solidified loess decreased approximately linearly, and the dry density increased; the curing reaction was rapid that occurred mainly at an early stage. The UCS increased with the increase in reactive MgO content and also increased approximately linearly with the increase of the dry density. The ratio of deformation modulus E50 to peak strength qu varied between 30 and 110. The UCS of the biocarbonized sample was approximately seven times higher than that of the original loess sample. The solidification effect of biocarbonized loess was also significantly higher than that of cement-solidified loess. The SEM results demonstrated that with the increasing reactive MgO content, the filling and cementing effect of hydrated magnesium carbonates (HMCs) on the soil particles further improved, leading to a better solidifying effect and resulting in higher strength.

Mucky soft soil with weak properties was usually improved by the chemical stabilization owing to the high cost of the situ replacement, and the main chemical binder used was Portland cement (PC). However, with the needs of environmental protection, energy conservation and emissions reduction, the research of low-carbon and high-efficiency soft soil treatment technology will be the development trend of modern engineering construction. The combined binder of reactive magnesia (MgO) and carbon dioxide (CO2) was regarded as a new curing agent instead of traditional PC, and the mass carbonation technology as an innovative idea was put forward to reinforce soft soil. In this view, the field mass stabilization was carried out and the single-point test was analyzed. Research results show that the temperature from the carbonation reaction is higher than that from the hydration reaction, and the temperature, strength and carbonation degree of the upper foundation soils increase significantly compared with the lower foundation soils, while the water content and pH value of the upper foundation soils are less than those of the lower foundation soils; the smaller the distance from the CO2 gas supply is, the higher the carbonation temperature, resilience modulus, penetration resistance, strength and carbonation degree are; the pipe-cover mass carbonation–stabilization technology can realize the carbonation and reinforcement of the on-site weak soils, but it should be noted that the CO2 should be injected at the bottom. The maximum ventilation radius should be 60 cm and the distance between the CO2 pipes should be less than 1.2 m in practical application. The bearing capacities of all regions are above 200 kPa, and the resilient moduli are about 20 MPa. The experimental research of mass stabilization based on MgO-CO2 carbonation technology has groundbreaking scientific significance and application prospects in the emission reduction and sustainable development of civil engineering.

Hydration of reactive magnesia cement (RMC) is limited by the formation of a Mg(OH)2 surface-layer on unreacted MgO particles. This study improved RMC hydration by using magnesium acetate (MA), along with hydromagnesite (H) and magnesite (M) as RMC replacements. While MA accelerated hydration and resulted in the formation of needle-like artinite, inclusion of M led to rosette-like crystals. The accumulated nucleation and growth of low-crystallinity Mg(OH)2 on H particles in a bird nest-like arrangement was observed for the first time in literature. This low-crystallinity Mg(OH)2 could be prone to carbonation. The replacement of up to 40% RMC with M in the presence of MA improved the compressive strength of RMC samples by 240%. This performance enhancement was supported by microstructure densification via the compact formation of hydrate and carbonate phases, defining M as a feasible partial RMC substitute.

Concrete made with Portland cement is by far the most heavily used construction material in the world today. Its success stems from the fact that it is relatively inexpensive yet highly versatile and functional and is made from widely available raw materials. However, in many environments, concrete structures gradually deteriorate over time. Premature deterioration of concrete is a major problem worldwide. Moreover, cement production is energy-intensive and releases a lot of CO2, this is compounded by its ever-increasing demand, particularly in developing countries. As such, there is an urgent need to develop more durable concretes to reduce their environmental impact and improve sustainability. To avoid such environmental problems, researchers are always searching for lightweight structural materials that show high performance during both processing and application. Among the various candidates, Magnesia (MgO) seems to be the most promising material to attain this target. This paper presents a comprehensive review of the characteristics and developments of MgO-based composites and their applications in cementitious materials and energy-efficient buildings. This paper starts with the characterization of MgO in terms of environmental production processes, calcination temperatures, reactivity, and micro-physical properties. Relationships between different MgO composites and energy-efficient building designs were established. Then, the influence of MgO incorporation on the properties of cementitious materials and indoor environmental quality was summarized. Finally, the future research directions on this were discussed.

This study investigated the long-term leachability of antimony (Sb) in a smelting residue (39519 mg/kg) solidified/stabilized by reactive magnesia (MgO). Different dosages of MgO (0% as control, 2%, 5%, and 10% on a dry basis) were compared, and the long-term performance was evaluated by an accelerated exposure test consist of 20 consecutive leaching steps with simulated strong acid rain (SAR, HNO3: H2SO4 = 1:2, pH = 3.20) as the extractant. Notably, the MgO treatments efficiently reduced the Sb leachability. Compared to the original slag (8.3 mg/L), the leaching concentrations based on a Chinese standard HJ/T299-2007 were reduced by 58%, 79%, 85%, and 86% at MgO dosages of 0%, 2%, 5%, and 10%, respectively. Because the studied slag was rich in oxides like SiO2, CaO, and MgO, the hydration reactions probably happened during the aging processes with oxic water. It was inferred that the formed hydration products have a self-solidification/stabilization function to suppress the Sb leaching from the solid phase. The mineralogical characterization results proved that the hydrated Mg(OH)2 played an essential role in the decrease of Sb leachability. Besides, the MgO addition promoted the hydration of this smelting slag and formed new hydrate gels that immobilize Sb in this slag. Our results confirmed that MgO-amended slags were resistant to continuous SAR corrosion. Compared to the control, the dosage of 5% MgO could effectively reduce the cumulatively released Sb by 57%, with only 0.46% of total Sb could be leached. The decomposition of Mg(OH)2 and hydrate gels determined the re-release of Sb in a long term. Our work has demonstrated that reactive MgO amendment could be potentially selected as an effective strategy for the treatment of Sb-containing smelting residues in field conditions.

This paper introduced a novel stabilization/solidification (S/S) method for the treatment of lead (Pb)- and zinc (Zn)-contaminated soils using reactive magnesia (MgO) and carbon dioxide (CO2). The MgO was first mixed with the Pb- and Zn-contaminated soils, and then the gaseous CO2 was introduced into the MgO-soil specimens for up to 120 h. The Portland cement (PC) was used as the control binder, and the PC-treated Pb- and Zn-contaminated soils were cured for 28 days. The treated contaminated soils were subjected to unconfined compressive strength test, one stage batch leaching test, mineralogical, and thermal analysis. The results indicated that the addition of Zn considerably slowed down the carbonation of MgO-stabilized soils, whilst Pb did not. For Pb-contaminated soils, MgO+CO2 yielded similar or higher strength compared to PC, but significantly reduced the Pb leachability. For Zn-contaminated soils, MgO+CO2 achieved similar Pb leachability compared to PC, but produced significantly higher strength. Magnesium carbonates were produced in the MgO+CO2-treated contaminated soils, which enhanced the strength of the soils. Additionally, lead carbonates and zinc carbonates were produced in the Pb- and Zn-contaminated soils, respectively, which contributed to the immobilization of Pb and Zn.

The thermodynamic equilibrium conditions of bicarbonate leaching were analyzed and used to develop a mathematical model for determining the parameters giving the highest extraction of MgO from the raw materials. Laboratory experiments on magnesite samples from Satkinsk deposit, Chelyabinsk Region, confirmed that the simulation results were adequate.

Reactive magnesium oxide (RMO) and circulating fluidised bed combustion (CFB) slags were used to prepare magnesium silicate cements using sodium hexametaphosphate (NaHMP) as a water reducer. The effects of curing condition and the initial levels of RMO and silica fume (SF) were studied for up to 90 d. The mechanical properties of the prepared pastes were evaluated through measurements of compressive strength. Mercury intrusion porosimetry (MIP) was employed to investigate the pore structure. X-ray diffraction, thermogravimetric analysis, mercury penetration analysis and scanning electron microscopy (SEM) were carried out to investigate the reaction products and final products. The results indicated that the final products were mainly magnesium silicate hydrate, hydrotalcite phases and hydromagnesite. Steam curing yielded higher strength, increased reaction products and closure of the macropores. SEM analysis showed that the product after steam curing exhibited abundant nanolattice structures (length ≤100 nm). MIP analysis showed that steam curing led to lower porosity and fewer macropores. The specimen prepared with 64 wt% CFB slag, 16 wt% SF and 20 wt% RMO exhibited the highest compressive strength (105 MPa).

Stabilization/solidification (S/S) is commonly applied to treat heavy metal-contaminated soils through the use of lime and ordinary Portland cement (OPC). Recently, reactive magnesia (MgO) has emerged as a novel binder for S/S of heavy metal-contaminated soils; however, a comprehensive comparison between MgO, lime (CaO), and OPC for S/S application is still missing. This study compares the S/S efficiency of MgO, CaO, and OPC for soils contaminated by six individual heavy metals (Pb, Cu, Zn, Ni, Cd, and Mn) through unconfined compressive strength (UCS) test, one stage batch leaching test, and microstructural analysis. The addition of binders can transform soluble heavy metal salts to insoluble hydroxides and their complexes, and hence the leachability of heavy metals decreases. However, the level, to which the leachability can be reduced, is highly pH dependent. Contaminated soils treated with MgO have pH of 9–10.5, at which the leachability of Pb and Zn is much lower than that of OPC- or CaO-treated soils with pH of 10.5–13; for example, the leached Pb and Zn from MgO-treated soils are only 0.1%–3.3% and 0.1%–9.4% of those from OPC-treated soils, respectively. On the other hand, the leached Cd and Mn from OPC-treated soils are 0.1%–28.5% and 0.1–10.7% of those from MgO-treated soils, respectively, due to the high pH and the formation of calcium silicate hydrate (CSH) in OPC-treated soils. OPC and CaO are more effective than MgO in decreasing the Ni leachability at high original concentrations, but less effective at low original concentrations. For all soils except those contaminated by Zn, the OPC generally produces a much higher UCS, up to two orders of magnitude, than the CaO and MgO. The results of study indicate that no single binder can treat all types of heavy metal-contaminated soils perfectly, and the selection of binder is a site-specific problem. Nanyang Technological University The authors appreciate the grant (M4081914) from Nanyang Technological University, Singapore and the assistance from Shunli Shen, Jian Wei Lui, and Thiam Loong Lim.

Reactive magnesia (MgO)-activated ground granulated blast furnace slag (GGBS) is a newly developed binder for soil stabilization/solidification. It can be used as an alternative of conventional hydraulic binders for the stabilization/solidification of heavy metal-contaminated soils. Acid rain exposure has been found to weaken the heavy metal immobilization in solidified/stabilized soils in the long term. However, very limited studies have been conducted to investigate the effect of acid rain on the physical and strength properties of GGBS–MgO-stabilized heavy metal-contaminated soils. In this study, GGBS–MgO-stabilized lead (Pb)-contaminated soils are subjected to the semi-dynamic leaching test using the simulated acid rain (SAR) as leachant, which has pH values of 2.0, 3.0, 4.0, and 5.0. Deionized water (pH 7.0) is also used for comparison. Dry density, soil pH, needle penetration depth, and unconfined compressive strength (qu) of the solidified/stabilized soils are measured after the leaching test. The results show that exposure to SAR yields reduction in qu and soil pH, whereas the needle penetration depth increases and dry density changes only marginally. The qu of the solidified/stabilized soils increases noticeably with increased GGBS–MgO content. It is also found that the stabilized Pb-contaminated soil exhibits lower qu than its clean soil counterpart. Furthermore, a quantitative relationship is proposed to correlate the normalized needle penetration resistance with the normalized qu. Finally, a multiple linear regression is performed to statistically reveal the dependence of soil strength on three independent variables, which are SAR pH, Pb concentration and binder content.

Reactive magnesia-based cement (RMC) is an emerging group of alternative binder to Portland cement. Recently, the first fiber-reinforced RMC-based strain-hardening composites (SHC) have been developed by the authors. The current work investigated the feasibility of the PC-free RMC-based SHC formulations to engage autogenous healing. Results showed that crack sealing and significant mechanical recovery can be realized through proper environmental conditioning. The presence of water is necessary to engage autogenous healing and elevated CO2 concentration leads to the formation of HMCs that can seal larger crack. However, ample supply of CO2 results in fast sealing of crack on the near surface region, which blocks the pathway for further carbonation and healing of interior region of cracks. Microstructure analysis reveals that the healing products are hydrated magnesium carbonates (HMCs) and different conditioning regimes lead to different types of HMCs as the healing products. Ministry of Education (MOE) The authors would like to acknowledge the Ministry of Education, Singapore for the financial support of this research by the MOE Academic Research Fund Tier 2 project No. MOE2017-T2-1-087 (S).

The ongoing tendency to create environmentally friendly building materials is nowadays connected with the use of reactive magnesia-based composites. The aim of the presented research was to develop an ecologically sustainable composite material based on MOC (magnesium oxychloride cement) with excellent mechanical, chemical, and physical properties. The effect of the preparation procedure of MOC pastes doped with graphene nanoplatelets on their fresh and hardened properties was researched. One-step and two-step homogenization techniques were proposed as prospective tools for the production of MOC-based composites of advanced parameters. The conducted experiments and analyses covered X-ray fluorescence, scanning electron microscopy, energy-dispersive spectroscopy, high-resolution transmission electron microscopy, sorption analysis, X-ray diffraction, and optical microscopy. The viscosity of the fresh mixtures was monitored using a rotational viscometer. For the hardened composites, macro- and micro-structural parameters were measured together with the mechanical parameters. These tests were performed after 7 days and 14 days. The use of a carbon-based nanoadditive led to a significant drop in porosity, thus densifying the MOC matrix. Accordingly, the mechanical resistance was greatly improved by graphene nanoplatelets. The two-step homogenization procedure positively affected all researched functional parameters of the developed composites (e.g., the compressive strength increase of approximately 54% after 7 days, and 37% after 14 days, respectively) and can be recommended for the preparation of advanced functional materials reinforced with graphene.

To achieve the goal of carbon neutrality, carbon capture and storage (CCS) is considered to be an effective approach. This study investigated the microstructural development of reactive magnesia cement-industrial waste (i.e., pulverized fly ash and ground granulated blast-furnace slag) formulations under accelerated carbonation conditions. The density and isothermal calorimetry analyses were supported with microstructural analysis performed. Results showed that pulverized fly ash and ground granulated blast-furnace slag could be activated by reactive magnesia cement, resulting in the formation of phases such as magnesia silicate hydrate, hydrotalcite and hydromagnesite, whose formation was enhanced in the presence of accelerated carbonation. Associated with a low initial pH, samples with pulverized fly ash outperformed samples with ground granulated blast-furnace slag counterparts in terms of their strength development. The study led to the determination of a formulation containing the reactive magnesia cement and pulverized fly ash with a higher mechanical performance than the control group, also highlighting the need for the revision of the adopted carbon footprint calculation to incorporate several critical factors. Ministry of Education (MOE) The authors acknowledge the financial support from the Singapore Ministry of Education (MOE) Academic Research Fund Tier 1 (RG 95/ 16). This work is also supported by the Fundamental Research Funds for the Central Universities (No. 2021QNA4023).

This study investigates production scaling up of MgO cement synthesized from reject brine and evaluates the mechanical properties and microstructures of the resulting MgO cement. Hydration process and carbonation degree of the resulting MgO mortar were investigated and compared with commercial MgO cement. Compressive strength, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), particle size analysis, acid reactivity, and thermogravimetric and differential thermal analysis (TG/DTA) were engaged for sample characterization. Results showed that MgO cement synthesized from reject brine possesses higher mechanical strength of 32.5 MPa at the age of 28 days as compared to the commercial MgO cement (20.1 MPa). This is mainly attributed to the high reactivity of the synthesized MgO cement as revealed by the microstructure study. Master of Engineering (CEE)

In some porous unreinforced concrete products carbonation could be employed to deliver significant benefits, both in terms of mechanical strength and carbon footprint. While much work has focused on the carbonation of lime, recent studies have shown that magnesium-based cementitious systems, including slags and magnesia, can be employed as the main strength development process in carbonated reactive magnesia cement products. In this chapter, the design, performance and applications of carbonated reactive magnesia in different compositions and scenarios are presented and discussed. A number of hydrated magnesium carbonates form upon the carbonation of magnesia, with different controlling factors for their formation, resulting in different mechanical properties as well as CO2 sequestration capacity. Pressed semi-dry magnesia-based masonry blocks are the first application example presented. The results of both laboratory and full-scale production of a range of magnesia-based products are discussed in terms of the variable considered. Experimental work was complemented with computer simulations to model the mechanism of the carbonation process. The use of carbonated reactive magnesia was also considered for ground improvement applications. Forced carbonation of soils mixed with dry reactive magnesia was employed both in triaxial samples and with the use of a model auger that simulated field applications of the carbonation process. Blended magnesia mixes were investigated. In all the studies reported here, the investigations considered the dosage of the magnesia as well as the carbonation environment followed by mechanical and microstructural analyses. Different accelerated carbonation regimes were also considered. This chapter reports details of the experimental and field work carried out on carbonated reactive magnesia cements highlighting the carbonation mechanisms and resulting properties as well as potential benefits and environmental impacts.

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. National Research Foundation (NRF) Accepted version

Alkali-activated cement (AAC) is one of the sustainable alternatives to OPC due to the large proportion of waste materials in its mix composition. The reaction kinetics and mechanisms of the alkali and aluminosilicate materials are highly dependant on the chemical/mineral compositions of the raw materials, while the role of the inherent magnesium oxide (MgO) content in ground granulated blast-furnace slag (GGBS) has attracted much attention in the past decade. Meanwhile, the activation capability of using only earth alkali compounds for GGBS are proved feasible and more sustainable than these caustic alkalis. This chapter critically examined the role of inherent MgO and additive reactive (i.e., caustic) MgO in improving the mechanical performance and durability of AAC with different compositions. The hydration mechanisms and products formed using reactive magnesia as either a sole activator or an additive for GGBS-bearing AAC were discussed in detail. The state-of-the-art progress on the development of MgO-modified AAC and the effect of reactivity and dosage of MgO on the various properties of this type of cement and concrete products are reviewed. In addition, recent development of using sustainable MgO-GGBS blends in a number of geotechnical/geoenvironmental applications such as ground improvement, contaminated soil stabilisation/solidification and low permeability barriers are presented and discussed. It is concluded that more research is warranted to better understand the interaction between MgO and aluminosilicate materials and fully explore the merits of MgO-modified AAC in different applications.

The authors have developed a new binder, KMP, which is made from oxalic acid-activated phosphate rock, monopotassium phosphate (KH2PO4), and reactive magnesia (MgO). This study explores the acid neutralization capacity, strength characteristics, water-soaking durability, resilient modulus, and pore size distribution of KMP stabilized soils with individual Zn, Pb, or coexisting Zn and Pb contaminants. For comparison purpose, Portland cement (PC) is also tested. The results show that KMP stabilized soils have a higher acid buffering capacity than PC stabilized soils, regardless of the soil contamination conditions. The water stability coefficient and resilient modulus of the KMP stabilized soils are found to be higher than PC stabilized soils. The reasons for the differences in these properties between KMP and PC stabilized soils are interpreted based on the stability and dissolubility of the main hydration products of the KMP and PC stabilized soils, the soil pore distribution, and concentration of Mg or Ca leached from the KMP and PC stabilized soils obtained from the acid neutralization capacity tests. Overall, this study demonstrates that the KMP is effective in stabilizing soils that are contaminated with Zn or Pb alone and mixed Zn and Pb contaminants, and the KMP stabilized soils are better suited as roadway subgrade material.