Preparation of Low-Cost Magnesium Oxychloride Cement Using Magnesium Residue Byproducts from the Production of Lithium Carbonate from Salt Lakes

Magnesium oxychloride cement (abbreviated as MOC) was prepared using magnesium residue obtained from Li₂CO₃ extraction from salt lakes as raw material instead of light magnesium oxide. The properties of magnesium residue calcined at different temperatures were researched by XRD, SEM, LSPA, and SNAA. The preparation of MOC specimens with magnesium residue at different calcination temperatures (from 500 °C to 800 °C) and magnesium chloride solutions with different Baume degrees (24 Baume and 28 Baume) were studied. Compression strength tests were conducted at different curing ages from 3 d to 28 d. The hydration products, microstructure, and porosity of the specimens were analyzed by XRD, SEM, and MIP, respectively. The experimental results showed that magnesium residue’s properties, the BET surface gradually decreased and the crystal size increased with increasing calcination temperature, resulting in a longer setting time of MOC cement. Additionally, the experiment also indicated that magnesium chloride solution with a high Baume makes the MOC cement have higher strength. The MOC specimens prepared by magnesium residue at 800 °C and magnesium chloride solution Baume 28 exhibited a compressive of 123.3 MPa at 28 d, which met the mechanical property requirement of MOC materials. At the same time, magnesium oxychloride cement can be an effective alternative to Portland cement-based materials. In addition, it can reduce environmental pollution and improve the environmental impact of the construction industry, which is of great significance for sustainable development.     

Research and Engineering Application of Salt Erosion Resistance of Magnesium Oxychloride Cement Concrete

Aiming at the problem that ordinary cement concrete is subjected to damage in heavy saline soil areas in China, a new type of magnesium oxychloride cement concrete is prepared by using the gelling properties of magnesium oxychloride cement in this study, and the erosion resistance of the synthesized magnesium oxychloride cement concrete in concentrated brine of salt lakes is studied through the full immersion test. The effects of concentrated brine of salt lakes on the macroscopic, microscopic morphology, phase composition and mechanical properties of magnesium oxychloride cement concrete are investigated by means of macro-morphology, erosion depth, SEM, XRD and strength changes. The salt erosion resistance mechanism of magnesium oxychloride cement concrete is revealed. The results demonstrate that under the environment of full immersion in concentrated brine of salt lakes, there is no macroscopic phenomenon of concrete damage due to salt crystallization, and the main phase composition is basically unchanged. The microscopic morphology mostly changes from needle-rod-like to gel-like. Due to the formation of a new 5·1·8 phase on the surface layer and the increase in compactness, its compressive strength has a gradual increase trend. Based on the engineering application of magnesium oxychloride cement concrete, it is further confirmed that magnesium oxychloride cement concrete has excellent salt erosion resistance and good weather resistance, which provides theoretical support for future popularization and application.     

Co-Doped Magnesium Oxychloride Composites with Unique Flexural Strength for Construction Use

In this study, the combined effect of graphene oxide (GO) and oxidized multi-walled carbon nanotubes (OMWCNTs) on material properties of the magnesium oxychloride (MOC) phase 5 was analyzed. The selected carbon-based nanoadditives were used in small content in order to obtain higher values of mechanical parameters and higher water resistance while maintaining acceptable price of the final composites. Two sets of samples containing either 0.1 wt. % or 0.2 wt. % of both nanoadditives were prepared, in addition to a set of reference samples without additives. Samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, and energy dispersive spectroscopy, which were used to obtain the basic information on the phase and chemical composition, as well as the microstructure and morphology. Basic macro- and micro-structural parameters were studied in order to determine the effect of the nanoadditives on the open porosity, bulk and specific density. In addition, the mechanical, hygric and thermal parameters of the prepared nano-doped composites were acquired and compared to the reference sample. An enhancement of all the mentioned types of parameters was observed. This can be assigned to the drop in porosity when GO and OMWCNTs were used. This research shows a pathway of increasing the water resistance of MOC-based composites, which is an important step in the development of the new generation of construction materials.     

The Impact of Graphene and Diatomite Admixtures on the Performance and Properties of High-Performance Magnesium Oxychloride Cement Composites

A high-performance magnesium oxychloride cement (MOC) composite composed of silica sand, diatomite powder, and doped with graphene nanoplatelets was prepared and characterized. Diatomite was used as a 10 vol.% replacement for silica sand. The dosage of graphene was 0.5 wt.% of the sum of the MgO and MgCl₂·6H₂O masses. The broad product characterization included high-resolution transmission electron microscopy, X-ray diffraction, X-ray fluorescence, scanning electron microscopy and energy dispersive spectroscopy analyses. The macrostructural parameters, pore size distribution, mechanical resistance, stiffness, hygric and thermal parameters of the composites matured for 28-days were also the subject of investigation. The combination of diatomite and graphene nanoplatelets greatly reduced the porosity and average pore size in comparison with the reference material composed of MOC and silica sand. In the developed composites, well stable and mechanically resistant phase 5 was the only precipitated compound. Therefore, the developed composite shows high compactness, strength, and low water imbibition which ensure high application potential of this novel type of material in the construction industry.     

Effect of Modified Magnesium Oxide on the Properties of Magnesium Phosphate Cement under a Negative Temperature Environment

As a rapid repair material, magnesium phosphate cement (MPC) can be used under various environmental temperature conditions, but different temperatures significantly impact its strength and working performance. In this study, based on the surface modification of magnesium oxide, the working and mechanical properties of samples were investigated at an ambient temperature of −5 °C, and the hydration properties and microstructure of MPC were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TG), mercury-in-pressure (MIP), and scanning electron microscopy (SEM). The results show that the modified magnesium oxide at a negative temperature prolongs the setting time of MPC from 10 min to more than 30 min, and fluidity can still be maintained or increased after half an hour. From 1 d to 28 d, the compressive strength growth rate of the reference group was 257.0% compared to 723.8% for the 10 wt% water-glass-modified MgO sample. K-struvite transformed from a blocky growth to a needle-like growth with the modified sample filling the pores and cracks inside the matrix. Compared with the unmodified sample, MPC’s porosity decreased from 9.62% to 9.23% for 10 wt% water-glass-modified MgO. Therefore, the surface modification of magnesium oxide not only prolonged the setting time but also further benefited mechanical performance, which provides the prerequisites for MPC construction in negative-temperature environments.     

Effect of Waste Glass on the Properties and Microstructure of Magnesium Potassium Phosphate Cement

Waste glass is a bulk solid waste, and its utilization is of great consequence for environmental protection; the application of waste glass to magnesium phosphate cement can also play a prominent role in its recycling. The purpose of this study is to evaluate the effect of glass powder (GP) on the mechanical and working properties of magnesium potassium phosphate cement (MKPC). Moreover, a 40mm × 40mm × 40mm mold was used in this experiment, the workability, setting time, strength, hydration heat release, porosity, and microstructure of the specimens were evaluated. The results indicated that the addition of glass powder prolonged the setting time of MKPC, reduced the workability of the matrix, and effectively lowered the hydration heat of the MKPC. Compared to an M/P ratio (MgO/KH₂PO₄ mass ratio) of 1:1, the workability of the MKPC with M/P ratios of 2:1 and 3:1 was reduced by 1% and 2.1%, respectively, and the peak hydration temperatures were reduced by 0.5% and 14.6%, respectively. The compressive strength of MKPC increased with an increase in the glass powder content at the M/P ratio of 1:1, and the addition of glass powder reduced the porosity of the matrix, effectively increased the yield of struvite-K, and affected the morphology of the hydration products. With an increase in the M/P ratio, the struvite-K content decreased, many tiny pores were more prevalent on the surface of the matrix, and the bonding integrity between the MKPC was weakened, thereby reducing the compressive strength of the matrix. At less than 40 wt.% glass powder content, the performance of MKPC improved at an M/P ratio of 1:1. In general, the addition of glass powders improved the mechanical properties of MKPC and reduced the heat of hydration.     

Effect of Al Content on Microstructure Evolution and Mechanical Properties of As-Cast Mg-11Gd-2Y-1Zn Alloy

In the present paper, the Mg-11Gd-2Y-1Zn alloys with different Al addition were fabricated by the gravity permanent mold method. The effect of Al content on microstructure evolution and mechanical properties of as-cast Mg-11Gd-2Y-1Zn alloy was studied by metallographic microscope, scanning electron microscope, XRD and tensile testing. The experimental results showed that the microstructure of as-cast Mg-11Gd-2Y-1Zn alloy consisted of α-Mg phase and island-shaped Mg₃ (RE, Zn) phase. When Al element was added, Al₂RE phase and lamellar Mg₁₂REZn (LPSO) phase were formed in the Mg-11Gd-2Y-1Zn alloy. With increasing Al content, LPSO phase and Mg₃ (RE, Zn) phase gradually decreased, while Al₂RE phase gradually increased. There were only α-Mg and Al₂RE phases in the Mg-11Gd-2Y-1Zn-5Al alloy. With the increase of Al content, the grain size decreased firstly and then increased. When the Al content was 1 wt.%, the grain size of the alloy was the minimum value (28.9 μm). The ultimate tensile strength and elongation increased firstly and then decreased with increasing Al addition. And the fracture mode changed from intergranular fracture to transgranular fracture with increasing addition. When Al addition was 1 wt.%, the maximum ultimate tensile strength reached 225.6 MPa, and the elongation was 7.8%. When the content of Al element was 3 wt.%, the maximum elongation reached 10.2% and the ultimate tensile strength was 207.7 MPa.     

Effect of Ordinary Portland Cement on Mechanical Properties and Microstructures of Metakaolin-Based Geopolymers

Geopolymers have been considered a sustainable alternative to ordinary Portland cement (CEM I) for its lower embodied carbon and ability to make use of industrial by-products. Additionally, its excellent engineering properties of high strength, low permeability, good chemical resistance, and excellent fire resistance also strike a chord in the minds of researchers. The goal of this study is to clarify the effect of calcium sources on the mechanical properties and microstructures of the geopolymers. CEM I was chosen as the sole calcium source, while metakaolin was used as the source material. Five distinct geopolymers were prepared, having various ratio of CEM I: 0%, 5%, 10%, 20%, and 30%. The alkali-activator was a mixture of 12 M sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃), utilizing compressive strength and flexural strength to evaluate the changes of the geopolymers’ mechanical properties. SEM, XRD, and FTIR were used to examine microscopic features, evaluate internal morphology, and analyze changes in components of the geopolymers containing different amounts of CEM I. The experimental results indicated that the optimal incorporation of CEM I was 5%. Under this dosage, the compressive strength and flexural strength of the geopolymers can reach 71.1 MPa and 6.75 MPa, respectively. With the incorporation of CEM I, the heat released by cement hydration can accelerate the geopolymerization reaction between silica-alumina materials and alkaline solutions. Additionally, the coexistence of N-A-S-H gel from components of an aluminosilicate mix and C-S-H gel from the CEM I promoted a more densified microstructure of the geopolymers and improved the geopolymer’s strength. However, as the amount of CEM I in the mixture increased, the geopolymer matrix was unable to provide enough water for the CEM I to hydrate, which prevented excessive CEM I from forming hydration products, weakening the workability of the matrix and eventually hindering the development of geopolymer strength.     

High-Temperature Resistance of Modified Potassium Magnesium Phosphate Cement

To study the high-temperature mechanical properties of potassium magnesium phosphate cement mortar and the high-temperature resistance of its laminates. Potassium magnesium phosphate cement (MKPC) was prepared by using heavy-burning magnesium oxide and potassium dihydrogen phosphate as the main raw materials, borax as the retarder, and compounded with a certain amount of fly ash and silica fume. The effect of the mass ratio of magnesium to phosphorus (M:P), compounded fly ash and silica fume on the setting time and mechanical properties of MKPC was investigated. Furthermore, based on the better M:P, the compressive strength of MKPC mortar was studied after 3 h of constant temperature at 400 °C, 600 °C, and 800 °C, and the effect of fly ash and silica fume on the high-temperature resistance of MKPC was analyzed. The high-temperature resistance of MKPC was further evaluated by analyzing the temperature variation of potassium magnesium phosphate cement laminate during a constant temperature of 650 °C for 3 h. The results showed that the mechanical properties of potassium magnesium phosphate cement were influenced by different raw material ratios, and the mechanical properties of potassium magnesium phosphate cement were optimal when M:P was 2:1, fly ash was 5% and silica fume was 15%. The internal temperature of MKPC laminate increased slowly with time, and its high-temperature resistance was better.     

Perspective of Using Magnesium Oxychloride Cement (MOC) and Wood as a Composite Building Material: A Bibliometric Literature Review

The building industry is known as one of the biggest consumers of natural resources and an important producer of CO₂ emissions. The biggest greenhouse gas emissions are recorded in the production of cement and metallic building materials. The purpose of this paper is to investigate if magnesium oxychloride cement (MOC) can be used as an alternative to the ordinary Portland cement in the mixture of wood–cement composite building materials in order to decrease the negative impact of the construction industry on the environment. The research methodology includes bibliometric literature research, a scientometric analysis and an in-depth discussion. The data used for the research were obtained by interrogating the ISI Web of Science database, selected using the guidelines of the PRISMA method and processed with the help of VOSviewer and Bibliometrix software. The research results indicate an increasing interest in this topic; for example, in the last five years, three times more articles were published on the subject of MOC cement than the number of all articles collected in previous years. Compared to ordinary Portland cement, MOC cement presents a good match with wood, so MOC can be a substitute for ordinary cement to manufacture wood-cement particleboard, especially for the wood species that have high incompatibility with ordinary cement.     

Effect of Fly Ash and Metakaolin on Properties and Microstructure of Magnesium Oxysulfate Cement

To improve the mechanical performance and lower the production cost of magnesium oxysulfate cement (MOSC), this article investigates the effects of single and compounded addition of metakaolin (MK) and/or fly ash (FA) on the setting time, mechanical strength, water resistance, hydration product, composition, and microstructure of the resulting cement. MOSC samples with different proportions, ranging from 0 to 30 wt.%, of FA and/or MK substituting magnesium oxide (MgO) were prepared. The microstructure was explored by scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry. The findings suggest that adding FA can delay the setting of MOSC; however, the effect of adding MK to MOSC was reversed. Furthermore, the phase composition of the MOSC hydration products was unaltered upon adding FA and/or MK, but thicker and longer 517 phase crystals were observed. FA and MK can effectively fill the large pores of MOSC through filling and nucleation effects, reduce the pore size, and form a denser microstructure, thereby improving its mechanical properties. The optimal MOSC sample was found by substituting 10 wt.% of both FA and MK, resulting in a cement that exhibited a short setting time and an incredibly high mechanical strength and density. These findings will further the development of stronger, more cost-efficient, and more water-resistant MOSC products.     

Effect of Thermal History on Microstructures and Mechanical Properties of AZ31 Magnesium Alloy Prepared by Friction Stir Processing

Hot-rolled AZ31 (Mg-2.57Al-0.84Zn-0.32Mn, in mass percentage) magnesium alloy is subjected to friction stir processing in air (normal friction stir processing, NFSP) and under water (submerged friction stir processing, SFSP). Thermal history of the two FSP procedures is measured, and its effect on microstructures and mechanical properties of the experimental materials is investigated. Compared with NFSP, the peak temperature during SFSP is lower and the duration time at a high temperature is shorter due to the enhanced cooling effect of water. Consequently, SFSP results in further grain refinement, and the average grain size of the NFSP and SFSP specimens in the stir zone (SZ) are 2.9 μm and 1.3 μm, respectively. Transmission electron microscopy (TEM) examinations confirm that grain refinement is attributed to continuous dynamic recrystallization both for NFSP and SFSP. The average Vickers hardness in the SZ of the NFSP and SFSP AZ31 magnesium alloy are 76 HV and 87 HV. Furthermore, the ultimate tensile strength and the elongation of the SFSP specimen increase from 191 MPa and 31.3% in the NFSP specimen to 210 MPa and 50.5%, respectively. Both the NFSP and SFSP alloys fail through ductile fracture, but the dimples are much more obvious in the SFSP alloy.     

Study on the Properties and Mechanism of Magnesium Oxychloride Adhesive Particleboard Modified by Fly Ash

Magnesium oxychloride adhesive (MOA) is a kind of inorganic adhesive with a low energy consumption and environmental protection. The modified particleboard with magnesium oxychloride adhesive (MOPB) has the advantages of no volatiles and has a high mechanical strength. In this study, MOA and poplar shavings were used to prepare MOPB, fly ash (FA) was used to modify and enhance the properties of MOPB; the influence mechanism on the mechanical of MOPB was studied. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were used to analysis the bonding mechanism of MOA and poplar shavings, and the composite system model of MOPB was constructed. The mechanism of modified MOPB with FA (MOPB-FA) was clarified. The results show that the use of FA effectively improved the mechanical strength of MOPB, and when the ratio of FA addition was 15 wt%, MOPB-FA’s modulus of rupture (MOR) value was 16.32 MPa, an increase of 24.5% more than before (13.11 MPa). The modulus of elasticity (MOE) value was 4595.51 MPa, an increase of 76.7% more than before (2600 MPa), and the thickness of swelling (TS) value was 0.35%, a decrease of 85.2% less than before (2.36%).     

Effect of Nano-SiO2 on the Microstructure and Mechanical Properties of Concrete under High Temperature Conditions

The aim of the research was to determine how the admixture of nanosilica affects the structure and mechanical performance of cement concrete exposed to high temperatures (200, 400, 600, and 800 °C). The structural tests were carried out on the cement paste and concrete using the methods of thermogravimetric analysis, mercury porosimetry, and scanning electron microscopy. The results show that despite the growth of the cement matrix’s total porosity with an increasing amount of nanosilica, the resistance to high temperature improves. Such behavior is the result of not only the thermal characteristics of nanosilica itself but also of the porosity structure in the cement matrix and using the effective method of dispersing the nanostructures in concrete. The nanosilica densifies the structure of the concrete, limiting the number of the pores with diameters from 0.3 to 300 μm, which leads to limitation of the microcracks, particularly in the coarse aggregate-cement matrix contact zone. This phenomenon, in turn, diminishes the cracking of the specimens containing nanosilica at high temperatures and improves the mechanical strength.     

Effect of Iron Phase on Calcination and Properties of Barium Calcium Sulfoaluminate Cement

In this paper, the effect of iron phase content on the calcination and properties of clinker and barium calcium sulfoaluminate cement was studied. The compressive strength of the samples was tested and combined with an XRD and SEM-EDS analysis, and the microstructure and composition of the barium calcium sulfoaluminate clinker and hydrated samples were characterized. The results showed that the oval-shaped particles were C₂S minerals, and the hexagonal plate-shaped or rhombohedral dodecahedral particles were C_2.75B_1.25A₃S¯. The Ba element was mainly distributed in the barium calcium sulfoaluminate region, and some of it was dissolved in C₂S; the Fe element was distributed between C_2.75B_1.25A₃S¯ and C₂S crystal grains in the form of an iron phase solid solution, which acted as a solvent. When the iron phase composition was C₄AF and the iron phase content was 5%, the early hydration and later strength were better, and the compressive strength after curing for 1, 3 and 28 days was 73.2 MPa, 97.9 MPa and 106.9 MPa, respectively. A proper amount of the iron phase can reduce the eutectic point of the sintered mature material system, increase the amount of liquid phase, reduce the viscosity of the liquid phase, effectively accelerate the migration of mineral ions and promote the formation and growth of minerals.     

Mechanical Properties and Durability of CNT Cement Composites

In the present paper, changes in mechanical properties of Portland cement-based mortars due to the addition of carbon nanotubes (CNT) and corrosion of embedded steel rebars in CNT cement pastes are reported. Bending strength, compression strength, porosity and density of mortars were determined and related to the CNT dosages. CNT cement paste specimens were exposed to carbonation and chloride attacks, and results on steel corrosion rate tests were related to CNT dosages. The increase in CNT content implies no significant variations of mechanical properties but higher steel corrosion intensities were observed.     

Effect of Ion Corrosion on 517 Phase Stability

The main hydration product and source of strength of magnesium oxysulfate cement is 5Mg(OH)₂·MgSO₄·7H₂O (known as the 517 phase). Hardened pastes containing 92.38% of the 517 phase were synthesized in this study, and the influence of different types of chloride solutions on the stability and compressive strength of the 517 phase was investigated. X-ray diffraction and the Rietveld method were used to investigate the 517 phase transition in chloride solutions. Ion chromatography and inductively coupled plasma spectrometry were used to analyze the ion concentrations of the chloride solutions. Scanning electron microscopy and mercury injection porosimetry were used to investigate the effect of ion erosion on the microstructure and pore size distribution. The results showed that the crystal structure of 517 phase remained stable upon immersion in chloride solutions (except for the CaCl₂ solution) up to 28 days, and there was no discernible attenuation in the compressive strength of the hardened pastes. Immersion of the 517 phase in CaCl₂ solution for 28 days caused Ca²⁺ ions to combine with SO₄²⁻ groups to generate CaSO₄·2H₂O, thereby decomposing the 517 phase. An increase in the concentration of magnesium and sulfate ions in the immersion solutions confirmed the decomposition of the 517 phase. Gel-like Mg(OH)₂ was observed in the microstructure of the decomposed 517 phase, and the decomposition of the 517 phase increased the porosity of the hardened pastes.     

Physical and Mechanical Properties of Novel Porous Ecological Concrete Based on Magnesium Phosphate Cement

Ecological concrete could reduce the environment impacts of the tremendous construction of infrastructures due to its favorability to plant growth. Nonetheless, the alkalinity of the ecological concrete is usually too high when using ordinary Portland cement (OPC). To solve this problem, the magnesium ammonium phosphate cement (MPC) was used to prepare a novel porous ecological concrete instead of OPC. The pH value and compressive strength of MPC were analyzed and the pore structure was evaluated. The chemical composition and morphology were investigated by an X-ray diffraction test and scanning electron microscope observation. In addition, the void ratio, compressive strength and planting-growing characteristic of MPC-based porous ecological concrete were also studied. The pH value of the MPC suspension ranged from 6.8 to 8.5, which was much lower than that of OPC. The pH value of MPC gradually increased with the increment of phosphorus/magnesium molar ratio (P/M) and the compressive strength reached a maximum value of 49.2 MPa when the P/M value was 1/4. Fly ash (FA) and ground blast furnace slag (GBFS) could improve the pore structure and compressive strength; however, the pH value was slightly increased. As the paste-to-aggregate ratio increased, the void ratio of concrete gradually decreased, while the compressive strength gradually increased. The meadow grass was planted in the MPC-based ecological concrete, and the seeds germinated in one week and showed a better growth status than those planted in the OPC-based ecological concrete.     

MOC Doped with Graphene Nanoplatelets: The Influence of the Mixture Preparation Technology on Its Properties

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.