Experimental and Numerical Simulation on the Penetration for Basic Magnesium Sulfate Cement Concrete.

The penetration resistance of the new material Basic Magnesium Sulfate Cement (BMSC) is studied through comprehensive application of an experimental and numerical simulation method. This paper consists of three parts. The first part introduces the preparation of Basic Magnesium Sulfate Cement Concrete (BMSCC) and the study of its dynamic mechanical properties. In the second part, on-site testing was carried out on both BMSCC and an ordinary Portland cement concrete (OPCC) target, and the anti-penetration performance of the two materials was analyzed and compared from three aspects: penetration depth, crater diameter and volume, and failure mode. In the last part, the numerical simulation analysis was carried out based on LS-DYNA, and the effects of factors, such as material strength and penetration velocity on the penetration depth, are analyzed. According to the results, the BMSCC targets have better penetration resistance performance than OPCC under the same conditions, mainly manifested in smaller penetration depth, smaller crater diameter and volume, as well as fewer cracks.

Solidification/stabilisation behaviours of Zn2+ in magnesium potassium phosphate cement: Experiments and density functional theory study.

The solidification/stabilisation behaviours of Zn2+ in magnesium potassium phosphate cement (MKPC) have not been thoroughly investigated. Herein, a series of experiments and a detailed density functional theory (DFT) study were conducted to investigate the solidification/stabilisation behaviours of Zn2+ in MKPC. The results showed that the compressive strength of MKPC reduced with the addition of Zn2+ because the formation of MgKPO4·6H2O (the main hydration product in MKPC) was delayed with the addition of Zn2+, as discovered by the crystal characteristics, and because Zn2+ exhibited a lower binding energy in MgKPO4·6H2O compared to Mg2+, as revealed by DFT results. Additonally, Zn2+ had little influence on the structure of MgKPO4·6H2O, and Zn2+ existed in MKPC as the formation of Zn2(OH)PO4, which was decomposed in the range of around 190-350 °C. Moreover, there were a lot of well-crystallised tabular hydration products before the addition of Zn2+, but the matrix was comprised of irregular prism crystals after adding Zn2+. Furthermore, the leaching toxicity of Zn2+ of MKPC was much smaller than the requirements of Chinese and European standards.

Optimization of Magnesium Potassium Phosphate Cements Using Ultrafine Fly Ash and Fly Ash.

The effect of ultrafine fly ash (UFA) and fly ash (FA) on the physical properties, phase assemblage, and microstructure of magnesium potassium phosphate cement (MKPC) was investigated. This study revealed that the UFA addition does not affect the calorimetry hydration peak associated with MKPC formation when normalized to the reactive components (MgO and KH2PO4). However, there is an indication that greater UFA additions lead to an increased reaction duration, suggesting the potential formation of secondary reaction products. The addition of a UFA:FA blend can delay the hydration and the setting time of MKPC, enhancing workability. MgKPO4·6H2O was the main crystalline phase observed in all systems; however, at low replacement levels in the UFA-only system (<30 wt %), Mg2KH(PO4)2·15H2O was also observed by XRD, SEM/EDS, TGA, and NMR (31P MAS, 1H-31P CP MAS). Detailed SEM/EDS and MAS NMR investigations (27Al, 29Si, 31P) demonstrated that the role of UFA and UFA:FA was mainly as a filler and diluent. Overall, the optimized formulation was determined to contain 40 wt % fly ash (10 wt % UFA and 30 wt % FA (U10F30)), which achieved the highest compressive strength and fluidity and produced a dense microstructure.

Immobilization of Zn(â ¡) and Cu(â ¡) in basic magnesium-sulfate-cementitious material system: Properties and mechanism.

To solve the environmental problems caused by heavy metal pollution, a new cementitious material (basic magnesium sulfate cement, BMSC) was developed for the solidification of Cu2+/Zn2+. First, the effects of different amounts of Cu2+/Zn2+ on the properties (compressive strength, setting time, pH, and leaching toxicity) of the BMSC were investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the effects of different amounts of Cu2+/Zn2+ on the phase and microstructure of BMSC. The results showed that Cu2+/Zn2+ inhibited the hydration of BMSC, reduced compressive strength, and prolonged the setting time. The results of the leaching tests showed that the BMSC system exhibited high immobilization efficiency (up to 99%) for Cu2+/Zn2+. Further, the BMSC solidification matrix exhibited excellent acid resistance (compressive strength >40 MPa after 28 days of immersion). The physical phase analysis showed that the main phases of BMSC were the 5Mg(OH)2-MgSO4-7 H2O (5-1-7) phase and Mg(OH)2, and the crystal structure refinement analysis suggested that Cu2+/Zn2+ ions were substituted with Mg2+ in the 5-1-7 phase. It was confirmed that the solidification mechanism of BMSC on Cu2+/Zn2+ is mainly performed by chemical complexation and ionic substitution.

Preparation of magnesium potassium phosphate cement using by-product MgO from Qarhan Salt Lake for low-carbon and sustainable cement production.

Herein, to reduce CO2 emissions and energy consumption and to promote the recycling of waste resources, two types of boron-containing MgO by-products, which were obtained by lithium extraction from Qarhan Salt Lake, China, were used as substitutes for dead-burned MgO to prepare magnesium phosphate potassium cement (MKPC) as a rapid repair material. First, the phase composition and particle-size distribution of the MgO by-product were investigated. The effects of different MgO sources, molar ratio of MgO to KH2PO4 (M/P), and curing age on the setting time and mechanical properties of MKPC were then studied. Based on the results, the mix proportion of MKPC was optimized. Finally, X-ray diffractometry, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), differential thermogravimetric (DTG) analysis, and mercury intrusion porosimetry were used to characterize the phase and microstructure evolution of MKPC prepared with different MgO contents. The results demonstrated that the by-product MgO prolonged the setting time of MKPC to more than 40 min. In addition, in the initial stage of hydration, the compressive strength of the MgO by-product was slightly lower than that of the dead-burned MgO; however, with increasing age, the mechanical properties of MKPC prepared by by-product MgO were excellent (up to 60 MPa). The phase and microstructure results revealed that the main hydration product of MKPC prepared using the three types of MgO was MgKPO4·6H2O. Combined with the physical and chemical properties of the raw materials, it was confirmed that the larger particle size and the coexisting impurities from the salt lake were the main reasons for the longer setting time of the MKPC prepared by the by-product MgO. We believe that this research will be of great significance for the preparation of low-carbon, low-cost, and high-performance MKPC materials.

Designing novel magnesium oxysulfate cement for stabilization/solidification of municipal solid waste incineration fly ash.

Municipal solid waste incineration (MSWI) fly ash is a typical hazardous waste worldwide. In this study, an innovative magnesium oxysulfate cement (MOSC) binder was designed for stabilization/solidification (S/S) of MSWI fly ash, focusing on the interactions between MOSC binder and typical metallic cations (Pb2+)/oxyanions (AsO33-). Experimental results showed that Pb and As slightly inhibited the reaction of high-sulfate 5MS system but significantly suppressed the reaction process of low-sulfate 10MS system. The 5MS binder system exhibited excellent immobilization efficiencies (99.8%) for both Pb and As. The extended X-ray absorption fine structure spectra suggested that Pb2+ coordinated with SO42-/OH- in the MOSC system and substituted Mg2+ ion sites in the internal structure of 5Mg(OH)2·MgSO4.7H2O (5-1-7) phase. In contrast, the AsO33- substituted SO42- sites with the formation of inner-sphere complexes with Mg2+ in the large interlayer space of the 5-1-7 structure. Subsequent MSWI fly ash S/S experiments showed that a small amount of reactive Si in MSWI fly ash interfered with the MOSC reaction and adversely influenced the immobilization efficiencies of Pb, As, and other elements. Through the use of 33 wt% tailored MOSC binder for MSWI fly ash treatment, a satisfying S/S performance could be achieved.

Properties and microstructure of basic magnesium sulfate cement: Influence of silica fume.

Silica fume (SF) as an important supplementary cementitious material has been widely used in Portland cement, but few published articles have reported on the effect of SF on the performance and hydration mechanism of basic magnesium sulfate cement (BMSC). In the present work, the properties, microstructure and hydration mechanism of BMSC influenced by SF was studied systematically. The results show that the setting time and compressive strength of BMSC may increase with the increase of SF content, while the hydration heat will decrease with the increase of SF content. Mercury intrusion porosimetry (MIP), X-ray computed tomography (X-CT), scanning electron microscope- Energy dispersive spectrometer (SEM-EDS) results show that SF exhibits filling effect in the BMSC matrix, which makes the microstructure of BMSC matrix with SF more compact. In addition, solid-state magnetic resonance (NMR) and SEM-EDS analysis indicate that the activity of SF was excited in the BMSC matrix, resulting in the formation of M-S-H gel.

High-efficiency and low-carbon remediation of zinc contaminated sludge by magnesium oxysulfate cement.

Electroplating sludge is classified as a hazardous waste due to its extremely high leachability of potentially toxic elements. This study concerns the use of magnesium oxysulfate cement (MOSC) for the stabilisation/solidification (S/S) of Zn-rich electroplating sludge. According to X-ray diffraction and thermogravimetric analyses, Zn was mainly immobilised through both chemical interaction and physical encapsulation in the MOSC hydrates of 5Mg(OH)2·MgSO4.7H2O (5-1-7) phase. The crystal size analysis, elemental mapping, and extended X-ray absorption fine structure (EXAFS) analysis proved that the Zn2+ was also incorporated in the structure of 5-1-7 phase. Unlike Portland cement system, hydration kinetics, setting time, and compressive strength of the MOSC system were only negligibly modified by the presence of Zn, indicating its superior compatibility. Subsequent S/S experiments demonstrated that the MOSC binder exhibited an excellent performance on immobilisation efficiency of Zn (up to 99.9%), as well as satisfying the requirements of setting time and mechanical strength of sludge S/S products. Therefore, MOSC could be an effective and sustainable binder for the treatment of the Zn-rich industrial wastes.

Investigation on the Corrosion Behavior of Steel Embedded in Basic Magnesium Sulfate Cement Concrete: An Attempt and Challenges.

Basic magnesium sulfate cement is a type of green high-performance cementitious material. In order to exert its performance advantages and expand its application field, it is urgent to study the problem of steel corrosion in basic magnesium sulfate cement concrete (BMSCC). In this paper, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) were used to study the corrosion behavior of steel bars in different strength grades of BMSCC in seawater. Based on the relationship between the corrosion current density and the immersion time, the corresponding time-varying model was obtained. The LPR and EIS results show that the corrosion potential and polarization resistance of steel bars in BMSCC decreased with the immersion time in the seawater environment. The fitting analysis indicated concordance between the corrosion rates with the logarithmic function time-varying model. Furthermore, the cracking time of the protective layer of BMSCC was analyzed based on the cracking time prediction model of Portland cement concrete and the mechanical properties of BMSCC.

Study on the injectability of a novel glucose modified magnesium potassium phosphate chemically bonded ceramic.

A novel magnesium potassium phosphate chemically bonded ceramic (MKPCBC) was prepared as a byproduct of boron-containing magnesium oxide (B-MgO) after extracting Li2CO3 from salt lakes. In this work, the influence of glucose on the properties of MKPCBC, such as the setting time, compressive strength and hydration heat, was investigated. In addition, we studied the effect of the magnesium-phosphate ratio (M/P) and liquid-solid ratio (L/S) on the injectability of MKPCBC. The pH change in glucose modified MKPCBC paste was also investigated. The phase composition and microstructure were studied in detail by using X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS). The results show that the optimal content of glucose is 6wt%. The optimum proportions of M/P and L/S for MKPCBC are 1.5 and 0.25, respectively. The properties of the novel MPCBC can meet the requirements of biomaterials. In addition, the retardation mechanism of glucose on MKPCBC and the hydration mechanism of novel MKPCBC were studied in detail through the continuous monitoring of the phase composition and microstructure.