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Effect of Molar Ratio of Phosphate to Magnesium on Properties of Magnesium Phosphate Cement Prepared by Natural Brucite
LI Yue, LONG Shiru, WANG Zigeng, WANG Nan
Materials Reports
2024,38(17 ):23120159 -6. DOI:10.11896/cldb.23120159
Abstract
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In this work, magnesium phosphate cement (MPC) was prepared by natural brucite without calcination. The influence of molar ratio of phosphate to magnesium (P/M) on the workability, mechanical properties, phase compositions and microstructure of the MPC was mainly investigated. The results showed that with the decrease of P/M, the fluidity of the MPC paste decreased gradually, the setting time extended, and the compressive strength increased first and then decreased. When the P/M was 1/4, the comprehensive performance of MPC was promising, which the fluidity was 110 mm, the setting time was 7 min, and the 1 d and 28 d compressive strengths were 20.0 MPa and 43.7 MPa, respectively. The microscopic test results showed that the hydration products of the MPC were MgKPO
4
·6H
2
O and Ca
10
(OH)
2
(PO
4
)
2
. After 28 d hydration reaction of the MPC, brucite did not completely reacted. With the decrease of P/M, the structural compactness of hardened matrix increased, the total porosity decreased.
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Study on Mechanical and Microscopic Properties of Magnesium Phosphate Cement-based Canvas
LIU Xiongfei, WANG Nan, HAO Yifei, LI Hui
Materials Reports
2024,38(17 ):23090003 -6. DOI:10.11896/cldb.23090003
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A new type of emergency protection material of MPC canvas was developed based on the characteristics of magnesium phosphate cement (MPC), fast hardening, high early strength, and excellent bonding strength. The effects of magnesium/phosphate molar ratio (M/P=3—8) and water-cement ratio (W/C=0.15—0.5) on the compressive and flexural strengths of MPC canvas were studied. The hydration morphology, product, and pore structure of MPC in canvas were analyzed by XRD, SEM, and X-CT, respectively. The optimization design method of MPC canvas was proposed. The experimental results showed that M/P significantly affected the mechanical properties of MPC canvas. When the molar ratio of M/P was 6, the compressive and flexural strengths of the MPC canvas reached the highest values of 26.65 MPa and 6.20 MPa at 28 d of age, respectively. W/C determined the hydration degree of MPC in canvas. Low or excessive W/C cannot ensure the integrity of MPC in canvas. 0.35 was the optimum W/C for MPC canvas, the compressive and flexural strengths of the MPC canvas reached the highest values of 26.90 MPa and 6.30 MPa at 28 d of age. The hydration products (K-struvite) reached the maximum content, and the interface between MPC and canvas fabric was compact. The porosity reached the minimum value of 7.63%, and the integrity of the MPC canvas was the optimum.
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Study on Mechanical Properties of Ultra-high Toughness Magnesium Phosphate Cement-based Composites Under Compression
FENG Hu, MIN Zhishuang, GUO Aofei, ZHU Biyang, CHEN Bing, HUANG Hao
Materials Reports
2024,38(17 ):23090058 -12. DOI:10.11896/cldb.23090058
Abstract
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Magnesium phosphate cement (MPC) is a new type of inorganic cementitious material with many advantages such as short setting time, high early strength and good bonding performance. However, MPC-based composite materials have brittle properties and low strain capacity. Engineered cementitious composites (ECC) is prepared by using high-performance fibers to improve the toughness of cement-based composites. Through fiber toughening technology, ultra-high toughness magnesium phosphate cement-based composites (UHTMC) with excellent properties of MPC and ECC can be prepared. Here, through the axial tensile test, it is confirmed that the UHTMC has excellent tensile performance, strain hardening and obvious multi-cracking behavior. The effects of fly ash (FA) (substitution amounts:0%, 15%, 30% and 45%) and curing ages (14 d and 28 d) on the mechanical properties of UHTMC under compression were analyzed by the axial compressive strength, ultimate compressive strain, compressive elastic modulus and Poisson's ratio of the specimens. The results show that the UHTMC specimens exhibit good compressive toughness. With the increase of FA substitution amount and curing age, the axial compressive strength and compressive elastic modulus also increase, but the ultimate compressive strain decreases, and the Poisson's ratio changes little. Through the axial compression stress-strain curve of UHTMC, the constitutive relation model of axial compression is proposed and established. Finally, the influence mechanism of FA substitution amount and curing age on the macroscopic mechanical properties of UHTMC under compression was analyzed at the micro level.
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Study on the Mechanism of Fly Ash in Magnesium Phosphate Cement
MENG Xiangrui, LIU Yuantao, CHEN Bing, WANG Liyan
Materials Reports
2024,38(17 ):24010084 -7. DOI:10.11896/cldb.24010084
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Fly ash (FA) was applied to modify magnesium phosphate cement (MPC) mortar by replacing a portion (≤30%) of magnesium oxide, and the reactivity and reaction mechanism of fly ash in the MPC system were investigated, the effect of FA on the physico-mechanical properties and microstructure of MPC was analyzed. The results showed that FA embodied complex physical and chemical effects in the MPC system. The spherical fly ash particles have the function of rolling bearings, which optimizes the fluidity of MPC; FA not only fills the micropores and microcracks of MPC, but also exhibits the ‘nucleating effect', which provides nucleation sites for the hydration of MPC, promotes the generation of the hydration product-struvite (MgNH
4
PO
4
·6H
2
O), densifies the microstructure and improves the mechanical properties of MPC. In addition, after the activation of hydration heat, Al
2
O
3
from FA would participate in the reaction and generate amorphous aluminum phosphate gels.
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Study of Flexural Strengthening and Toughening Effect of Basalt Minibar on Magnesium Phosphate Cement Repair Mortar
LI Wei, XIE Jian, TONG Chenglong
Materials Reports
2024,38(17 ):23120021 -9. DOI:10.11896/cldb.23120021
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In order to study the strengthening and toughening modification of basalt minibar on magnesium phosphate cement repair mortar (MPCRM), the effects of different test parameters (fiber type, fiber volume fraction, water-binder ratio and curing conditions) on flexural strength and toughness of MPCRM were analyzed by four-point bending test. The results show that the basalt minibar has excellent flexural strengthening and toughening properties for MPCRM, and its toughening effect is better than that of traditional basalt monofilament fiber. Under the condition that the volume fraction of basalt minibar is not less than 1.0%, the flexural strength and toughness of MPCRM increase with the increase of volume fraction of basalt minibar. The flexural toughness of basalt minibar reinforced MPCRM decreases with the increase of water-binder ratio, while its flexural strength increases first and then decreases. The bending performance of basalt minibar reinforced MPCRM with water-binder ratio of 0.18 and the volume fraction of 1.5% is the best. The low temperature curing conditions cause the deterioration of the bending performance of MPCRM, the incorporation of basalt minibar can partially compensate for the frost damage of MPCRM.
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Research Progress on High-temperature Performance of Magnesium Phosphate Cement
LI Xiao, ZHAO Yingying, ERBUDUREXITI Guliziba, JIA Xingwen, QIAN Jueshi
Materials Reports
2024,38(17 ):23120217 -8. DOI:10.11896/cldb.23120217
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Magnesium phosphate cement (MPC) with excellent high-temperature performance has good application prospects not only in steel structure protection, radioactive and hazardous waste solidification fields, but also in high-temperature scenarios such as high-end equipment thermal protection and aircraft thermal barrier coatings. The paper summarizes the hydration mechanism of MPC and its physical and mechanical properties at high temperatures, as well as the evolution law of hydration products. Then the basic research progress and existing problems of the application of MPC in steel structure fire prevention, radioactive and hazardous waste solidification are summarized, and the suggestions for further improving the high-temperature performance of MPC are proposed from three main directions, including insulation performance, mechanical performance, and high-temperature volume stability, so as to provide reference for promoting the practical application of MPC in high-temperature scenarios.
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Mechanical Properties and Freeze-Thaw Cycling Resistance of Magnesium Phosphate Cement Mortar Prepared at Low Temperatures in Highland Regions
CHEN Xin, LIU Wen, CUI Anqi, ZHENG Haitao, HUANG Xin, YANG Wencui, GE Yong
Materials Reports
2024,38(17 ):23120019 -9. DOI:10.11896/cldb.23120019
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Magnesium phosphate cement mortar is particularly suitable for repairing damaged concrete structures in high-and-cold regions due to its outstanding strength development potential at low temperatures and its characteristic of not requiring heat retention and moisture preservation for curing. The study utilized on-site experimental methods, employing variables such as molding temperature, magnesium-phosphate mass ratio, and curing mode, to investigate the flexural and compressive strengths of magnesium phosphate cement mortar under various conditions at different ages, thereby clarifying the development patterns of its mechanical properties in high-altitude and low-temperature conditions. Further, the study identified the factors affecting the freeze-thaw cycling resistance of magnesium phosphate cement mortar formed at low temperatures in high-altitude regions, using molding altitude, temperature, and magnesium-phosphate mass ratio as variables, and air-void structure and water saturation as intermediate indicators. The scaled mass of the surface exposed to salt solution during unilateral freeze-thaw cycles and changes in relative mass and dynamic elastic modulus during freeze-thaw cycles were used as direct indicators. The findings reveal that natural curing, compared to constant low-temperature curing, is beneficial for early strength development but has a negative impact on later strength development. Low-temperature forming benefits the later strength of magnesium phosphate cement mortar under natural curing. Each of the molding altitude, temperature, and magnesium-phosphate mass ratio of mortar negatively correlates with the air content, but positively correlates with the air-void bubble spacing coefficient and water saturation. Under experimental conditions, lower values of these three variables enhance the mortar's freeze-thaw cycling resistance. A molding temperature of -10 ℃ with a magnesium-phosphate mass ratio of 4 is most beneficial for mortar strength, while a ratio of 3 is most advantageous for freeze-thaw cycling resistance. These research outcomes provide technical recommendations and theoretical support for the application of magnesium phosphate cement mortar in high-and-cold regions.
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Review of Magnesium Cementitious Material:Current Developments and Low Carbon Path
ZENG Tian, CHEN Xiaoyang, WANG Nan, ZHANG Tingting, GUAN Yan, BI Wanli, CHANG Jun
Materials Reports
2024,38(17 ):24010170 -15. DOI:10.11896/cldb.24010170
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Magnesium cementitious material has the characteristics of light weight, high strength, low alkali, high temperature resistance and low carbon dioxide emissions, it is widely used in prefabricated construction, road repair, nuclear waste fixation and other fields. Here systematically reviews the main research results of magnesium cementitious material in recent years, focuses on the raw materials, hydration mechanism, phase transition and crystal microstructure of magnesium oxychloride cement, magnesium oxysulfide cement, magnesium silicate hydrate cement and magnesium phosphate cement. Then summarises the research progress in carbon sequestration, mechanical performance and durability. Furthermore, provides a comparative analysis of the performance and current applications of these four types of cement. Finally, considering the shortages and advantages of magnesium cementitious material, highlight the research thoughts and directions in the future. China has the most abundant magnesia resource in the world. As an important part of magnesite industry, magnesite cementitious material has unique properties, and will certainly have great potential in the utilization of low-grade magnesite tailings as building materials.
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Influence of Mg(OH)
2
on the Hydration Process and Properties of Magnesium Phosphate Cement
CHEN Jiawei, ZHANG Yunqiao, CHEN Zhuofan, LIU Zhi, LI Jun, LU Zhongyuan, LAI Zhenyu
Materials Reports
2024,38(17 ):24010085 -7. DOI:10.11896/cldb.24010085
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The magnesium raw material for magnesium phosphate cement (MPC) is mainly dead-burned MgO. However, the high-temperature calcination required for producing dead-burned MgO results in high costs and excessive carbon emissions. magnesium hydroxide (Mg(OH)
2
) is abundant, has low production costs, and is environmentally safe and pollution-free. Mg(OH)
2
is a rich source of low-cost, non-polluting, safe and environmentally friendly, which is of great significance to alleviate the problem of high cost of use and carbon emissions if it can be adopted in large quantities. Therefore, in this work, Mg(OH)
2
was used instead of partially dead-burned MgO, by means of tests such as X-ray diffraction, thermal analysis, scanning electron microscopy and energy spectrum analysis, the effects on workability, hydration process, strength development and microstructure were investigated. The results showed that the incorporation of Mg(OH)
2
accelerated the hydration process of MPC, the fluidity of the slurry was reduced and the setting time was shortened. In this paper, the working properties are improved by retardation means such as water glass heat treatment modification. The hydration temperature and pH of magnesium phosphate cement decreased significantly with the incorporation of Mg(OH)
2
, increased number of K-struvite crystals in the later 28 d, compressive strength decreases with increasing Mg(OH)
2
content. When Mg(OH)
2
is doped within 50%, its late strength decreases in the range of 15%, with the increase of Mg(OH)
2
doping, more hydration products are formed by the reaction of Mg(OH)
2
. When Mg(OH)
2
doping accounted for 66.6%, the reactivity of dead-burned MgO was reduced from an average value of 22% to 7%. In conclusion, the preparation of magnesium phosphate cement with large amount of Mg(OH)
2
is feasible and can ensure that the later mechanical properties of the material can meet the requirements of engineering applications. It has potential applications for reducing the cost of magnesium phosphate cement use and carbon emissions.
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Effect of Sodium Gluconate on the Basic Properties of Magnesium Silicate Potassium Phosphate Cement
YANG Yizhe, LIN Xujian, XU Xiaoying, LIN Hengzhou, CHEN Weiyu, YE Caifa
Materials Reports
2024,38(17 ):23080008 -6. DOI:10.11896/cldb.23080008
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Magnesium silicon potassium phosphate cement (MSPPC) is an innovative high-performance magnesium phosphate cement, which is different from ordinary magnesium potassium phosphate cement (MKPC) using acidic potassium dihydrogen phosphate. Its necessary raw materials include basic dipotassium hydrogen phosphate, magnesium oxide and silica fume. MKPC is difficult to prepare without retarder, while MSPPC can be prepared without retarder due to its long setting time. After hardening, MSPPC has better mechanical properties than MKPC, but its working performance has not been able to meet the needs of practical engineering applications. The setting time and fluidity can be adjusted by the amount of retarder. Borax is a commonly used retarder for magnesium phosphate cement, but its potential toxicity problems forces the need to find a safe and effective alternative. This study investigated the influence of sodium gluconate (SG) content on the performance, compressive strength, hydration temperature, pH value, phase composition, porosity, and microstructure of MSPPC, and established a model for the retarding mechanism. The results demonstrated that a higher SG content led to a more pronounced retarding effect. At the SG dosage was 6%, the initial setting time was extended from 12 min to 30 min, and the cement paste fluidity improved from 84 mm to 142 mm. The proportion of pores larger than 0.1 μm was reduced, effectively improving pore size distribution. Although the type of hydration products did not change, there was an adverse effect on the 3 d compressive strength, from 64.3 MPa to 53.9 MPa, a decreased by 16.2%, while the 56 d compressive strength decreased by 6.99% and the adverse effect was reduced.
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