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Article Content
Abstract
This study explores the potential of MPC in terms of high resistivity and insulation performance, addressing the safety and reliability requirements of modern electrical equipment and building materials under harsh conditions, such as high temperature and humidity. High-resistivity magnesium phosphate cement (HRMPC) was prepared by using high-insulation electrofused magnesia as raw materials and incorporating vinyl acetate-ethylene copolymer (VAE), the resistivity reaches up to 109 Ω·cm, while the compressive strength reaches 49.9 MPa. The phase transformation and microstructural evolution of magnesium phosphate cement (MPC) during hydration were investigated, along with the relationship between resistivity, strength, and hardness. The mechanism by which VAE enhances resistivity was also elucidated. During early hydration, HRMPC primarily consists of magnesium ammonium phosphate hexahydrate (MNP) crystalline regions, magnesium oxide, silica, and amorphous hydration products. As hydration progresses, water combines with magnesium ammonium phosphate to form MNP crystals, while excess water evaporates. This reduction in water content significantly increases resistivity, while the growth of MNP crystals improves strength and hardness. The added VAE forms a thin film coating the inner pore walls, obstructing electron transfer and enhancing resistivity. These findings demonstrate that HRMPC achieves high resistivity and mechanical performance through controlled hydration and VAE incorporation, making it suitable for applications in harsh environments. The discovery of MPC’s high electrical resistance facilitates its application in buildings requiring excellent insulation, such as explosion-proof and fire-resistant structures.
Introduction
In modern engineering and construction materials, the performance of insulating materials is critical [1]. Traditional insulators, such as polymers and ceramics, exhibit significant limitations under high voltage [2], [3], [4], high-frequency conditions, and harsh environments. Consequently, developing advanced insulating materials to meet stringent performance demands has emerged as a key research focus in materials science and engineering. MPC is suitable for environments requiring good insulation properties, locations with strong chemical corrosion, and projects that need rapid power restoration or quick construction due to its low conductivity, excellent acid [5], [6] and alkali resistance [7], fast setting time [8], [9], and environmentally friendly characteristics [10], [11]. Additionally, MPC exhibits fireproof properties [12], [13], which enhance the safety of electrical equipment and reduce the risk of fire. The insulation performance of magnesium phosphate-based materials is a crucial characteristic, primarily influenced by the struvite phase [14], [15], [16], the amorphous phase [17], [18], [19], and the contents of crystalline water [20], [21] and free water [22], [23] present in the material. These factors collectively determine the effectiveness and reliability of the material in electrical insulation applications.
Current research primarily focuses on the low thermal conductivity and fire resistance of MPC. Dongsoo Lee et al. [24]. found that the electrical resistivity, calculated from the measured electrical resistance, consistently increased with the curing time for all the mixtures, reflecting the evaporation of water and microstructural changes due to hydration. Pedro de Almeida Carísio et al. [25]. concluded that a minimum amount of CNT, above the percolation region, can minimize the effects of moisture variations on electrical properties. Yue Li et al. [26]. found that the appropriate incorporation of hollow glass microspheres (HGM) can enhance the compressive strength of struvite-based MPC, reduce dry density, extend structural fire resistance, and lower thermal conductivity. Fang Yuan et al. [27]. pointed out that MPC exhibits excellent fire resistance, and the addition of expandable polystyrene (EP) can further enhance this property while extending the solidification time and improving bonding strength. Xiaobing Dai et al. [28]. found that foamed MPC exhibits high porosity and low thermal conductivity, which can delay heat transfer during a fire through the release of CO2 from the pores and the dehydration of hydration products, thereby demonstrating superior fire prevention performance. Although lower thermal conductivity indicates better insulation performance, this aspect has received limited attention in the research on the insulation properties of MPC.
In cement-based materials, organic fillers are added to fill the pores of the cement, thereby enhancing their properties. Shiao Yan et al. [29]. found that the addition of VAE extended both the initial and final setting times of the grouting material while increasing the yield stress and plastic viscosity. This modification resulted in a decrease in compressive strength but improved toughness. In addition, Wang Min et al. [30]. showed that there are two types of polymer latex in the field of modified cement: one type covers the surfaces of hydration crystals and fills the cracks and pores of the cement, while the other type can react with the hydration products to form a three-dimensional network structure. These organic fillers can not only enhance the performance of the cement but also exhibit beneficial insulation properties.
However, there is a relative scarcity of studies on MPC, particularly concerning its electrical insulation properties. Existing studies primarily focus on the mechanical properties and durability of MPC, while discussions regarding its characteristics as insulating materials require further exploration. Therefore, research on magnesium phosphate-based ultra-high resistance cement holds significant academic value and presents new opportunities for its application in the field of electrical insulation. By adding organic fillers, it may be possible to further improve the insulating properties and overall application effect of MPC.
Section snippets
Materials
Magnesite, a magnesium oxide ore calcined at 2800 °C (China Haicheng Fengchi Refractory Co., Ltd.), served as the primary material in this study. Its chemical composition determined by X-ray fluorescence (XRF) analysis is presented in Table 1. The magnesite was crushed and ground according to the procedure illustrated in Fig. 1(a), followed by measurements of X-ray diffraction (XRD) and particle size. Analytical grade dihydrogen phosphate (NH4H2PO4) and borate (Na2B4O7·5 H2O), both with a
Resistivity and strength
Fig. 2 shows the change pattern of resistivity and compressive strength of HRMPC over time. As hydration time increased, both the resistivity and compressive strength of the samples increased. By the 28th day, the resistivity reached 109 Ω·cm, meeting the insulation standard for cement materials (Chinese National Standards (GB/T 2015–2017 “Electrical Insulating Cement for Refractory Materials”)). Additionally, its compressive strength exceeded 40 MPa after 7 days of hydration, demonstrating
Conclusion
- 1.
The MNP content inside HRMPC gradually increases over hydration time, resulting in a significant increase in resistivity, as well as the compressive strength and hardness of HRMPC. The resistivity reached 109 Ω·cm, the compressive strength reached 49.9 MPa, and the hardness was maintained at around 45–56 HV on the 28th day. The dense and interconnected microstructure of MNP crystals not only enhances mechanical properties but also improves electrical insulation, while the reduction in free
CRediT authorship contribution statement
Jun Chang: Conceptualization. Jing Li: Project administration, Methodology, Investigation, Data curation, Conceptualization. Xiaoyang Chen: Methodology, Data curation, Conceptualization. Yingxue Teng: Writing – review & editing, Writing – original draft, Data curation, Conceptualization. Enyu Sun: Formal analysis, Data curation, Conceptualization. Jianbin Wang: Writing – review & editing, Writing – original draft, Formal analysis, Data curation, Conceptualization. Hansen Li: Formal analysis,
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jing Li reports financial support was provided by the National Key R &D Program of China. Jun Chang reports was provided by the National Key R &D Program of China. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
