Magnesium phosphosilicate cement (MPSC) is a novel phosphate bonded cement, which consists mainly of magnesia, phosphate and silicate minerals. The traditional magnesium phosphate cements (MPCs) usually composed by ammonium phosphate, and gaseous ammonia will emit during mixing and in service. There is no noxious ammonia released from MPSC, furthermore, it can recycle a large volume of the non-hazardous waste. The goal of this research is to investigate the composition, reaction products, reaction mechanism, microstructure, properties, durability and applications of the MPSC....[ Read more ]
Magnesium phosphosilicate cement (MPSC) is a novel phosphate bonded cement, which consists mainly of magnesia, phosphate and silicate minerals. The traditional magnesium phosphate cements (MPCs) usually composed by ammonium phosphate, and gaseous ammonia will emit during mixing and in service. There is no noxious ammonia released from MPSC, furthermore, it can recycle a large volume of the non-hazardous waste. The goal of this research is to investigate the composition, reaction products, reaction mechanism, microstructure, properties, durability and applications of the MPSC.
MPSC sets rapidly and has high early strength. It reacts better with solid industrial waste when compared to Portland cement. Many solid industrial wastes, such as fly ash, steel slag, coal gangue, red coal gangue, red mud, barium-bearing slag, copper slag, silica fume, and ground granulated blast furnace slag, have been used as the main component (40% by weight) in MPSC. The research has found that these aluminosilicate (or ironsilicate, or calciumsilicate) minerals with an amorphous or glass structure can enhance the performance of MPSC. The disorganized internal structure of amorphous materials may make it possess higher reactivity compared to the crystalline phases. Chemical reaction between phosphate and these minerals may form an amorphous gel, which is favorable to the cementing. Borax, boric acid and sodium tripolyphosphate have been used as retardants in the MPSC system. It is found that boric acid has a higher retarding effect on the setting of cement, than borax does. However, sodium polyphosphate accelerates the reaction of MPSC. The hydration of MPSC is exothermic reaction. The heat evolution may prompt hydrates formation, and shorten the setting process.
Modern materials characterization techniques, XRD, DSC, TG-DTA FTIR, XPS, MAS-NMR, SEM, TEM, MIP, etc. were used to analyze the phase composition, micro morphology, and microstructure of hardened MPSC. The main hydration product in MPSC is MgKPO4⋅6H2O (MKP), which has both crystalline and amorphous phases. There are many unreacted magnesia grains in the hardened MPSC paste. They act as nucleus of the hardened framework. The hydrates grow around the magnesia grains rims, fill in the voids among the magnesia grains and bond unreacted magnesia part into a solid continuum. The reaction mechanism, such as setting, exothermic reaction and bonding are explained according to the crystallization chemistry, formation thermochernistry, reaction kinetics of MgKPO4⋅6H2O, and solid chemistry. The structure of MgKPO4⋅6H20 consists of PO43+ tetrahedra, Mg(H2O)62+ octahedra, and K+ groups, held together by hydrogen bonds. Research shows that the formation of MgKPO4⋅6H20 is especially sensitive to pH value, and the induction and nucleation period seemed very short in MKP formation. During hardening of the cementitous materials, the electrical conductivity of the paste will change with the microstructure formation. In the hydration of MPSC, the conductivity change was monitored by an electrodeless resistivity meter recently developed in HKUST. The results show that the conductivity of MPSC decreases quickly during hydration, which is caused by the rapid setting and hardening process. The smaller the conductivity of the cement paste, the denser is the microstructure, and the higher is the strength development.
A micro structural model of the hardened MPSC paste was put forward, according the comprehensive analysis. MPSC has a very low open porosity and low water saturation, but many discrete and closed pores. The special microstructure makes the mechanical properties and durability of MPSC better. Comparing to Portland cement, MPSC has superior deicer freezing-thawing scaling resistance, excellent resistance to high temperature and abrasion, slight expansive in volume instead of shrinkage, rapid strength development under negative temperature, very good chemical corrosion resistance in sulfate solutions and in sea water. Therefore, it is a high performance cementitious material for the sustainable development. MPSC has the similar elastic modulus with Portland cement, very suitable for rapid repair of damaged Portland cement concrete structures. It is also very suitable for shotcrete applications with dry mix process. MPSC can be expediently utilized in civil engineering, military affairs, and environmental protection, etc.