Experimental Study On The Magnesium Potassium Phosphate Cement Prepared By The Byproduct Of Magnesium Oxide After Producing Li₂CO₃ From Salt Lakes

Magnesium phosphate cement(MPC) is a kind of air hardening cement. MPC possesses many excellent characteristics, such as very rapid setting, high early strength, high bond and compatibility with Portland cement concrete, low shrinkage and very good frost-resistance and so on, which lead to more applications today. But the cost of conventional MPC is very high, which limits their application in industry. In this paper, it’s the first time to prepare MPC with magnesium oxide containing boron after producing Li2CO3 from salt lakes, rather than dead-burned magnesium oxide.Firstly, magnesium oxide containing boron was heat treated at various temperatures, and the physical and chemical properties of them after heat treating were measured. Then according to systemic experiments, the effect of heat treating temperature, M/P ratio, and the kind of phosphate on the properties of MPC was investigated. According to the contrast of these properties, best conditions were gained, and the phase composition and morphology were also analyzed. Furthermore, mixed minerals were added at best conditions. By contrast the change of properties of MPC after curing at air, fresh water and simulated seawater, and the change of the phase composition and morphology, the suitable quantity of mixed minerals was also obtained. The MPC prepared with these raw materials not only decreased the cost, but also achieved the cyclically using of resource in salt lakes. The main work is as following:(1) Boron-containing magnesium oxide was heat treated at various temperatures. The phase composition and morphology of boron-containing magnesium oxide at various temperatures were tested using SEM and XRD-Topas. Their particle size distribution, specific surface area and total pore volume were also measured by LSPA(Laser particle size analyzer) and SNAA(Static nitrogen adsorption analyzer). The content of activated magnesium oxide was tested using hydration method. The results indicated that the temperature of heat treating had an effect on the phase composition of boron-containing magnesium oxide. A large amount of Mg(OH)2 was in raw material before heat treating, but Mg(OH)2 almost vanished and the content of Mg O increased after heat treating, which indicated that all Mg(OH)2 transformed to Mg O. In addition, the defect of Mg O crystal decreased, so the specific surface area, deformation ratio, total pore volume and the content of activated Mg O decreased with the increase of the heat treating temperature.(2) By setting time, compressive strength at early and later periods, the cost, the effect of heat treating temperature of boron-containing magnesium oxide, M/P ratio and the kind of phosphate on the properties of MPC was systematically investigated. The phase composition and morphology of MPC at various times prepared by boron-containing magnesium oxide after heat treating were analyzed by SEM and XRD-Topas. The results indicated that the optimized condition to prepare inexpensive and high performance MPC were that the heat treating temperature was 1000℃~1200℃, the M/P ratio was 6 and the kind of phosphate was KH2PO4.(3) According to the optimized conditions for preparation, the effect of various contents of fly ash and slag on the setting time, compression strength, water resistance and corrosion resistance at seawater of MPC were obtained. The results indicated that the setting time of MPC expanded with the increase of the content of mixed minerals. The reason was that the mixed minerals decreased the contact point between Mg O and KH2PO4. When the content of fly ash was not more than 30%, and the content of slag was not more than 10%, the later compression strength of MPC remained a constant. While with the increase content of fly ash and slag, the compression strength of MPC decreased, but the water resistance and corrosion resistance at seawater of MPC increased. Among these conditions, when the content of fly ash was 40% and the content of slag was 20%, the HPMPC not only satisfied the demand of quick-repair material, but also obviously improved the water resistance and corrosion resistance at seawater. After the HPMPC was immersed in water and simulated seawater for 60 d, the softening coefficiency and seawater corrosion resistance coefficiency were 0.83-0.81 and 0.97-0.86, respectively. In addition, the cost of MPC can be obviously reduced by adding mixed minerals.(4) The phase composition and morphology were analyzed.(a) The phase composition and morphology of HPMPC prepared at optimized conditions were measured by SEM and XRD-Topas. The results indicated that the main hydration products of HPMPC was Mg KPO4·6H2O(MKP). With the curing time expanding, the content of MKP increased, while the content of Mg O and KH2PO4 decreased, which indicated that hydrate reaction proceeded with the expanding of curing time. The morphology showed that the hydration product was bulk crystal when the HPMPC was hydrated at 3 h. It indicated that the hydration product was not formed. A large amount of MKP crystal formed when the curing was 28 d.(b) There was an effect of mixed minerals on the hydration product and morphology of HPMPC. After adding mixed minerals, the hydration product was MKP. With the curing time expanding, the content of MKP increased, which indicated that the hydrate reaction proceeded. The morphology indicated that a large amount of bulk MKP gel formed when the curing time was 3 h. The mixed minerals showed the eruption effect here, so that MKPAS gel formed and covered the surface of the mixed minerals. With the curing time expanding, a large amount of tenuis stick MKP crystal formed and the surface of MKP crystal was smooth, which indicated that MKP and MKPAS gel had transformed to MKP crystal.(5) According to the phase composition, the content of MKP increased with the curing time expanding at fresh water and simulated seawater. The hydrate reaction of mixted minerals HPMPC still proceeded. The morphology showed that there were many pore on mer after the mixed minerals HPMPC paste was immersed for 60 d. The main reason was that some immature gel was resorpted in fresh water and seawater. While the HPMPC was very compact after immersed for 60 d. The reason of this phenomenon was the mixed minerals not only lead to eruption effect, but also exerted microaggregate and adsorption effect. There were some shapeless MKP and MKPAS in mer. With the immersing time of the shapeless gel expanding, core formation took place and immature MKP crystal formed. So mixted minerals HPMPC had high water resistance and seawater corrosion resistance. The reason about why the water resistance and seawater corrosion resistance of slag HPMPC was better than the water resistance and seawater corrosion resistance of fly ash HPMPC was that the C-S-H gel formed by the hydration of the aspect of slag. The C-S-H gel further increased the water resistance and seawater corrosion resistance.