###### Abstract  
Salt lake magnesium phosphate cement (SLMKPC) was prepared by mixing potassium dihydrogen phosphate (KH\({}_{2}\)PO\({}_{4}\)) with lithium-extracted magnesium residue (MR). The hydration-hardening process and the variation in the phase composition and microstructure of hydration products with the change of the water-cement-ratio (W/C) were studied by measuring the setting time, hydration exothermic temperature, and compressive strength of the hardened slurry. It was found that W/C ratios had significant effects on the setting time, hydration exothermal temperature curve, and compressive strength of SLMKPC. With the increase of W/C, the setting time was prolonged significantly. The exothermic hydration peak temperature first increased and then decreased, and the number of exothermic peaks gradually changed from one to two. The optimal compressive strength was obtained when W/C = 0.2-0.3. Hydration products changed significantly with the increase of W/C, the MgKPO\({}_{4}\)6H\({}_{2}\)O (MKP) in the matrix was more stable when W/C = 0.2 and 0.3 than when W/C = 0.25, 0.35, and 0.4. When W/C = 0.3, SLMKPC had a compact structure with a certain operating space and better mechanical properties. magnesium residue; magnesium potassium phosphate cement; water-to-cement ratio; preparation; property +
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Similar to periclase MgO-based binding materials, MKPC is capable of yielding quick hardness and early strength [8; 9; 10]; thus, it is extensively used in rapid repairing and biological bone repairing applications, structural coatings, porous ceramics, and waste curing [11; 12; 13]. MgO used in MKPC is dead-burned magnesia produced from magnesite ore calcined at 1500-1700 \({}^{\circ}\)C [14]. The demand for high-quality magnesite has increased greatly in recent years [15], and the production of dead-burned magnesia requires higher energy. The high cost of dead-burned MgO and the shortage of raw ore make MKPC more expensive and limit its practical applications [16]. Moreover, boric acid, alkali, and alkaline earth metal plasma are generally found in salt lake brine [17]; however, it is difficult to completely extract boron [18]. Boric acid (H\({}_{3}\)BO\({}_{3}\)) or borax (Na\({}_{2}\)B\({}_{4}\)O\({}_{7}\)\(\cdot\)10H\({}_{2}\)O) [19; 20] can significantly regulate acid-base reactions to improve the strength or workability of an MKPC system [21]; therefore, boric acid or borax are used as reaction retarders for MKPC [22]. When MR is used as a magnesium oxide raw material of MKPC, no additional retarder is required, and the retarded time can be regulated by changing the calcination temperature of MR [23; 24; 25]. The preparation of MKPC using MR as a raw material consumes a large amount of salt lake by-products to alleviate environmental pollution and reduce the product cost and also makes full use of the characteristics of boron-rich by-products. Numerous studies have been performed to explore the effects of water-to-cement ratios (W/C) [26; 27], magnesia-to-phosphate ratios (Mg/P) [9; 28; 29], retarders and additives on the properties [21; 30; 31; 32; 33], and thermal Stability [34; 35] of MKPC. It is generally noticed that W/C has important effects on the mechanical properties, hydration products, and workability of MKPC [36; 37]. The MgO raw material used in these studies was usually commercially available MgO, which has higher purity and stability, and relatively higher price. [PERSON] and [PERSON] et al. [23; 24; 25] reported that salt lake Magnesium Phosphate Cement (SLMKPC) was prepared by MR and good mechanical properties were obtained. The above studies indicate that it was feasible to use MR instead of Normal MgO for the formation of MKPC materials and the utilization of raw materials. Meanwhile, the economic cost of materials will be greatly reduced due to the reuse of waste resources. Moreover, the MgO/KH\({}_{2}\)PO\({}_{4}\) mass ratio also has a significant influence on the hydration heat curve, compressive strength, and porosity of SLMKPC.