Greenhouse gas emissions are a critical problem nowadays. The cement manufacturing sector alone accounts for 8% of all human-generated emissions, and as the world’s population grows and globalization intensifies, this sector will require significantly more resources. In order to fulfill the need of geomaterials for construction and to reduce carbon dioxide emissions into the atmosphere, conventional approaches to soil reinforcement need to be reconsidered. Calcium phosphate compounds (CPCs) are new materials that have only recently found their place in the soil reinforcement field. Its eco-friendly, non-toxic, reaction pathway is highly dependent on the pH of the medium and the concentration of components inside the solution. CPCs has advantages over the two most common environmental methods of soil reinforcement, microbial-induced carbonate precipitation (MICP) and enzyme induced carbonate precipitation (EICP); with CPCs, the ammonium problem can be neutralized and thus allowed to be applied in the field. In this review paper, the advantages and disadvantages of the engineering uses of CPCs for soil improvement have been discussed. Additionally, the process of how CPCs perform has been studied and an analysis of existing studies related to soil reinforcement by CPC implementation was conducted.

Many new soil reinforcement techniques have recently emerged, the most popular of which are microbial-induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP). They are environmentally friendly and more sustainable than conventional methods for soil stabilization, however during carbonate (e.g., calcite) precipitation, ammonia (NH3) and ammonium (NH4 + ) emissions are released into air and groundwater, which are hazardous. There are techniques for removing NH4 + from the soil, however, the ammonia problem remains to be addressed. By using calcium phosphate compounds (CPCs), ammonium emissions can be eliminated by more than 90%. The precipitation of calcium phosphate occurs when the calcium and phosphorus sources interact at increasing pH of the medium. Deposition type of CPCs depends on pH of environment and Ca/P ratio of solution. The most common precipitation methods are: 1) mixing calcium and phosphorus sources directly; and 2) mixing urea and acid urease or acidic bacteria with Ca and P sources. Deposition takes place in between sand particles enhancing their contact and, therefore, strengthens the soil. Given the relatively low popularity and lack of research on CPCs for soil improvement, this review discusses soil improvement methods using CPCs, their prospects, and their limitations. In addition, it will also show differences in products when using different methods of obtaining CPCs and merits of using CPCs.

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