State-of-the-Art Review on Engineering Uses of Calcium Phosphate Compounds: An Eco-Friendly Approach for Soil Improvement

Abstract

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.

Soil Improvement Using Calcium Phosphate Compounds as a Novel Sustainable Method: A Review

“…This has resulted in the legislation in Japan where the maximum concentration of ammonium in industrial wastewater <100 mg/L [12]. As a byproduct of MICP and EICP, 10.500 mg/L and 11.200 mg/L of ammonia and ammonium pollute environment, respectively [13]. These emissions are 100 times higher than the limits, which has led to only a handful of studies conducted in large-scale applications [14].…”

“…In the initial stage, the aqueous solution becomes saturated with calcium and phosphorus ions, followed by reaching thermodynamic equilibrium. Thermodynamic equilibrium is the state at which, at constant pH, the solution is completely saturated without any excess calcium and phosphorus ions, enabling precipitation of CPCs [13].…”

“…From eleven well-known CPCs with a molar ratio of Ca/P in the range from 0.5 to 2.0 (Table 1), there are only six compounds that can be precipitated from aqueous solutions at temperatures below 100 o C: MCPM, DCPD, OCP, ACP, CDHA and HA. Other compounds required high temperature and can harden as a result of solidphase reaction [13].…”

“…Table 2 Major properties of CPCs [13] Compound According to the final product of deposition, CPCs can be divided into two groups: apatitic and brushite CPCs. The desired end product can be predicted depending on the solubility of CaP precursor and the pH of setting reaction.…”

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“…It could have a wide range of practical applications in the future [ 47 ]. There are two main types of biocement technology: MICP and EICP [ 48 ]. The mechanism of MICP is the use of the urea decomposition capacity of microorganisms to metabolize urea, producing ammonium and carbonate.…”