# Optimization and Hydration Mechanism of Ecological Ternary Cements Containing Phosphogypsum
## 4 Evolution Mechanism of the Hydration Reaction  
Figure 12 shows the ternary phase diagram of the main hydration products in the ECP system. The hydration reaction mechanism of the various raw materials in the ECP can be characterized as follows: at the initial stage of the reaction, C\({}_{3}\)A and C\({}_{4}\)AF are the first to be hydrated. Calcium sulfate dihydrate reacts with CaO and Al\({}_{2}\)O\({}_{3}\) to generate AFt needle-like crystals, which evolve from cement particles to form bridges. Subsequently, a large amount of Ca(OH)\({}_{2}\) and low-density C-S-H gels are generated via the hydration of C\({}_{3}\)S and \(\beta\)-C\({}_{2}\)S. Parts of CH are precipitated and filled in the crystal space structure formed by the low-density C-S-H gel and PG particles. Finally, the low-density C-S-H gels are further hydrated to form plate-like high-density C-S-H and C-A-S-H gels.  
The results of this study demonstrate that PBM is a three-phase composite material, where hydration is the consequence of the co-hydration of multiple minerals. The nucleation process is a reaction involving solid, liquid and gaseous phases, which passes through four stages.  
I: Physical and chemical mixing and modification treatment. The PG, PC and FA are pulverized in a certain proportion to form a slurry and dissolved to provide a good hydration environment.  
II: Modification and hydration with low calcium content. The hydration of the three materials produces plasma like [SiO\({}_{4}\)]\({}^{2-}\), Ca\({}^{2+}\), [AlO\({}_{4}\)]\({}^{5-}\), [CaSiO\({}_{4}\)]\({}^{2-}\) and OH\({}^{-}\), breaking any chemical bonds, and repolymerizing to form new hydrates on the surface particles, which adhere to the solid, gradually forming an irregular surface layer and pore structure. This phase mainly involves ions of [SiO\({}_{4}\)]\({}^{2-}\), dissolved by C\({}_{3}\)S and C\({}_{2}\)S, which react with calcium aluminate to form AFt, and then combine with Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\), dissolving the FA and producing C-S-H gels, which form the ettrinigite framework. The growth of C-S-H, the product of hydration in this phase, occurs mainly on the surface of the mineral mud particle.  
III: Secondary hydration. The hydration products react with free ions to gradually form C-S-H and irregular crystals that bridge each other to form a layer and network  
Figure 12: (**Left**) Modelled phase stability fields for the major hydrate phases within the ternary system for full binder hydration. (**Right**) Positions of the five investigated systems within the ternary system [30].  
structure and then combine to form a new silicate framework. Part of the crystals and unreacted particles are filled into the framework and pore structure, resulting in a certain strength of the materials. This phase occurs mainly because the vitreous structure of C\({}_{3}\)A in the PC is stimulated to depolymerize by the Ca(OH)\({}_{2}\) in the hydration solution. Since the C-S-H growth is intertwined at different nucleation sites on the surface of the slurry mineral particles, the inter-gel pores are formed between the products of hydration.  
IV: High calcium condensation and geoplymenation. As the hydration progresses, the insufficiently reacted plasma of Ca\({}^{2+}\), [AlO\({}_{4}\)]\({}^{5-}\), [CaSiO\({}_{4}\)]\({}^{2-}\) and OH\({}^{-}\) in the slurry and segment of the C-S-H repolymerize to form C-A-S-H. Eventually, the PBM slowly solidifies and forms with new mineral crystals and C-S-H.