Preparation of acid aluminum phosphate solutions for metakaolin phosphate geopolymer binder

This work assessed the potential of synthetic acid aluminum phosphate solutions for the enhancement of the characteristics of metakaolin phosphate geopolymer binders obtained at room temperature. The main parameters dealt with are the concentration of the initial phosphoric acid solution (40 wt%, 50 wt%, and 60 wt%) and the molar ratio Al/P (1/3 and 1.4/3) of the synthesized acid aluminum phosphate solutions. The prepared solutions have different contents and types of mono aluminum phosphate compounds (MAP) and their reactivity is pH-dependent. This is because of the continuous neutralization of the protons due to the dissolution of aluminum hydroxide, which raises the pH and decreases the conductivity. Thus the acid aluminum phosphate solutions with molar ratio Al/P of 1/3 are the most reactive. They have significantly enhanced the compressive strength of the resulting phosphate geopolymer binders. But, when compared to phosphate geopolymer obtained with pure phosphoric acid of the same concentration, the highest rate of compressive strength improvement is recorded for acid aluminum phosphate solutions having an initial concentration of phosphoric acid of 40 wt%. Thus, the modification of the composition of the phosphoric acid with the addition of the appropriate amount of aluminum is beneficial for enhancing the characteristics of phosphate geopolymer binder at any age.