Synergy effect of synthetic ettringite modified by citric acid on the properties of ultrafine sulfoaluminat cement-based materials

In our previous study, ultrafine ettringite was prepared in the presence of citric acid by using a nucleation and aging separation method. Due to the seed effect, ultrafine ettringite enhanced the hydration rate and compressive strength of sulfoaluminat cement-based materials at all ages. However, when the particle size of sulfoaluminat cement-based materials became smaller, ultrafine ettringite retarded the hydration rate of ultrafine sulfoaluminat cement-based materials (USCM). In this paper, ettringite was prepared under different citric acid concentrations and characterized by XRD, FT-IR, TG-DSC, PSD and TOC measurements. The effect of ettringite on the properties of USCM was experimentally studied. Results showed that citric acid can absorb on the surface of ettringite. The average particle size of the citric acid-ettringite composites（CECs）were in the range of 100–300 nm, and increasing the concentration of citric acid in composites lead a smaller size of CECs. When present in the alkaline solution of pH 13, the higher citric acid concentration used in the preparation of CECs, the more amount of tartaric acid released from CECs. CECs retarded the cement hydration, modified the pore structure and enhanced the compressive strength of USCM pastes at all ages due to the synergistic effect of nano crystal nucleus and citric acid released from CECs.