Synthesis of belite sulfoaluminate-ternesite cements with phosphogypsum

Phosphogypsum (PG) is an industrial byproduct from phosphoric acid production. In order to facilitate the use of PG, belite sulfoaluminate-ternesite cements were produced with high amounts of PG. This work regards the laboratory production of belite sulfoaluminate-ternesite cements by using a new synthesis method. Using a secondary heat treatment step, the method achieves the coexistence of ye'elimite and ternesite. Quantitative X-ray powder diffraction and electron microscopy were used to analyze the phase composition of clinkers. The secondary heat treatment between 1100 and 1200 °C can significantly facilitate the formation of ternesite in the clinkers. The presence of large amount of ternesite results in a decrease in early strength, but the hydration of ternesite at later ages can increase the strength after 56 days of hardening. The hydration products of pure ternesite and the clinker containing ternesite were also investigated. The results show that pure ternesite can hydrate at a slow rate, and the addition of ye'elimite promotes the hydration of ternesite and the formation of ettringite. The ettringite formation for the clinker containing ternesite mainly depends on the dissolution of gypsum originating from the hydration of ternesite.