Considerations on the mechanical behavior and hydration process supersulfated cement (CSS) formulated with phosphogypsum

ABSTRACT

Supersulfated Cements (SSC) are composed from blast furnace slag (90%), calcium sulfate (10-20%) and a small amount of alkali activator (up 5%). Gypsum is a conventional source of calcium sulfate, however, it can be replaced by phosphogypsum (PG), a byproduct from the production of phosphoric acid (H₃PO₄) with similar chemical and mineralogical composition of the gypsum. In Brazil, the production of this material is about 4.5 million tons per year. Because the SSC contains a higher calcium sulfate content (20%) in relation to Portland cement, a higher consumption of phosphogypsum is possible. The goal of this study was to investigate the phosphogypsum (PG) as an alternative source of calcium sulfate in order to obtain CSS. In addition to use of PG, the effects of both calcium sulfate and alkali activator content on the process of hydration were investigated. The results showed that SSC made with phosphogypsum met the minimum compressive strength required by the European standard for SSC (EN 15743/2010). Low heat of hydration rates mainly influenced by the low alkali activator content was observed. The excess of alkali activator (KOH) had different influences according to calcium sulfate content. In pastes made with low content (10%), 0.8% of KOH reduced the compressive strength, while in those with a high calcium sulfate content (20%) the high alkaline content resulted in the instability of ettringite.

Keywords:
supersulfated cement; phosphogypsum; calcium sulfate; alkaline activator; ettringite