Improving the properties of phosphogypsum-based clinker-free cement system by in-situ precipitation of ettringite seeds: strength, hydration, microstructure and sustainability

Excess-sulphate phosphogypsum slag cement (ESC) is a clinker-free binder with low carbon footprint, low embodied energy and good corrosion resistance, but slow strength development, known as one of the most serious issues, limits its application in practical engineering. In this regard, in-situ precipitation of ettringite by adding NaAlO 2 (NA) to act as nucleation seeds was proposed in this study. ESC consisting of 45% granulated blast-furnace slag (GBFS), 45% phosphogypsum, and 10% steel slag was designed, and 0.1~4% NA was added as chemical admixture to produce ettringite. Effects of NA on compressive strength, hydration process, microstructure and shrinkage were discussed. Results suggested that 0.1~4% NA significantly improved the compressive strength of ESC paste after 7 d, and 1% NA enhanced the strength by 87.6% at 28 d and 77.5% at 56 d. The reason was that NA could supply Al(OH) 4 - to react with the ions in pore solution of ESC to quickly precipitate ettringite seeds, thus inducing the hydration of ESC and formation of hydrates, and especially the formation of ettringite was greatly promoted. However, NA hardly affected early strength of ESC, since NA reduced liquid alkalinity during ettringite seeds formation, and this is not conducive to early precursor dissolution and hydrate formation.