Metakaolin

Engineering

Portlandite

Cementitious

Metallurgy

Materials science

Raw material

Slag (welding)

Chemical engineering

Phosphogypsum

Organic chemistry

Portland cement

Cement

Microstructure

Composite material

Ettringite

Chemistry

Nucleation

Compressive strength

Journal of Cleaner Production

Landfilled phosphogypsum would cause severe environmental issues, but the waste can be recycled for preparing excess-sulphate cement, an eco-friendly alternative to conventional cement. This paper investigates the effect of metakaolin (0–50%) on early hydration, phase assemblages and mechanical properties of the excess-sulphate phosphogypsum cementitious materials (ESPCMs). Results indicate that metakaolin is related to a new exothermic peak and significantly shortens the induction period. Setting time of ESPCM pastes is reduced by 13%–38% with 10%–50% dosage of metakaolin. More ettringite and highly disordered C-(A)-S-H gel are characterised when metakaolin dosage is below 20%, leading to 70% increase in 28-d compressive strength. With above 20% metakaolin dosage, portlandite consumption at early stage is promoted and hydration degree at late state is reduced. It turns out that within 20% metakaolin dosage is efficient to optimise setting time and strength development of ESPCMs, where slag and metakaolin synergistically promote the formation of ettringite and C-(A)-S-H gel to bind the unhydrated cement particles effectively.

Recycling utilization of phosphogypsum in eco excess-sulphate cement: Synergistic effects of metakaolin and slag additives on hydration, strength and microstructure