By-product of sulfur recovery from phosphogypsum as concrete aggregate

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A sulfur-recovery process from phosphogypsum produces a by-product slag that can potentially be used as aggregate in portland cement concrete. The phase composition and microstructure of samples of slag from a bench-scale (Florida Institute for Phosphate Research [FIPR]) and a pilot-scale (campaign 5) run of the process were characterized. Both slags have a frothy appearance. The FIPR slag consists of elongate, and often radiating crystals of gehlenite (Ca2Al(Si,Al)O7), enclosing larnite (β-Ca2SiO4). Some amounts of iron sulfide and magnetite are also present. Campaign 5 slag does not contain gehlenite since quartz is present. Its microstructure is also different, consisting of an intimate, heterogeneous mixture of calcium silicates and opaque phases. Residual gypsum is present in both slags but in greater amounts in campaign 5; the latter also contains some ettringite and native sulfur. The FIPR bench-scale run produced a completely burned residue while the pilot-scale run residue was underburned. However, mineralogically, both are suitable as aggregate in concrete as long as the amount of sulfate phases is not too high (<4.5%). © ASCE.

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Journal of Materials in Civil Engineering

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