The Effects of Seawater on the Dissolution Potential of Phosphogypsum:Cement Composites

Abstract

ABSTRACT

Polarized optical microscopy, scanning electron microscopy, and X-ray microprobe analysis were used to investigate the effect of sea water on phosphogypsum (PG):cement composites. Thin surface sections were taken from 85%: 15% PG:cement composites that had been submerged under natural seawater conditions for 1 month and 70%:30% PG:cement composites that had been submerged under natural seawater conditions for 1 year. The optical microscopy results revealed a crystalline layer of carbonates on the surface of the 70%:30% composites, which was absent on the 85%:15% composites. Microprobe analyses indicated that the carbonates, in the form of CaCO₃, were formed from Ca²⁺ and CO₃²⁻ contained in the seawater, not from the composites. This CaCO₃ coating may act as a physical barrier to seawater intrusion into the composites, preventing block degradation. The polarized optical and scanning electron microscopy images of the 85%:15% composites showed surface disruption, which is hypothesized to be caused by the interaction of seawater and the phosphogypsum. Ruptures and ettringite crystals, formed through the interaction of dissolved sulfate ions and Ca₃Al oxides, were also found throughout the composites. The absence of a CaCO₃ outer layer and the subsequent dissolution of PG and the formation of ettringite are potentially responsible for the degradation of the 85%:15% PG:cement composites. The identification of the CaCO₃ layer on the 70%:30% PG:cement composites may lead to the development of techniques to increase the integrity of PG:cement composites used for seawater applications.

Key words:

Phosphogypsum; cement stabilization; seawater degradation