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

Abstract

Polarized optical microscopy, scanning electron microscopy, and X-ray microprobe analysis were used to investigate the effect of seawater 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% PGicement 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 CaCOj, were formed from Ca2+ and C032contained in the seawater, not from the composites. This CaCOa 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 Ca.^Al oxides, were also found throughout the composites. The absence of a CaCO^o uter 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 CaC03 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.

Determination of Calcium Diffusion Coefficients as an Estimator of the Long-Term Dissolution Potential of Phosphogypsum:Cement:Lime Composites

“…Conversely, the cement content must remain low enough to maintain the economic feasibility of such an approach. Recent studies have indicated the presence of a calcium carbonate coating on the surface of composites that showed no signs of deterioration following saltwater submergence (2). Calcium carbonate requires high pH conditions to form.…”

Stabilized Phosphogypsum:Class C Fly Ash:Portland Type II Cement Composites for Potential Marine Application

“…Diffusion Model. Other research by Guo et al (11) indicates that the reactants (Ca 2+ and CO3 2-) for the formation of the CaCO3 coating originated from saltwater and not from the PG composite. Thus, Ca 2+ release from the PG composite can be considered nonreactive.…”

“…Initial research showed PG composites that remained physically intact after submergence in saltwater contained a calcium carbonate coating. Conversely, this coating was absent in severely degraded composites (11).…”

“…This cycle continues until the composite has completely dissolved. Previous research, however, indicates that the formation of a CaCO3 precipitate on the composite surface protects the block from the dissolution process (11).…”

“…To produce PG composites that can compete economically with other resource materials (mostly limestone based), the portland type II cement content must be less than 5% on a dry weight basis (10). A demonstration study showed that 70%:30% PG:portland type II cement test composites survived in natural saltwater conditions at the Grand Isle Oyster Culture Facility, LA, for more than 1 yr (8) while the 85%:15% PG:portland type II cement test composites exhibited severe degradation after 1 month (11). These results indicate that reducing portland type II cement to 5% to meet the economic target must be offset by adding another mixture at a cost lower than cement.…”