Hot acid resistance of polymer-modified calcium aluminate/fly ash/polyphosphate cements

To confirm the resistance of sodium polyphosphate-modified class F fly ash/calcium aluminate blend (SFCB) cements to an H₂SO₄ solution (pH 1·6) at 90°C, various different organic polymer dispersions were incorporated into the cements. It was found that acrylic-styrene copolymer (ASC), with a pH ∼4, was the most effective in mitigating acid erosion of the cements. The reasons for making them less susceptible to acid attack are due to the following two major roles of ASC. The first is ASC's chemical affinity for the Ca²⁺ ions liberated from monocalcium aluminate (CA) and calcium dialuminate (CA₂) in the calcium aluminate cement (CAC) reactant, so forming Ca-complexed carboxylate compounds in the interfacial contact zone between ASC and CAC. The other role is related to the coverage of the reaction products by the ASC film and the unreacted CAC and fly ash particles, generating a network structure. These roles significantly restrain the formation of gypsum yielded by the reaction between H₂SO₄ and CAC. Gypsum is detrimental to the cements because it expands, making them vulnerable to acid erosion, because of its expansion nature. The loss in weight of ASC-modified SFCB cements after exposure for 25 days to hot acid was only 2·6%, compared with 27·3% in unmodified cements after the same exposure time.