Citric acid as a set retarder for calcium aluminate phosphate cements

Citric acid added as set retarder significantly contributed to enhancing the setting temperature and to extending the thickening time of a calcium aluminate phosphate (CaP) geothermal cement slurry consisting of calcium aluminate cement (CAC) as the base reactant and sodium polyphosphate (NaP) solution as the acid reactant. The set-retarding activity of citric acid was due to the uptake of Ca²⁺ ions from the CAC by carboxylic acid groups within the citric acid. This uptake led to the precipitation of a Ca-complexed carboxylate compound as a setretarding barrier layer on the CAC grains' surfaces. However, this barrier layer was vulnerable to disintegration by the attack of free Ca²⁺ ions from CAC, and also to degradation at elevated temperature, thereby promoting the generation of exothermic energy from acid—base reactions between the CAC and NaP after the barrier was broken. The exothermic reaction energy that was promoted in this way minimised the loss in strength of the citric-acid-retarded cement. The phase composition assembled in both retarded and non-retarded cements after autoclaving at 180°C encompassed three reaction products, hydroxyapatite (HOAp), hydrogrossular and boehmite, which are responsible for strengthening the autoclaved cement. The first two reaction products were susceptible to reactions with sulphuric acid and sodium sulphate to form crystalline bassanite scale as the corrosion product. The boehmite phase possessed a great resistance to acid and sulphate. Although the bassanite scales clinging to the surfaces of the cement were the major factor governing the loss in weight, they served to protect the cement from further acid and sulphate corrosion until their spallation eventually occurred. Nevertheless, the repetitive processes of HOAp and hydorgrossular → bassanite → spallation played an important role in extending the useful lifetime of CaP cement in a low pH environment at 180°C.