Determination of the sodium monofluorophosphate in a hardened cement paste by ion chromatography Original Research Article

In order to prevent the corrosion of steel reinforcements in concrete, sodium monofluorophosphate (MFP) is proposed as a corrosion inhibitor. When directly applied onto the concrete surface, the inhibiting action of MFP depends on the amount of product that reaches the steel. Until now, the only approach considered for determining MFP concentration has been based on water extraction; this method allows identifying the active inhibiting aspect of MFP. Nevertheless, the results obtained using this method have yielded values close to zero, hence no information is available either on the total amount of MFP applied onto the concrete surface or on the nature of MFP-insoluble compounds. This paper proposes a rapid method for quantifying the total amount of MFP in cementitious materials; this method is based on acid extraction followed by ion chromatographic analysis. During a first step, a study is carried out on pure MFP in order to determine the total hydrolysis condition of this compound. Ion chromatograms display the total MFP hydrolysis in HPO42− and F− ions for a pH close to 1. Acid extraction with 4% HNO3, as is typically used in cement related analysis, enables meeting this condition. In a second step, a 30 wt.% MFP solution is applied to a hardened cement paste (HCP). Quantification of the HPO42− chromatographic peak after acid extraction reflects the total MFP amount, with a detection limit of 5 mg of MFP per gram of HCP. Moreover, it appears that the other anions resulting from cement dissociation do not interfere with HPO42− quantification as long as the proportion of HPO42− potentially present in the cement has been taken into account. Chromatograms performed following water extraction lead neither to HPO42− nor MFP peaks, as previously noted, which indicates that the total MFP amount is precipitated as an insoluble compound. A complementary study shows that MFP reacts with the Ca(OH)2 present in HCP; the resulting compound is an apatite identified by X-ray diffraction.