Precipitation mechanism of soluble phosphates in mortar

The penetration of three phosphate compounds in mortar bulk matrix, sodium monofluorophosphate (Na2FPO3) (MFP), disodium hydrogen phosphate (Na2HPO4) (DHP) and trisodium phosphate (Na3PO4) (TSP) has been evaluated using two water: cement ratios (0.5 and 0.6). The mortar samples were immersed in a 5% MFP, DHP or TSP solution for a period of 40 days. Microprobe line profile analysis for phosphorus (P) and fluorine (F) showed similar behaviour for the three soluble phosphates, which penetrated the mortar to a depth of more than 2 mm, below which the P and F contents dropped to almost zero. The stability of the phosphate ions vs. pH was calculated using thermodynamic data to obtain the following stability ranges: pH 2-4 for HFPO -3 ion, pH 5-9 for FPO32- ion, pH 3-6 for H2PO4- ion, pH 8-12 for HPO42- ion, and pH above 11 for PO43- ion. The use of the MFP, DHP and TSP as corrosion inhibitors presents some limitations in the case of total or partial immersion.