Precipitation mechanism of soluble phosphates in mortar

Abstract

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 ${\text{HFPO}}_3^{-}$ ion, pH 5−9 for ${\text{FPO}}_3^{2-}$ ion, pH 3–6 for ${\text{H}}_2{\text{PO}}_4^{-}$ ion, pH 8–12 for ${\text{HPO}}_4^{2-}$ ion, and pH above 11 for ${\text{PO}}_4^{3-}$ ion. The use of the MFP, DHP and TSP as corrosion inhibitors presents some limitations in the case of total or partial immersion.

Keywords:

• Cement mortar
• phosphates
• penetration
• stability
• corrosion inhibitor

Acknowledgements

The authors are grateful to Projects BIA2008–05398 of the MINECO and MAT2015-71761-R of the MINECO/FEDER, Spain, for financial support. M. Criado expresses her gratitude to the Spanish Ministry of Science and Innovation for the Juan de la Cierva Contract. D. M. Bastidas gratefully acknowledges funding from the Ramon & Cajal Program of the Spanish MINECO.