Corrosion prevention of steel rebar embedded in the cement mortar under accelerated conditions: Combined effects of phosphate and chloride ions

Abstract

The present work focuses on the role of different amounts i.e. 0.295 and 2.95 kg/m3 ammonium phosphate monobasic (NH4H2PO4: APM) inhibitor and 1.2 kg/m3 NaCl as admixtures in cement mortar under wetting-- drying accelerated condition to mitigate the corrosion of steel rebar. The results show that phosphate ions (from APM) react with NaCl and ordinary Portland cement (OPC) constituents to form Ca3(PO4)2 and Ca5(PO4)3Cl as hydration products, which led to make the cement mortar matrix dense and compact, and thus, improved the compressive strength after 28 days of water curing. Consequently, the phosphate ions in the cement mortar initiate the oxidation of embedded steel rebars immersed in 3.5 wt. % NaCl solution due to the formation of Fe (H2PO4)2, and FeHPO4 as intermediate. These could be transformed into stable and protective tertiary iron phosphate (FePO4) if optimum amount of phosphate ions is present but in the case of 0.295 kg/m3 APM, mostly phosphate ions consumed in the formation of Ca3(PO4)2 and Ca5(PO4)3Cl. Therefore, this sample became vulnerable to corrosion. On the other hand, 2.95 kg/m3 APM, the phosphate ions are significant and, thus, it helps to properly form the Ca3(PO4)2, Ca5(PO4)3Cl, FePO4 and maghemite (gamma-Fe2O3).