Impact of calcium nitrite corrosion inhibitor on the constitution of cement hydrates exposed to chloride.

摘要

Calcium nitrite acts as a corrosion inhibitor for embedded steel. Additionally it is used as an accelerating admixture to increase early strength development or counteract slow strength gain of concrete in cold conditions. Hydrated cements contain mainly portlandite and a gel-like phase, a calcium silicate hydrate termed C-S-H while the alumina combines with water and calcium, etc. to form AFt and AFm phases. Experimental work was undertaken to determine the impact of calcium nitrite. It strongly interacts with AFm. The layered double hydroxide corresponding to Ca4Al2(NO2)2(OH)12·4H2O, was synthesized and characterised by XRD and TG/DTG. Solubility was determined experimentally. Ion concentrations were measured after several months of equilibration (water/solid ratio around 30). Ionic speciation and log Ksp was calculated using the GEMS-PSI code. Thermodynamic modelling was applied to predict phase changes from added Ca(NO2)2. A key purpose of the calculation was to determine the binding power of AFm for nitrite in competition with sulfate, carbonate and chloride at the high pH characteristic of cement. Relevant experiments on phase assemblages were performed to verify thermodynamic predictions. The implications for retention of nitrite in cement undergoing attack in service environments are presented. Cements containing nitrite exhibit ‘smart’ behaviour. Nitrite can be stored within the AFm phase even in competition with hydroxide, sulfate and carbonate but is released in response to chloride ingress: chloride displaces nitrite from the AFm and is bound in by forming Ca4Al2Cl2(OH)12·4H2O (Friedel’s salt). Nitrite ions thus released are concentrated in the aqueous phase and result in increased [NO2-]/[Cl-] ratio which is important to assure that steel reinforcements remain passivated. This mechanism is described and quantified and suggestions for optimising its ‘smart’ behaviour are made.