Mechanism of Friedel's salt formation in cements rich in tri-calcium aluminate

Abstract

In the present investigation, based on pore solution analyses, the mechanisms of Friedel's salt formation and the related chemical processes taking place in mortar due to chloride binding are dealt with. Pore solutions from mortars containing NaCl and CaCl 2 added during mixing were analyzed. Based on the results it is proposed that in the presence of NaCl the Friedel's salt forms by two separate mechanisms; an adsorption mechanism, and an anion-exchange mechanism. In the adsorption mechanism, Friedel's salt forms due to the adsorption of the bulk Cl − ions present in the pore solution into the interlayers of the principal layers, [Ca 2Al(OH −) 6.2H 2O] +, of the AFm (Aluminate Ferrite mono) structure to balance the charge. In the anion-exchange mechanism, a fraction of the free-chloride ions bind with the AFm hydrates (C 4AH 13 and its derivatives) to form Friedel's salt by an anion-exchange with the OH − ions present in the interlayers of the principal layer, [Ca 2Al(OH −) 6.nH 2O] +. As a result of Friedel's salt formation by the adsorption mechanism, an amount of Na + ions equivalent to the adsorbed chloride ions (in moles) are removed from the pore solution to maintain the ionic charge neutrality. The Na + ions thus removed from the pore solution, bind with the calcium silicate hydrate (C-S-H) gel lattice to balance the charge arising due to the replacement of Si 4+ ions by Al 3+ and Fe 3+ ions. In contrast, the Friedel's salt formation by the anion-exchange mechanism involves the release of OH − ions from the AFm hydrates into the pore solution, thereby increasing the pH of the pore solution. The above mechanisms are also valid for CaCl 2 salt introduced in a similar way. Although the addition of CaCl 2 salt to the mortar lowers the net pH of the pore solution, the total bound chlorides were significantly higher in comparison with the equivalent Cl − added as NaCl salt. The higher chloride binding with equivalent CaCl 2 addition is explained by the lower degree of competition offered by the OH − ions during the bulk free-chloride adsorption into the interlayers of the principal layers.