Synergic effect of Friedel’s salt from pozzolan and from OPC co-precipitating in a chloride solution

Abstract

Two prior papers on this subject have shown, with XRD and SEM techniques, that almost all pozzolanic additions can induce the rapid formation of Friedel’s salt in quantities in keeping with the reactive alumina, Al2O3r- (tetra- or penta-coordinated alumina) content of the pozzolan. The formation rate of Friedel’s salt of pozzolan origin has also been shown to be higher than the rate in the slower forming compound, whose origin is the C3A in OPC. Consequently, the plate-like hexagonal crystals are smaller and less perfectly shaped in fast- than in slow-forming Friedel’s salt.To describe the inter-relationships between these two non-expansive processes, a terminological analogy is drawn between the rapid and slow formation of Friedel’s salt and drug interaction. A common development in the treatment of certain diseases, drug interaction may be quantitative or qualitative and, depending on the end result, is typed under one of the following headings: additive synergy, partial antagonism, competitive antagonism, potentiating synergy, non-competitive antagonism or physiological and functional antagonism. Borrowing from this classification, the present study sought to determine whether the joint formation of Friedel’s salt from pozzolan and from OPC in a common chloride solution is synergic, additive, antagonistic or able to invert the expected outcome.To this end, 14 binders, 2 PC (1 OPC and 1 SRPC) and 12 blended cements containing 20% or 30% of one of six pozzolans, were analysed with XRD technique. Water resistance, capillary absorption and total porosity were also determined, along with the chemical composition and physical properties of some cement tested.The experimental results showed that fast- and slow-forming Friedel’s salt precipitated in a common chloride solution not separately but inter-dependently and the closer the pozzolan particles were to the cement particles, the greater was that inter-dependence. Moreover, the joint precipitation – co-precipitation – of the Friedel’s salt from the Al2O3r- present in pozzolans and the Friedel’s salt from the C3A present in OPC was, to use drug interaction terminology, consistently more synergic than additive. Furthermore, depending on the parameter considered and from a purely technological point of view, the practical implications of the Synergic Effect (SE) between the two types of Friedel’s salt were always beneficial. The experimental results showed, in addition, that the pozzolanic activity of three of the pozzolans tested, C, M1 and M0 specially, once again proved to be more specific than generic in chloride and water environments. This would induce speedier chloride hydration of all or part of the C3A in the OPC fraction than when the OPC was hydrated without the pozzolan. Moreover, the Friedel’s salt from the Al2O3r- in these pozzolans was the chief direct and indirect cause of the SE, in conjunction with the Friedel’s salt from the C3A in PC, due to their very specific pozzolanic activity in such chloride media. In contrast, when the pozzolan used was silica fume (SF), its pozzolanic activity is not also more specific than generic for the same result but for the contrary result, that is, SF is unspecific for the same result.