Sulfate attack : co-precipitation of rapid and slow forming ettringite . Consequence : Expansive Synergic Effect

Abstract

Several prior papers on this subject have shown, with XRD and SEM techniques and Le Chatelier-Ansttet (L-A), ASTM C 452-68 and RT-AL tests, that almost all pozzolanic additions can bring about the rapid formation of ettringite, ett-rf, a process dependent upon their reactive alumina, Al2O3r-, content, which may be vitreous or amorphous. It has likewise been found that the formation rate, Vf, of this ettringite from pozzolans origin is higher than the Vf of slower forming ettringite, ett-lf, originating from the C3A in OPC; consequently, the size of the ett-rf is, logically, approximately 10-fold smaller. In this context, the key questions relating to the development of the two types of ettringite when forming together in a common plaster-bearing solution, are: What will the outcome of the expansive effects be? What type of effect will they ultimately produce? Addition? Synergism? Antagonism? or perhaps Inversion of final expansive action? To reply to these questions, 20 cements -5 Portland cements and 15 blended cements containing 20%, 30% or 40% metakaolin- were tested using the ASTM C 452-68 test and several direct parameters were measured, as follows: Increase in length, ΔL and sulfate content in the curing water of the specimens, SO4=cw;, for ASTM C 452-68 specimens. In parallel, concrete specimens also were prepared with and without excess gypsum (15.05% or 45.16%), and the following parameters were measured: compressive strength and indirect tensile strength ("brazilian" test). The experimental results show that the joint precipitation -co-precipitation- in the same plaster-bearing solutions, of the ettringite from the Al2O3r- present in pozzolans, and the ettringite from the C3A present in OPC, was always more synergic than additive. On the other hand, the pozzolanic activity of MK has proved to be more specific than generic in gypsum and water environments, prompting greater, speedier and earlier gypsum-mediated hydration of the C3A than of the C3S in the OPC, and as a result, a stronger or weaker ESE according to its C3A content, mainly. Therefore, the ett-rf from the Al2O3r- in MK is the chief direct and indirect cause of the greater or lesser ESE generated in conjunction with ettringite from the C3A in PC in all the ASTM C 452-68 specimens (and in concretes and mortars on construction sites where gypsum and water are present as well), due to its very specific pozzolanic activity in such gypsum media. This very specific pozzolanic behaviour makes it possible to give the title to the MK of pozzolan with aluminic chemical character. Finally, pursuant to the fundamentals of the ESE generated by ettringite from pozzolan and OPC, the ett-rf identified in this study may have had at least two origins: the Al2O3r- in pozzolans and the C3A in OPC, if blended with appropriate aluminic pozzolans in chemical character -and in suitable quantities-, due to the very specific pozzolanic activity of their Al2O3r-. Ett-lf, on the contrary, has had only one origin: C3A present in OPC.