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Expansive synergic effect of ettringite from pozzolan (metakaolin) and from OPC, co-precipitating in a common plaster-bearing solution. Part II: Fundamentals, explanation and justification

Construction and Building Materials
Publication Date
DOI: 10.1016/j.conbuildmat.2010.09.006
  • Gypsum Attack
  • Metakaolin
  • Portland Cements
  • “Rapid” And “Slow” Forming Ettringites
  • Synergies
  • Chemistry


Abstract In Part I, already published about this topic, Expansive Synergic Effect (ESE) between both types of ettringite was shown by means of cement pastes and mortars. In this Part II, ESE will also be shown by means of cement mortars and concretes, but above all, will be explained and justified. For this purpose, the same cementitious materials (OPC, SRPC and metakaolin, MK) plus OPC P-31, silica fume, SF, and diatomite, D, the same blended cements plus the corresponding POZC of SF and D, and the same ASTM C 452-68 test than in Part I were used, and specimens of the most significant types of cement were made for ASTM C 452-68 testing, and once again, several direct and indirect physical, chemical and mechanical strengths parameters were measured, as follows: increase in length, Δ L (%), and pozzolanic activity index of pozzolans. In parallel, concrete specimens also were prepared with (15.05% or 45.16%) and without excess gypsum, and the following parameters were measured: compressive strength and indirect tensile strength (“brazilian” test). Finally, other complementary determinations were specific physical properties, chemical analysis of some cement tested and SEM-EDX analysis of ettr-rf and ett-lf formed. The experimental results have once again shown that, the joint precipitation in the same plaster-bearing solutions – co-precipitation – of the ettringite from the Al 2 O 3 r - present in pozzolans, and the ettringite from the C 3A present in OPC, was, to use drug interaction terminology, always more synergic than additive. Furthermore, depending on the parameter considered and from a purely technological point of view, the practical implications of Expansive Synergic Effect (ESE) between the two types of ettringite can be classified as beneficial, adverse or indifferent (for more detail, see Abstract in Part I). On the other hand, the experimental results have also shown that the pozzolanic activity of MK has proved to be once again more specific than generic in gypsum and water environments, prompting greater or lesser but speedier gypsum hydration of all or part of the C 3A (than of the C 3S) content of the OPC fraction than when the OPC in question was hydrated in the same manner but without MK, and as a result, a stronger or weaker ESE, being moreover, the ett-rf from the Al 2 O 3 r - in MK its chief direct and indirect cause in conjunction with the ettringite from the C 3A in PC, due to its very specific pozzolanic activity in such gypsum media; and when it was hydrated with SF – in this case, with at least 20% of such pozzolan SF – gypsum-mediated hydration of the C 3A was obstructed, thereby confirming that, as expected, this pozzolan protects its corresponding POZC from gypsum attack, that is, 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.

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