Expansive Synergic Effect of ettringite from pozzolan (metakaolin) and from OPC, co-precipitating in a common plaster-bearing solution: Part I: By cement pastes and mortars

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-ΔL tests, that almost all pozzolanic additions can bring about the rapid formation of ettringite, ett-rf  , a process dependent upon their reactive alumina, or Al2O3r-, content, which may be vitreous or amorphous (tetra- or penta-coordinated). 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 ≈ 10-fold smaller.To describe the interrelationships between their expansive processes, a terminological analogy is drawn between the rapid and slow formation of ettringite, on the one hand, and drugs interaction, on the other. A common development in the treatment of certain diseases and as a result of hospital practice, drugs interaction may be quantitative or qualitative and is denominated, based on the final outcome, to be additive synergy, partial antagonism, competitive antagonism, potentiation synergy, non-competitive antagonism or physiological and functional antagonism. 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, 16 cements – 4 Portland cements and 12 blended cements containing 20%, 30% or 40% metakaolin (M pozzolan) – were tested using the L–A, ASTM C 452-68 and EN-196-1 tests. Specimens of all 16 types of cement were made for L–A, ASTM C 452-68 and EN-196-1 testing and several direct and indirect parameters were measured, as follows:–Increase in diameter, ΔØ (%); diameter growth rate, Vcø(=ΔØ (%)/day); and Vicat Needle Penetration, VNP (mm), for L–A specimens, and–Flexural and compressive strengths for ASTM C 452-68 and for EN-196-1 specimens.Finally, other complementary determinations were specific chemical analysis and physic properties of some cements tested, being particularly interesting their XDA patterns of gypsum to quantify it as well.The experimental results show that the joint precipitation in the same plaster-bearing solutions, co-precipitation, of the ettringite   from the Al2O3r- present in pozzolans, and the ettringite from the C3A present in OPC, was, to use drug interaction terminology, always, quantitatively speaking, 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. It may be, however, that these judgements cannot be sustained when two or more related parameters are considered together. Finally, when 7.0% SO3, equivalent to 15.05% of gypsum, was added to the M pozzolan-containing Portland cement and tested with the ASTM C 452-68 method, it was not found to behave aggressively but rather as “setting regulator” because the increase in mechanical strength over time and setting times in these mixes were, therefore, similar to the pattern observed in any PC. However, when the gypsum content was raised to slightly more than double that amount (33.33% for the L–A test), it behaved aggressively. In both cases, logically, ett-rf, and/or ett-lf were involved in the resulting beneficial or adverse behavior. Therefore, in the first case, the amount of gypsum added can be regarded as being suitable for some POZC containing this particular M pozzolan, and indeed, all or most of these POZC met the criteria to be classified as “expansive hydraulic cements” set out in standard ASTM C 845-90. In higher amounts, on the contrary, gypsum behaved aggressively towards OPC P-1 and P-2 and their respective POZC with this M pozzolan, mainly, and towards the POZC of the SRPC PY-4 or PY-6 with this M pozzolan, as well. This latter behaviour was very intense, aggressive and rapid, and for this reason, it justified to be named “rapid gypsum attack”.