Calcined clay has become the supplementary cementitious materials with the greatest potential to reduce the clinker/cement. In this research, the mechanical strengths and sulphate resistance of blended cements with a high content of calcined clay as a pozzolanic addition were evaluated to demonstrate that these cements could be designed as CEM (cement) type IV/A-SR and IV/B-SR cements by the current European standard UNE-EN 197-1: 2011. The blended cements were prepared by two Portland cements (P1 and PY6) with different mineralogical compositions and a calcined clay. The level of replacement was greater than 40% by weight. The results obtained confirm the decrease in the mechanical strengths and the increase in the sulfate resistance of the two Portland cements when they are replaced by calcined clay at a level of replacement greater than 40%. These results are a consequence of the chemical effect from the pozzolanic activity of the calcined clay. Therefore, there is an important decrease in portlandite levels of paste liquid phase that causes the increase in sulfate resistance and the decrease of the mechanical strengths.
The rheological behaviour of fresh Portland cement (PC) pastes with different chemical and potential mineralogical compositions was analysed when were blended with crystalline mineral additions (fillers). These two fillers were very different: a siliceous, Q, a-quartz type, and another limestone, C. Trials were conducted during latent hydration at 25 °C. The findings showed that apparent viscosity was higher in the PC with low C 3 A and high C 3 S content, consequently, the Na 2 O eq .(%) content of everyone also contributed very significantly. Moreover, it was also observed that the incorporation of each filler affects differently the rheological behaviour of fresh cement pastes. The different influence is due to the different physical and chemical properties of each filler (nature, chemical composition and character and texture intimate of the particles), and also of the type of PC to interact. Finally and at these very early ages, both fillers stimulated the hydration of the PC which they were mixed by direct and non-direct way, and in the case of C filler by indirect way as well, but without any pozzolanic activity. Despite the fact that blended cement P1/Q 60/40 has proved to be a ''false positive" in Frattini test.
In this paper the rheological behavior of fresh Portland cement pastes mixed with different calcined clays was analyzed. For this purpose, two samples of Portland cement with different mineral composition (one with low C 3 A and high C 3 S content and another with low C 3 S and high C 3 A) were used combined with different replacement percentages of two calcined clays (two different metakaolin). Grounded aquartz was used as control as well. Both mineral admixtures had different crystallinity and morphology: the a-quartz is fully crystalline while metakaolin is completely amorphous with a very small crystalline fraction giving very high pozzolanic properties in the vitreous state. All determinations were performed in the hydration latency period. The results show that the fresh Portland cement paste with low C 3 A(%) content and high C 3 S(%) content present great shear strength and the replacement with calcined clays affects the rheological behavior of the fresh pastes depending on the greater or lower pozzolanic reactivity of the mineral addition.
The Le Chatelier-Anstett test was used to study the sulfate resistance of cement pastes containing calcined paper sludge waste (MC). MC was blended with two types of portland cement (PC): P1 with and P2 without C 3 A (ordinary portland cement, OPC, and sulfateresistant portland cement, SRPC, respectively). The study of thaumasite formation required the utilization of a third PC, P3. The results showed that sulfate resistance always decreased at higher ratios of PC replacement by MC. This behavior was attributable to the expansive synergic effect (ESE), originated by the coprecipitation of ettringite from two resources in a gypsum and water medium. Part of the ettringite formed rapidly from the Al 2 O r− 3 in MC, and rapidly or slowly from the C 3 A in P1, depending on whether its early pozzolanic activity was sufficient or not. Rapid-forming ettringite of both origins was induced by the sufficient specific, fast, and early pozzolanic activity resulting in the indirect stimulation of sulfate-mediated P1 hydration (its C 3 A content especially) by the metakaolin present in the MC. Where such activity was insufficient, however, slow-forming ettringite was generated, reducing the ESE. The substantial calcite content in MC also played a very significant role in ESE, the result of the stimulation of such hydration, concomitant sulfate attack, and thaumasite formation. Stimulation was both direct (due to initial particle moistening by the mixing water) and nondirect (due, at the very outset, to the positive and negative electrostatic charge acquired by their particles during grinding and/or mixing, and subsequently, to the zeta potential originated as PC hydration progressed). Lastly, calcite (natural or synthesized in portlandite carbonation) was shown to be needed for thaumasite to form, a process not confined solely to low-temperature environments.
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