12 13 Hydration reactions of C 3 A with various amounts of calcium sulfate hemihydrate, 14 gypsum or a mixture of the two, were investigated by isothermal microcalorimetry, 15 and a monitoring of the ionic concentrations of diluted suspensions. This study 16shows that sulfate type used modifies the early C 3 A-CaSO 4 hydration products and 17 the rate of this hydration. The fast initial AFm formation observed before ettringite 18 precipitation in the C 3 A-gypsum system is avoided as soon as hemihydrate is 19 present in the suspension. This was attributed to.higher super saturation degrees and 20 then higher nucleation frequency with regard to the ettringite obtained in the 21 presence of hemihydrate. Moreover, replacement of gypsum by hemihydrate also 22 leads to an increase of the ettringite formation rate during at least the five first hours 23 under experimental conditions. 24 25
It is well-established that a competitive adsorption exists between polycarboxylate superplasticizers (SP) with soluble sulfate ions solubilized in interstitial solution of cement paste, which may causes a loss of dispersing properties depending on the cement composition. Early reports suggested that this incompatibility is due to competitive weak ionic interaction between functional acrylate groups and solubilized sulfate ions with cement grains. In this study, SPs including trialkoxysilane functional groups have been synthesized via radical copolymerization of methacrylic acid, poly(ethylene glycol) methyl ether methacrylate and trimethoxysilyl propyl methacrylate. Adsorption and dispersing properties of these new SPs were evaluated in cement paste at different concentration of Na 2 SO 4 added in aqueous solution. Partial substitution of acrylate groups by trialkoxysilane in the polymer composition significantly enhances the compatibility with sulfate ions. We suggested that the high adsorption capacity of these SPs results from the formation of strong bonds between hydroxysilane groups and calcium silicate hydrate phases present at the surface of C 3 S. The improved compatibility of these new silylated SPs with cements more or less rich in sulfate alkaline has been demonstrated through the formulation of two different concrete equivalent mortars.
The interaction mechanism between polycarboxylate-type superplasticizer (PCP) and cement hydration is not fully understood and incompatibilities between concrete and additive are sometimes observed. In some cases, the fluidity tends to increase ("overfluidification") few minutes after mixing. This is a problem because the overfluidification leds to bleeding of the concrete which could be critical on job site. Our study consisted first in highlighting the phenomenon of "over-fluidification" by slump flow tests on mortar. Next, the time evolution of the rheological behaviour of cement pastes in the presence of PCP was analysed thanks to a rheometry protocol in order to quantify the phenomenon. Later on, a parametric study was undertaken using this methodology. The operating conditions such as temperature and mixing process were studied as well as the effect of PCP structural parameters and the chemical characteristics of cement. In order to understand the origin of the phenomenon, adsorption measurements of PCP on cement particles were performed in the same conditions as those in the rheological measurements. Indeed, the phenomenon of "over-fluidification" could be related to the rate of the initial adsorption and the adsorption kinetics, both of which depend on the parameters of the process, the PCP structure and the cement reactivity.
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