Sébastien Georges

Changes in Cement Paste and Mortar Fluidity after Mixing Induced by PCP: A Parametric Study

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.

Synthesis of poly(N-acryloylmorpholine) macromonomers using RAFT and their copolymerization with methacrylic acid for the design of graft copolymer additives for concrete

Methacrylate end-capped poly(N-acryloylmorpholine) macromonomers (PNAM-MA) of number-average molar mass of about 2400 g mol−1 have been synthesized by post-modification of PNAM chains obtained by RAFT. The good control of the NAM polymerization in the presence of a 2-cyano-2-thiothiopropylsulfanyl propane (CTPP) RAFT agent allowed the synthesis of highly thiocarbonylthio end-functionalized chains (89%). Two sequences of reactions have then been carried out in order to introduce a methacrylate functionality: the reduction of the thiocarbonylthio function into a thiol function by hydrazine followed by the reaction with isocyanatoethyl methacrylate (IEMA) or the one-pot aminolysis/Michael addition onto acryloyloxyethyl methacrylate (AOEMA), resulting in 77% and 86.5% of methacrylate functionality, respectively. The resulting macromonomers were successfully copolymerized with methacrylic acid (MAA) at a MAA : PNAM macromonomer molar ratio of 3 : 1 to afford graft copolymers that exhibit all the features of superplasticizers for concrete. Their stability towards alkaline conditions and high ionic strength usually met in concrete pore solutions has been assessed. The ester linkage resulting from the aminolysis/Michael addition sequence led to highly hydrolyzable side chains, which are therefore not appropriate for this application. The thiourethane linkage coming from the thiol–isocyanate chemistry, resisted the alkaline conditions of the interstitial concrete media and could potentially be used for novel concrete formulations.