Annabelle Phelipot-Mardelé

Thermal and mechanical properties of gypsum–cement foam concrete: effects of surfactant

The mineral foams studied were mainly composed of a reactive, partially anhydrous calcium sulphate (90%) and Portland cement (10%), mixed with a low water-to-binder ratio (W/B = .35) leading to a highly concentrated suspension. The foaming method used allowed a wide foam density range (250 < ρ < 820 kg/m3) to be obtained after hardening and drying, with the same mineral suspension composition. To ensure that foam formed and remained stable until the material had set, six surfactants were chosen. The influence of the surfactants on paste properties in the fresh state was evaluated and the thermal and mechanical performances of gypsum–cement products (foamed and unfoamed) were analysed. The thermal conductivity of gypsum–cement foams evolved linearly with density but compressive strength values were widely dispersed and depended on the porous structure, which was strongly dependent on surfactant type and content. The performance levels of gypsum–cement foams were compared with those of lightweight foam concrete from litterature. The most efficient surfactants and the optimum content range leading to the best thermomechanical compromise were identified. They ensured a minimal compressive strength of 2 MPa, and thermal conductivity lower than .2 W/(m.K).

Quasi-static bubble in a yield stress fluid: elasto-plastic model

The influence of the yield stress of Carbopol® gel dispersions on the behaviour of quasi-static bubbles was investigated. Many fluids, from many different industrial fields, have yield stress behaviour. Most of them contain gas bubbles. To study bubble behaviour in such suspensions, a transparent model fluid (dispersion of Carbopol® in water) was used. The experimental device allowed to quasi-statically increase bubble internal pressure with small pressure step to reach a maximum target internal pressure and the pressure setpoint was inverted to return to the initial pressure. Hysterical behaviour of the bubbles was highlighted as they did not regain their initial shape because of yield stress. We show that the rheological behaviour is related to the internal pressure, bubble geometry and yield stress in quasi-static conditions. A modification of the Laplace law depending on the yield stress of the fluid and bubble sphericity was proposed and validated.

Development of a tribological test for asphalt

The measurement framework allowing for analyses on standard products, such as dense hot-mix asphalt, must be completed to analyse the behaviour of porous cold-mix asphalts. Such soft materials need time to stabilise its performances and short-term loading can drive to large deterioration of the applied layer. The effects of wheel tread turning and rubbing on asphalt surface have to be understood. In order to do that, the tribological behaviour of an asphalt surface is studied using an apparatus that can load a sample surface simultaneously in compression and torsion. The paper describes the technical details and the design of the apparatus, followed by a description of the adopted experimental test protocol. Tests are realised with imposed normal load and rotation rate of a disc. The adhesion and friction stresses are calculated from torque measurement. The displacement of sample surface during torsion is recorded. The value of this parameter at the end of the test appears as a useful degradation indicator. Performing tests during ageing of sample enlighten us on the stabilisation of the performances. The interpretation of results is illustrated by tests carried out on porous cold-mix asphalt that is particularly sensitive.

Mineral foams obtained with super sulfated cement

The purpose of this study is to evaluate the performances of Super Sulfated Cement (SSC) foams, focusing on structural, thermal and mechanical characteristics. The studied set of SSC foam samples is obtained with the same slurry. The chosen foaming method allows an interesting density variation: from 489 kg/m3 to 1793 kg/m3. Thanks to a CCD camera, the visual study of foam pore structure reveals two kinds of bubbles distribution and associated connectivity. This may partially explain the obtained thermal and mechanical behavior. Results show that SSC foams with low density (< 550 kg/m3) are usable as thermal insulator for non-loadbearing walls. Mean density SSC foams (550 kg/m3 < - < 640 kg/m3) can be used as slight-loadbearing and thermal insulating products in housing and SSC foams with high density (640 kg/m3 < - < 1200 kg/m3) as loadbearing products staying within lightweight class.

Mechanical Performances of Super Sulfated Cements

Tested super sulfated cements (SSC) are composed with various amounts of ground granulated blast furnace slag, gypsum or activated gypsum and Portland cement. Our initial results on the optimization of the composition and of the curing conditions of such cements are presented herein. Tests are conducted on the basis of standard mortar with two different gauging ratios according to old and new standards for SSC. Four types of curing are used. Mechanical performances are assessed at 2, 7, 28, 60 and 90 days on standard mortar samples (binder/sand mass ratio = 1/3). This study shows that, thanks to an optimization of the Portland cement content, it is completely possible to obtain super sulfated cement with a strength class 32.5N, and always in conformity with the new European standard. For a given SSC formulation, it is clearly demonstrated that the origin of the calcium sulfate addition that affects the development of sulfo-aluminous compounds in these mixtures constitutes a key point in the phenomena of setting and hardening. The use of heat-activated gypsum as a sulfate activator appears preferable.