(8c) Mortar Paste Structure Evolution during Ram Extrusion Process: an Experimental Investigation

Extrusion is a current forming process for a wide range of materials such as polymers, metals, foods and ceramics pastes. For those materials, the extrusion stress and the flow typology are well known and have been the subject of numerous studies. The extrusion process is not a common way to form cement-based materials in an industrial context. This is explained by the heterogeneity of such material and the difficulty to formulate the right composition that enables to flow. However, it has already been studied by some authors under different point of view. Zhou et al., Srinivasan et al. used the extrusion geometry as a capillary rheometer to invetigate the flow properties cement-based fiber-reinforced composites. Li and Xhou, Peled and Shah showed that extrusion process makes hardened cement composites with high mechanical properties. Stang and Li developed a method to extrude cement composites tube using an oscillatory translation of the extruder body. Those studies focused on adjuvanted cement pastes reinforced with different type of fibers. Such formulated cement composites present optimized mechanical properties but present an expensive cost. Recently, Toutou et al. developed a method to identify the rheological behaviour of cement-based material and to evaluate the extrusion ability of adjuvanted cement pastes and mortars. The authors showed that the extrusion ability of a cement based material can be evaluated by the study of its rheological behaviour and its potential fluid filtration. Moreover, they showed that material must have a sufficient yield stress to retain it shape. This impose a material composition with low water content. The squeeze tests performed by Toutou and al. showed that extrudible cement based materials present a frictional plastic behaviour and are very sensitive to drainage and fluid filtration at low shear rate and long sollicitation time. Those conditions are inherent to the extrusion process and induce a multiphasic flow. Ancey and Coussot showed that suspensions with low water content and submitted to low shear rate are flowing in a frictional regime. The water filtration phenomenon has already been highlighted in extrusion of low water content extrusion of ceramic and polymer pastes by Götz et al., Burbidge et al. and Li et al.. It has never been studied in the case of cement paste of mortars. With such materials, the granular effect and drainage effect are expected to be high du to a large equivalent diameter (around 1 mm) of the granular part of the solid material and because the solid volum fraction of the granular part is extremely high (about 80%). In this case, the extrusion flow induces a local consolidation which influences the flow typology and the quality of the extrudates. Here, we first focus on the effect of extrusion process parameters (ram velocity, ratio of extrusion, billet length) on fluid filtration and the effect of the fluid filtration on flow typology. An experimental study is established and a extrudable mortar is formulated according to extrusion criteria given by Toutou et al. We performed extrusion tests with a given triplet of ram velocity, ratio of extrusion and billet length values. In order to check extrusion parameters influence, the extrusion force is recorded during the test and the extrudates quality is studied. In order to study the link between flow typology and consolidation with time, we developed a paste hardness measurement. All extrusion tests were performed in the same conditions and were stopped after a different time of extrusion. The part of the paste inside the extruder is then analysed. This cylinder is cut into two hemi-cylinder and the local hardness is measured all along the cut surface. This technique allows us to study the evolution of the paste consolidation and gives new informations to qualify the flow typology. Measurements of the local paste density in the extrusion process and the flow visualisations of mortars complete the study. The results give some new indications for extrusion flow study of multiphasic material with high solid volume fraction. In the range of used velocity ram, fluid filtration induces a zone of preferential flow and consolidated areas, requiring an extrusion model adaptation.