Master 2 student in Materials sciences

Title: The durability of dikes reinforced by the soil mixing method - The study of the clay harmfulness in the soil-cement mixtures

• Location: Ifsttar, Department GERS, Laboratory SRO, Marne la Vallée (France)
• Duration: 4-6 months internship
• Starting date: 01/02/2018 - 01/03/2018
• Tutors : Dr Myriam Duc

Context and objectives

The method of soil treatment named “deep soil mixing” is used in geotechnical engineering. It involves the mixing of in situ soil (no excavation) with cement either in the form of anhydrous powder (dry method) or in the form of grout (wet method). This technique is used to mechanically strengthen the soil or reinforce structures such as dikes to improve their sealing properties. Given current practices, soil-cement mixtures and their performance as well as their durability over time may vary from one point to another of the reinforced structure because of the variation of added water to remain within the workability range (it allows the progression of the mixing device within the mixture), the homogenization degree of the soil-cement mixture (with more or less soil pellets or cement-rich zones) and finally, more or less sandy or clayey particles in in-situ soil.

The purpose of the internship is to improve the understanding of the behavior of soil-cement mixtures and in particular the effect of different types of clays mixed with CEM III C 32.5 cement. This study is important to better understand the composition and the microstructure (porosity) of soil-cement mixtures, which has a direct impact on the sustainability of mixtures over the long term. In concrete domain, the presence of clay is forbidden because clays are considered harmful (sand are washed in quarries and controller filler may be added). In our case, we must take into account the presence of clays in in-situ soils that have to be treated and put in evidence the origin of their harmfulness (or highlight this origin from probable old literature).

Main steps

Particular attention will be paid to the hydration mechanism of CEM III C cement containing more than 65% of slag. It will be compared to the clinker-based CEM I hydration mechanism. Then, we will try to understand how the introduction of clay modifies this mechanism that impacts the material properties after curing and its durability over the long term via the microstructure.

The behavior of a mixture of sand with cement (mortar) will be used as a reference compared to that of cement mixtures with “pure” clays such as illite, or kaolinite or montmorillonite (some clays with increasing water sensitivity). The internship consists not only to understand the mechanisms of hydration of the cement but also in taking into account the properties of clays as their cation exchange capacity (CEC) related to their ability to adsorb ions such as Ca²⁺ (necessary for the growth of cement hydrates such as CSH), their ability to adsorb water which causes their swelling, or to buffer the alkaline medium by dissolution/adsorption reactions, which are at the origin of their pozzolanic properties (ability to release species which will allow the formation of additional CSH in the long term). As fine particles, clays also have a role of "active filler" that will coat the larger grains and potentially screen phenomena at the interfaces (impact of particle size distribution).

To carry out this work, the intern will have to show his taste for lab experiments. After a literature review, and the acquisition of bases on clay properties and cement hydration, he will use his knowledge and apply different techniques in a more or less autonomous way: X-ray diffraction, scanning electron microscopy and mercury porosimetry (water porosimetry), experimentation on dilute clay/cement suspensions (zetametry and titrimetry), pozzolanicity test. The measurements of electrical conductivity (using a dedicated cell) on paste and mortar will be the object of a wider development in order to discriminate the behaviors of différent mixtures. The use of reaction calorimetry is also considered. Samples matured at 7, 14, 28 or 90 days or more have already been prepared and then lyophilized. They will be used as a basis in order to start our study. Other mixtures (especially for tests on pastes at early edge) will also be made.

Profile

Master 2 students in the domain of material, physico-chemistry, (mineral) chemistry…

Competences

Main skills: material sciences (knowledge in geomaterials or materials for civil engineering will be a plus), basis in experimental techniques cited previously, multi-scale approach, chemical reactivity at interfaces…  
Kow-how: Autonomy, sense of initiative, curious, rigor, taste for experiments

Contacts

Candidatures (cover letter + CV) should be send by email to myriam.duc(at)ifsttar.fr
Phone. 0033 (0)1 81 66 82 53
IFSTTAR Marne La Vallée, Bâtiment Bienvenue.