Description of the thesis topic

Rheology and hydration of silico-magnesia cements

I. INTRODUCTION

I.1 Context.
Portland cement, the primary component used as a binder in concrete and other building materials is the most widely used manufactured product globally, with an annual worldwide production exceeding 4 billion tons. Its production accounts for approximately 8% of global anthropogenic CO2 emissions. Decarbonizing the cement industry to achieve net-zero is particularly challenging because a significant fraction of CO2 emissions stems from the intrinsic chemical processes involved in its production. Clinker, the main component of Ordinary Portland Cement (OPC), is produced by calcining at 1450°C in a rotary kiln a mineral mixture composed of approximately 80% limestone (mainly calcium carbonate: CaCO3) and 20% clay (a silico-aluminate mineral). Consequently, a large part of CO2 emissions originates from the decomposition of limestone at about 900°C (CaCO3 → CaO (lime) + CO2). Therefore, even if fully decarbonized energy is used for the manufacturing process and transportation, a lower limit footprint of about 600 kg of CO2 per metric ton of clinker remains, originating from limestone decarbonation. This limit cannot be overcome without a supplementary technology, which would imply additional costs that are expected to be challenging to manage, given the relatively low price per metric ton of cement. Most current research and development activities in the cement and concrete sectors are dedicated to the development of new low-CO2 binders and associated products. In most reported studies, the primary approach to reducing the carbon footprint of cementitious materials is to partially replace Portland clinker with supplementary cementitious materials (SCMs) and/or minimize its use in the formulation of construction materials like concrete. However, these technologies will not allow the cement industry to achieve net-zero emissions, as Portland clinker remains an essential component in these low-carbon cements.

I.2 Objective.
This project aims to explore hydraulic binders based on magnesium (instead of calcium), as this element is abundant in the widely available silico- magnesian ultramafic rocks, offering the potential to eliminate the CO2 emissions associated with the decarbonation of raw materials during the production process. Silico-magnesian cements have been known for a long time, and their performance is comparable to that of OPC (1, 2). The main challenge to their widespread use is scalability. We have developed a groundbreaking, scalable process for producing silico-magnesian CO2-free cements from ultramafic rocks (CNRS pending patent). If decarbonized energy is used during production, the overall carbon footprint of this binder can even be net-negative, as magnesium hydroxide formed during hydration can sequester atmospheric CO2 throughout the service life of the cementitious product via carbonation.
The objectives of this PhD project are as follows:
1. Investigate the rheological properties of pastes made from this novel silico-magnesia cement, and in particular consider the working mechanisms of the common superplasticizers and retardants, when Ca is replaced by Mg in the system.
2. Investigate the hydration mechanisms in the presence of various mineral and organic additives of this new cement in order to improve its performance, in particular its strength development and durability.

Work Context

This PhD will be carried out within the framework of a European contract (Horizon Europe, EIC). The work will be shared between two laboratories located on the Saclay Plateau: the Laboratoire de Mécanique Paris-Saclay (LMPS) at ENS Paris-Saclay/École Centrale/CNRS, and the Laboratoire de Physique des Solides (LPS) at Université Paris-Saclay/CNRS.
The LMPS is a joint research unit of ENS Paris-Saclay, École Centrale, and CNRS. It conducts cutting-edge research at both the national and international levels in the field of mechanics and materials. The LMPS is organized into four main research teams (around 50 members each). The PhD work will be conducted within the “OMEIR” team (Structures, Materials, Environment: Interactions and Risks). The laboratory will bring in particular its expertise in cementitious materials, especially regarding their mechanical behavior.
The LPS is a joint research unit of CNRS and Université Paris-Saclay. It carries out high-level research in the field of condensed matter physics. The LPS is organized into three main research areas. This PhD will be affiliated with the “Soft Matter and Physics-Biology Interface” area, specifically within the “TICE” team (Tissues, Cells, and Biofilms).
Both laboratories are equipped with most of the facilities required to develop this PhD project: bacterial culture, rheology, surface charge measurements (zeta potential), thermal analysis, X-ray diffraction (XRD), infrared spectroscopy, mechanical behavior testing, materials analysis by electron microscopy, and more.

The position is located in a sector under the protection of scientific and technical potential (PPST), and therefore requires, in accordance with the regulations, that your arrival is authorized by the competent authority of the MESR.