MSCA-COFUND-CLEAR-Doc-PhD Position#CD22-50: CO2 storage in soil treated with low carbon footprint alternative binders

In the context of climate change, the reduction of greenhouse gases, the saving of resources and energy sobriety in accordance with the adaptation to climate change are urgent concerns. The collective awareness can be favorable for the reduction of traffic in cities. This change in practice will lead to a decrease in the performance levels required for the design of low traffic structures.

However, the carbon footprint of earthworks is known to be significant. This footprint is mainly related to the transportation of materials during earth moving works. Soil treatment is known to be the second largest contributor to CO2 emissions after transportation. Currently, soil stabilization is performed with conventional binders, usually with hydraulic binder. The common binders used in earthworks are generally materials with a high carbon footprint, from the extraction of the raw materials to the realization of the works.

In recent years, climate hazards have demonstrated the need to increase the permeability of urban soils to allow the soil to absorb the flow of rainwater more easily.

The different findings mentioned above indicate a fundamental change that must be implemented in earthworks. For example, unpaved tracks will become an interesting alternative to the usual bituminous pavement. The practice of soil stabilization in its current form will need to be improved by other more environmentally friendly treatment alternatives. The alternative solutions must provide an acceptable level of permeability and at the same time be a means of CO2 storage.

Recent research work has shown that the organization of the compacted soil microstructure is shaped by the wetting path of the soil studied. The structure of the soil particle aggregates is formed during the wetting of the soil. These aggregates are then stacked and deformed by compaction without impacting the porosity level (Sediki et al. 2016).

At the laboratory scale, the study conducted by Ranaivomanana et al. 2016 showed that compaction of natural soil allows to decrease the pores corresponding to the size of the inter-aggregate space. In the case of treated soil, the compaction action promotes hydration and contact between the aggregates constituting the soil matrix and the binder (Ranaivomanana et al. 2018).

Under the kneading compaction effect, the tortuosity value of the soil matrix is strongly impacted by the compaction state of the soil matrix (Das et al. 2022c, Ranaivomanana et al. 2021, 2022b). These previous studies show that the mechanical performance of compacted soil is intimately linked with its microstructure induced by compaction. The microstructure itself can be regulated by the quality of implementation.

As for the carbonation response, Deneele et al 2021 showed the positive effect of carbonation on soil behavior. According to Das et al 2022d, the evolution of the natural carbonation front observed on a real dike exposed under climatic loading is a slow process. Carbonation appears to generate a stable barrier layer that limits the evolution of the carbonation front in the soil matrix.

Previous research work shows the possibility of CO2 storage in soil compacted with hydraulic binders.

The objective of this thesis is to study the different possibilities of optimizing CO2 storage in compacted, treated and permeable soil consolidated with alternative binders with low ecological footprint. The thesis work can be divided as follows: A first phase of geotechnical and mineralogical characterization of materials, stabilization with binders of nature to be defined. Then, a thorough microstructural study to highlight the level of CO2 storage. This part of the thesis will focus on the investigation of the microstructure (MIP, BJH, SEM, XRF, XRD, ATG...) and thus deduce the mechanism that governs the carbonation. The identified mechanism will then be modeled by ratiometric Laser Induced Fluorescence to study the durability and/or the degree of reversibility of CO2 storage by the soil.