The cement industry contributes around 5-7% of man-made CO2 emissions globally because of the Portland Cement (PC) production. Therefore, innovative reactive magnesia cement, with significant sustainable and technical advantages, has been proposed by blending reactive MgO and hydraulic binders in various proportions. MgO is currently produced from the calcination of magnesite (MgCO3), emitting more CO2 than the production of PC, or from seawater/brine which is also extremely energy intensive. Hence this research aims to investigate an innovative method to produce MgO from reject brine, a waste Mg source, through carbon sequestration, by its reaction with CO2, to provide a comparable low carbon manufacturing process due to the recycling of CO2. The produced deposits are then calcined to oxides with potential usage in construction industry. The entire system is a closed loop to achieve both environmental optimisation and good productivity. This research focuses on the chemical manufacturing process, integrated with material science knowledge and advancements, instead of concentrating purely on chemistry evaluations. Six series of studies were conducted, utilising MgCl2, CaCl2, MgCl2-CaCl2, MgCl2-CaCl2-NaCl, and MgCl2-CaCl2-NaCl-KCl to react with CO2 under alkaline conditions. The precipitates include hydrated magnesium carbonates, calcium carbonates and magnesian calcite. Generated carbonates were then calcined in a furnace to obtain MgO, CaO or dolime (CaO•MgO). All six series of carbonation processes were carried out under a controlled pH level, to study the constant pH’s effect on the process and resulting precipitates. Other controllable factors include pH, temperature, initial concentration, stirring speed, and CO2 flux rate. In conclusion, the optimum parameters for the production of the carbonated precipitates are: 0.25MgCl2 + 0.05CaCl2 + 2.35NaCl + 0.05KCl, 700rpm stirring speed, 25oC room temperature, pH=10.5, and 500cm3/min CO2 infusion rate. Reaction time is within a day. These parameters are chosen based on the sequestration level, particle performance morphology and the operational convenience. The optimum calcination parameters are at 800oC heating temperature with a 4h retention time.

Les sols constituent le principal réservoir de carbone, avec près de deux fois plus de carbone que le pool atmosphérique. Afin de pouvoir prédire et anticiper le devenir du carbone dans le contexte actuel de changement climatique et de changement d'usage des terres, il apparaît nécessaire de mieux comprendre les processus qui régulent la décomposition des matières organiques dans les sols. Cette thèse se propose donc d'étudier deux types de régulateurs de la dynamique du carbone du sol : les propriétés de l'habitat microbien et celles des communautés microbiennes. En effet, puisque directement affectées par les changements climatiques d'une part, et les changements d'usage des terres et de pratiques culturales d'autre part, l'habitat microbien et les communautés microbiennes apparaissent comme des régulateurs clés de la dynamique du carbone du sol. Des dispositifs expérimentaux permettant de faire varier les propriétés de l'habitat microbien et celles des communautés microbiennes de façon indépendante ou simultanée ont été mis en place. Dans un premier temps, des microcosmes dont la structure du sol a été manipulée afin d'obtenir des gradients de déstructuration, ont été incubés. Dans un second temps ce sont des microcosmes mettant en jeu des gradients de diversité microbienne qui ont été incubés. Enfin, une incubation utilisant les différences naturelles de propriétés de l'habitat microbien et de communautés microbiennes a été mise en place pour tenter de hiérarchiser ces régulateurs de la décomposition des matières organiques du sol. Les résultats obtenus ont mis en évidence que l'activité microbienne de décomposition du carbone organique du sol semble plus contrôlée par les conditions environnementales (comme le pH, la texture et l'approvisionnement en substrat) que par la structure des communautés microbiennes ou leurs capacités métaboliques. En plus de cela, la fonction de minéralisation ne semble être affectée que dans le cas d'une très grande érosion de la biodiversité suggérant la présence d'un effet seuil, et que l'importance de la redondance fonctionnelle n'est pas toujours aussi grande que ce que le suggère de nombreuses études. D'autre part, dans des conditions d'aération suffisante, les mécanismes qui réguleraient la dynamique du carbone organique des sols se passeraient à des échelles très fines. / Soils represent the principal reservoir of carbon with two times as much carbon as is found in the atmospheric pool. In an effort to better predict and anticipate how soil carbon dynamics will be affected by environmental changes and by the evolution of cropping systems, it is necessary to better understand the processes that regulate soil organic matter decomposition. This study aims to investigate two regulatory mechanisms of the soil carbon dynamic: the properties of the microbial habitat and the ones of the microbial communities. Because they are directly affected by the climatic changes and by the rapid evolution of cropping systems these two mechanisms appear to have a key role in the regulation of soil carbon decomposition. Experimental designs were setup allowing the variations, independent or simultaneous, of the properties of microbial habitat and the ones of the microbial communities. First, to assess the relative importance of soil structure, microcosms with different gradient of disaggregation were incubated. Then, to assess the relative importance of diversity erosion, microcosms with microbial diversity gradient were incubated. Finally, using contrasted soils varying in their habitats and their microbial communities properties, we aimed to hierarchize these two carbon decomposition regulatory mechanisms.The obtained results indicate that microbial activity of soil organic carbon decomposition seems to be more controlled by environmental conditions (such as pH, texture and also substrate supply) than by the microbial community structure or metabolic profiles. Then we observed that organic carbon mineralisation was impacted only when the levels of diversity were very low suggesting the existence of a threshold, and that the functional redundancy is maybe not as great as numerous studies suggest. Moreover, our work showed that when conditions of aeration in the pore system are sufficient, mechanisms regulating the dynamic of soil organic carbon take place at fine spatial scales.

This research investigated carbon dynamics, water dynamics and peat formation at Wellington Plain peatland in the Victorian Alps. The properties of bog peat and dried peat were measured, and the ensuing results are outlined below. The carbon chemistries of both bog peat and dried peat displayed changes with depth consistent with an increase in the extent of decomposition of the organic material. Representative changes in the alkyl:O-alkyl ratio down the profile were 0.14 to 0.96 for bog peat and 0.28 to 1.07 for dried peat. Laboratory incubations on the influence of chemistry, particle size, water content and sample preparation indicated that, in the absence of confounding factors, peat chemistry was the most important factor in determining the size of the mineralisable carbon pool. Water content was the most important factor in determining the rate of carbon mineralization. In the field, both bog peat and dried peat emitted an average of 2 g CO2/m2/d from the surface. Carbon mineralisation was related to both soil temperature and soil water content, and this relationship was used to model peat mineralisation under a range of possible future climate scenarios. Below the surface, however, I measured lower rates of decomposition in the dried peat than in the bog peat. The water-holding capacity of peat was measured in the laboratory, as was the rate of water movement through peat. Specific yield decreased down the profile in both bog peat (0.88 to 0.45 cm3/cm3) and dried peat (0.36 to 0.11 cm3/cm3). Hydraulic conductivity also decreased down the profile in both peats: 5.1x10-4 to 3.0x10-6 m/s in bog peat, and 1.0x10-4 to 7.0x10-6 m/s in dried peat. Relationships between the hydrologic properties of peat and its physical and chemical properties were identified. In the field, fluctuations in the watertable were monitored in concert with rainfall. These laboratory and field measurements enabled me to develop models of the hydrology of bog peat and dried peat. Radioisotope dating indicated that both bog peat and dried peat began forming around 3300 years ago. The bog peat appeared to have drained to form dried peat between 131 and 139 years ago. Since that time, erosion appeared to have contributed more to the loss of organic material from dried peat than carbon mineralisation had.