Production of Magnesium-Based Cements by Means of Solar-Driven Calcination of MgCO3-Containing Natural Sources

Extensive R&D is in progress to exploit the huge amount of solar energy falling on Earth. Concentrating Solar Power (CSP) systems offer, beside the possibility of producing firm electrical energy through the integration with energy storage systems and thermodynamic cycles, the possibility of producing solar fuels and/or chemicals by exploiting the concentrated solar energy to drive endothermic chemical reactions. Integration of CSP in the mining/cement industry is currently attracting interest. Concrete is the second most consumed raw material on Earth after water, and it is by far the most widely used building material worldwide. The key component of concrete is cement, whose production is a highly energy-intensive manufacturing process due to the consumption of high quantities of fuels (mainly fossil fuel and pet coke). Cement production is also one of the largest industrial sources of CO2 emission. For ordinary (Portland) cements, limestone calcination is the stage responsible for most of CO2 emissions and energy requirement. Alternative cements such as those based on magnesium show several environmental benefits, as discussed below. Moreover, the use of solar energy to sustain the calcination of the MgO precursors (MgCO3-containing natural materials) could provide additional and unquestionable advantages on both the economic and environmental aspects of the cement production. In this work the possibility of using rich-MgO sources calcined in a directly irradiated FB reactor for the synthesis of Magnesium-Based cements (MBCs) was assessed. MBCs represent a sustainable alternative to Portland Cement (PC) inasmuch as they have different environmental benefits such as: i) the lower calcination temperature, ii) the absorption of CO2 by the key-hydration product of magnesia cements (Mg(OH)2), iii) the incorporation of industrial by-products and iv) their potential suitability for the encapsulation of heavy-metal-containing wastes. To this aim, different MgO sources in the form of carbonates were calcined in a lab-scale 0.1 m ID Fluidized Bed (FB) reactor, heated through a 12 kWel simulated solar furnace (Figure 1). Simulation of the solar radiation was performed through an array of three short arc Xe-lamps coupled with elliptical reflectors, yielding a peak flux of nearly 3000 kW m–2 and a total power of nearly 3 kWth incident on the bed surface. The obtained MgO-based samples were used together with different amounts of selected silico-aluminates-based by-products for the manufacture of novel “green” MBCs. The FB calcination reaction was performed using air as fluidizing gas at different temperatures and times, in order to investigate the influence of the thermal history experienced by MgO sources during the direct solar FB calcination on the MgO reactivity behaviour. In a previous work, we successfully demonstrated the technical feasibility of solar-driven calcination of limestone in a fluidized bed to produce lime usable in the clinkerization process of PC. This work proceeds along the laid path and applies a similar methodology to investigate the production of solar magnesium-based cements.