# Eco-cement

Eco-Cement is a brand-name for a type of cement which incorporates reactive magnesia (sometimes called caustic calcined magnesia or magnesium oxide, MgO), another hydraulic cement such as Portland cement, and optionally pozzolans and industrial by-products, to reduce the environmental impact relative to conventional cement. One problem with the commercialization of this cement, other than the conservatism of the building industry, is that the feedstock magnesite is rarely mined.

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Ordinary Portland cement requires a kiln temperature of around 1450 °C. The reactive magnesia in Eco-Cement requires a lower kiln temperature of 750 °C,[1] which lowers the energy requirements, and hence the use of fossil fuels and emission of carbon dioxide (CO2).

Eco-Cement sets and hardens by sequestering CO2 from the atmosphere and is recyclable. The rate of absorption of CO2 varies with the degree of porosity and the amount of MgO. Carbonation occurs quickly at first and more slowly towards completion. A typical Eco-Cement concrete block would be expected to fully carbonate within a year.

Eco-Cement is able to incorporate a greater number of industrial waste products as aggregate than Portland cement as it is less alkaline. This reduces the incidence of alkali-aggregate reactions which cause damage to hardened concrete.[2] Eco-Cement also has the ability to be almost fully recycled back into cement, should a concrete structure become obsolete.

To make truly zero CO2 and pollutants emission cement, MIT researchers have come up with a very innovative approach. The Figure shows the cement production process of this new approach.[3] First of all, the new approach can replace the use of fossil fuels in the heating process with electricity from clean, renewable sources. At present, we have many ways to obtain clean electricity, such as solar cells, wind power, nuclear power and so on. Also, in many regions, renewable electricity is the cheapest energy source we have today, and its cost is still falling. In the new process, crushed limestone is dissolved in acid at one electrode and releases high-purity CO2, while Ca(OH)2 is precipitated as a solid at the other electrode. The sum of the electrochemical reaction occurring in this process is

${displaystyle {ce {2CaCO3 + 4H2O -> 2Ca(OH)2 + 2H_2 + O_2 + 2CO_2}}}$

The Ca(OH)2 can then be processed in another step to produce cement. And then, we can easily capture the high purity CO2, O2 and H2 produced by this process. The high purity CO2 can be used to produce value-added product, and the O2 and H2 may be used to generate electric power via fuel cells or combustors. This approach can also significantly reduce the water consumption of cement production. In this approach, half of this water would be recovered upon the dehydration of Ca(OH)2. If H2 was used to fuel the kiln, the other half of the water could be condensed from the flue gas. In principle, all of the water used for electrolysis could be recycled.[3]

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