Embodied energy is the amount of energy that has gone into the making of a material or thing made with materials. A very high percentage of the world’s energy is derived from fossil fuels which when burnt release vast amounts of CO2. As the production of energy from fossil fuels is environmentally unfriendly, materials and things that have a lower embodied energy are more sustainable than those with a higher embodied energy
The Embodied Energy of Concrete
Contrary to lay opinion concrete has relatively low embodied energy.
Embodied Energy of Building Materials 
However concrete is the most widely used material on earth with over two tonnes
per person on the planet being used. Associated high usage in construction results
in higher total emissions than any other material.
Embodied Energy in Buildings 
 Tucker, Selwyn (2001). "The Embodied Energy in Buildings" www.dbce.csiro.au/ind-serv/brochures/embodied/embodied.htm (last accessed 07 March 2000).
 Reactive magnesia is also variously known as caustic calcined magnesia, caustic magnesia or CCM. The temperature of firing has a greater influence on reactivity than grind size as excess energy goes into lattice energy.