Reactive Magnesia Cements
Cements containing reactive magnesia that can be and often are blended in any proportion with other hydraulic cements such as Portland cement because they will hydrate sufficiently rapidly so as to not cause dimensional distress." They could also be defined as cements containing reactive magnesia but such a lame definition would give no hint of the fundamental requirement - that they hydrate quickly and can be blended hydraulic cements. In most cases this would mean that they have the ability to hydrate in the same rate order as Portland cement. Technically magnesia is reactive if it has a high proportion of unsaturated co-ordination sites (and thus relatively low lattice energy) and this can be argued thermodynamically to be related to low temperature calcination. See technical.reactive_magnesia.php.
The blend depends on the engineering requirements and formulations range from high strenth concrete with very little reactive magnesia added to gas permeable masonry units with significant amounts added. At TecEco we call our higher strength concretes Tec-Cements and our Carbonating Concretes as Eco-Cement if they are gas permeable and Enviro-Cements if they are not.