MgO enters into solid solution in all the cement clinker minerals to different degrees. In these solid solutions it essentially substitutes for CaO. The convention is that up to 2% MgO can be accommodated in solid solution in the clinker minerals. Beyond 2% there is the likelihood that free MgO will be present and then there is the possibility of long-term durability problems due to expansion when the cement is used in concrete.

Dr Michael Clark