The free MgO in clinker at the clinkering temperature of 1400–
1500 C is in a dead-burnt state in the form of periclase crystals. In
normal conditions of exposure to ambient conditions of humidity
and temperature it may take several years for the periclase to
hydrate [1]. As the conversion of MgO to Mg(OH)2 involves a molar
volume expansion of 117% [2], the presence of excessive amounts
of MgO in hardened cement-based materials may lead to expansion
and crack formation. However, the expansive forces can be
used to advantage in the chemical prestressing of concrete.