The presence of the following two oxides adversely affects the quality of cement:
(1) Alkali oxides K2O and Na2O
(2) Magnesium oxide MgO.
If the amount alkali oxide exceeds 1 per cent, it leads to the failure of concrete made from that cement. Similarly, if the content of magnesium oxide exceeds 5 per cent, it causes cracks after mortar or concrete hardens. It is due to the fact that magnesium oxide, burned at a temperature of about 1500ºC slakes very slowly, when mixed with water.