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What is the Effect of  MgO on soundness of cement?

A cement can also be unsound due to presence of MgO, which reacts with water in a manner similar to CaO. However, only Periclase(crystalline MgO) is deleteriously reactive, while MgO present in glass is harmless.

The effect of excess crystalline free MgO (periclase) on cement soundness and concrete durability under actual service conditions is often considered questionable as there is very little data correlating the  percentage of MgO (or periclase), autoclave expansion test results, and the actual field performance of concrete. 

In a study of sound and unsound cements, Narang et al. (1981) demonstrated the effects of cement fineness on autoclave expansion. A high MgO-content cement ground to a fineness of 225 m²/kg showed an autoclave expansion of 7.06%. When the cement was ground to a higher fineness of 350 m²/kg, the autoclave expansion dropped to 1.39%, and finally when the fineness was increased to a value of 400 m²/kg, the resulting autoclave expansion was only 0.24%. It was also found that cement ground from a rapidly cooled clinker gave a low autoclave expansion. 

Thus, one of the factors that influence the amount of expansion that will occur when a cement is autoclaved is the fineness or specific surface area of the cement being tested. Coarser ground cements have always exhibited a greater amount autoclave expansion, all other things being equal.

It has been suggested that the autoclave expansion test is highly conservative  and will exclude cements that would otherwise demonstrate acceptable field performance characteristics with no reduction in long-term durability. In fact, there have been no reported cases of concrete failure within the last 60 to 100 years in the U.S. that can be attributed to a free MgO-induced unsoundness condition. However, this could well be the  result of having such a rigorous autoclave expansion test requirement in the ASTM C 150, Standard Specification for Portland Cement.