A MgO expansive agent was prepared using different calcination methods. The effect of the calcination method on the hydration of the MgO expansive agent and its linear expansion was studied and the best method identified. The properties of concrete containing the MgO expansive agent that had been prepared by this method were investigated. The results showed that the MgO expansive agent delayed and reduced the expansion and could also compensate for the shrinkage. Delayed autoclave expansion was used to evaluate the soundness of the concrete on the basis of expansion rate or strength changes after autoclaving and was shown to be a better method with which to assess the soundness of concrete containing the MgO expansive agent. The overall pore structure distribution tended to be more homogeneous when the MgO expansive agent had been added.

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