Not All "Lime" is Burnt Equal

Petrographers can see which portion of a lime particle or which mineral phase in the lime hydrates and contributes to expansion. In a recent MgO-caused popout project, large MgO (periclase) crystals have partially hydrated and are surrounded by the hydration product brucite, with very different optical characteristics from periclase. Hydration rates of lime and the potential to cause distress depend on burning temperature (e.g., hard-burnt vs. soft-burnt), crystal size, particle size, cover depth, and of course moisture availability. In a previous project where thousands of popouts occurred due to soft burnt dolomite, the expansion was due to complete hydration of CaO in the lime. MgO in the same particles did not exhibit any evidence of hydration.