Reappraisal of the Autoclave Expansion Test

This report re-examines the technical basis for the AET as presented in PCA Research Department Bulletin RX45 as well as more recent research. The focus is on the potential for disruptive expansions caused by crystalline MgO in portland cements hydrated under various conditions, including the AET. Much of the data indicates that the AET would be a good test for unsoundness caused by free CaO alone, but that expansions in the AET caused by MgO and C(3)A do not correlate well with disruptive expansions under ordinary conditions. Consideration of expansive reaction mechanisms and the published data indicate that MgO continues to hydrate slowly under a wide range of conditions and that stress-relief mechanisms and stabilization processes apparently operate to prevent disruptive expansions in moist environments. Since these mechanisms may not operate as well in dry environments, there are undetermined risks with respect to reliance on performance tests that do not cause hydration of MgO. Investigation of these mechanisms is recommended so that portland cement soundness and performance can be assured, perhaps without reliance on the AET.