(1) Neil Beningfield & Associates Ltd., UK
(2) WHD Microanalysis Consultants Ltd., UK
(3) CPI Ltd, Republic of Ireland


This paper uses a series of laboratory investigations, to study mortar pore structure then discusses a theoretical, proposition that is confirmed by the practical work outlined in the paper. The research confirmed that microscopy is a valuable tool in elucidating the pore structure of hydrated cementitious materials, typically concrete and mortar but, particularly in the case of the latter, it is fallacious to equate the total void space in the hardened mortar with the initial air content. Indeed, any relationship is tenuous.

This work used scanning electron microscopy of polished sections of mortar to elucidate the pore structure and investigated the changes in pore structure as a function of unit type and suction, curing condition and initial air content. The effect of mortar joint orientation was also briefly considered. The results showed that the total hardened mortar pore volume, which approximates to the porosity, differs greatly from the initial air content in the wet or plastic state, and showed that measurement of the former, the total hardened pores, cannot be used to state a value for the initial plastic air content.