The opaline silica-rich sedimentary rocks of Milos Island, Greece and their behaviour as pozzolanas in the manufacture of cement

In the Pliocene–Pleistocene succession of Milos Island, Greece, siliceous rocks of biogenic origin (diatomites) occur interbedded with volcanic tuffs. Locally lava sills penetrate the diatomaceous rocks causing their diagenesis. In order to identify the nature of lava and related tuffites and the influence that lava intrusion had on the diatomaceous rocks, X-ray diffraction, scanning electrom microscopy, X-ray fluorescence and light microscopy analyses were performed on representative samples. The original diatomaceous rocks are mainly composed of opal-A in the form of broken diatom frustules and well preserved silicoflagellate skeletons. The affected diatomite beds are characterised as porcelanites containing neoformed opal-CT lepispheres, in expense of opal-A. To utilise the diatomites and porcelanite as pozzolanas, measurements on the Blaine, water demand and compressive strength of cements were carried out. Slightly lower strength was measured in porcelanite cement, due to the higher crystallinity of opal-CT compared to opal-A. However, both rocks improve the pozzolanic cement properties compared to that currently produced, using glassy tuffs.