Pore structure of cement silica fume systems - Presence of hollow-shell pores

Significant amounts of "hollow-shell" pores (i.e., Hadley grains) have been found in mature Portland cement-based systems containing silica fume. Hollow-shell pores were found directly by electron microscopy and were indicated by desorption isotherms. Hollow shells are an intrinsic type of pore, along with capillary pores and gel pores. They are large pores enclosed in cement gel and connected to the outside via gel and capillary pores. At low water-to-binder ratios they can be more than two orders of magnitude larger than the capillary pores, and they may constitute a porosity larger than that of capillary pores. Despite self-desiccation effects, hollow-shell pores seem to remain largely saturated with pore fluid. The smaller capillary pores dry before hollow-shell pores. The development of hollow-shell pores and their preservation at later ages in cement silica fume systems is discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.