A transmission electron microscopy study of interfaces and matrix homogeneity in ultra-high-performance cement-based materials

Ultra-high-performance cement-based materials produced under different cond itions have been characterized by transmission electron microscopy (TEM), s canning transmission electron Microscopy (STEM), high resolution transmissi on microscopy (HRTM) and chemical analysis. In addition to cement, these ma terials contain large amounts of crushed quartz and amorphous submicrometre silica. A post-set heat treatment was also applied in some cases. An abras ive thinning method combined with grazing angle ion etching allowed the pre paration of 100 nm thick specimens with wide observation surface areas whil e avoiding any water or CO2 contact which may cause changes. Clinker, silic a fume and crushed quartz reactivity as a function of the curing processes have been studied, as well as the interfacial zones with the hydrated matri ces. The Ca/Si ratio spatial distribution in hydrated products has been ana lyzed and shown to undergo strong local fluctuations. Nevertheless, the com position fluctuations were less pronounced and the average Ca/Si ratio was lower than in silica-free cement paste. HRTM lattice imaging shows the coex istence of nanocrystalline phases and mesoscale ordered regions within an a morphous matrix. A d-spacings analysis of the nanocrystalline phase suggest s a tobermorite-like structure for the calcium silicate hydrates, whereas t he mesoscale order might reflect modulations in the water content. (C) 2001 Kluwer Academic Publishers.