ABSTRACT
Different HR-SEM imaging techniques are neces sary for the characterization of the hydration proc ess and studying the development of microstruc ture of UHPC: ESEM in WET mode for the early hydration process, NanoSEM for the examination of the extremely dense microstructures of hardened samples and BSE imaging analysis for polished microsections. A strong retardation of the hydra tion process caused by a high amount of superplas ticizer can be observed. Furthermore, the growth of ettringite is influenced. The length of ettringite crystals is smaller than 600 nm whereas under “normal” cement hydration conditions a length of up to 3 µm is visible. Clinker grains smaller than 2 µm had often been completely dissolved, resulting in hollow shell grains in the UHPC microstructure. The bonding between the aggregates and the UHPC matrix is very strong. The interfacial region shows no gaps between these components. Indi vidual silica fume agglomerates up to a diameter of 250 µm are visible in the UHPC microstructure irrespective of the storage conditions of the samples. Locally concentrated initiation of ASR can be observed but the typical crack formation occurs only around the silica fume aggregates. No damages due to ASR occur. Investigations on the durability in a climate simulation chamber show that the expansion of UHPC reference mixtures M2Q is below the threshold value of 0.4 mm/m regardless of water storage, heat treatment of intentional pre-damaging. In heat treated and predamaged samples a secondary ettringite formation in micro-cracks with a crack width smaller than 10 µm can be observed whereas in wider cracks a signifcant carbonation occurs. Nevertheless the expansion threshold value is not exceeded.
