Investigations on the workability and microstructure development of UHPC; part 2: Influence of admixtures and curing on the microstructure of ultra-high strength concretes

The priority programmme 1182 .,Building with Ultra High Performance Concrete" was initiated by the German Research Foundation in the year 2005 for a period of six years. The aim of the programme is the development of technical and scientific basis for a wide application of ultra high performance concrete (UHPC). This paper is the second part of a two paper series (part 1 published in CEMENT INTERNATIONAL 8 (2010) No. 4) which summarizes the most important results of the group .,development". j In this contribution the topics binder optimization, internal curing using super absorbent polymers (SAP) as well as hydration and microstructure development will be presented. The composition of a standardized ultra high performance concrete mixture (M2Q) was modified as follows: substitution of ordinary Portland cement by ground granulated blast furnace slag (GGBS) or fly ash, substitution of quartz flour by fly ash and substitution of silica fume by sifted fly ash as well as addition of SAP with or without extra water. The influence j of admixtures and curing on microstructure of UHPC was investigated using the following methods: different high resolution electron microscopy imaging and analytical techniques (FE-SEM) as well as a high pressure mercury intrusion porosimetry (MIP). FE-SEM investigations showed a strong retardation of the hydration process caused by the high amount of superplasticizers. They also revealed a high amount of incompletely reacted clinker phases caused by the very low water-cement ratio of 0.22 and the high amount of ordinary Portland cement in the M2Q mixture. For this reason GGBS and fly ash were used as an alternative binder constituent, maintaining the workability and mechanical properties. As revealed from MIP tests, some shifting in the volume of gel and capillary pores is to be expected when SAP is introduced to UHPC. The extent of particular changes is observed to be dependent on amount of SAP and extra water added, age of concrete and presence of artefacts. During the hardening process an extremely dense and compact UHPC microstructure is formed. Therefore an ultra high resolution FE-SEM with low vacuum imaging capabilities under low voltage conditions must be deployed in order to achieve a contrast optimization.