Title: Early Hydration Process of a Low Shrinkage and High Strength Packing Density Optimised Mortar-Mixture Made of an Alternative NPC-CSA-CA Blend
Author(s): Tim Schade and Prof. Dr. Bernhard Middendorf
Publication: Symposium Paper
Appears on pages(s): 446-465
Keywords: Calcium Aluminate Cement, Calcium Sulfoaluminate Cement, ettringite formation, packing density optimised concrete, setting behaviour, shrinkage
Compared to normal concrete, packing density optimised Ultra High Performance Concretes have a high shrinkage up to 1 mm/m due to their high cement content. Especially in the first 24 hours approximately 80 % of the final shrinkage is reached which reduces the early strength due to microcracks. Instead of additives within the scope of this research work, parts of normal portland cement (NPC) were substituted by Calcium Sulfoaluminate (CSA) Cement and Calcium Aluminate (CA) Cement with the aim to reduce shrinkage of UHPC-mixture as well as a fast setting. CSA-cements with low CO2 footprint are characterised by their fast strength development and expansion behaviour due to early ettringite formation. X-ray diffraction was used to study the phase development. The influence on the shrinkage value was measured by shrinkage tests. In addition, the development of the microstructure was investigated by scanning electron microscopy. Finally, the influence on the strength development was correlated by ultrasonic measurement. These techniques allow a prediction of the setting process in the early stages. Finally, an environmentally friendly NPC-CSA blend could be developed which, in addition to high early strength, also achieves low shrinkage. Furthermore, the influence of the ettringite formation on the microstructure could be determined.