Limits of the Binder Composition of Durable Fly Ash Concretes
Project Number 315
Since the 1970s, a lot of research work has been done in the field of using fly ash in concrete. Therefore, all fundamental issues regarding strength and durability could be clarified for the mix design relevant in present-day practice. Recently, however, fly ash rich concretes have been designed which shall be applied under permanent exposure. The use of fly ash is led to its technical limits thus raising again the question of the limits of applicability.
The use of fly ash has different effects on the durability of the concrete. The formation of different hydration products causes an increase in strength and hence in the structural resistance against all mechanical attacks. A further effect consists in the change of the pore structure. This leads to an increased total pore content caused by the incomplete and delayed reaction of the fly ash. There is, however, a significantly finer pore size distribution which causes a higher resistance against the penetration of concrete damaging substances as e. g. sulphates.
Concretes with higher fly ash contents have been examined at the Institute of Building Materials Research of RWTH Aachen University (ibac). The focus of these investigations was directed at the determination of the reaction progress to assess the alkalinity as well as on the pore structure under sealed conditions. It was ascertained that, even with a very high fly ash content in the binder (up to 60 % by mass), the reserve alkalinity is not exhausted even after very long reaction times. In combination with blast furnace slag cement, however, the remaining absolute portlandite content in the hardened cement paste is low. Therefore, the carbonation velocity is problematic because it may have a negative effect on the strength of the cover zone and on the resistance against frost and freeze-thaw cycles.
In this research project, in a first step tests on hardened cement paste shall be conducted in which the fly ash-to-cement ratio as well as the water-to-binder ratio shall be varied in a target-oriented way. A customary Portland cement and two fly ashes with different finenesses are used. The produced test specimens are examined regarding their carbonation depth at defined times. In addition, porosity, thermogravimetric and chemical analyses are carried out. It is the aim of these investigations to determine reference values for the limits of the binder composition of durable fly ash concretes and to gain a better understanding of the carbonation processes.
After a successful conclusion of the first step, the investigation programme shall be expanded to examine further types of cement and concrete mixes with good prospects in a long-term test. The research programme serves to increase the use of fly ash in concrete in compliance with the durability requirements. The results are a basic precondition for the use of higher amounts of fly ash in concrete.
This project is carried out under the direction of Prof. Wolfgang Brameshuber at the Institute of Building Materials Research of RWTH Aachen University (ibac) from August 2008 until October 2009 and it is accompanied by the VGB Technical Committee
Power Plant By-products
. It is assigned to the VGB research programme “Waste Management of Coal-fired Power Plants and Waste Incineration Plants” (ERKOM).