Mineral Additions for the Inhibition of Delayed Ettringite Formation in Concrete: The Role of Limestone Filler

Abstract

One of the most popular preventive measures to minimize the occurrence of chemical expansive reactions, namely the internal sulphate reaction (ISR), in hardened concrete is the use of mineral additions. This pathology is due to the formation of expansive ettringite (delayed ettringite formation - DEF) inside the material and is very difficult to deal with, because presently there is no efficient method to repair concrete structures affected by DEF. Hence, there is an urgent need to find preventive methods that may enable the inhibition of DEF in new constructions. Nowadays, it’s recommended the use of mineral additions to sustain this type of degradation. Moreover, their effect depends on the chemical and mineralogical composition and also the cement replacement. The research work presented in this paper deals with the influence of limestone filler, a type I mineral addition, in the inhibition of DEF (Santos Silva et al., 2010a, 2010b), and is part of an extensive study to elucidate the role that the mineral additions have in the mechanism of inhibition of DEF in concrete. For this purpose different concrete mixes were produced by using the same cement type (CEM I 42.5Ra with 3.11% SO3 and 6.4% C3A) and water/cement ratio (0.45), incorporating different amounts of mineral additions, like fly ash, metakaolin, blast-furnace slag, silica fume and limestone filler. The filler influence was followed by expansion and microstructure evaluation of concrete mix at several ages. These studies showed an interesting behaviour of limestone filler, which motivated new concrete formulations with different cement types (CEM I 42.5Rb and CEM I 52.5) that differ in SO3 and C3A contents, in order to investigate its influence in development of DEF. This research includes also a concrete composition with a cement type II (CEM II A-L 42.5R). The results obtained were compared and the conclusions were extracted. It was found that the concrete mixes with limestone filler showing higher expansions than the control composition. According to these results it seems that the limestone filler does not inhibit rather it promotes the expansion due to DEF. Thus, for concretes subjected to high temperatures in early ages, the limestone filler is not adequate to sustain DEF development rather it may even increase the expansion behaviour in concrete mixes. According to these findings, it was proposed that the ISR recommendations must prohibit this kind of mineral addition.