Abstract
Concrete for agricultural or industrial applications is often subject to intense acid attack. Most affected structures are sewage structures and biogas plants, natural draught cooling towers or silage silos. Widely independent from acid type, in most cases the acid attack on concrete runs the same way, starting with dissolution of easily soluble calcareous phases like calcium hydroxide. With ongoing attack, calcium-silicate-hydrate crystals (CSH) are also affected by acidic media. In contrast, siliceous phases like silicon-dioxide (SiO2) are widely unaffected by acid attack. While the dissolution of the matrix is increasing with ongoing attack, quarzitic aggregates remain unchanged. Beside the use of coarse SiO2-aggregates, the resistance against acid attack is mainly increased by a minimization of the porosity. For this purpose on one hand, a low water/cement-ratio has to be sought, on the other hand also the fines should be distributed with an optimized grading curve (e.g. Fuller-principle). In practice, this results in a combination of various fine and ultra-fine components, e.g. fly ash, GGBS, silica fume or metakaolin. Such binder compositions lead to a particularly dense microstructure, especially at pore sizes below 1 micron, and a higher chemical resistance due to a lower Ca(OH)2 content. This paper gives an overview on typical acid-resistant concretes, most common applications as well as the effects of the related acid attack and points out the potential of granulated blast furnace slag addition to such concretes.
Highlights
In agriculture as well as in industrial and wastewater engineering plants, concrete structures may be exposed to various chemical attacks
While the dissolution of the matrix is increasing with ongoing attack, quarzitic aggregates remain unchanged
For this purpose on one hand, a low water/cement-ratio has to be sought, on the other hand the fines should be distributed with an optimized grading curve (e.g. Fuller-principle)
Summary
In agriculture as well as in industrial and wastewater engineering plants, concrete structures may be exposed to various chemical attacks. One type of these attacks is based on the action of strong acids. Biogas and sewage plants can be exposed to a sulfuric acid and in silos used for agriculture acetic and lactic acid can be formed. To increase the resistance of concretes, priority is given to reducing the porosity. This is done on the one hand by the choice of a low w/b-ratio and on the other hand by the use of an optimized grain size distribution up to the finest aggregates. With increasing w/b-ratio, more and more capillary pores are formed, causing a connected network of capillary pores at a w/b-ratio of approx. 0.60 [1,2,3,4]
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