Bioflygaska i betong: inverkan på hållfasthet och beständighet : En laborativ studie där bioflygaskans inverkan på betong utvärderas genom kemisk exponering för svavelsyra

Sammanfattning

Concrete is an attractive building material when it comes to structures that require high strength, long life and durability. Despite concrete's good properties when it comes to strength and durability, it can still be affected by degrading environments. Concrete can be broken down in various ways in unfavorable environments, one of which is exposure to chemicals. In the water treatment industry, constructions of concrete are exposed to various chemicals through purification processes or by chemicals that can be formed due to improper conditions. In extreme cases, very acidic and highly corrosive sulfuric acid can be formed. Sulfuric acid attacks concrete by chemically dissolving the cement that acts as a binder for the aggregate grains and is the component that gives concrete its characteristic properties. To improve and change the properties of concrete, various types of additives are used, of which the residual product fly ash is one of them. Fly ash can replace cement to a certain extent and, among other things, improves the durability of concrete in chemically exposed environments. In this report, fly ash extracted from bio-based heating plants is in focus as the combustion of fossil fuels is being phased out. Biobased fly ash is a less explored area and is therefore interesting as a potential replacement for coal fly ash. The effects of bio-based fly ash on concrete were investigated through a laboratory experiment. A total of 25 concrete specimens were cast and divided into test groups of five cubes each, in which 0 %, 10 %, 25 %, 33 %, and 50 % of the cement was replaced with fly ash. The specimens were immersed in a 5 % sulfuric acid bath, and each test group was assigned a specific exposure period. Following exposure, mass loss, surface characteristics, and compressive strength were evaluated. Five specimens were not exposed to the acid bath and were instead used as reference specimens for comparison during the assessment. The results in the report show decreasing mass loss as the replacement level/fly ash content increases. Bio-based fly ash thus has a positive effect on concrete exposed to sulfuric acid. In the testing regarding compressive strength, an uneven and unclear result was given, which does not show any improving properties of concrete with fly ash, but rather the opposite. This could possibly be explained by the quality of the fly ash or the experimental method. The conclusion is that bio-based fly ash can function as a replacement material instead of cement if the aim is to improve resistance to chemical attacks such as sulfuric acid. From a compressive strength point of view, results are given that undermine the theory of improving properties, but deeper analysis and testing are required for confirmation. Further research in the subject may possibly develop the understanding of the result and the properties of fly ash.