Use of alternative ash in concrete. Workability and setting

The compressive strength of the mortar decreases when wood-ash from Køge Kraft-Varmeværk is partly substituting cement. When all cement is substituted with ash the mortar obtains no compressive strength, but mortar is able to set. Compared to the reference mortar there is no difference in setting time when ash is used as partly cement replacement. Slump test shows, that extra water is required when larger amounts of wood-ash is used. The slump test of a mortar with a 5 % wood-ash substitution complies with [DS/EN 450-1, 2012]. Porosity increases and density decreases, the more wood-ash is applied in the mortar. Over time, the porosity decreases, and the density increases. The capillary action increases the more ash in the mortar. There is no clear impact on the capillary action when a small amount of wood-ash is used. The wood ash has a high content of Cl (7-9 wt%), and the influence of Cl and its impact on workability and hardening is researched. To be able to study the effect of Cl, all salts is washed out and NaCl is added. The mass of nitrate, sulphate and chloride is calculated based on the molar mass of these salts. The equivalent amount of chloride is added to the washed ash and mixed into the mortar. Furthermore, an extreme case is tested, where 5 times the NaCl is added to the mortar. This is done to provoke a lager deviation in the results. It shows, that the compressive strength increases when NaCl is added, although only to a certain point (the compressive strength does not increase when an extreme amount of NaCl is added). The compressive strength for both mortar with washed ash, and washed ash with NaCl, is higher than the compressive strength for a 5 % cement substitution with the ash as received. This indicates that sulphate or nitrate is the reason for the lower compressive strength. Mortar with 5 % cement substitution with washed ash sets faster, than the mortar with 5 % cement substitution with ash as received. NaCl seems to have no impact on the setting time.

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