Hansen, E.Ø

The report is divided into two parts. Part 1 is about replacing cement with wood ash in lightweight concrete, and part 2 is about using thermal runaway to produce lightweight aggregate from sewage sludge ash.

For part 1 for the major part of the experimental work a wood ash from Amagerværket was used. It ash was prewashed due to a relatively high water solubility of 13 %. A mix design from commercial producer was adjusted to the lab equipment. As the water requirement for wood ash is not the same as that for cement the water amount was adjusted. The casting of the LWA blocks was done on a small-scale block machine. Blocks were cast with cement replacement from 15 % to 40 % with a 5 % increment. Using commercially available blocks, the test methods for the different analysis were refined, and the blocks were tested for compressive strength, porosity and density, capillary suction and leaching.

Up to 25 % cement replacement with wood ash gave minimal effect on the compressive strength The heavy metals found in the wood ash were bound in the concrete to a degree, where the block was harmless. The ash did increase the capillary suction, but as this is not regulated, it was not deemed a problem.

Part 2 started out as failed attempt at drying out some 0-2 mm lightweight aggregate in a microwave oven, which let the wood ash to sinter together and expand. It was found that the possible phenomenon of thermal runaway was to blame for the heating, and carbonate for the voids. As thermal runaway mostly occur in a small number of metal-oxides, the concentrations of these metals were found in the LWA, and compared to those of a sewage sludge ash. As there was a higher concentration of all of the different metals, especially iron, tests were conducted on the sewage sludge ash. It was found that it too was capable of going into thermal runaway, and creating something that resembles LWA.

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