Ash in concrete – new problems and possibilities. Use of bottom ash from straw and wood

Bottom ash from co-combustion of wood and straw pellets from the incinerator at Vattenfall Amagerværk, is used in the experimental work of this project. The ash has a high soluble fraction and by washing it in water approximately 25 % of the mass is lost. The compressive strength for 7-days samples with replacement of cement with the ash as received (5%, 10% and 20%) decreases from 46 MPa for the reference to 42 MPa, 43 MPa and 32 MPa. After washing the ash in water the strength of the corresponding samples is 48 MPa, 34 MPa and 30 MPa, respectively. By further processing the ash through grinding it with the cement, an increase in compressive strength is found: 48 MPa, 43 MPa and 40 MPa relative to the reference sample cast with milled cement of 49 MPa. The pozzolanic properties of the ash is examined with Bolomeys formula and the ash had none or very little pozzolan properties. The heat development of the cement paste is examined showed that ash has a marginal (negative) effect on the rate of which heat is developed relative to the grinded reference sample (basis cement). Furthermore the evolved heat decreases drastically with the amount of ash in the cement paste. The ash shows retarding effects on the setting time shown with a vicat-apparatus. The setting time determined from heat development support this finding.

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