Alternate ash in concrete. Application for sewage sludge ash

Milled sewage sludge ash from Avedøre Spildevandscenter is used in this thesis. TEM analysis showed that the ash has a relative course form on the microscopic level and a crystalline structure on the atomic level. The most abundant elements of sewage sludge ash were Fe, Ca, P, Al, Mg, Si and Zn. Coal fly ash essentially consist of Ca, Fe, Al and Si which implies, based on the elementary content, that sewage sludge ash have pozzolanic potentiel. The setting time increased in the order Reference < 5% cement replacement < 10% sand replacement. The capillary suction followed the same order and for all three samples it increased from 7 days cured samples to 28 days samples. Sludge ash is added to the reference mortar composition instead of replacing cement or sand (as in previous reports). The ash is added systematically with 2.5% ash by mass cement up to 15% (from 11g to 68 g of ash to a mortar mixture containing 450 g cement) to 7 days samples. In every case the compressive strength was higher than for the reference mortar. The highest compressive strength was obtained with 34 and 45 g of ash and here the compressive strength reached 52 MPa compared to 44 MPa for the reference sample. Sewage sludge ash has as coarse, porous structure which may increase the water demand in order to have sufficient water for cement hydration. The optimal water-cement/powder ratio is investigated systematically from 0.42-0.52 and found to be 0.43 where a 7 days compressive strength of 56 MPa was obtained. All mortar compositions by normalizing with respect to Bolomey's calculated strengths was found to be stronger than reference composition, which strongly indicate strengthening properties of the ash, however, it is difficult to conclude if they are puzzolanic or compacting.

Download report