Alternative Ashes in Concrete — New Technical and Aesthetical Performance

Abstract

The research findings presented in this thesis Alternative Ashes in Concrete – New Technical and Aesthetical Performance contribute to the discussion on utilisation of sewage sludge ash (SSA) as a secondary resource. SSA is the result of incineration of sewage sludge, which is mainly applied at water treatment plants in dense areas to safely handle large amount of sludge. Today SSA is in most cases sent to landfill, which raises the question: is it possible for SSA to change status from waste to resource by utilising SSA in the production of cement based materials? So far SSA has not commercially been applied in concrete production. This is even though research has shown that SSA has some potential as secondary resource to partially replace cement in blended cements or concrete. One main reason might be the fact that SSA has a relatively high content of phosphorous, which is an irreplaceable nutrient essential for crop growth. Since phosphorous is regarded as a scarce resource, options to keep phosphorous in the nutrient cycle are increasingly becoming an object of attention. Consequently, the present research has focused on SSA utilisation as resource for production of cement based materials without losing the potential source for phosphorous used in fertilizer production. The present research included three studies in which different degrees of processed SSA was used in mortar to partially replace cement. The three processes used to treat the SSA were: milling, acid washing and electrodialytic treatment. The treatments were applied for two purposes: 1. optimizing the performance of the mortar, and 2. to recover the phosphorous available in the SSA. The main purpose of the experimental work was to unfold the aesthetical and technical potentials of mortar with SSA by showing how basic properties of mortar were affected when 20 % of cement was replaced with untreated and treated SSA. The outcome of the research project has both a linguistic and non-linguistic part, which collectively forms the thesis of a practiced based research. The non-linguistic part of the thesis is represented by four series of physical samples that show how ordinary mortar transforms when different degrees of processed SSA are used as partial cement replacement. The physical output signifies the potentials of using SSA as a resource, whereas the technical and aesthetical potentials are unfolded by correlating the physical output with quantitative measurements of compressive strength and flow value of the mortars. The research covers a general assessment of the feasibility to utilise SSA-containing concrete, and the experiments were designed to investigate the material behaviour in an open framework. The research findings of the present study support findings of previous research. Numerous studies have shown that the compressive strength and workability decrease when SSA is used as partial cement replacement in mortar and concrete. However, when SSA is milled to obtain finer particles it is possible to reach compressive strength and flow value comparable to ordinary mortar. Up until now only one previous study has combined phosphorous recovery and an investigation of phosphorous extracted SSA in mortar, and only one previous demonstration project Biocrete has reported on the influence of SSA on the colour of concrete. Therefore have I chosen to focus on these two parameters in order to address these issues that challenge the use of SSA as resource in cement based materials. The experimental work of the present research showed that the colour intensity increased parallel to an increase of the compressive strength and workability when the particle sizes of raw SSA decreased (obtained by increasing durations of milling). Furthermore, it was shown that the properties of mortar were notably affected when phosphorous was extracted before the SSA was used in mortar as partial cement replacement. Especially the visible changes were even more evident for mortars with SSA after phosphorous extraction, both acid washed and electrodialytic treated. The colour of mortar with either acid washed SSA or electrodialytically treated SSA changed from the familiar grey colour of ordinary mortar into two similarly saturated reddish colours. The colour tones of the mortar did not gradually increase when the two types of treated SSA were milled to obtain finer particles as seen for raw milled SSA. The compressive strength found for mortar with acid washed SSA was slightly below the compressive strength of ordinary mortar, but increased when the treated SSA was milled to finer particle sizes and reached to the level of the reference. Like the compressive strength the workability increased, however, without reaching the level of ordinary mortar. For mortar with electrodialytically treated SSA, the milling of the treated SSA did not have any significant effect on the performance of mortar, which initially was below ordinary mortar. The findings of the present research can be used to point out future possibilities to utilise SSA as resource in cement based materials in specific cases by displaying the behaviour of mortar with the different types of processed SSA. The knowledge obtained is attended for professionals from different disciplines in engineering and architecture and the result of the research may serve as common ground for future research on SSA utilisation within the scope of resource efficiency.