The necessity of recovering soluble phosphorus from sewage sludge ashes before use in concrete based on concrete setting and workability

Abstract

By replacing cement with alternative ashes, such as sewage sludge ashes (SSA) from mono-incineration plants, it is possible to reduce CO2-emmision from the production of cement. SSA contains a large amount of phosphate which can be extracted before addition in concrete. The Danish Standard DS/EN 450-1 states an upper limit for total phosphorus in concrete, but it does not account for the solubility. The aim of this study is to determine an upper limit for soluble phosphorus in concrete, thus minimizing the formation of calcium phosphate to an acceptable level. The analysis will be based on the setting and workability. When adding soluble phosphorus salts to a mortar the setting process is extended with no pronounced difference between different soluble phosphorus salts. It is therefore assumed, that the soluble phosphorus (SP) influences the concrete setting. A logarithmic relationship between the increased addition of SP and the initial setting time is seen. By comparison with the limit for an initial setting time established in DS/EN 450-1 it is possible to establish a limit for SP of 0.54 wt% cement. When studying the workability an objective limit for SP of 0.16 wt% cement can be established. SSA from the Danish mono-incineration plant at Spildevandscenter Avedore is examined. At a pH-value of 13 it is possible to replace 55% and 16% of the cement, based on the set limits, with SSA from Spildevandscenter Avedore, before it is necessary to extract SP from SSA before adding to the concrete mixture. Former studies have shown a replacement of up to 20% of the cement with SSA is feasible regarding compressive strength, and therefore it is necessary to consider the limit for SP of 0.16% wt% cement.