Sustainable Utilisation of Raw Sewage Sludge (RSS) as a Water Replacement in Cement-Based Materials Containing Unprocessed Fly Ash

Abstract

Prior to the implementation of the European Union Urban Waste Water Treatment Directive (91/271/EEC) in 31 Dec 1998, around a quarter of the sewage sludge produced in the UK was either discharged to surface waters via pipes or disposed from ships at sea. Discontinuing this route together with the quality requirements of the European Waste Water Directive, led to the generation of significant quantities of sewage sludge. It has therefore become required to treat this waste effectively before it can be sent back to the environment. Consequently, this added greater challenges for the environmental agencies, as well as local authorities. The treatment process comprises costly and energy consuming applications including physical, chemical, biological and thermal. In addition to the sewage sludge, the power generation industry produces massive quantities of fly ash from burning coal. In the UK, there is about 5,300,000 tonnes of fly ash that are generated annually, which require to be processed and classified in order to meet the standard requirements before it can be used in the construction applications. The classifying process also involves a series of costly and energy consuming mechanical and physical applications. This research programme has introduced an innovative alternative to the traditional re-use and disposal routes of Raw Sewage Sludge (RSS) and unprocessed fly ash. It has suggested the utilisation of RSS and unprocessed fly ash as raw ingredients for the production of sustainable construction materials. This research programme has therefore examined the performance of cement-based materials containing Raw Sewage Sludge (RSS) as a water replacement and unprocessed fly ash as cement replacement. Mortar and concrete mixes incorporating these materials were tested for their flowability/workability, density, Total Water Absorption (TWA), Ultrasonic Pulse Velocity (UPV), compressive strength, flexural strength, drying shrinkage, sulphate attack and leaching properties. Three series of cement-based materials were studied including mortar mixes with RSS and unprocessed fly ash (Series 1), mortar mixes with RSS and large proportions of unprocessed fly ash (Series 2), and concrete mixes with RSS and unprocessed fly ash (Series 3). The outcomes of the investigation were encouraging in that cement-based materials containing RSS and unprocessed fly ash that were produced demonstrated relatively good engineering, durability and environmental properties in comparison to the control mixes. The inclusion of unprocessed fly ash significantly reduced flowability/workability; however it improved long-term compressive strength for both mixes with RSS and water. The best compressive strength results were recorded when cement was replaced with 10-20% unprocessed fly ash by weight of total binder. The results also showed that sulphate attack resistance improved when fly ash was included. Moreover, safe concentration levels of heavy metals and free ions were detected when leaching test was performed. However, it must be kept in mind that more environmental tests must be performed before any large scale use is undertaken.