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Drict discharge of domestic wastewater(sewage) to the environment or into absorbing wells has caused many problems including surface and groundwater pollution. To reduce such problems, the number of wastewater treatment plants has increased significantly in Iran during the last two decades. During wastewater treatment, a significant amount of sludge, composed of organic and mineral material, is produced. This sludge, if not handled and disposed properly, can create serious environmental and health issues. One environmentally attractive way of dealing with such wastes is to use them in different types of applications. In this regard, many economical and beneficial methods have been developed to reuse sludge. Incineration of sludge for energy recovery or the use of sludge ash in cement-based construction materials are among these methods. Sludge incineration produces considerable amount of ash which should be disposed. However the ash can be used as cement substitude in procuction of cement-based material. The subject of using sludge ash as cement substitude has been investigated by a few researcher with the conclusion that the usage of ash can affect the final cement-based product quality. Based on their experimental results, the use of sludge ash tends to decrease the compressive strength of mortar or concrete. However, it should be mentioned that no research has yet been done to investigatethe the effects of sludge ash replacement on mechanical and durability properties of concrete. The main aim of this study was to investigate the effects of sludge ash usage as cement substitude on physical, mechanical and durability properties of concrete. For this purpose, the effects of three key parameters: replacement level ( 0-20%, by weight), curing times (7, 28, 91 and 180 days) and water-cementitious material ratio (0.35, 0.45 and 0.55) were investigated. The sludge used in this research was obtained from one of the local wastewater treatment plants, which subsequently was dried and then was incinerated at 800oC to produce ash, The ash was in general, made up of irregular grains which were aggregates of smaller particles. Also, the ash was composed mainly of calcium, silica and aluminium oxides. The results showed that increasing the amount of sludge ash induced higher mortar setting times as compared to the control samples, using Vicat test. The effect of ash content on mechanical properties of concrete samples was carried out by compressive strength tests. Results indicated that for 7 and 28 days curing time, concrete samples containing a mixture of sludge ash and cement yielded lower compressive strength values than those samples using only cement (without any ash content). However, for curing times greater than 28 days, the increase in ash content of concrete samples (0-15% by weight) led to an increase in compressive strength. Water absorption and electrical resistivity tests were conducted to determine the durability of concrete containing sewage sludge ash. As blending percentages of ash content increased fom 5% to 20%, electrical resistivity of concrete samples decreased for regardless of the applied curing times. This phenomenon might be the result of increased porosity and material ionization.

Keywords: Municipal sewage sludge ash, concrete, compressive strength, durability, Water Absorption

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