Mechanical and physical properties of cement blended with sewage sludge ash
Introduction
The term “green concrete” refers to concrete designed to reduce environmental impacts (Glavind and Munch-Petersen, 2002). An environmentally sustainable concrete structure minimally impacts the environment over its entire life cycle. One method that is currently being used to develop green concrete (Sarkar and Roumain, 2003) is the addition of residual materials that may substitute, at least partially, for raw materials used in traditional concrete fabrication (basically sand and cement) with minimal effect on the concrete’s properties. The partial replacement of cement by pozzolanic materials has advantages from ecological, economic and technical points of view. The incorporation of these materials in concrete fabrication would result in a reduction of cement per unit volume of concrete, causing a reduction in CO2 emissions. Additionally, the addition of pozzolanic or hydraulic materials in cement production allows residual materials to be reused; in most cases little value would be added, and in others this practice would increase the strength and reduce the porosity of concrete.
The execution of the European Regulation on urban sewage treatment (9/271/EEC) is causing a rapid expansion in sewage treatment plants in Europe. Large quantities of organic sludge are produced as a by-product; according to the previously cited regulation, these cannot be dumped into seas. Researchers have therefore been searching for possible ways of reusing sludge (Valls and Vázquez, 2000, Valls and Vázquez, 2002, Malliou et al., 2007) apart from its traditional use in agriculture (Casado-Vela et al., 2006). However, since much of the sludge contains heavy metals, and due to the strictness of European disposal laws, there is an increasing interest in incineration as an alternative method of sludge management. One of the results of sewage sludge incineration is a 90% reduction in material volume, making further management an easier task. In this process, the organic components of the sludge are burned and eliminated, generating energy and producing a powdered material referred to in this paper as sewage sludge ash (SSA). SSA may be used in brick fabrication (Wiebusch and Seyfried, 1997), as a fine sand in mortar (Bhatty and Reid, 1989), in the preparation of synthetic sand (Wainwright and Cresswell, 2001), in the preparation of pavement materials (Lin et al., 2006), as raw cement material (Lin and Lin, 2005), as material to produce lightweight aggregate (Chiou et al., 2006, Chen et al., 2006) or in asphalt paving material (Al Sayed et al., 1995). In any case, one of the applications of greatest interest is the reuse of SSA as a mineral component of mortars and concrete used in construction. Previous studies have demonstrated that Portland cement mortars made with SSA exhibit good mechanical properties (Monzó et al., 1996, Monzó et al., 1999). When cement was replaced with SSA in amounts of up to 10% by mass, mortar corrosion performance was comparable to the behaviour observed in control mortar (García-Alcocel et al., 2006). An observed decrease in workability can be offset by the addition of plasticizing agents (Monzó et al., 2003).
The main aim of this study is to evaluate the suitability of SSA for blending with various commercially available cements. Physical and mechanical properties of mortars made from various cements with various percentages replaced by SSA (10–30%) were studied. Mortar properties were evaluated by comparison with mortars containing no SSA (commercial cements, 0% replacement).
Section snippets
Materials
Distilled water, siliceous sand, and different blended cements were used for mortar preparation. The blended cements were prepared by mixing various amounts of commercial cement and sewage sludge ash (SSA). The following types of Portland cement were used for blending: CEM I 52.5R, CEM I 42.5R, CEM II/B-M (V-LL) 42.5R and CEM II/ B-L 32.5N. All the cements meet the requirements of European standard UNE-EN 196-1. Table 1 shows the admixture composition of the cements used. The SSA was collected
Workability
Table 3 summarizes the results of the physical tests and workability studies on the mortars fabricated with CEM I 52.5R Portland cement (0%) and the corresponding 10% SSA-blended cement. The replacement of 10% of the CEM I 52.5R cement by SSA did not modify the time at which the mix began to set. In addition, the end of the setting period was only slightly longer for SSA-blended cement. In order to obtain normal consistency in the pastes, the water/cement ratio was increased from 0.34 for pure
Conclusions
The blended cements prepared by 10% substitution of cement for SSA meets the requirements of the European standard EN 197-1 in terms of workability. Mortars fabricated with 10% SSA replacement meet the mechanical requirements of the European standard in terms of early age compressive strength and nominal compressive strength.
Long-term mechanical strengths for mortars fabricated with cement containing fly ash and the addition of several percentages of SSA are higher than or similar to those
Acknowledgements
The authors thank the Conselleria d´Empresa, Universitat i Ciència of the Generalitat Valenciana for its financial support to the excellent work group (GRUPOS04/58) of the Section of Research on Concrete Science (Unidad de Investigación en Ciencia del Hormigón, UNICH), which, in turn, sponsored this work.
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