Physicochemical properties of activated sludge ash as an additive in mortars

Departamento de Ingeniería Civil en Obras Civiles, Universidad de Santiago de Chile, Avda. Ecuador 3659, Estación Central, Santiago, Chile

^* Corresponding author: patricia.mery@usach.cl

Abstract

Wasted activated sludge is a by-product of the treatment of wastewater, that historically been considered a waste and disposed in landfill. This has resulted in the search for alternatives in its disposal and possible reuses of these wastes. This study focuses on the use of activated sludge ashes from wastewater treatment plants as an additive in mortars and the impact on its mechanical resistance. For this purpose, it was determined the physical and chemical properties of the ashes and the mechanical properties of different percentage of ash content used in the fabrication of mortar prisms. The physical-chemical properties were centred in the percentage of ash versus temperature and the pozzolanic characteristics of the ash generated. Therefore, the determination of the ash percentage by thermogravimetric analysis was performed. Two temperatures were employed in the generation of ash (550 ºC and 750 ºC) and measured its pozzolanic characteristics by a rapid determination of pozzolanic activity of materials method. As a result, the ash generated at a higher temperature had a stronger pozzolanic characteristics, than the ash generated at the temperature used for determining volatile content of the sludge. Afterwards, sludge was calcinated in presence of oxygen at 750 ºC, and the ash was used in the manufacture of mortar prisms. Due to the variation in the size of the particles generated in the calcination processed, it was proceeded to separate them into two sizes (mesh sieve Nº 4 and Nº 100). The percentage by weight of the ash employed in the fabrication of the prism were 0%, 5, 10 and 15%. The prisms were tested for flexure and compression at 10, 21 and 28 days, and compared with a standard mortar. With regards of the consistency of the mortar, it was observed that the addition of ash had an impact on this property. The greatest loss of consistency was observed with the 15% of ash content and mesh sieve Nº 4, decreasing by 26%, while the consistency of the mortar with 15% of ash content and mesh sieve Nº 100 decreased up to 16.6%. With regards of the mechanical properties, it was observed that the addition of ash as an additive for mortar, observing an increase in flexural strength of up to 11% at 28 days for the mixture of 10% ash content and mesh sieve Nº 4. In the case of compression, the resistance after 21 days of curing, increased by 82% for additions of 10% and 15% of ashes content for the mesh sieve Nº 100. Therefore, it is possible to use ashes from wasted sludge of activated sludge wastewater treatment plant as an additive in mortars, and this can be employed as an alternative to disposal in landfills.