Power plants ashes recovery in eco-friendly mortar compositions

The paper treats the possibilities to recover the waste from coal combustion in some power plants in Romania. The greenhouse gas and the ashes have a huge impact on environment and the living species. The using of ash – recovered wastes – induces decreasing of the demand of natural resources. They also reduce the energy - intensive production of other concrete ingredients, leading to energy saving and decreasing the “greenhouse gas” emission. Replacing one tone of cement with fly ash it would save enough electricity to power an average home for 24 days, and reduce carbon dioxide emissions equal to a two months use of an automobile.
   During the study, in the experimental work were compared the properties of five different compositions of masonry mortars were prepared replacing the cement with different amounts of ash. The setting time and the workability were determined on the fresh mortar. After 28 days of hardening in standard conditions (5 days in moulds at 20°C and 90% humidity; 2 days without moulds at 20°C and 90% humidity; 21 days without moulds at 20°C and 65% humidity) the density and water absorption of the mortars were determined using the methods indicated in the European standards. The flexural and compressive strength of the compositions were determined after 28 and 56 days of hardening.
   The fresh and hardened mortars characteristics were investigated. The compositions (cement, Zalau ash, sand and water, in different proportions) were prepared by forced mixing using a laboratory mixer. The fresh mortar was cast in metallic moulds obtaining 160x40x40 mm prisms which were subjected to testing in hardened state.
   The study demonstrated that it is possible to use ashes in the mortar compositions, by replacing a part of the cement by ashes. The replacement of cement in proportion of 5, 10, 20 and 30 wt% was experimented. Thus, the setting times of the mortars increases. The difference between the initial setting time of the composition without ash and the composition in which 5 wt% of cement was replaced by ash is only 5 minutes. The differences are bigger for higher ash content; it reaches 80 minutes for composition 5 in which 30 wt% of cement was replaced by ash.
   The differences are more evident in the case of the final time of setting, where replacement of 5 wt% cement lead to a 30 minutes longer final setting time and replacement of 30 wt% cement with ash a 310 minutes longer time, which means an increase of 1,5 times.
In the case of mortars workability no differences were observed between the composition with no ash and the composition in which 5 wt% of cement were replaced, after that every 10wt% of cement replaced by ash brings 5 minutes in plus.
   The density increases slowly by replacing 5% of cement, after that a decrease is observed, every sample densities being under the density of the standard composition. The water absorption is in agreement with the results obtained for the densities. The water absorption decreases from 8.96 % (in the standard composition) to 8.34% (for composition 2 with 5wt% ash). For the other compositions the value of absorption increases to 12.87%, while the ash proportion was increased to 30 wt%.
   The values for the mechanical strength state the observations at the density and absorption determination. For the composition with 5 wt% ash was observed an increasing of flexural and compressive strength. While the ash content was increased the mechanicalstrength decreased and it is situated below the standard composition strength. The mechanical test after 56 day of hardening shows that the strength increase is higher for the compositions with ash.
   It can be concluded that the Zalau power plant ash can be used in mortar compositions 5 wt% replacement of cement by ash brings both economical and qualitative benefits.