The use of shale ash in dry mix construction materials

The research was made to determine the use of shale ash usage in dry mix construction materials by replacing part of cement amount. Cement mortar ZM produced by SIA Sakret and two types of shale ashes from Narva Power plant (cyclone ash and electrostatic precipitator ash) were used. Fresh mortar properties, hardened mortar bulk density, thermal conductivity (λ10, dry) (table value) were tested in mortar ZM samples and mortar samples in which 20% of the amount of cement was replaced by ash. Compressive strenght, frost resistance and resistance to sulphate salt solutions were checked. It was stated that the use of electrostatic precipitator ash had a little change of the material properties, but the cyclone ash significantly reduced the mechanical strength of the material.


Introduction
Nowadays the most widely used building materials in the world are cement mortar and concrete [1]. Only concrete consumption alone is approximately 10 billion tonnes per year [2]. Despite the fact that basic principles of technologies remains the same, it is apparent significant concrete and mortar technology development that makes it possible to obtain materials with better physical and mechanical properties and durability [3]. These products are made of binders (cement, lime hydrate), several types of aggregates (sand, crushed granite, crushed dolomite, dolomite sand, limestone, etc.) and various chemical additives used to improve material physical and chemical properties (plasticizers, superplasticizers, antifreeze additives, air participatory additives, etc.) [1]. Often, in order to improve physical, mechanical and process binding properties of material, silica dust (micro silica) or fly ash are added to cement [3].
Cement is one of the main raw material in the production of dry building materials. With the development of the construction industry, building materials has been evolved too, including cement. In recent years, more and more are being sought various additives for cement, which could improve the properties of it. Taking into account today's issues at stake, the potential for reducing CO2 emissions are sought materials that could partially replace cement, because cement production gives approximately 5% of total global CO2 emissions. Such materials could be such as pozzolans, ash, etc. Number of preconditions are for ash use. First, the ash is by-product and using or disposing this waste in some kind of industry, it is possible to reduce the amount of ash -to protect the environment. In 2008. Kingston (USA) the ash discharge happened, resulting 4.2 million tons of ash got into the river and nearest neighbourhoods. The water of river was polluted, but the factory adjoining areas were coated with a 1.8 m thick layer of ash. In order to avoid such disasters, ashes should be disposed of or processed [4]. Second, the use of ash in the cement industry allows to reduce the demand for portlandecement, leading to reduction of CO2 emissions into the air. According to American scientists, if all the ash that is produced during the year would be used in the industry of cement, the CO2 emissions would be reduced by 25% [5]. Third, the use of ash range is very wide: • cement industry, • brick industry, • ceramic industry, • road and dam construction, • dry mixtures, • a variety of floor coverings and so on. [6].

Materials and methods
The samples from SIA SAKRET supply of dry building mixtures were used in this analysis: cement mortar with industrial name ZM and two types of shale ash from Estonian Narva power plant -ash obtained using electrostatic precipitator (active surface area -8.620e+00 m²/g), and ash, generated by using cyclone-type filters (active surface area -2.469e+00 m²/g). Ashes by their chemical composition, are the same [7]. Experiments were carried out with three types of samples: • ZM (reference sample); • ZM-e (20% of the amount of cement replaced by the electrostatic precipitator ash); • ZM-c (20% of the amount of cement replaced by a cyclone ashes). Table 1 shows dry mix composition (% by weight):

Fresh mortar properties
Discharge on Hagerman flow-table, density and porosity of fresh mortar samples were evaluated - Table  2. The amount of water to reference sample was added by the manufacturer's recommendations, in experimental samples amount of water was adjusted to achieve equal flow ability for all samples. Less amount of water was necessary for the samples with the ashes in the composition in order to achieve the same flow ability.
There is coherence between fresh mortar density and pore size range. The samples with electrostatic precipitator ashes had the highest density (1903kg/m 3 ) and they had the smallest amount of pores -16% and according the sample with cyclone ashes had the lowest density (1856kg/m 3 ) and the largest pore content -19.5%.
Such results can be explained by the fact that the electrostatic precipitator ash is very fine and is able to fill the pores of the material so the mass formed was denser, but the cyclone ash is coarser -formed mass was more porous with a lower density.

Hardened mortar bulk density and thermal conductivity coefficient
Hardened mortar sample bulk density and thermal conductivity coefficient was determined using 5 samples of each mortar. Thermal conductivity coefficient values are the values taken from table of the standard -LVS EN 1745: 2003 " Masonry and masonry products -Methods for determining design thermal values " [11]. Average results are shown in Table 3: The thermal conductivity coefficient depends on bulk density of the samples, the higher the density, the higher the coefficient of thermal conductivity and materials has lower heat insulation properties.
Electrostatic precipitator ash didn't change the thermal conductivity of mortar. There was observed decrease in bulk density in samples with cyclone ashes and consequently these samples are better heat insulators.

Compressive strength
The results of mechanical strength of all three types of mortar samples are shown in Fig.1, 2.  Flexural strength of reference sample reaches 4.5 N/ mm 2 compressive strength -14 to 15 N/mm 2 . Samples with electrostatic precipitator ash show very similar, if not slightly better mechanical properties results as reference samples, but samples with cyclone ash show 1 N/mm 2 worse flexural strenght results and about 5 N/mm 2 decrease in compressive strenght. Such results can be explained by the variety of active surface area because of great dispersity of electrostatic precipitator ash formed it denser and therefore mechanically more durable mass.

Resistance to sulphate salt solutions
The results of the resistance to sulphate salt solutions of mortar samples are shown in Fig.3. One cycle in saline solution lasts for 24 hours, samples are soaked in 14% Na2SO4 for 2 hours then dried at the temperature 103 0 C for 19 hours and chilled for 3 hours, at the end of each cycle samples are weighted. The first 12 cycles all samples passed equally. After 12 th cycle, the samples showed rapid weight loss. The highest weight loss occurred in samples with electrostatic precipitator ash, but the best resistance to sulphate salt solutions had the reference samples without any ash. It's possible that ash chemical composition reduces materials resistance to soluble sulphates.

Frost resistance
The results of frost resistance are shown in Fig. 3. Before being tested the samples were dried at the temperature 60 o C for 24 hours until they gained constant weight then saturated with water for 48 hours. Each cycle of frost resistance lasted for 24 hours -8 hours in a freezer at temperature -15 o C to -20 o C and 16 hours of thawing in water at the room temperature (20 +/-5 o C).

Conclusions
1. The research was made to determine the use of shale ash usage in dry mix construction materials by replacing part of cement amount 2. There is coherence between fresh mortar density and pore size range. The samples with electrostatic precipitator ashes had the highest density (1903kg/m 3 ), the smallest amount of pores -16%, accordingly the sample with cyclone ashes had the lowest density (1856kg/m3) and the largest pore content -19.5%. 3. Electrostatic precipitator ash didn't change thermal conductivity of mortar. There was observed decrease in bulk density in the samples with cyclone ashes and consequently these samples were better heat insulators. 4. Cyclone ash reduced the mechanical properties of the material, but the electrostatic precipitator ash -did not affect (even slightly improved) material compressive and flexural strength, due to their high dispersity. 5. Shale ash reduced the material resistance to soluble sulphate salt solutions, but did not affect the material frost resistance. Cement mortar is a structural building material, so the main criteria for mortar recipe development is acquiring a material with a certain mechanical strength. From the results obtained it can be seen that shale ash can be used to replace amount of cement in constructive building materials. The optimal results were obtained using dispersive electrostatic precipitator ash, but cyclone ash impairs mechanical properties which were studied in this research.