Evaluation of secondary aluminium dross in cement and concrete production

In this study, the usability of aluminium dross which is released and disposed as a result of secondary aluminium production by melting from scrap in calcium aluminate cement production was investigated. The recycling and disposal of dross, released by the aluminium industry, is amongst the most challenging problems in the world. Dross is solid waste that can cause serious environmental pollution and is hazardous to public health. A large proportion of the released dross is disposed to landfills, causing a loss of valuable metals and polluting ground water. About 20 % of this dross is black dross and the rest is white dross released from primary aluminium production. About 95 % of this waste is landfilled each year. It is declared that almost five million tons of white and black dross are produced worldwide per year. In addition, the reaction of aluminium dross with moisture or water vapour may produce dangerous flammable and poisonous gases such as CH4, NH3, PH3, H2, H2S, etc. For these reasons, it is necessary to create a recycling mechanism for the aluminium dross produced in these industries. The secondary aluminium dross was subjected to grinding processes and brought below 20 µm. According to the XRF analysis of the ground raw dross, it was determined that the alumina content was 37.74% and it was quite low in the dross. Oxide forms of elements such as Cr, Mn, Cu, Zn, Ba, Ti, Na, K and Fe were also found in the dross. In addition, according to XRD analysis, it has been determined that the dross contains high amounts of salts such as NaCl and KCl. The chlorine content caused by the salts adversely affects the strength of the cement to be obtained. Chlorine was determined as 22.57% in XRF analysis. This limits the direct use of dross as a pre-treatment cement raw material. Since the high chlorine and sulfur content of the dross to be evaluated in cement production will cause brittle fractures and cracks in the concrete, it is undesirable and must be reomoved. For this reason, as a result of washing processes for 60 minutes, the chlorine content reduced from 22.57% to 0.033%. The alumina content of the dross also increased from 37.74% to 68.03 and during this washing period, the percentage of oxide forms such as TiO2, SiO2, MgO, Fe2O3, CaO, which are insoluble in water, also increased in the dross. After the washing process, since the chlorine and sulfur content in the dross decreased to the desired level, it was seen that 60 minutes of washing was sufficient.Since the alumina levet in the secondary aluminium dross washed for 60 minutes was at a suitable level for the production of low alumina CA (Calcium Aluminate) cement, but the calcium oxide level was not sufficient, mixing with commercial quality quicklime was carried out and cement compositions were within the limit set in TS EN 14647 standard. A blend within the values was obtained. Instead of using secondary aluminium dross as an additive to portland cement due to its high alumina content, it is aimed to be used in calcium aluminate cement, which is known to be more expensive than portland cement. In this way, the added value of the dross is increased. Due to the amount of alumina, calcium oxide and some oxide phases in the dross, it was determined that it would not be suitable for use in the production of medium alumina and high alumina CA cement. According to the XRD analysis, mayenite (C12A7), mono calcium aluminate (CA) and grossite (CA2), unreacted MgO, gehlenite (C2AS), larnite – belite (C2S), brownmillerit (C4AF) and spinel (MA) phases in calcium aluminate cement in the samples obtained at 1200°C of the mixtures for sintering times of 1, 3 hours and 1250°C sintering temperatures of the mixtures for sintering times of 3, 5 hours. Moreover, unreacted quicklime and wadsleyite phases were encountered at 1200°C sintering temperature. As a result of sintering at 1250°C for 3 and 5 hours, it was observed that the desired phases in calcium aluminate cement were sufficient and close to each other. As a result of the sintering process at 1300°C, it was observed that the mixture became glassy due to the low melting point compounds in the slag. The tests were carried out to examine the effect on the physical properties of the cement by adding 2.5, 5, 7.5, 10 and 12.5% by weight of the experimentally obtained waste cement to the commercial calcium aluminate cement. When the specific gravity test results of the prepared cement mixtures are examined. It is seen that the specific gravity value decreases partially with the increase in the waste substitution in the cement, but it is close to each other. The fact that the specific gravity value are close to commercial cement confirms that the phases and amount of waste CA cement similar to commercial cement. When the normal consistency test results of the mixtures are examined. It was observed that the workability decreased and the water requirement incresed due to the increase in the waste cement ratio in the paste. In the case of a higher level of waste CA added to the cement mixture, it is necessary to limit the amount of waste CA in the mixture, since the workability of the cement will decrease further. When the setting time test results of cement mixtures are examined, setting start and finish times in cement pastes decrease with the increase of waste CA cement additive in cement paste. Accordingly, it was observed that the increased waste cement content in the cement accelerated the hardening. According to the TS EN 14647 standard, it is a requirement that the initial setting time is not less than 90 minutes. Although the setting determinations of the results obtained for all mixtures are within the standard conditions, waste CA cement should be limited in the mixture depending on the desired usage conditions of the product. It was observed that the volume expansion amount increased depending on the increase in the waste cement ratio in the commercial cement. This situation was interpreted as the free MgO phases, which expanded as hydrated late after hardening of the concrete, made a negative contribution to the volume expansion. According to the TS EN 14647 standard, when the compressive strength of calcium aluminate cement was tested after 6 hours and 24 hours, the compressive strength decreased with the increase of waste CA substitution in the cement mixture. In cement mixtures prepared with waste cement substitute from 0% to 12.5%, the compressive strength of concrete decreased by 65.2% in 6-hour samples and 58.9% in 24-hour samples. In addition, the compressive strength of calcium aluminate cement should not be less than 18 MPa for 6 hours and 40 MPa for 24 hours when tested after 6 hours and 24 hours according to the EN 196-1 standard. The mixture with 7.5% waaste CA substituted has 20.6 MPa for 6 hours and 42.33 MPa for 24 hours and the concrete minimum requirements in the TS EN 14647 standard were met. When the flexural strength results of the concrete samples were examined. It was revealed that the flexural strength properties were negatively affected with the increase in the waste cement substitution level in the mixture, similar to compressive strength results, and the necessity to limit the amount of waste CA was revealed. As a result from this study, It is confirmed that it is possible to use secondary aluminium dross as the material added to cement with low alumina after washing and sintering it with quicklime at optimum conditions. The use of a high volume of the cement additive prepared from aluminium dross is not appropriate because of its high water-absorption capacity. An increase in the cement additive content causes a decrease in both the initial and final setting times of the cement. This may be due to a higher surface area of the cement additive. The shortening of these times reduces the workability of the mortar and therefore the addition of the cement additive should be limited. The compressive strengths decline with the increasing cement additive proportion in the cement mortar. Replacements of the commercial cement by the cement additive of up to 7.5 w/% provide for the compressive strengths of the concrete determined by the international standard requirements.