Ecologization of Waste from the Steel Industry in Civil Engineering Works

The massive industrialization of the last century has come with the drawback of creating large waste deposits. Over the past century, steel mills have generated significant quantities of steel slag, deposited in heaps that occupy vast land areas. The desire to recycle steel slag arises from both ecological and economic interests. To achieve this goal in the steel industry, it was necessary to recycle steel slag (waste from the technological process of steel production) by using them as aggregates in civil constructions. In this work, the optimal option will be determined based on the mathematical modelling of the main physical-mechanical characteristics of the asphalt mixtures used in wear layers made with steel slag aggregates in comparison with classic asphalt mixtures.


1.
Introduction The processing of steel slag is a technique practiced for over 30 years in Europe or the USA, in fact the transformation of these solid wastes into valuable mineral granular materials is a concern, worldwide.Ferrous metallurgical slags are perhaps the only materials, in the wider category of metallurgical slags, which, through use, do not have negative influences on the environment.The processing technology is part of the "clean technologies" because it does not involve chemical transformations on the slags, but only physical-mechanical ones, and this does not differ much from natural rock processing facilities.The processing of ferrous metallurgical plant slag brings benefits from the point of view of environmental protection, from the community or agricultural point of view, as well as from the technical point of view.Ferrous metallurgical plant slags, being non-toxic materials, but having physical-mechanical properties similar to rocks, are a viable alternative, in certain applications even more valuable than natural variants for the construction field.Steel slags with varied chemical composition are processed as lumps or as fine-grained products.The processing steps during the production of these slag granules correspond to the processing applied to natural stone.Crushed steel slag is used in constructions for traffic (parking platforms, runway strips, bicycle tracks, beltway, etc.).In addition, steel slag is also used for road construction and for mineral seals.In our country, blast furnace slag has been used in road construction for almost 30 years, with a pozzolanic role in the construction of road systems [1].It has been found, particularly in developed countries concerned with limiting industrial waste, that steel slag left over after the extraction of ferrous waste, sorted and cleaned of impurities, successfully replaces natural aggregates specific to road and/or hydro-engineering works [1].In road construction, steel slag aggregates are used either as a foundation layer, base layer or, in recent years, mainly for bituminous surfacing.It has been estimated that worldwide, since 1978, more than half of the steel slag production has been used in road construction, as follows: -base layers: 57%; -foundation layers: 6%; -bituminous layers: 13% -deposited in landfills 24% [1].In terms of production, approximately 700 000 tonnes/year of new slag from steel production alone is processed annually.In the dumping ground, excavation is done only for the search of iron to be recovered.
Steel slag, as a waste by-product of steel-making procedure, mainly comprises CaO, SiO2, Al2O3, MgO, and Fe2O3.Although it is prone to swelling due to the hydration of free lime, some of its properties such as hardness, wear resistance, adhesiveness, roughness, and toughness have made it a raw material in civil engineering applications such as embankment fills, road construction, and concrete building.Substitution of natural aggregates with steel slag aggregates can improve the properties of the asphalt mixture and reduce steel production wastes [2].However, 13 million tonnes of slag are "produced" annually in Romania from steel and cast iron production processes.Their disposal in dumping grounds poses serious environmental problems, with large areas of land taken up and unstable slag layers that can be tens of metres thick.There has always been a concern in steel to use this product as efficiently as possible.These efforts have led to the almost total use of metallurgical slag, especially as a mineral material in construction and as a fertiliser.Through various technologies, waste from slag can become primary approved products used in various branches of the economy such as roads, land improvements, and railway infrastructures [3].The recycling percentage of this waste makes it one of the most recoverable materials used in modern industry.

2.
Mechanical and physical characteristics for the steel slag Slag or metallurgical by-products are chemically and mineralogical different from natural rocks, but have similar physical and mechanical (technical) characteristics.The widely varying chemical compositions of slags, which result in differences in their physicalchemical and mineralogical properties, make their processing very difficult.The short solidification interval means that steel slag is mainly processed in the solid state.Following sampling of the Buituri slag dumping ground, 10 samples were considered significant and have been presented in table1 [4].[5].These asphalt mix formulas required the following types of aggregates: -crushed slag aggregate 0-4 mm -0-4 mm chippings -crushed slag aggregate 4-8 mm -4-8 mm chippings -crushed slag aggregate 8-16 mm -chippings 8-16 mm -natural sand 0-4 mm -filler.For determining the grain size of natural and artificial aggregates, the following test sieves are used in accordance with SR EN 933-2-2020 [6], [7].Following the screenings, the obtained sieve passages are shown in Tables 2-9 [5], [7].

Assessment of the asphalt mix recipes
Thus, according to Table 10 and 11 [1], the following percentages of aggregates were established for the manufacturing of asphalt recipes BA16 with chippings and BA16 with steel slag aggregates, taking into consideration the granularity of each aggregate as a result of the laboratory sieving:

Assessment of the asphalt mix density
The assessment will be conducted in accordance with SR EN 12697-6 /2020 [9].This testing method is used to assess the bulk density of bituminous asphalt after compaction.The test is conducted on laboratory-compacted specimens.
To determine the bulk density for the two types of asphalt mixtures, tests were carried out for each mixture with 5 different bitumen dosages.For each percentage of bitumen 6 Marshall specimens were made.The average bulk density values for each bitumen dosage are shown in Tables 12 and 13.As seen in Figures 1 and 2, the findings were analyzed using the mathematical analysis tool Curve Expert Professional, based on a variety of mathematical functions, to calculate the ideal value of the bitumen % in terms of bulk density [7].The standard error is 0, meanwhile the correlation coefficient is 9.999999999999998E-01.It can be seen that the maximum bulk density value corresponds to a bitumen dosage between 6.00% and 6.30%.After interpreting the data, we find that, for asphalt mixtures containing steel slag, the polynomial equation of degree 4 best describes how the values of bulk densities evolve with respect to bitumen %.The polynomial equation of degree 4 is as follow: 1723.69 -1102.47*x+ 264.35*x 2 -28.12*x 3 +1.12*x 4 = 0 The standard error is 0, meanwhile the correlation coefficient is 9.999999999999998E-01.

Figure 2. Interpretation of bulk density evolution mathematically for BA16 with steel slag
We note that the maximum bulk density value corresponds to a bitumen dosage between 6.00% and 6.30%.

Calculating the water absorption
When an asphalt mix specimen is held in water under vacuum, the amount of water absorbed by its externally exposed voids is measured and reported as a percentage of the specimen's initial mass or volume.This process is known as water absorption.Tables 14 and 15 show the average water absorption values for each bitumen dosage [7].In terms of water absorption, the results were interpreted using a mathematical analysis program (Fig. The standard error is 0, meanwhile the correlation coefficient is 1.
We note that with increasing bitumen percentage, the water absorption value decreases, but we observe that for bitumen dosage higher than 6.25% its influence on water absorption is insignificant.The standard error is 0, meanwhile the correlation coefficient is 1.It has been noted that the water absorption value reduces as bitumen percentage rises.Steel aggregates have less of an impact on water absorption than chipped aggregates do, and at bitumen concentrations greater than 6.00%, water absorption is negligible.

Assessment of Marshall stability
The assessment method is according to SR EN 12697-34:2020 -Asphalt mixtures.Test methods.Part 34: Marshall test [9].The standard error is 0, meanwhile the correlation coefficient is 9.999999999999962E-01.It is observed that the ideal range for the bitumen percentage needed to obtain Marshall Stability values in accordance to SR EN 12697-34:2020 is between 5.90% and 6.60% [9].The standard error is 0, meanwhile the correlation coefficient is 9.999999999999962E-01.It is observed seen that the optimum range of the percentage of bitumen required to obtain Marshall flow values according to SR EN 12697-34:2020 is between 5.75% and 6.40% [9].The standard error is 0, meanwhile the correlation coefficient is 1.To obtain Marshall S/I ratio values according to SR EN 12697-34:2020 the percentage of bitumen must be less than 6.75% [9].The standard error is 0, meanwhile the correlation coefficient is 9.999999999999971E-01.It is observed that the optimum range of the percentage of bitumen required to obtain Marshall Stability values according to SR EN 12697-34:2020 is between 6.00% and 6.50% [10].The standard error is 0, meanwhile the correlation coefficient is 1.It is observed that the optimum range of the percentage of bitumen required to obtain Marshall flow values according to SR EN 12697-34:2020 is between 5.75% and 6.00% [7,10].The standard error is 0, meanwhile the correlation coefficient is 9.999999999999980E-01.It is noted that the optimum range of bitumen percentage required to obtain S/I Marshall Ratio values according to SR EN 12697-34:2020 is between 6.25% and 6.75% [10].

Figure 1 .
Figure 1.Interpretation of bulk density evolution mathematically for BA16 with chippings Using a variety of mathematical functions and the mathematical analysis software Curve Expert Professional, the findings were evaluated to compute the ideal bitumen % in terms of bulk density.The polynomial equation of degree 4 is as follows: 207 -130.34*x+ 30.93*x 2 -3.25*x 3 +12.80*x 4 =0The standard error is 0, meanwhile the correlation coefficient is 9.999999999999998E-01.It can be seen that the maximum bulk density value corresponds to a bitumen dosage between 6.00% and 6.30%.After interpreting the data, we find that, for asphalt mixtures containing steel slag, the polynomial equation of degree 4 best describes how the values of bulk densities evolve with respect to bitumen %.The polynomial equation of degree 4 is as follow:1723.69-1102.47*x+ 264.35*x 2 -28.12*x 3 +1.12*x 4 = 0 The standard error is 0, meanwhile the correlation coefficient is 9.999999999999998E-01.

3 ,
Fig. 4) to determine the optimum percentage of bitumen.The polynomial equation of degree 4 best reflects, the evolution of the water absorption values in relation to the asphalt percentage for asphalt mixtures with steel slag.The polynomial equation of degree 4 is as follows: 19705.84-12197.74*x+ 2831.40*x 2 -292.07*x 3 + 11.30*x 4 =0

Figure 3 .Figure 4 .
Figure 3. Mathematical interpretation of water uptake evolution for BA16 with chippings Table 15.Determination of water absorption for BA16 with steel slag bitumen percentage, % bitumen type water absorption,% Condition AND 605 5.75

Figure 5 .Figure 6 .
Figure 5. Mathematical interpretation of Marshall Stability evolution for BA16 with chippings

Figure 7 .
Figure 7. Mathematical interpretation of the S/I ratio evolution for BA16 with chippings

Figure 8 .
Figure 8. Mathematical interpretation of the Marshall Stability evolution for BA16 with steel slag The polynomial equation of 4 degree that most accurately depicts the development of Marshall Stability values in relation to the asphalt percentage for asphalt mixtures with steel slag is: 82285.40-52620.27*x+ 12603.07*x 2 -1339.73*x 3 + 53.33*x 4 =0The standard error is 0, meanwhile the correlation coefficient is 9.999999999999971E-01.It is observed that the optimum range of the percentage of bitumen required to obtain Marshall Stability values according to SR EN 12697-34:2020 is between 6.00% and 6.50%[10].

Figure 9 .
Figure 9. Mathematical interpretation of the evolution of the flow for BA16 with steel slag The polynomial equation of degree 4 best reflects, the evolution of the Marshall flow values in relation to the asphalt percentage for asphalt mixtures with steel slag is: 3616.06 -2422.13*x+ 606.07*x 2 -67.09x 3 + 2.77*x 4 =0The standard error is 0, meanwhile the correlation coefficient is 1.It is observed that the optimum range of the percentage of bitumen required to obtain Marshall flow values according to SR EN 12697-34:2020 is between 5.75% and 6.00%[7,10].

Figure 10 .
Figure 10.Mathematical interpretation of the evolution of the S/I ratio for BA16 with steel slag The polynomial equation of 4 degree best reflects, the evolution of the values of the S/I ratio in relation to the asphalt percentage for asphalt mixtures with steel slag is: 19054.64-12080.53*x+ 2869.17*x 2 -302.51x 3 + 11.95*x 4 =0The standard error is 0, meanwhile the correlation coefficient is 9.999999999999980E-01.It is noted that the optimum range of bitumen percentage required to obtain S/I Marshall Ratio values according to SR EN 12697-34:2020 is between 6.25% and 6.75%[10].

Table 1 .
The chemical composition of the slag from the Buituri slag dumping ground

53 11.39 5.94 6.66 18.38 6.83 6.53 35.30 9.20 0.20 0.56 2.08 3. Laboratory tests Laboratory
tests were conducted to determine the mechanical and physical properties of the two types of asphalt mixtures.The tests were performed at the Road Laboratory of the Faculty of Civil Engineering and Building Services from Iasi.

Table 9 .
Grading characteristics for filler

Table 10 .
Analysing the mineralogical mixture of asphalt mix BA16 with chippings

Table 11 .
Analysing the mineralogical mixture of asphalt mix BA16 with steel slag aggregates

Table 12 .
Bulk density assessment for BA16 with chippings

Table 13 .
Determination of bulk density for BA16 with steel slag

Table 14 .
Determination of water absorption for BA16 with with chippings

Table 16 .
Marshall test for BA16 with chippings tool was used to interpret the results in terms of the Marshall test.Multiple mathematical functions were established to best follow the evolution of these values, as indicated in figures 5, 6, and 7 for BA16 with chippings and figures 8, 9, and 10 for BA16 with steel slag.The polynomial equation of 4 degree that best reflects, the evolution of Marshall Stability values in relation to the asphalt percentage for asphalt mixtures with steel slag is as follows:

Table 17 .
Marshall test for BA16 with steel slag