Performance of green concrete paving block imbibed with industrial scrap steel mill scale for sustainable construction

The utilization of industrial waste materials in concrete compensates the shortage of natural resources by not only solving the problem due to disposal of wastes but also by developing alternative solutions to protect the environment as well as reduction in the area requirement for landfill. The concrete made with such wastages using less energy during its production and eco-friendly is called as Green Concrete. Variety of industrial wastes is employed as whole or partial substitution for coarse or fine aggregate. Steel mill scale is one such kind of waste materials produced as a result of hot process of rolling of steel in steel companies with rich source of iron content with least impurities. This research study investigates the viability of adopting steel mill scale as a partial substitute material for fine aggregate (M-sand). The current study investigates the influence on fresh and hardened concrete paving blocks and its properties, when M-sand is replaced at 0%, 20%, 40%, 60%, 80%, and 100% with steel mill scale using a mix ratio of 1:1.6:2.1 at sustained water-cement ratio value of 0.5 and target strength at 28 days of 30 Mega Pascal. Physical and chemical characterisation of the materials, concrete compressive strength, concrete split tensile strength, water absorption, and also micro-structural examination of hardened paving blocks are experimentally investigated. Results suggest that 60% of the replacements outperformed the originals. The research findings point towards the feasibility of producing paving blocks from scrap steel mill scale for enhancing environmentally friendly construction practices and sustainable pavement infrastructure.


Abstract
The utilization of industrial waste materials in concrete compensates the shortage of natural resources by not only solving the problem due to disposal of wastes but also by developing alternative solutions to protect the environment as well as reduction in the area requirement for landfill. The concrete made with such wastages using less energy during its production and eco-friendly is called as Green Concrete. Variety of industrial wastes is employed as whole or partial substitution for coarse or fine aggregate. Steel mill scale is one such kind of waste materials produced as a result of hot process of rolling of steel in steel companies with rich source of iron content with least impurities. This research study investigates the viability of adopting steel mill scale as a partial substitute material for fine aggregate (M-sand). The current study investigates the influence on fresh and hardened concrete paving blocks and its properties, when M-sand is replaced at 0%, 20%, 40%, 60%, 80%, and 100% with steel mill scale using a mix ratio of 1:1.6:2.1 at sustained water-cement ratio value of 0.5 and target strength at 28 days of 30 Mega Pascal. Physical and chemical characterisation of the materials, concrete compressive strength, concrete split tensile strength, water absorption, and also micro-structural examination of hardened paving blocks are experimentally investigated. Results suggest that 60% of the replacements outperformed the originals. The research findings point towards the feasibility of producing paving blocks from scrap steel mill scale for enhancing environmentally friendly construction practices and sustainable pavement infrastructure.

Introduction
In the recent days, every part of the world fighting with extreme problems is regarding waste production and dealing with the management of produced waste. Every city's periphery became a dumpyard, where the daily wastes produced are in tonnes [1]. On the other hand, scarcity of river sand followed by the rise in price of river sand, which is considered as one of the major material constituents used in conventional concrete production, was also reported. The volume of natural resources utilised globally in buildings and infrastructure for transportation facilities has shown 23-fold increase between 1900 and 2010 [2]. Sand along with crushed rock and gravel referred as solid construction aggregates which are the resources extracted majorly for construction practices [3]. Resources in common-pool are vulnerable to major tragedies due to the difficulties in regulating their consumption, extraction by people self-centredly without taking into account the long-term effects, ultimately leading to overexploitation or degradation. Though sand mining is well regulated, there occurs typically widespread unlawful extraction and trafficking [4]. Yearly, the demand for sand in emerging countries rises; as a result, globally sand has been extracted between a range of 32 and 50 billion tonnes [5]. Sand mining is a massive industry that has negative social and environmental consequences, ultimately affecting freshwater eco-systems [6]. In order to overcome the constant demand arose for river sand and to reduce the requirements for landfill area [7], researchers and construction practitioners have spotted some alternatives namely slag, fly ash, manufactured sand, glass aggregate, recycled concrete aggregate, blast furnace slag, siliceous stone powder, limestone powder etc in producing Green Concrete [8].
Frequent increase in mining of river sand has prompted the Construction Engineers to look into an alternative fine aggregate suitably, and one of this kind of alternative resource is found as manufactured sand i.e., M-sand [9]. Next to manufactured sand, one of the recent kind of alternative resources is steel mill scale produced by a variety of trash, including sludges and scales from oil mill. Steel mill scale is typically a by-product from industries for steel fabrication [10]. These are produced at the rolling mill during the cooling and rolling processes of hot steel [11]. Every year, generation of steel mill scales is approximately 13.5 million tonnes globally [12]. Certain scraps from metallurgy and steelmaking have found ubiquitous application for the construction industry. Granulated slag obtained from the pig iron manufacturing process is being used as aggregate substitute materials in concrete. Majority of studies demonstrated that concrete made of slag aggregates outperform well compared to the concrete made with natural aggregates in terms of strengths such as compressive, tensile, flexural, and modulus of elasticity [13,14].
Various iron and steel manufacturing processes generate 500 kg/tonne of wastes in solid forms roughly; steel mill scale is another one of those wastes, accounting for approximately 2% of steel produced [15]. In terms of compressive strength, replacing steel mill scales at 40% instead of sand is the optimum composition for mortars made of cement [16]. The results improved when 60 percent of the river sand in the concrete was replaced by steel mill scale. So, it can be recommended to be used as a constituent of concrete to improve both mechanical and micro-structural properties [17].
Few researchers studied various properties of concrete and compared it with steel mill scale, and more researchers have done the same research on the properties of concrete made with M-sand. There are no researches identified for concrete paving blocks with M-sand, and steel mill scale. As an addendum, this research study is intended to scrutinise the viability of the utilisation of steel mill scale and M-sand substituted in place of river sand in concrete and its related properties with varying proportions in the production of paving blocks.

Experimental program
The primary goal in this research is the performance assessment of the paving blocks that comprised of fine aggregate (M-Sand) partially replaced by steel mill scale. In order to evaluate the performance of paving blocks, various experiments were conducted including, compression strength test, split tensile strength test, water absorption test and SEM analysis. This section describes the materials utilised, mix proportions, specimen preparation, and the experimental procedure employed throughout this investigation.
Presently, there are industrial by-products which are considered as materials for replacement, mainly used as an alternative for conventional construction materials. Waste materials from industries are recommended majorly for low cost constructions and where higher strength in construction is needed [18]. One such latest waste material is steel mill scale, mainly collected from dumpyards of steel industries. Steel mill scale wastes are collected depending upon the required particle size, to meet the general criteria and recommendations given in IS: 10262:2009 [18]. Cement content of 385 kg m −3 , along with an average water cement ratio of 0.5 was adopted for all the trail mixes to get medium degree of workability. The mix proportions of both controlled mix concrete as well as steel mill scale waste concrete in different partial replacement ratios is tabulated in table 2. The controlled concrete grade used in this experimental work was M30, which is expected to exhibit same or increase in strength for the steel mill scale waste green concrete. Both M-sand and steel mill scale wastes behaves like river sand belonging to zone II according to IS 383: 1970, resembling the properties of natural river sand.

Cement
Locally purchased branded ordinary Portland cement of 43 grade according to IS: 4031-1991, with specific gravity value as 3.15 and fineness of 3.5 percent, was utilised.

Steel mill scale
Steel mill scale was hoarded from the dumpyards of steel industry located in Coimbatore, Tamilnadu, India. The steel mill scale has properties like specific gravity with a value of 4.1, a loose bulk density of 2478.22 kilogrammes per cubic metre and a compacted bulk density of 2857.14 kilogrammes per cubic metre. Due to the higher values in specific gravity and bulk density of steel mill scale, it is evident that slag particles having higher specific gravity are stronger and more stable [20]. Physical properties of steel mill scale determined through various laboratory tests are listed in table 1. Results of particle size distribution of steel mill scale and M-Sand after conducting sieve analysis test, is enlisted in table 2. The results depict that, the pattern and range of distribution of steel mill scale particles adhere with the values obtained for M-sand. Maximum particles are distributed within the sieve sizes ranging from 4.75 mm to 0.6 mm, which clearly shows that steel mill scale can be actively used as a substitute material for fine aggregates.

Coarse aggregate
According to IS:15658(2006) criteria, natural, locally accessible, clean angular-shaped coarse aggregates with a size of 10 millimetres were employed. Tests on determination of physical properties of coarse aggregates used in this study was done based on the recommendations given in IS: 2386-1963 [19]. The specific gravity value of coarse aggregate obtained was 2.61; the impact value obtained as 10.1 percent; and the water absorption rate was 3%.

Water
For casting and curing the concrete specimens, clean water free of pollutants such as solid particles, oil, and acids was employed.
2.5. Microstructural SEM analysis of fine aggregates 2.5.1. SEM analysis of fine aggregates The SEM analysis provides enlarged details of size, shape, crystallography, composition, and other chemical and physical properties of materials through image outputs [24]. The shape, dimension, distribution of particles along with the topographical flaws like voids and fissures have utmost influence on the inherent microstructure features [17]. SEM pictures were examined for M-sand, and also steel mill scale. Figures 1(a), and (b), depict the form of particles of M-Sand, and steel mill scale. The photos demonstrate that particles of M-sand are more angular and abrasive. The angularity of the particles of M-Sand in concrete can able to increase its flexural and compressive strength [25]. Abrasive particle's cementation with the cement paste leads to improvement in strength. In comparison to concrete made with natural sand, concrete made with M-sand has higher unit consumption of water, stronger strength on compression, superior permeability, and resistance against frost due to grain form and M-sand's surface state [26]. The steel mill scale particles are shaped irregularly and exhibit a rough surface [27]. This feature of steel mill scale may aid in improving particle bonding in concrete [17], due to the irregular shape with sharp points and the straight line pattern exhibited throughout the structure.

EDAX analysis of fine aggregates
The materials' elemental composition can be examined through energy dispersive x-ray analysis, often known as EDS or EDAX. With this analysis, Materials along with product research, deformation, troubleshooting and related applications are possible. EDX systems, as an attachment to electron microscopy instruments use imaging capability of microscope to identify the specimen of interest. Analysis through EDX produces spectra with peaks related to the components comprising the sample's real composition being studied. Analysis of Image and mapping of element of a particular sample too are available [28]. EDAX pictures of M-sand and steel mill scale are examined. Figures 2(a) and (b), depict the form of composition of M-Sand, and steel mill scale particles,

Mix proportion
Paving blocks were made utilizing M30 grade concrete mixed with M-sand in the proper proportioning ratios. To replace M-sand, steel mill scale was added in various mix combinations of 20%, 40%, 60%, 80%, and 100%. The materials were chosen in accordance with IS: 15658-2006 [20] recommendations. The mix design was carried out with extreme carefulness in accordance with IS 10262-2009 [29] recommendations, without altering its core qualities. The study's design mix proportion was arrived as 1:1.6:2.1 (Cement: Fine aggregate: Coarse aggregate), adopting w/c ratio as 0.5. Table 2 exhibit the specimen mix combinations. Details of Concrete mix propotion is given in table 3.

Preparation of specimen
Laboratory tests were conducted on hardened concrete paving block specimens to evaluate specified strength parameters, abrasion resistance, water absorption, and microstructure. The specimen was prepared at 250 × 123 × 80 mm, according to IS 15658 (2006) [30]. Figure 3 depicts the finished concrete paving block. Six specimens were casted for each ratio of the total six mixes, within which three specimens were tested at 14 days and the remaining 3 specimens were subjected to testing at 28 days of curing, for a total of 108 specimens for the water absorption and strength tests. According to the provisions of BS 1881 [31,32], hardened concrete paving block samples were subjected to get tested for split tensile and compressive strengths [33].   figure 4. From figure 4, it was observed that from 0 percent to 100 percent, the workability of the concrete is continuously reduced. From 0 percent to 60 percent of replacement, the slump values lie from 50 to 100. So it shows that 0 to 60 percent replacements have medium workability. The slump value is between 25 and 50 percent above 60 percent replacement. As a result, more than 60% of replacements have low and poor workability because of less water absorption by steel mill scale, as the particles are devoid of voids. This is particularly due to the densely packed structure of steel mill scale particles depicted in figure 1(b). M-sand concrete possess higher unit water consumption, because of the grain form and M-sand's surface state [17]. The decrease in workability with rising steel mill scale replacement levels might be related to steel mill scale particles' fineness, which may reduce the water absorption by steel mill scale particles inspite of the necessity to supply required water content over the increased specific surface area of the steel mill scale and M-sand blended matrix. The presence of M-sand and steel mill scale increases the need for water consumption, which in turn reduces the workability of the concrete [35,36].

Compaction factor test
For measuring the fresh concrete's workability for various volumes of steel mill scale replacements, test on compaction factor was carried out as per IS: 1199-1959 [37] codal procedure. Figure 5 depicts outcomes due of the compaction test.
Results from the graph shows that the reduction of compaction factor values of concrete takes place with steel mill scale addition at various replacement levels. Compaction factor reduces from a starting value of 0.99 at  0% replacement to 0.93 at 100% replacement. Compaction factor test exhibits results similar to slump cone results, depicting the fact that steel mill scale addition in concrete affects workability. Generally, medium workable conventional concrete have compaction factor values ranges between 0.92 and 0.94; and workability occurs for the compaction factor value greater than 0.95 [38]. Based on the above statement and from the results obtained for compaction factor tests, it is evident that upto 60% replacement of steel mill scale, workability lies under high workable range, and subsequently steel mill scale concrete shows medium workability after 60% replacement levels. This reduction in workability is due to the less water absorption by steel mill scale particles with the increased addition in percentage replacement levels of steel mill scale, especially after 60% replacement instead of M-sand.
From the workability results obtained from both Slump test and Compaction factor test, due to poor workability of concrete mix leads after 60% replacement levels, voids may increase after hardening, due to segregation of concrete mix, which may affect the strength and durability properties of the paving blocks.' Figure 6 shows the evolution of strength on compression of hardened concrete paving blocks. The compressive strength increased upto 60 percent steel mill scale substitution. However, as the steel mill scale amount grows beyond 60 percent of replacement, interlocking concrete paving block units' characteristic compressive strength decreases.

Compressive strength
On the other hand, the usage of 80 percent and 100 percent steel mill scale replacement reduced compressive strength of concrete paving blocks after 14th and 28th days of curing. This improvement in strength upto 60% replacement levels is because of good adhesion between the mill scale and the cement paste, which increases  strength [39]. Furthermore, the rougher surface roughness and steel mill scale's high angularity increase binding capacity with cement, resulting in extremely thick concrete, that definitely boosts strength against compression [35,39,40]. The minimum compressive strength of paving blocks is 30MPa, according to IS 15658 [30]. At all replacement levels, the 20 to 100 percent M-sand replaced concrete with steel mill scale produced more than 30 Mega Pascal.
Compressive strength is calculated by using the formula: Where, F = compressive strength of paving block specimen (Mega Pascal), P = Maximum load applied in N, A = Cross-sectional area of paving block specimen in mm 2 The results from figure 6 indicate that compressive strength undergoes an increased strength pattern upto 60% both at 14 days and 24 days curing and started decreasing till it reaches 100% replacement. But, it is found that compressive strengths attained for concrete mix from 20% to 100% replacements are greater than that of 0% conventional mix without steel mill scale replacement. Hence it is clear that optimum steel mill scale content of 60 percent will be significant for the concrete paving blocks of the specific size and thickness considered in present study, after which the dormant appearance of steel mill scale materials in the concrete mixes, which could have been responsible for lower strength relatively at increased percentage replacement viz. 80 and 100 percent steel mill scale [38]. Another factor includes the drop in workability of steel mill scale concrete after 60% replacement levels, leading to reduced binding effect between the ingredients of concrete. Furthermore, lesser pore sizes of steel mill scale particles absorbs less amount of water at higher replacement levels giving rise to less workable concrete with reduced strengths. This condition with reduction in strength may be improved by the addition of super-plasticizers to improve workability. Figure 7 represents the change in split tensile strength in concrete paving block caused by steel mill scale addition after 14th day of curing and 28th day of curing. According to figure 7, value of tensile strength increased as the percentage substitution of M-sand with steel mill scale increased from 0% to 60%. The strength increase could be attributed to the reaction takes place chemically between the SiO 2 content present in steel mill scale and also in cement content, which results in creating calcium silicate hydrate gel [17].

Split tensile strength
Where, F = force applied in N, D = specimen's diameter in mm L = specimen's length in mm The split tensile test done on concrete paving block units shows strength results found decreased at 80 and 100 percent levels of replacement. The decreasing performance of the concrete paving block unit samples with respect to tensile strength is similar to the fall in compressive strength. Increase in split tensile strength upto 60% replacement of steel mill scale is due to the angular shaped particles exhibiting well packed bonding with the binder along with the adequate water absorption by M-sand. With higher levels of steel mill scale addition viz. 80 percent and 100 percent, absorbing less water along with lesser percentage of M-sand addition, decrease in workability is assessed upto 100% replacement of steel mill scale. This may again leads to less binding ability of concrete constituents and further leads to reduction in split tensile strength. Split tensile strength decreases passably, without showing drastic decline at 80% and 100% replacement levels and the values obtained for split tensile strength for the above mentioned replacement levels exhibited almost nearer values obtained for 60% replacement of steel mill scale in concrete. This again proves that, the addition of steel mill scale above 60% replacement levels will not accelerate any progressive gain in strength. Figure 8 exhibits the water absorption behaviour of concrete paving blocks made of M-sand with partially replaced steel mill scale. Water absorption decreased as the proportion of steel mill scale increased from 0% to 60%. Water absorption is quite poor when M-sand is replaced by steel mill scale to 60 percent due to the reduced porosity properties and voids ratio exhibited by the steel mill scale materials.

Water absorption test
The proportion of water absorption in the control specimen is 3.97%, whereas it is 3.67% in the 60 percent replacement specimen. It is mostly due to the concrete's packing density steadily increasing when the steel mill scale is added incrementally. The concrete surface with 60% steel mill scale replacement was found to be exceptionally solid and free of porosity [17]. The rough surface, angular shape, affinity of M-sand towards water, as well as the rough surface and irregular shape exhibited by steel mill scale, generate good bonding with cement particles and due to this, high workable composition, concrete is more dense at 60 percent M-sand replacement. After 60 percent replacement, the voids in the paving blocks may increase due to reduction in M-sand addition, which may absorb more amount of water, depending upon the quantity added to concrete, workability may decrease leading to decrease in binding ability of the concrete matrix leading to increase in number of pores. SEM image of 100 percent replacement is evident for this statement. Also, the strength of paving blocks reduces after this increase in number of pores after 60% replacement level.

Micro structural analysis
All of the above-mentioned testing results demonstrate that replacing M-sand with steel mill scale at 60% levels of replacements delivers the best performance. Better particle packing has been recognised as the key explanation for the enhanced behaviour of concrete containing 60 percent steel mill scale and 40 percent M-sand. To confirm the foregoing inference, a scanning electron microscope was used for microstructural examination. After curing for 28 days, the samples were taken deeply from the hardened specimens' cores. Figure 9 shows the Micro-structural images of concrete paving block specimens. Micro-structural images of concrete paving block specimens were taken using SEM testing for 0 percent, 60 percent, and 100 percent replacement. For better comparative results, the same magnification of 20 microns was maintained in all the images. The pore size of the SEM images is measured using an SEM testing machine. At 0 percent replacement sample has average pore size of 12.72 microns, meanwhile 60 percent replacement sample has pore average size of 3.18 microns, and the 100 percent replacement sample has average pore size of 8.15 microns. When we compare void zero of a 0 percent replacement specimen with a 60 percent replacement specimen, the 60 percent replaced specimen has very low average voids. Likewise, when we compare 60 percent replacement levels with 100 percent replacement levels, 60 percent has a very low average void size. When 0 percent replaced specimen is compared with the 100 percent replaced specimen, the 100 percent replaced specimen has a void size is comparatively high. Due to the reduced void sizes, dense packing of steel mill scale concrete is possible with increased mechanical strength possessing optimum water requirements and workability, same like required for conventional concrete upto the replacement level (60 percent) which shows good workability. Also, at 100% replacement levels, void sizes are higher than other replacement levels, and thus may entertain the ingression of natural agencies into the hardened concrete that may affect durability.

Conclusion
The performance of steel mill scale as a substitution for M-sand for manufacturing concrete paving blocks is evaluated in this study. This study leads to the following conclusions: The steel mill scale's density is higher when compared to that of density of M-sand. Specific gravity of steel mill scale is 4.1, which is far greater than the values obtained for M-Sand and river sand. This leads to a clear view that the steel mill scale may be considered as stronger aggregates therefore adhering with the physical properties of conventional fine aggregates used in concrete. But, due to higher density of steel mill scale, self-weight of concrete may increase and therefore, care should be taken while if steel mill scale is utilized in structural reinforced elements. Water absorption values obtained for steel mill scale, while assessing its physical properties was found less i.e. 0.74, which is less when compared to that of M-sand and traditional aggregates.
The results reveal that a consistent rise in the split tensile strength and compressive strength of the paving block has taken place, when the proportion of steel mill scale is increased up to 60% replacement. with a former value of 2.82 Mega Pascal at 14 days and 3.32 Mega Pascal at 28 days and the later shows an ultimate compressive strength of 28.28 Mega Pascal at 14 days curing and 37.47 Mega Pascal at 28 days curing. Following then, compressive and split tensile strength is reduced for higher replacement levels. Though, 100% replacement level shows less strength of about 26.27 Mega Pascal at 14 days and 33.1 Mega Pascal at 28 days than the compressive strength attained at 60% replacement, 100% steel mill scale replaced concrete exhibited mechanical strength greater than the 0% replacement levels (26.48 Mega Pascal and 32.81 Mega Pascal both at 14 and 28 days respectively). The above mentioned statement fits with the assessments made for split tensile strength of the specimens also.
The decline in both compressive strength and split tensile strength may be due to less water absorption properties of Steel mill scale, which may lower the workability, and in turn affects the binding ability of aggregate-binder at higher replacement levels. Porosity may be increased due to the lesser binding of the concrete matrix, furthermore reduces the overall strength of the specimens after 60 percent replacement levels of steel mill scale.
Water absorption is greater in 0% steel mill scale replaced concrete specimens than in M-sand and steel mill scale blended paving block specimens. At 60% replacement levels, water absorption is relatively low, leading to a densely packed structure of steel mill scale and the higher level of water absorption by M-Sand. After 60 percent replacement, water absorption of hardened paving blocks started increasing from 3.71 percent for 80 percent steel mill scale replacement to 3.83 percent at 100 percent replacement levels. This may be resulting due to the lesser quantity of M-Sand, to the mix along with lesser water absorption by the steel mill scale which thereby leads to inadequate water content to bind the concrete constituents initially during plastic state before hardening takes place. This reduced consumption of water in the concrete mix may lead in initiating voids in the concrete matrix after hardening phase starts, which later on may be creating paths for the water to seep through, thereby increasing the water absorption. Therefore, workability enhancing admixtures may be required after 60% replacement levels.
When compared to other percentages of replacement, the paving block produced with 60% replaced steel mill scale and M-sand combination has better micro-structural outcomes. The concrete prepared with steel mill scale at 60% replacement level was packed densely with a small average pore size and hence performed better in tests.
However, due to the higher iron content in 100% replacement, a detailed research is required to learn about the behaviour of 100% steel mill scale replaced paving blocks relating durability in terms of corrosion.
This study reveals that scrap steel mill scale material can effectively be used in making eco-friendly concrete paving blocks of higher densities than the standard concrete paving blocks thereby reducing waste accumulation and pollution. The study also clearly shows that steel mill scale may be used as one of the alternative novel construction materials with concrete constituents for achieving durable infrastructure build out and greener environment, wherever IS 15658 codal recommendations are adopted. Therefore, more insights may be a requisite on durability properties, which can be obtained by conducting comprehensive durability tests, before adopting for practical applications.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files). The data that support the findings of this study are available upon reasonable request from the authors.