COMPARATIVE STUDY OF STEEL SLAG AND STAINLESS STEEL SLAG AS A REPLACEMENT OF CEMENT MATERIALS – REVIEW PAPER

Abstract:

Slag waste from stainless steel production is studied in this research to determine whether or not it can be used as a cement substitute and whether or not different treatments are effective at improving the slag's mechanical properties. Utilizing by-products from the stainless steel industry in cement manufacturing saves money on raw materials while also generating revenue. This study aimed to analyze the cementation and pozzolanic reaction characteristics of stainless steel slag waste in order to determine its strength activity index and environmental impact. Cement was replaced with varied percentages of untreated stainless steel slag waste and slag waste that had been processed by crushing, burning, or both to determine the appropriate replacement ratio based on the mechanical properties of the resulting concrete. A research based on multivariate factor analysis was developed to compare and contrast the mechanical characteristics of these processed wastes. The selection mechanism is based on a rating system that applies a feature extraction method to the wastes used as cement substitutes.

Keywords: Stainless steel slag, steel slag, cement materials, concrete, cement substitutes

1. Introduction:

In order to better understand the potential of steel slags as cementitious materials, we compared the chemical, mineralogical, and morphological characteristics of carbon slag from a basic oxygen furnace with stainless steel slag from an electric arc furnace. We also looked at how the amount of slag replacement impacted the fluidity, compressive strength, regular consistency, setting time, and impact of blended cement mortar (Duraman, 2020). The trials' findings show that BOF C has more alkalinity, a higher pH, and more hydraulic phases than EAF S. Both types of slag have a substantial impact on water usage due to their fineness. Early on, flash setting was observed in pure BOF C paste, and the setting time of blended cement was sped significantly, particularly at high slag percentages. EAF S nonetheless reduced the time needed for mixed cement to cure, even at low slag percentages (Bin, 2012).

The pH of cement that has been combined with either half BOF C or EAF S is lower than that of neutral paste and regular cement. Compressive strength of the material reduced as slag content rose (Yeong, 2013). BOF C mortar will live longer if the EAF S mortar and BOF C mortar have the same age and replacement ratio. BOF C and EAF S slags both meet the requirements for Grade 1 and Grade 2 steel slag as defined by the Chinese National Standard, as assessed by their respective activity indices (Qiang, 2011).

The production of essential construction materials is still greatly impacted by the current economic and commercial climate in China. According to the US Geological Survey's Mineral Commodity Summaries for February 2015, China produced 60% of the 4.0 billion tons of cement consumed worldwide in 2014. Furthermore, China produced roughly 50% of the world's output of both crude steel and stainless steel in 2014. Extremely high carbon dioxide emissions from the mass manufacturing of steel and cement have an extensive impact on both humans and the environment (Qiang, 2011). Waste products from steel manufacturing, however, have the potential to persuade the sector to protect the environment and achieve the goal of reducing CO2 emissions when added to the production of cement and concrete. Blast furnace slag is used in cement factories to significantly reduce their carbon dioxide emissions (Tossavainen, 2007).

This mineral combination is still quite uncommon in many regions of China. Slags generated during the steelmaking process are being given more thought. China produced between 55 and 78 percent more steel slag than was needed (ASTM, 2014). Unrecycled steel slag disposal can contaminate groundwater and landfills. The sustainability of the steel and concrete industries would increase if more steel slags were utilized in cement and concrete production. The chemical and mineralogical characteristics of steel slags play a significant role in their use as a cementitious material (AENOR, 2006).

2. Stainless steel slag:

Slag is created as a by-product of producing stainless steel when scrap iron is melted in an electric furnace arc. Between a quarter and a third of a ton of slag wastes, including both oxidizing and reducing slag from the steel-making and steel-refining processes would be produced for every ton of stainless steel produced. Both the dust and the first, a by-product of the production of aggregate, have mean particle sizes of about 3 m. In 2018, more than 50.7 million tons of stainless steel was produced, resulting in the production of 12.67 to 16.90 million tons of steelmaking slags globally (AENOR, 2006).

Slags are occasionally dumped as rubbish in huge quantities and transferred to landfills, where they can cause serious environmental damage. While steel slag can be used to produce aggregates for concrete production or roadbed material after stabilization treatments, ground blast furnace slag (BFS) is only occasionally employed as a mineral ingredient for blended cement. As a result, a lot of researchers are interested in finding ways to use steel slag in the building industry in a way that would be less harmful to the environment (Duraman, 2020).

Figure.1. Material structure of stainless steel slag (Ref: Duraman, 2020)

A growing but mostly untapped resource comes from the slags produced by the steel industry. In particular, distinctive slags are generated during the production of stainless steel, which may inspire a plan directed at their application in non-traditional recycling settings (Das, 2007). According to these findings, stainless steel slag can be utilized in place of limestone filler in self-compacting concrete. After being subjected to different treatments, the physical and chemical properties of slags were analysed (MCS, 2015). However, research into self-compacting concrete has focused primarily on the material's mechanical qualities and durability (Tang, 1973). Surprisingly, the findings point to the possibility of using stainless steel slag as a building material, which would have a favourable impact on waste minimization, environmental friendliness, and the circular economy.

Table.1. Mechanical properties of stainless steel slag (Ref: Yeong, 2013)

Grade

304

304L

304H

Tensile Strength (MPa)

540 - 750

520 - 700

-

Proof Stress (MPa)

230 Min

220 Min

-

Elongation A50 mm

45 Min %

45 Min %

-

?

3. Steel slag:

By-product In steel-making furnaces, a by-product known as “steel slag” is created as impurities are filtered out of the molten steel. Slag is the complicated mixture of silicates and oxides that forms when molten metal cools. Today, steel is typically produced in either massive integrated mills employing some variant of the basic oxygen process or in more modest electric arc furnace mills. Open hearth furnaces are only on display in a select number of museums. During the basic oxygen process, charges of hot liquid blast furnace metal, slag, and fluxes such lime (CaO) and dolomitic lime are introduced to a converter (Zhu, 2013).

The converter is injected with a lance of high-pressure oxygen. After reacting with the oxygen, the contaminated charge is cleansed. Slag is made up of impurities such carbon (in the form of carbon monoxide gas), silicon, manganese, phosphorus, and some iron (in the form of oxides), lime, and dolime. The liquid steel is tapped (poured) into a ladle once the refining process is complete, while the steel slag is left in the vessel and tapped into a separate slag pot at a later time (Qiang, 2011).

Figure.2. Material structure of steel slag (Ref: Qiang, 2011)

The characteristics of the slag may differ significantly depending on the steel quality. Steel grades are divided into high, medium, and low categories based on their carbon content. High carbon content is a characteristic of premium steels. By introducing a lot of oxygen when the steel is being made, the carbon content can be lowered. For the production of slag and the purging of steel impurities, more lime and dolime must be added (Bin, 2012).

The process of manufacturing steel results in the production of many kinds of slag. Pit slag, cleanout slag, synthetic slags from the ladle, and furnace slag are a few names for this substance (Duraman, 2020). Figure.2 shows graphically how various slags are produced and distributed in a contemporary steel plant.

Slag created in the initial stages of steel manufacture is referred to as tap slag or furnace slag. The main raw material for steel slag aggregate is this. The molten steel is moved with the aid of a ladle from the furnace to the refinery, where it undergoes additional processing to get rid of any impurities that may still be present (AENOR, 2006). Anything refined in the ladle remains in the transfer ladle at all times. When steel is refined with a ladle, more slags are produced during the melting process (Yeong, 2013). To help absorb deoxidation products (inclusions), provide thermal insulation, and protect the refractories in the ladle, these slags are blended with the leftover slag from the furnace. Raker and ladle slags are the slags created at this step of the steel-making process.

There are two other types of slag that are frequently seen in the steel-making process: slag from the ladle after tapping and slag that accumulates on the plant floor at various phases of operation (Zhu, 2013). The substantially differing properties of synthetic slag from furnace slag often make it unsuitable for use in the production of steel slag aggregate. The fact that ladles refining typically necessitates very large flux inputs is the cause of this. It is essential to separate these different slags from furnace slag in order to keep contaminants out of the final slag aggregate (ASTM, 2014).

Metals are also recovered from the liquid furnace slag and ladle slags by processing them to remove any lingering ferrous components. The steel manufacturer is required to carry out this metals recovery operation (using a magnetic separator on the conveyor and/or a crane electromagnet) because the metals can be recycled back into the steel plant and used as blast furnace feed material to create iron (Yeong, 2013).

3.1. Mechanical Properties:

Table.2. Typical mechanical properties of steel slag (Ref: MCS, 2015)

Property

Value

Los Angeles Abrasion (ASTM C131), %

20 - 25

Sodium Sulfate Soundness Loss (ASTM C88), %

<12

Angle of Internal Friction

40° - 50°

Hardness (measured by Moh's scale of mineral hardness)*

6 - 7

California Bearing Ratio (CBR), % top size 19 mm (3/4 inch)**

up to 300

Hardness of dolomite measured on same scale is 3 to 4. * Typical CBR value for crushed limestone is 100%.

?

The processed steel slag has useful mechanical properties for aggregate use, including high bearing strength, low water absorption, and high abrasion resistance. Table.2 displays some of the more common mechanical qualities of steel slag (Qiang, 2011).

4. Conclusion:

Slag from all stages of the steelmaking process, even the stainless ones, can be recovered and used again as part of a comprehensive strategy for managing industrial waste. Slag from electric arc furnaces and ladle furnaces are included in this. Making cement and concrete is one method that could lead to value development. In addition to EAF and LF slags, BOF slag can be used as a cementitious material at a substitution ratio of 10–20 weight percent due to its higher alkalinity and increased reactivity. The rough texture of BOF slag when crushed makes it perfect for use as aggregates in concrete.

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