LOW GHG Emission Footprints
High Abrasion Resistance & low Hydration Heat Requirement make it Ideal For Road Construction
This article introduces a commercialised energy-saving cement made by co-grinding OPC clinker with steelmaking slag (steel-slag) and blastfurnace slag that represents a different approach to cutting greenhouse-gas (GHG) emission footprints and conserving virgin natural resources. The cement uses only 15-30% of OPC clinker, 30-40% of steel-slag and 40-50% of blastfurnace slag (BFS), allowing for great reductions in a GHG emissions, consumption of virgin natural resources and energy use by between 70% and 85%.
Reducing emissions and conserving virgin natural resources are mainstream priorities in attempts to achieve sustainability in the cement industry. Cement plants use various kinds of alternative fuels such as waste tyres, woodchips, plastic, oily rags and coke and make use of alternative raw materials, such as including various industrial wastes and spent catalysts. The application of different supplementary cementitious materials (SCM) in cement production has already been proven as the most effective way to reduce greenhouse gas (GHG) emissions from cement production by replacing the same amount of clinker. Producing 1t of clinker emits about 1t of GHG. Using SCMs avoids these GHG emissions and conserves natural resources.
Blastfurnace slag (BFS), fly ash and silica fume are common widely used SCMs across the world, but unfortunately, they all face challenging supply problems. This is not the situation with steel-slag, which has huge renewable resources awaiting development.
Steel-Slag
It is estimated that 115-180Mt of steel-slag is poured out annually worldwide and in addition to this previous accumulation of the material has created mountains of steel-slag. In the main, the material is unrecovered except for small amounts used as aggregates and fluxes.
Types
Steel-slag is a by-product of the steelmaking process, which is formed when iron and scrap metals are melted together with fluxes such as lime and dolomite under oxidising conditions by injecting large amount of air or oxygen. Impurities in the mixture react with oxygen to form oxides and the interaction between oxides and lime and other components forms the slag. Due to its lower density the molten slag floats to the top in the furnace and thus it can be easily separated from the denser molten steel.
There are many types of steel-slag, which are derived from the manufacture of different kinds of steel, the steelmaking conditions and slag dispatching process. Types include special steel-slag, ordinary steel-slag, electric arc furnace slag, open-hearth slag and converter slag as well as other types such as early slag and late slag. Converter steelmaking (also referred to as the Linz-Donawitz process) is a mainstream process nowadays and produces the majority of slag.
Characteristics
Different stages of slag and different types of slags have different compositions. Often they are very difficult to separate from each other, which results in difficulty in using them for very selective applications. Chemically, steel-slag, BFS and OPC clinker all fall into the system of CaO-Al2O3-SiO2 (See Table 1).
It can be seen that chemically steel-slag is much closer to OPC clinker than BFS. Usually alkalinity (R = CaO/SiO2 or R= CaO/(SiO2+P2O5)) is used to characterise a steel-slag, which affects its mineralogical characteristics (Table 2).
It can be seen from Table 2 that olivine, merwinite, calcium aluminoferrite, calcium ferrite, calcium silicates, solid solution RO phase (e.g. (Fe,Mn,Mg)O), free MgO and free CaO are the main minerals in steel-slag. Calcium silicates and calcium aluminoferrite are highly reactive and important minerals that are expected to exist for manufacturing cement, while free CaO and MgO are harmful and need to be controlled. The others are inert. Figure 1 shows some optical microscopic pictures of steel-slag.
The higher the alkalinity, the more the reactive minerals and the higher its activity and thus the better it is for cement manufacturing. Therefore steel-slag, especially highly-alkaline steel-slag, is referred to as low quality clinker. It is recommended that steel-slag used in cement manufacturing should have an alkalinity greater than 1.8.
Energy-Saving Steel-Slag Cement
Energy-saving steel-slag cement is made from steel-slag, clinker and others (such as BFS), where steel-slag should not be less than 15%. Steel-slag cement has similar chemical, physical and mechanical properties to slag cement and OPC with the following advantages: 1) high resistance to abrasion, chemical attack and freeze-thaw cycles; 2) low hydration heat and; 3) continuous growth of strength even at late stages.
Strength and Savings
Table 3 shows industrial production samples with their proportions, strengths and the comparison against the corresponding standard strength requirements.
Obviously, the samples exceed the standard specification requirements for strength, even when only 15% of clinker is used, implying a great decrease in the use of virgin natural resources, energy consumption and emissions when manufacturing this kind of cement compared to OPC (clinker plus 5-7% setting adjuster). Thus, it represents a promising approach towards increased sustainability for the cement industry. Producing 1t of this cement only requires 0.15-0.3t of clinker, 0.3-0.4t of steel-slag and 0.4-0.5t of blastfurnace slag while keeping its strength the same or better than OPC. Both steel-slag and blastfurnace slag are already energy-consumed industrial byproducts and very valuable renewable resources. Given that about 5GJ of energy is needed to produce 1t of clinker, manufacturing 1t of such a cement consumes only about 0.75-1.5GJ of energy. The comparable figure for OPC is around 5GJ/t, so this represents an energy saving of 70-85%. Meanwhile, it only consumes about 0.2-0.5t of virgin raw materials against about 1.4t for 1t of OPC. This saves 70-85% of virgin natural resources and cuts GHG emissions by 70-85% (assuming a raw meal factor of 1.54 and clinker factor of 0.90).
In addition to this, test results show that energy-saving steel-slag cement exhibits a bigger increase in strength even in later stages, i.e., after 180 days, than conventional OPC. Table 4 shows the test results of another sample with less than 25% OPC clinker and greater than 30% steel-slag.
Applications
Other properties were also tested, incuding the amount of water for normal consistency (about 25-30%), the resistance of the material to chemicals (similar to slag cement) and resistance to freeze-thaw cycles. After 100 cycles, strength increased by 1.04% and weight lost was 0.12% based on concrete made with energy-saving steel-slag cement. Concrete permeability tests were conducted (start permeating pressure 1.73 MPa, compared to 1.0MPa for OPC) as well as hydration heat (28.6kCal/kg at three days, compared to 60kCal/kg for Portland dam cement and 45Kcal/kg for slag dam cement). Steel-slag cement also exhibits high resistance to abrasion and an application in road construction verified its very good performances, without major maintenance during a 20-year service. Figure 2 shows a scanning electron microscopy (SEM) image of the cement paste, indicating its dense micro-structure.
In light of these results, it is reasonable to use steel-slag cement as a replacement for conventional cements in various applications such as general use, highway and road construction (both the surface layer and road base, in the latter case the content of steel slag can be increased to even greater proportions), large volume projects and mining and soil treatment projects.
Conclusion
1. Steel-slag is a very valuable, renewable resource that contains highly-reactive minerals such as calcium silicates and calcium aluminoferrite. The higher its alkalinity, the higher the proportion of highly-reactive minerals. Steel-slag with alkalinity greater than 1.8 is usually recommended.
2. Steel-slag can be used to produce energy-saving cement by co-grinding with OPC clinker and blastfurnace slag. In such preparations OPC clinker is about 15-30%, steel-slag around 30-40% and blastfurnace slag is around 40-50%.
3. Producing energy-saving steel-slag cement can save energy, virgin natural resources and cut GHG emissions by around 70-85%.
4. Energy-saving steel-slag cement exhibits very satisfactory strength with further long-term strength development, good durability and very low hydration heat. It can replace Portland cement in various applications and is especially suitable for projects where a low heat of hydration is required.
Reference :1. Mingshu, T. ‘The Possibility of Making Cement from Steelmaking Slag Based on Mineralogical Characteristics,’ Nanjing Institute of Chemical Technology
