Utilization of By-Products and Waste GGBFS Materials from Steel Processing Industry to Produce Sustainable Green Pervious Concrete in Indonesia

Pervious concrete is a reliable solution for environmentally friendly concrete pavements, especially for densely populated urban areas. Some of the benefits of pervious concrete to the environment are mainly increasing the absorption of water into the soil, so that it can quickly purify rainwater runoff, recharge groundwater, reduce rainwater runoff, and when combined with well-designed drainage system it can minimize the potential of flooding. The uniqueness of this concrete is that it uses few or no fine aggregate at all, with the main composition consisting of coarse aggregate, binder (cementitious/pozzolanic materials), water and admixture. The development of pervious concrete with steel slag as raw material as coarse aggregate and Ground Granulated Blast Furnace Slag (GGBFS) as a partial replacement for cement is carried out to address environmental problems caused by impervious pavement that is commonly used in urban areas, as well as to increase the added value of material by-products of the steel processing industry. The aim of this research is to obtain a pervious concrete mixture with an optimum compressive strength and porosity utilizing locally available by-products and waste materials from steel processing industry to produce a more sustainable construction material. Evaluation based on the experimental results shows that the specimen with well graded or blended-size of steel slag coarse aggregates, with a maximum of 10% steel slag fine aggregate, and GGBFS as 32% cement substitute material (CSM) is found to be an optimized mixture in achieving high compressive strength of 20.24 MPa at which both density and porosity values complying ACI 522R-10.


Introduction
Pervious concrete is a reliable solution for environmentally friendly concrete pavements, especially for densely populated urban areas.The benefits of pervious concrete for the environment are permeable at which it increases the absorption of water into the soil.By permeable layer of pervious concrete in the 1245 (2023) 012017 IOP Publishing doi:10.1088/1755-1315/1245/1/012017 2 pavement, it may quickly purify rainwater runoff, replenish groundwater, reduce rainwater runoff [1].When such pervious concrete pavement is combined with a properly designed drainage system, it may further minimize the potential for flooding.The pervious concrete may also be advantageous in terms of sound absorption and noise reduction of a busy highway [2].
Pervious concrete has its own uniqueness.First, it uses only a small portion or even without fine aggregate.Its main composition consists of coarse aggregate, binder (cementitious/pozzolanic materials), water and admixture.In this study, with the aim to provide society with a more sustainable construction material, as well as to increase the added value of material by-products of the steel processing industry, the development of pervious concrete with steel slag as raw material as coarse aggregate and GGBFS as a partial replacement for cement is carried out.
Steel slag and GGBFS, by-products of steel production, have been considered as industrial wastes due to the challenges associated with its accumulation and disposal.In Indonesia, there are at least three prominent steel industries, namely PT Krakatau Steel, PT Krakatau Posco in Cilegon, Banten, and PT DSI in Morowali, Central Sulawesi, that produce steel slag and GGBFS through the smelting of iron ore into pig iron using blast furnaces.Extensive research has been conducted on the potential utilization of steel slag as a substitute for natural aggregates [3].However, the exploration and utilization of steel slag as a construction material especially in the surroundings of these steel industries in Indonesia, particularly in the context of pervious pavement, have not been widely explored and implemented.ACI 522R-10 guideline regulated minimum compressive strength' requirement for the pervious compressive strength of concrete ranging from 2.8 to 28 MPa with a range of void content from 15% to 35% as an effective permeability measure of the pervious concrete.The properties of pervious concrete are primarily dependent on its porosity (void content), cementitious content, ratio of water to binder (w/cm), compaction level, and aggregate gradation and quality [4].The relatively high compressive strengths of pervious concrete mixtures are possible to achieve.However, such a high strength of pervious concrete is achieved only by the reduction of void content.Therefore, in order to obtain an optimum mixture of pervious concrete, efforts need to be made in balancing the compressive strength and porosity in regard to void content.Previous study had shown that a minimum porosity of approximately 15% was required to achieve significant percolation.To generate larger-sized pores in the material, larger aggregate sizes are recommended [4] as they may reduce chances of pore-clogging [5].

Material
The main ingredient of this research was steel slag obtained from PT. Krakatau Steel, Cilegon that was used as coarse aggregate as a replacement of natural aggregate.The steel slag that has been previously separated from the metal impurities and shown in Figure 1(a).Variation of sizes of the steel slag aggregate (SSA) studied herein may either be uniformly single-size of either 4.75-9.5 mm or 9.5-19 mm, or blended-size coarse aggregate system with both sizes, with or without the addition of steel slag fine aggregate (0.15-2.36 mm).In this study, a blended size of aggregates was carried out, as replacing smaller-sized aggregates with a higher percentage of larger-sized ones increases the pore size.This is mainly due to the coarser particle may not be able to fit in the void left by the removed finer particle [6,7].The main components of steel slag are iron, magnesium, aluminium, calcium, and other oxides [2].The content of oxides and elements of SSA in this study are shown in Table 1.
The second material was Ordinary Portland Cement (OPC) Type 1, used as the main binder in the pervious concrete mix.OPC obtained from PT. Semen Jakarta as shown in Figure 1(b).The oxide content and elements of the OPC cement used are shown in Table 1.
The third raw material incorporated into the mix was Ground Granulated Blast Furnace Slag (GGBFS) from PT. Krakatau Steel, Cilegon, Indonesia.GGBFS as a by-product of iron production in blast-furnace which mainly consists of silicate, aluminosilicate and calcium oxide of melted calcium (Table 1) which possess both pozzolan and cementitious properties, thus can work as cement substitute material (CSM).Its chemical compositions depend on the raw materials used in the production of iron while the physical characteristics of Granulated Blast Furnace Slag (GBFS) depend on the cooling process of the molten iron.To work as cementitious material, GBFS material needs to be grind until it reached a fineness the size of cement grains as shown in Figure 1(c).Fineness of GGBFS cement is considered an important parameter affecting the strength development of pervious concrete.However, the percentage of use of GGBFS needs to be limited to less than 50% OPC replacement to ensure the effectiveness of the resulting compressive strength of pervious concrete.Other materials that were considered key ingredients were water and stabilizer.The ratio of w/c in this study was limited to 0.28, corresponding to the range of 0.26-0.4recommended by [4].An excessive amount of cement paste content, and/or fine aggregate may result in a 'clogged' void of the concrete and, consequently, the porosity is expected to be reduced.
All the raw materials were then identified with regards to the physical characteristics, i.e. specific gravity, water absorption, density, air content, water content, fineness modulus, abrasion and soundness, content of friable particles and organic material, as well as physical gradation.These data are necessary to decide and proportionate the mixture of pervious concrete.

Mix Proportion
In this study, three groups of specimens as given in Table 2 are varied by the aggregate grading, of single and blended coarse aggregate effects, use of steel slag fine aggregate and increased percentage of GGBFS as a partial replacement for cement.The proportion of pervious concrete mixture is shown in Table 3.

Mixing, samples, and testing procedures
To obtain a consistent mix, dry mix method was first carried out to blend the aggregates with the binder.Water and stabilizer that had been mixed with were then poured into the aggregate and binder mixture gradually until homogenously mixed.This method is carried out to prevent the accumulation of paste in certain parts only and to ensure the paste smears aggregates thoroughly.Two indications for right mixture are the slump value of fresh pervious concrete that is close to zero and the mixture can be made as lump as shown in Figure 2.These indicate that the pervious concrete mixture contains sufficient water (neither dry nor too wet) and is homogenous.Homogeneity greatly affects the formation of pores and the rate percolation of the hardened pervious concrete.
Cylinder specimens of 100 mm diameter and 200 mm height were casted.To achieve consistent compaction on all specimens, compaction was carried out using a Standard Proctor Hammer.ACI 522R-10 [4] stated that the compressive strength of pervious concrete is strongly influenced by the mix proportion and compaction method during concrete casting.
After casting, the specimens were then cured by air curing of 20 ± 2°C and 95 ± 5% relative humidity for a day, demoulded and water cured to maintain the hydration process.The specimens were tested for its compressive strength conforming ASTM C39 requirements, and for density as well as porosity according to ASTM C1754 requirements.

Results and Discussion
The experimental results are summarized in Table 4 as follows.Variation of specimens following Tables 2 and 3 gives rise in compressive strength variation between 7.70 to 20.24 MPa, density between 2191.44 to 2540.42 kg/m 3 , and porosity between 18.37 to 31.18%.

Effect of aggregate grading
Compressive strength of pervious concrete is greatly influenced by aggregate grading as illustrate in Figure 3. Well graded coarse aggregate in a form of blended sizes of both 4.75-9.5 mm and 9.5-19 mm enhances the compressive strength of the specimen by 27.92% to 87.40%.Similarly, Figure 4 shows well graded coarse aggregate slightly increases the density of specimen by 4.44% to 5.77%.Consequently, it hence decreases the porosity 13.73% to 16.30%.This is in accordance to ACI 522R-10 [4].
The replacement of coarse aggregate by 10% fine aggregate to the well graded coarse aggregate mixture (SSPC IV) results in slightly higher compressive strength and density compared to SSPC III.Small amount of fine aggregate in the mixture maintains the porosity of the pervious concrete specimen.

Effect of GGBFS as CSM
GGBFS is incorporated into the mixture as cement substitute material as given in Tables 2 and 3. Figure 5 illustrates the increase of compressive strength as percentage of GGBFS as CSM in the mix increases.
Similarly, the density of pervious concrete increases as the percentage of GGBFS as CSM is higher as illustrated in Figure 6.Conversely does the porosity, as the percentage of GGBFS as CSM increases, the porosity is reduced respectively.

Conclusion and Further Study
From the experiment it may therefore be concluded that to obtain an optimum mix of sustainable green pervious concrete, there are at least five important factors to consider, i.e., utilization of well blended coarse aggregate, replacement of small amount of coarse aggregate with fine aggregate, optimization of the achieved compressive strength in regard to the porosity or void content, ensuring a consistent compaction work and also the optimization of GGBFS as CSM.In this study, the effect of well graded or blended-size steel slag coarse aggregate and inclusion of steel slag fine aggregate as much as 10% significantly increases the compressive strength and density of pervious concrete by conforming porosity as per ACI 522R.Finally, replacement of OPC by 32% GGBFS as CSM is found to be an optimized mix proportion to produce high compressive strength, high density and required porosity of a more sustainable yet green pervious concrete made by the by-products of steel processing industry.
Further study on the feasibility of the use of steel slag and GGBFS in producing green pervious concrete is currently carried out by evaluating the effectiveness and cost efficient of the mix.

Figure 2 .
Figure 2. Mixing of Pervious Concrete Materials

Figure 3 .Figure 4 .
Figure 3.Effect of the aggregate grading on the compressive strength of pervious concrete

Figure 5 .Figure 6 .
Figure 5.The effect of GGBFS as CSM on the compressive strength of pervious concrete

Table 1 .
Oxide Content of SSA, OPC and GGBFS

Table 4 .
Summary of Experimental Results