Compressive Strength and Microstructure Properties of Alkali-Activated Systems with Blast Furnace Slag , Desulfurization Slag , and Gypsum

,is study investigates the effect of desulfurization slag (DS) and gypsum (G) on the compressive strength and microstructure properties of blast furnace slag-(BFS-) based alkali-activated systems. DS is produced in a Kambara reactor process of molten iron produced in a steel production process. DS contains CaO, SiO2, Fe2O3, and SO3 and is composed of Ca(OH)2 and 2CaO·SiO2 as main compounds. In this investigation, the weight of BFS was replaced by DS at 5, 10, 15, 20, 25, and 30%. In addition, G was also applied at 9, 12, and 15% by weight of BFS to improve the compressive strength of the alkali-activated system with BFS and DS. According to this investigation, the compressive strength of the alkali-activated mixes with BFS and DS ranged from 14.9MPa (B95D5) to 19.8MPa (B90D10) after 91 days. However, the 28 days compressive strength of the alkali-activated mixes with BFS, DS, and G reached 39.1MPa, 45.2MPa, and 48.4MPa, respectively, which were approximately 78.8 to 97.5% of that of O100 mix (49.6MPa).,emain hydrates of the BFS-DS (B80D20) binder sample were Ca(OH)2, CaCO3, and low-crystalline calcium silicate hydrates, while the main hydration product of BFS-DS-G (B75D10G15) binder was found as ettringite. ,e use of BFS-DS-G binders would result in the value-added utilization of steel slag and provide an environmentally friendly construction material, and contribute to a reduction of CO2 in the cement industry.


Introduction
When producing 1 ton of ordinary Portland cement (OPC), approximately 900 kg of CO 2 is generated.Recognizing the sustainability of the environment as a top priority, many studies for reducing the amount of CO 2 generated by cement production and reducing energy consumption are reported [1][2][3][4].As part of this, various efforts have been made to reduce the amount of OPC by increasing the amount of cementitious materials for concrete such as fly ash, blast furnace slag (BFS), and limestone powder [5][6][7].In addition, many studies have been actively made on alkali-activated systems, which can be hardened without containing cement.
ese alkali-activated systems can be hardened without cement by incorporating an alkaline activator into cementitious materials rich in CaO and SiO 2 , such as BFS, fly ash, metakaolin, and kaolinite clays [8][9][10][11][12].In the case of the BFS, the incorporation of an alkali activator into the BFS facilitates the elution of Ca 2+ , Si 4+ , and Al 3+ from the BFS, allowing calcium silicate hydrates (C-S-H) and calcium aluminum hydrates (C-A-H) [13][14][15][16].For alkali activation in alkali-activated systems, NaOH, Na 2 SiO 3 , and KOH, which are based on alkali metals such as sodium (Na) and potassium (K), have been mainly used in many studies [2,[17][18][19][20][21].Although Na-and K-type alkali activators contribute to the formation of high concentration of hydroxide ions (OH-), which leads to quick setting in a short time, some disadvantages such as high cost, high toxicity (with a pH of 14 or higher), and alkali silica reaction have also been reported [22][23][24].
In this study, we investigated the applicability of desulfurization slag (DS) as an alkali activator of an alkaliactivated system.DS is produced in a Kambara reactor process of molten iron produced in a steel production process.In general, molten iron contains a small amount of sulfur (0.1-0.5%), and it has a negative effect on the quality of steel product.erefore, the desulfurization process is required to reduce the sulfur amount by approximately below 0.05%.
In this process, mechanical stirring is performed after the addition of a desulfurizing agent (CaO).Sulfur in molten iron reacts with CaO to form CaS-type compounds including CaS and CaSO 4 .Because the reaction rate of CaO is 30% or less, unreacted CaO remains.e specific gravity of CaS compound and unreacted CaO is low; therefore, they float to the top of the molten steel and mix into the slag layer.
e slag produced in the desulfurization process is DS [25,26].
e DS filled in a moving pot is transferred to the cooling yard.Next, water is sprayed to control scattering dust and achieve rapid cooling.After the water spraying, iron is recovered from the slag through the process of crushing and magnetic separation.e recovered iron is recycled as a substitute for iron ore or scrap in the iron and steelmaking process.e nonmagnetic DS, the final remnant in the process, consists of sand-shaped grains with a diameter of 2 mm or less.It is used only for road base or landfill with a mixture of a convert slag.Korean steel makers generate up to about 300,000 ton/year of the nonmagnetic DS, but it has not yet been utilized except as materials for civil works.
Since DS is cooled by water spray, most of CaO is converted to Ca(OH) 2 , which dissolves in water with Ca 2+ and 2OH − to form a high pH environment.In this study, the compressive strength and microstructure characteristics of alkali-activated BFS-based systems using DS and gypsum (G) as activators were examined.e ultimate goal of this study is to show that DS, an industrial byproduct, can be used more effectively and efficiently in alkali-activated systems.

Materials and Methods
2.1.Materials.ASTM type Ι OPC was used.BFS, DS, and G (natural anhydrous gypsum) were used as constituents of the alkali-activated system.e BFS was used as the powdered product.
In addition, the mix with only OPC and the mix with 50% OPC and 50% BFS were used for performance comparison of the alkali-activated system.DS was produced from P Company of Korea, and the Blaine Fineness was 404 m 2 /kg, whereas those for the OPC, G, and BFS used in this study were 341, 433, and 453 m 2 /kg, respectively.e chemical composition is presented in Table 1.e mortar specimen for the compressive strength test was manufactured with ISO standard sand (KS L ISO 679, standard sand for strength measurement) as indicated in Table 2. e absorption and specific gravity of ISO sand were 1.03% and 2.54 g/cm 3 , respectively.3 presents the experimental plan of this study.For the weight of BFS, DS was applied at 5, 10, 15, 20, 25, and 30%.In addition, G was also applied at 9, 12, and 15% by BFS weight to improve the compressive strength of the alkaliactivated system with BFS and DS.

Mix Proportions and Specimen Preparation. Table
e weight ratio of water : sand : binder in the mortar for evaluating compressive strength of the mixes was 0.5 : 3 : 1.
e compressive strength of the mortar was measured in compliance with ASTM C349.e mortar specimen with dimensions of 40 × 40 × 160 mm was made using a jolting machine and water cured at 20 ± 2 °C.
e water-binder ratio of the samples prepared for the microstructure property including hydration heat, X-ray diffraction (XRD), scanning electron microscopy (SEM), and porosity analyses was 0.5.
e hydration heat was measured for 7 days at intervals of 30 s using a calorimeter (MMC-511SV6, Tokyo Rico Corp.).

Chemical Properties of Raw Materials.
Figure 1 shows the XRD patterns and SEM images of the raw materials used in this study.DS contains CaO, SiO 2 , Fe 2 O 3 , and SO 3 and is composed of Ca(OH) 2. and 2CaO•SiO 2 as main compounds.Since water is sprayed during the cooling process, the unreacted CaO, which was added as a desulfurizing agent, is transferred to Ca(OH) 2 .e Ca(OH) 2 content in DS calculated by thermogravimetric analysis was approximately 25-30%.SiO 2 and Al 2 O 3 contents in DS are significantly lower than those in BFS, but it contains more CaO and Fe 2 O 3 than BFS.
e XRD patterns of DS show a very high peak of crystalline carbon.In general, cokes are used to separate oxygen from iron ores present as FeOx-type oxides in the steelmaking process.Unburned cokes, which are deoxidizers, also exist in molten iron, and most of the unburned cokes are incorporated into the slag layer.e DS thus contains approximately 4-5% of coke (a carbon component).
e main peaks of OPC show 3CaO•SiO 2 (alite) and 2CaO•SiO 2 (belite).It also contains small amounts of 3CaO e main peak of DS is Ca(OH) 2 , and it can act as an alkaline activator because it contains an OH − ion.Some peaks of CaCO 3 were also found by the drying process but free CaO was not found in DS.In addition, DS contains the peak of 2CaO•SiO 2 because some CaO added as a desulfurizing agent reacts with SiO 2 present in the slag to form a CaO-SiO 2 compound.e SEM image of DS shows rough surface characteristics because of the presence of Ca(OH) 2 and CaCO 3 particles formed by the water spray cooling and drying processes.In the case of the SEM image of BFS, an 2 Advances in Civil Engineering amorphous and very smooth surface was found because BFS was cooled through rapid watering in the molten state.).e compressive strengths of the alkaliactivated mixes with BFS and DS ranged from 14.9 MPa (B95D5) to 19.8 MPa (B90D10) after 91 days.When the amount of DS is less than 5% or more than 20%, the compressive strength of the mixes was lower than that of other mixes.erefore, the appropriate amount of DS in alkali-activated systems with BFS and DS is considered to be in the range of 10 to 15%.However, the compressive strength of alkali-activated systems with BFS and DS was relatively lower than that of the O100 and O50B50 mixes at all ages.e compressive strength of the alkali-activated systems with BFS and DS was approximately 25 to 36% of that of O100 and O50B50 at 91 days.

Compressive
In order to improve such low compressive strengths, the anhydrous G was applied in this study.Generally, it is considered that CaSO 4 can contribute to the formation of needle-shaped ettringite (3CaO which can decrease the porosity by packing micropores [33][34][35].
Figure 3 shows the variation in compressive strength of alkali-activated mixes consisting of BFS, DS, and G. e amount of DS was 10%, which was considered to prevent reduction of the workability through preliminary experiments, and the G was used to replace BFS at replacement ratios of 9, 12, and 15% by binder mass.
At 3 and 7 days, the compressive strength of alkaliactivated mixes with BFS, DS, and G was lower than that of O100 and O50B50 mixes.However, the 28 days compressive strengths of B81D10G9, B78D10G12, and B75D10G15 mixes reached 39.1 MPa, 45.2 MPa, and 48.4 MPa, respectively, which were approximately 78.8 to 97.5% of that of O100 mix (49.6 MPa).After 56 days, the increase in compressive strength of alkali-activated mixes with BFS, DS, and G appeared to be insignificant.
Figure 4 shows the relative compressive strength ratios of the alkali-activated mixes compared to that of O100 mix.In the case of the alkali-activated mixes with BFS and DS, the compressive strength was approximately 20 to 30% of that of O100 mix.In addition, it was found that the growth rate of compressive strength of the mixes according to age was very low.However, the increase in compressive strength of the alkali-activated mixes with BFS, DS, and G was noticeable until 28 days.In particular, the compressive strength ratios of B75D10G15 mix at 28, 56, and 91 days were 97.6%, 106.3%, and 104.1%, respectively, of those of O100 mix.e compressive strength ratios of B78D10G12 mix at 28, 56, and 91 days were 91.1%, 97.9%, and 97.8%, respectively, compared to those of O100 mix.erefore, the addition of G to the alkali-activated mixes with BFS and DS made it possible to produce alkali-activated systems that can increase the compressive strength by approximately 3 times compared to the alkali-activated mixes consisting of BFS and DS.It seems that it is possible to produce alkali-activated systems having compressive strengths similar to mixes with OPC.

Heat of Hydration.
Figure 5 shows the variation in hydration heat of the samples.e first peak is an exothermic reaction due to the hydration reaction of water and 3CaO•Al 2 O 3 and the formation of ettringite [36,37].e first peak appears within approximately 1 h at all mixes.e heat evolution rates of the first peak of O100 and O50B50 mixes were measured to be 1.2 and 0.5 J/h•g, respectively.e heat evolution rate of the first peak of B80D20 was 0.12 J/h•g, and the heat evolution rates of B81D10G9, B78D10G12, and B75D10G15 mixes were in the range of 0.075-0.08J/h•g.ese values were much lower compared to those of O100 and O50B50 mixes.
e first peak of the alkali-activated mixes with BFS, DS, and G was lower than the alkaliactivated mixes without G.
e second peak means the hydration exothermic reaction of 3CaO•SiO 2 .e hydration heat peak of O100 mix appeared between 13 and 14 h, whereas the hydration heat peak of O50B50 mix was between 18 and 19 h.However, the hydration heat peaks of the alkali-activated mixes with BFS, DS, and G ranged from 26 to 30 h, which were very late compared to those of O100 and O50B50 mixes.In particular, the hydration heat peak of the alkali-activated mix with BFS and DS was not noticeable, which means that the hydration reactivity is very low [38,39].

XRD and SEM Analysis.
Figure 6 shows the XRD patterns of O100, O50B50, B80D20, and B75D10G15 samples after 28 days.As shown in Figure 6, the main hydrates of O100 and O50B50 samples were Ca(OH) 2 and C-S-H, and alite (3CaO•SiO 2 ) and belite (2CaO•SiO 2 ) peaks were also 4 Advances in Civil Engineering observed.In O100 and O50B50 samples, the Ca(OH) 2 peak is approximately 18 °, 34 °, 47 °, 51 °, and 54 °, and the peak of the O50B50 sample is lower than that of O100 sample.e reason that the peak of the O50B50 sample is lower than the O100 sample is because the Ca(OH) 2 produced from OPC is used and decreased owing to the latent hydration of BFS. e main hydrates of the B80D20 sample were Ca(OH) 2 , graphite, CaCO 3 , and low-crystalline C-S-H, while main hydrates of B75D10G15 were found as ettringite because G is mixed in the sample.
In the O100 sample, ettringite and C-S-H were observed.In B80D20, ettringite and flat-shaped calcium hydroxide were observed.In the B75D10G15 sample, a relatively large amount of ettringite was observed compared to that of O100 and B80D20 samples.
e cumulative porosities of O100, O50B50, B80D20, B81D10G9, B78D10G12, and B75D10G15 samples are 0.064, 0.081, 0.127, 0.154, 0.160, and 0.167 ml/g, respectively.e cumulative porosity of the alkali-activated sample with BFS, DS, and G was larger than (239 to 261%) that of the O100 sample.In addition, the higher the amount of G, the higher the cumulative porosity.
e cumulative porosity of the B80D20 sample was also higher than that of the O100 sample.
Figure 8(b) shows the pore distribution characteristics of the total pores divided into three levels of 1 to 10 nm, 10 to 100 nm, and 100 to 10000 nm. e ratio of the pore size of 10 to 100 nm of the total voids was 75 to 83%.In the case of the pore size of 1 to 10 nm, the porosity of the B80D20 sample was the highest at 1.12 ml/g and the porosity of the O100 sample was the lowest at 0.36 ml/g.In the range of 10 to 100 nm pore size, the void contents of O100, O50B50, B80D20, B81D10G9, B78D10G12, and B75D10G15 samples were 2.43, 2.98, 4.34, 4.51, 4.82, and 4.84 ml/g, respectively.
e porosity of the alkali-activated systems with BFS, DS, and G was approximately 2 times higher than that of the O100 sample.
(5) e main hydrates of the B80D20 sample were Ca(OH) 2 , graphite, CaCO 3 , and low-crystalline C-S-H, while the main hydrates of B75D10G15 was found as ettringite because G is mixed in the sample.(6) e cumulative porosity of the alkali-activated sample with BFS, DS, and G was larger than (239 to 261%) that of the O100 sample.In addition, the higher the amount of G, the higher the cumulative porosity.
In addition, further studies are needed to establish the influence and relationships between the workability, strength and durability properties of alkali-activated systems, and the replacement ratio of DS and G.

Data Availability
e experimental investigation data used to support the findings of this study are available from the corresponding author upon request.

Figure 8 :
Figure 8: Porosity of samples by binder types.(a) Cumulative pore volume.(b) Distribution of pore size.
[30][31][32]n water is added to the mixture consisting of BFS and DS, OH − ions are eluted from Ca(OH) 2 contained in DS. e impermeable film around BFS is destroyed by the alkali activation of OH − ions, and hydration reaction occurs to produce hydration products such as xCaO-ySiO 2 -zH 2 O and xCaO-yAl 2 O 3 -zH 2 O[30][31][32].Figure2shows the compressive strength of alkaliactivated mixes with BFS and DS.At 28 days, the com-

Table 2 :
Sieve analysis of the sand.
2 , Fe 2 O 3 , and SO 3 and is composed of Ca(OH) 2 and 2CaO•SiO 2 as main compounds.Additionally, SiO 2 and Al 2 O 3 contents in DS are significantly lower than those in BFS, but it contains more CaO and Fe 2 O 3 than BFS.