Geopolymer Technologies for Stabilization of Basic Oxygen Furnace Slags and Sustainable Application as Construction Materials

Abstract

The basic oxygen furnace slag is a major waste by-product generated from steel-producing plants. It possesses excellent characteristics and can be used as a natural aggregate. Chemically, the basic oxygen furnace slag encloses free CaO and free MgO, which is the main reason for the expansion crisis since these free oxides of alkaline earth metals react with water to form their hydroxide yields. The objective of the present research study is to stabilize the basic oxygen furnace slag by using innovative geopolymer technology, as their matrix contains a vast quantity of free silicon, which can react with free CaO and free MgO to form stable silicate compounds resulting in the prevention of the basic oxygen furnace slag expansion predicament. Lab-scale and ready-mixed plant pilot-scale experimental findings revealed that the compressive strength of fine basic oxygen furnace slag-based geopolymer mortar can achieve a compressive strength of 30–40 MPa after 28 days, and increased compressive strength, as well as the expansion, can be controlled less than 0.5% after ASTM C151 autoclave testing. Several pilot-scale cubic meters basic oxygen furnace slag-based geopolymer concrete blocks were developed in a ready-mixed plant. The compressive strength and autoclave expansion test results demonstrated that geopolymer technology does not merely stabilize the basic oxygen furnace slag production issue totally, but also turns the slags into value-added products.

1. Introduction

The Basic Oxygen Furnace (BOF) slag is a left-behind residual waste generated from the steelmaking industry. There are 1.5 million tons of BOF slags produced annually in Taiwan [1]. This high quantity leads to environmental and ecological issues together with health concerns. This waste demonstrates excellent physical and mechanical attributes, such as far-above-the-ground hardness, a higher compressive strength, and a near-to-the-ground abrasion ratio. It is capable of substituting natural aggregates to produce structural materials, road pavements, and more. Unfortunately, the key setback of BOF slags is their expansion behavior because of their gigantic content of free lime, as a consequence, a hindrance in the context of BOF slag recycling and reuse takes place [2,3,4,5,6]. Most of the coarse BOF slags, larger than four mesh, are mixed with asphalt to produce asphalt-concrete which can be employed as road pavements. However, approximately 0.7 million tons of fine BOF slags, which is less than four mesh, are unable to be utilized effectively, especially used in the Portland cement system [7]. For that reason, the way in which a BOF slag can be reutilized as a fine aggregate has become an imperative issue.

Previous studies on BOF slags used as aggregates in concrete and their advantageous properties have been extensively reported [4,5,8,9,10,11]. However, the BOF slag contains free-CaO and free-MgO that can result in volumetric instability due to their reaction with water, yielding alkaline earth metal hydroxides. This must be addressed, employing appropriate treatment to prevent expansion. There are several ways to stabilize BOF slags, such as stabilization in the hot or molten stage [12,13,14], and stabilization by employing a carbonation process [12,15,16]. Singh et al. supplemented oxalic acid into cement mortar to improve the densification and mechanical properties of concrete [17], while Ding et al. utilized scrubbing attrition and chelating reagent treatment to remove free lime from BOF slags [5].

On the other hand, Lin et al. immersed BOF slags into the water to provide sufficient stabilization reactions [18]. According to previous research results, the adding up of silica sand in the molten stage can complete the solution and get rid of free lime crisis. This brings into question whether or not analogous reactions can occur at room temperature. If possible, they will entirely resolve the quandary of BOF slag expansion and achieve the goals of waste recycling following the principles of the Circular Economy.

Nowadays, innovative geopolymer technology is drawing the attention of researchers on account of not only its nine-times lower carbon footprint and six-times lower energy consumption as compared to the OPC (Ordinary Portland Cement) system [19], but also due to the demonstration of excellent properties by geopolymer composites, such as high initial strength, brilliant resistance to chemicals and freeze-thaw as well as thermal and fire calamities, sustainability, and strength and durability. This means simply that novel geopolymer technology can lend a hand in solving the great dilemma of global warming and the saving of natural resources of rocks and minerals. On top of that, it is cost-effective, since it can incorporate profusely accessible various wastes in the manufacture of geopolymer composites, which otherwise create problems of landfilling and pollution of the environment, soil, surface and subsurface waters besides health hazards [20,21]. Accordingly, it also extends to an organized solution of the disposal of these wastes. Inorganic geopolymers are analogous to natural zeolite minerals but possess an amorphous microstructure, they are classed as three-dimensionally networked materials synthesized through the reaction of rich aluminosilicate materials, such as precursors and alkaline solutions, as activators [22,23,24]. During the geopolymerization reaction, Si gel and Al gel were produced on the solid particle surface, and thus, they formed the Si-O-Al framework. SiO₄ and AlO₄ tetrahedra are linked to each other by sharing all O₂ atoms [25,26]. Their unique internal structure is the core reason for why geopolymer has superior chemical and physical properties such as high compressive strength, high durability, favorable structural integrity, and low permeability. These exothermic and complex geopolymer processes contain large amounts of free silicon. This free silicon can react with oxides of free lime and free magnesium in the BOF slag, and hence, the formation of stable compounds takes place, thereby inhibiting the expansion of the BOF slag.

In this original research, BOF slags, granulated blast furnace slag (GGBS) powder, coal fly ash (FA) and alkali activator solutions were employed as raw materials to manufacture BOF slag-based geopolymer mortar. The effect on compressive strength and expansion behavior from the GGBS/FA ratio and the SiO₂/Na₂O ratio in geopolymer mortar are presented in this investigation. For testing the expansion behavior of BOF slag-based geopolymer mortar, the application of the autoclave testing method as the accelerated test was made [27]. Lab-scale and ready-mixed plant pilot-scale recycling processes are also investigated in this study.

2. Assumption of the Mechanism for Stabilization BOF Slags in Geopolymer System

In general, the expansion phenomenon of the BOF slag is mainly due to the f-CaO, and f-MgO content presented. When the BOF slag makes contact with water, f-CaO will undergo a hydration reaction, causing volume expansion. The f-MgO hydration reaction is relatively slow, and about 30 days later, it will also cause volume expansion. After the BOF slag absorbs water, f-CaO will become calcium hydroxide “Ca(OH)₂”, the volume will expand by 127% [28], and f-MgO will become magnesium hydroxide “Mg(OH)₂”. The volume expansion is about 118% [29]. Therefore, this study conceived that utilizing free silicon to react f-CaO into a stable calcium silicate compound would prevent the expansion. The relevant reaction Equations (1) and (2) are as follows:

f-CaO + Si → CaSiO₃

(1)

f-MgO + Si → MgSiO₃

(2)

After the geopolymer reaction is completed, the excessively free silicon will remain in the matrix of BOF slag-based geopolymer. If other factors caused the geopolymer matrix and/or BOF slag aggregates to be broken, the f-CaO may be released again. At this time, if water infiltrates from the crack, the free silicon remaining in the geopolymer product will dissolve in the water, and then react with f-CaO to form a stable calcium silicate to prevent expansion. The reaction schematic diagram is shown in Figure 1.

3. Experimental

3.1. Materials

BOF slag aggregates with a particle size less than four mesh original fine with a D₅₀ at 0.75 mm obtained from CHC Resources Corporation were utilized. The chemical composition of fine BOF slags is depicted in

Table 1. Chemical composition of basic oxygen furnace (BOF) slag, granulated blast furnace slag (GGBS) and coal fly ash (FA).

	Composition	SiO2	CaO	Al2O3	Fe2O3	MgO	f-CaO	LOI.	Others
wt.%
BOF Slag		9.4	37.1	4.2	24.1	7.1	4.7	0.8	12.6
GGBS		27.7	57.9	11.2	0.4	–	–	–	2.8
FA		60.2	2.7	19.1	8.7	–	–	2.9	6.4

. The key composition in BOF slag was CaO and Fe₂O₃; it also contains 4.7 wt% of free lime. Additionally, GGBS and FA were obtained from CHC Resources Corporation in Taiwan. GGBS possesses a particle size between 0.6–135.7 μm with D₅₀ at 12.3 μm while those of FA range among 0.7–201.9 μm with a D₅₀ at 22.2 μm. The chemical composition of GGBS powder and FA were also represented in Table 1. Alkali activator solutions with various SiO₂/Na₂O molar ratios were prepared by mixing sodium silicate solution (9.5 wt% Na₂O, 29 wt% SiO₂) and 6M sodium hydroxide. The molar ratio for SiO₂/Al₂O₃ was kept at 50 and was controlled by sodium aluminate. The alkali solutions were prepared and ready to use the day before the experiment.

3.2. Methods

GGBS and FA were mixed at the designed weight ratio with an additional 3 wt% of wollastonite added during blending. Subsequent to pre-mixing for 3 min, the mixture was then activated with alkali solutions. After thorough mixing for 3 min, the fine BOF slags were then added into geopolymer paste for an additional 3 min blending to prepare geopolymer mortar with various geopolymer: BOF slag ratios. The geopolymer mortar was then cast into cylindrical molds of size (Φ50 × 100 mm). Following 24 h of de-molding, the samples were then cured at room temperature until certain days for testing. In order to measure the volume stability, all the BOF slag-based geopolymer mortar samples were tested using the autoclave testing method according to ASTM C151 standard.

Before and after autoclave testing, the length, diameter, and volume change rate of the cylindrical specimen were calculated by dividing the circle portion of the cylindrical specimen into three sections with the center point of the circle. A Vernier caliper was used for measuring the length varieties of the cylindrical sample. For diameter measurements, an arbitrarily selected marked two points at the middle of the height of the cylindrical test body was measured and another two points were made at a position rotated by 90°, which were also measured. The change in total volume is calculated from the above results.

4. Results and Discussion

4.1. BOF Slags Expansion Behavior and Powdering Rate

The powdering rate measurement was based on GB/T 24175-2009 (Test method for stability of steel slag). Before collecting an 800 g oversize part to put into an autoclave, the BOF slags were screened using a No. 4 sieve. The autoclave maintained the pressure at 20.8 ± 0.7 kgf/cm², temperature 215.7 ± 1.7 °C for 3 h. Subsequently after drying and sieving again by employing the No. 4 sieve, the powdering rates were calculated as illustrated in Equation (3).

Table 2. Powdering rate for different size range BOF slags after autoclave test.

Particle Size Range	Powdering Rate (%)
3/8 inch–#4 mesh	29.9
3/4 inch–3/8 inch	27.8
>3/4 inch	17.4

demonstrates the results of powdering rates for various size ranges of BOF slags after autoclave tests. The powdering rate increased from 17.4% to 29.9% with decreasing particle size ranges from >3/4 inch–3/8 inch–No. 4 mesh. This implies that the finer particle size range of BOF slags contained more free lime.

$$\mathrm{Powdering}\mathrm{Rate}=\frac{ Weight of less than 4 mesh part}{Total Sample weight}\times 100\%$$

(3)

In order to compare the performance of BOF slags (aggregate) in the Portland cement system and geopolymer system (binder), the binder/aggregate weight ratio was kept at 1:2.75 for the BOF slag expansion test. The expansion behavior of the BOF slag in the Portland cement system and geopolymer technology is presented in

Table 3. BOF slag expansion behavior in Portland cement system and geopolymer system after autoclave test.

BOF Slag in Different System		Before Autoclave Test	After Autoclave Test
Portland Cement System
Geopolymer System
	Ave height change	0.18%
	Ave diameter change	0.07%
	Ave volume change	0.35%

. Based on the experiment results, it is seen that in the case of the BOF slag in the Portland cement system, the sample totally collapsed after the autoclave test due to the steam, which accelerated the reaction of free lime, thus causing expansion. Nevertheless, in the case of the BOF slag in the geopolymer system, the sample still maintained its integrity after the autoclave test as displayed in Table 3. The average height, diameter, and volume changes recorded are 0.18%, 0.07%, and 0.35%, respectively. After the autoclave test, the samples were crushed, grinded and analyzed by XRD (X-ray Diffraction) (Figure 2). The mineral phase of calcium silicate was found, indicating that the geopolymer technique can stabilize the untreated BOF slag fines. This is ascribed to the enormous quantities of free silicon present in the geopolymer matrix. This free silicon reacted with free lime or free-MgO on the BOF slag surface to form a stable compound. When the BOF slag-based geopolymer is subjected to an external force and cracks crop up, the moisture will enter into the BOF slags. The free silicon in the geopolymer matrix will be dissolved and brought into internal BOF slag to react with free-CaO or free-MgO to form stable calcium silicate or magnesium silicate. This reaction can effectively inhibit the expansion of the BOF slags.

4.2. Effect of SiO₂/Na₂O Molar Ratio on the Properties of BOF Slag-Based Geopolymer Mortar

In order to understand the effect of the SiO₂/Na₂O molar ratio on the features depending on the BOF slag-based geopolymer mortar, the experimental parameters were adjusted, and the experimental results exhibit that the compressive strength depending on the SiO₂/Na₂O ratio of the BOF slag-based geopolymer mortar is augmented as the ratio of SiO₂/Na₂O increases, as portrayed in Figure 3. Both tend to enhance the compressive strength as the curing time increases. When the ratio of SiO₂/Na₂O was between 1.28 and 1.5, the compressive strength at the age of 56 days was about 32–38 MPa. Likewise, when the ratio of SiO₂/Na₂O was 1.6, the compressive strength achieved 53 MPa on the day 56. However, only the decrease of compressive strength after 56 days for SiO₂/Na₂O = 1.4 was found. The reason for this is large amounts of micropores generated in the structure of the geopolymer that hinder the development of compressive strength, and no such phenomenon is found in a ratio higher or lower than the SiO₂/Na₂O ratio of 1.4 [30]. Moreover, it is found that most of the specimens subjected to the autoclave expansion treatment according to the ASTM C151 standard possess a tendency to increase the compressive strength, and the strength on day 56 can obtain 40–55 MPa as illustrated in Figure 3b. A careful observation of the surface of the specimen after the autoclave test revealed that it still has a small amount of surface peeling, which may be due to the fact that the BOF slag particles on the surface of the specimen cannot be entirely covered by the geopolymeric slurry, or maybe because of the highly thin coating. Therefore, there is still a small part of the reaction expansion phenomenon. However, due to its substantial increase in compressive strength, it indicates that the overall performance of the test body subsequent to the autoclave expansion test is still very stable.

The effect of the SiO₂/Na₂O molar ratio on the curing age of the BOF slag-based geopolymer mortar was subjected to an autoclave expansion test specimen, and its linear expansion, diameter expansion, and bulk expansion characteristics were analyzed. The results are shown in Figure 4. As illustrated in Figure 4, the volume expansion ratio of each of the samples was 0.4% or less, of which, the expansion ratio of SiO₂/Na₂O = 1.4 and 1.5 is the lowest and is only 0.1% or less. Furthermore, the linear expansion ratio and the diameter expansion ratio were analyzed ahead, and the linear expansion ratio was mostly less than 0.1%, and the diameter expansion ratio was also 0.15% or less. This shows that the untreated BOF slag has high stability under the geopolymer system. According to the results of this experiment, the subsequent selection of the BOF slag-based geopolymer mortar was carried out with a ratio of SiO₂/Na₂O of 1.5.

4.3. Effect of GGBS/FA Ratio on the Properties of BOF Slag-Based Geopolymer Mortar

The results of the compressive strength of the BOF slag-based geopolymer mortar, with varying proportions of the GGBS and FA, are shown in Figure 5. The compressive strength increased with increased GGBS content (Figure 5a). This could be due to the fact that the structure of geopolymer mortar is denser and more increased with curing age. The BOF slag-based geopolymer mortar of different powder ratios was subjected to a compressive strength test after an autoclave treatment, and the results are shown in Figure 5b. According to the experimental results, it can be found that although the surface of the test piece has a small part of peeling after the autoclave test, the compressive strength is still significantly increased. This means that the test specimen is still very stable after the autoclave expansion test. The higher the content of fly ash, the significantly higher the strength of the sample after the autoclave test. This may be due to the high silicon content of fly ash in the system to suppress the BOF slag expansion [31].

The autoclave expansion test, ASTM C151, was carried out to understand the stability and volume expansion rate of BOF slag-based geopolymer mortar samples, with different proportions of GGBS:Fly ash, shown in

Table 4. The effect of GGBS/FA Ratio on the expansion ratio of BOF slag-based geopolymer mortar after autoclave test.

GGBS: FA	Diameter Expansion (%)			Length Expansion (%)			Volume Expansion (%)
	7d	28d	56d	7d	28d	56d	7d	28d	56d
6:4	0.18	0.18	0.18	0.24	0.22	0.15	0.62	0.59	0.53
5:5	0.03	0.02	0.02	0.01	0.02	0.03	0.07	0.05	0.10
3:7	0.24	0.21	0.26	0.16	0.19	0.19	0.66	0.63	0.71

. Table 4 highlights that in the BOF slag-based geopolymer mortar sample, the excessive GGBS or the excessively high FA content has a high expansion rate. Moreover, with a ratio of GGBS:FA = 6:4, the volume expansion rate is 0.53% after 56 days of curing, but the GGBS:FA = 3:7, after 56 days of curing, has an expansion rate as high as 0.71%. Only between the GGBS:FA = 5:5, an expansion rate of less than 0.1% exists. The linear expansion ratio and the diameter expansion ratio were less than 0.3%.

Similarly, in the ratio of GGBS:FA = 5:5, both the diameter and linear expansion rate were less than 0.03%. Although the FA in the system can provide more silicon to inhibit the expansion of the BOF slag, the powder itself has large shrinkage and insufficient strength, thus cannot add too much. On the other hand, in the sample with higher GGBS content, although the content of silicon—which can react with free lime—is reduced, it is still sufficient to inhibit the expansion of the BOF slag due to its own strength, and its volume expansion is still 0.53%. In the GGBS:FA = 5:5, the strength provided by the GGBS in the system and the FA that can react to inhibit free calcium reached the optimum amount, and consequently the expansion rate and age are the best, <0.1% for 7–56 days curing.

4.4. Laboratory Horizontal Double Shaft Mixer Tests

This phase of the test is mainly to simulate the large-scale test of the actual plant. GGBS and FA (5:5) are used as source materials. The alkaline liquid has NaOH concentration of 6M, the SiO₂/Na₂O molar ratio is 1.5, the SiO₂/Al₂O₃, as well as molar ratio, is 50. The moisture content of the BOF slag is controlled at 10%, where the BOF slag is not pretreated before the tests. To simulate the actual factory test, this experiment prepared a BOF slag-based geopolymer mortar with a horizontal biaxial mixing machine and poured a test specimen of Φ10 cm×20 cm for the compression test and the autoclave expansion test. The mixture proportion and process are shown in

Table 5. Simulation the large-scale experiment for BOF slag-based geopolymer mortar.

Mix No.	Source Materials		Binder: Aggregate	Wollastonite	L/S	BOF Slags Water Content
	FA	GGBS
GC-BOF10%	5	5	1:2.75	5%	0.49	10%

.

Table 6. Hardening time result of simulation of large-scale experimental for BOF slag-based geopolymer mortar.

Mix No.	Setting Time
	Initial	Final
GC-BOF10%	3 h 15 min	8 h 40 min

shows the hardening time of the BOF slag-based geopolymer mortar. According to the hardening time test results, the initial setting time is about 3 h; the final setting time is about 9 h.

The compressive strength of the simulated large-scale experimental BOF slag-based geopolymer mortar is shown in Figure 6. As the curing age increases, the strength of the test body increases. At seven days of age, the strength reached about 27 MPa; at 28 days of age, the strength reached 40 MPa. It is said that the simulation of the large-scale experimental of BOF slag-based geopolymer mortar has excellent strength performance.

The autoclave expansion test of the simulated large-scale experimental BOF slag-based geopolymer mortar is shown in

Table 7. Autoclave expansion test (seven days) results of simulating the large-scale of the experimental BOF slag-based geopolymer mortar.

Test Specimen GC-BOF10%	Before Autoclave Test	After Autoclave Test	Volume Changed
			0.27%
Compressive Strength (MPa)	28.4	35.6

. After the autoclave expansion test, the BOF slag-based geopolymer mortar sample is complete, and only a slight surface is peeling. The compressive strength test before and after autoclave found that the surface peeling did not affect the stability of the test specimen.

4.5. BOF Slag-Based Geopolymer Mortar Tests in Ready-Mixed Plant

4.5.1. Ready-Mixed Plant Small Scale Tests

The mixed proportion for Ready-mixed plant small scale test is shown in

Table 8. Mixture proportion of ready-mixed plant small-scale test.

NO.	L/S	Binder : Aggregate	Mixture Proportion (kg)				Total Weight (kg)
			GGBS	FA	BOF (15% water)	Alkali Solution
Test-1	0.41	1:2.90	15.12	15.12	100.85 (87.70)	12.50	143.59
Test-2	0.38	1:1.98	19.70	19.70	90.00 (78.26)	15.00	144.40

. The main difference between the two tests is the ratio of binder and aggregate, which is the additional amount of the BOF slags.

The fresh properties of the ready-mixed plant small-scale test mixture are shown in

Table 9. Fresh properties mixture after ready-mixed plant small scale test.

NO.	Initial		After 45 Min
Test-1	Slump	Slump flow	Slump	Slump flow
	265 mm	540*580 mm	250 mm	520*520 mm
Test-2	Initial		After 45 min
	Slump	Slump flow	Slump	Slump flow
	260 mm	470*480 mm	250 mm	440*440 mm

. According to the results, it is found that the BOF slags of the high-water content of BOF slag-based geopolymer mortar showed excellent workability. The initial slump flow of Test-1 is 540*580 mm, and it is allowed to stand for 45 min after, and the slump flow is reduced to only 520*520 mm. Test-2′s initial slump flow is 470*480 mm, and after allowing it to stand for 45 min, its slump flow is reduced to 440*440 mm. The results of the two tests were found to have low slump flow loss performance.

The compressive strength of the ready-mixed plant small scale tests is shown in

Table 10. Compressive strength of concerning plant small scale test.

NO.	Test Time	Compressive Strength (MPa)
		1d	Ave	3d	Ave	7d	Ave	28d	Ave	After Autoclave Test
Test-1	0 min	13.2	12.9	20.4	20.5	22.9	24.5	32.9	33.7	43.8
	45 min	12.7		20.5		26.1		34.5
Test-2	0 min	12.1	12.2	26.8	28.6	39.7	38.8	49.3	48.1	38.8
	45 min	12.3		30.4		38.0		47.0

and Figure 7. As the curing age increases, the strength of the test body increases. The strength of the late Test-2 is higher than that of Test-1 because the strength source of the geopolymer system is from the geopolymer slurry rather than an aggregate. Therefore, in Test-2, where the BOF fine aggregate is relatively low, the compressive strength will be higher than Test-1. However, after the autoclave test, the compressive strength decreases in Test-2. The reason for this is due to water release and formed cracks.

4.5.2. Ready-Mixed Plant Pilot-Scale Tests

Ready-mixed plant pilot-scale test parameters are shown in

Table 11. Mixture proportion of ready-mixed plant pilot-scale test.

NO.	FA : GGBS	L/S	Binder : Aggregate	Mixture Proportion (kg)						Total Volume and Weight
				Alkali Solution	GGBS	FA	BOF Slag (Wet)	BOF Slag Water Content	Water Added
Test-3	5:5	0.50	1:2.936	374	375	375	2,400	9.0%	20.0	1.5 m3 3544 kg
Test-4	6:4	0.52	1:3.575	349	267	400	2,560	7.4%	60.0	1.5 m3 3636 kg

. The ratios of FA and GGBS are 5:5 and 6:4, the ratios of binder and aggregate are 1:2.936 and 1:3.575. The total test volume is 1.5 cubic meters, and the total weight in each test is approximately 3.5–3.6 tons.

The fresh properties of the ready-mixed plant pilot-scale tests are shown in

Table 12. Fresh properties of ready mixed plant pilot-scale tests.

NO.	FA:GGBS	Slump	Slump Flow
Test-3	5:5
		260 mm	380*390 mm
Test-4	6:4
		270 mm	510*490 mm

. The test number of Test-3 and Test-4 have a slump flow of 380*390 mm and 510*490 mm, respectively. The reason for this difference is the increase in the use of FA in Test-4 samples, which is spherical in shape and contributes to its fluidity [32,33].

The compressive strength and their autoclave test of ready-mixed plant pilot-scale experiments are shown in

Table 13. Compressive strength and autoclave test of ready-mixed plant pilot-scale tests.

NO.	FA : GGBS	Compressive Strength (MPa)								Autoclave Test
		1d	Ave	3d	Ave	7d	Ave	28d	Ave	28d
Test-3	5:5	20.3	20.9	33.9	33.4	37.2	37.5	39.7	41.6	45.4
		21.5		32.8		37.9		43.4
Test-4	6:4	13.8	13.9	24.9	25.4	31.6	31.4	39.7	39.9	31.7
		14.1		26.0		31.1		40.4

and Figure 8. As the curing age increases, the strength of the test body increases. The compressive strength of Test-3 is slightly higher than that of Test-4. The main reason is the adjustment of the ratio of FA to GGBS. Test-4 is higher in the amount of FA, and the reactivity of FA itself is poorer than that of GGBS powder which causes its intensity to be slightly lower than Test-3. According to the results of the autoclave expansion test, as the curing age increases, the structure of the test body is more complete, and the test specimen has no break point after the autoclave expansion test. The expansion changes after the autoclave test for the ready-mixed plant pilot-scale tests are shown in

Table 14. Expansion changes after autoclave test for ready-mixed plant pilot-scale tests (curing time 28 days).

NO.	Expansion Ratio After Autoclave Test (%)
	Diameter Change	Length Change
Test-3	−0.41	−0.41
Test-4	−0.41	−0.41

. All the expansion test results can be controlled around −0.41% for diameter and liner changes.

5. Conclusions

The present study and its findings were piloted, and conclude that:

• Reduction concerning CO₂ emissions and consumption of copiously available BOF slag wastes is possible by employing geopolymer technology. Not merely that, it stabilizes the BOF slag production absolutely by turning them into valuable products.
• Stabilization of the BOF slag through geopolymer technology is successfully determined, since their matrix encompasses a large amount of free silicon (Si) which can react with free CaO and free MgO to form stable silicate compounds leading to the addressing of the quandary of BOF slag expansion.
• Lab-scale and ready-mixed plant pilot-scale experimental upshots unveiled that the compressive strength of fine BOF slag-based geopolymer mortar achieved the compressive strength of 30–40 MPa after 28 days, and increased compressive strength. The expansion can be controlled also, less than 0.5% after ASTM C151 autoclave testing.
• A systematic solution for the disposal of the waste accumulation of BOF slags is extended through this novel geopolymer technology since its incorporation is possible in manufacturing, user and eco-friendly green geopolymer composites, otherwise filling land spaces and causing the contamination of environments, ecology, soils, surface and subsurface waters as well as health hazards.