The Influence of the Assembly Unit of CO2-Cured Secondary Aluminum Ash and CO2-Cured Iron Tailings on High Performance Concrete’s Properties

Abstract

This paper aims to study the influence of the assembly units of CO2-cured iron tailings (IOT) and CO2-cured secondary aluminum ash (SAA) on the fresh high-performance concrete’s (HPC’s) slump flow and setting time. The mechanical properties including the flexural strength, compressive strength, the bonding strength and the dry shrinkage rate of the hardened HPC are measured. The amount of leached Cr and Zn after immersing in deionized water for 1 month~6 months is measured. The influence of the basalt fibers’ volume ratio and the aspect ratio of the high-performance concrete’s performance is considered. The scanning electron microscopy energy spectrums (SEM-EDS) are obtained. The results show that the slump flow and the setting time of fresh HPC are increased by the added CO2-cured SAA and IOT. The fresh HPC with 10% CO2-cured IOT and 20% CO2-cured SAA had the highest slump flow. The slump flow decreases in the form of cubic function with the placing time. The mechanical strengths and the dry shrinkage rate of HPC during the early curing ages (cured for 0.5 day~7 days) are decreased by the CO2-cured SAA and CO2-cured IOT, while the mechanical strengths at later curing ages (14 days~90 days) are increased by the added CO2-cured SAA and CO2-cured IOT. HPC with 10% CO2-cured SAA and 20% CO2-cured IOT shows the highest mechanical strengths. The amount of leached Cr and Zn is decreased by the CO₂ cured SAA and IOT. The relationship between the mechanical strengths and the curing time coincides with the cubic equation. The basalt fibers with a volume ratio of 2% and aspect ratio of 1000 show the highest mechanical strengths, the lowest dry shrinkage rate and the least amount of leached Cr and Zn. CO2-cured SAA and IOT can improve the compactness of HPC’s hydration products. HPC with 10% CO2-cured SAA and 20% CO2-cured IOT shows the highest compact hydration products.

1. Introduction

The application of solid waste in the preparation of cement-based materials is a hot research topic in today’s society [1,2]. However, many drawbacks exist when the single-solid waste material is used for manufacturing the cement-based materials [3,4]. Iron tailing is a type of industrial solid waste generated during the separation of valuable metals in iron ore [5]. Due to the rapid development of the steel industry and the increasing demand in the construction industry, the amount of iron tailings piled up is gradually increasing, and methods to recycle and reuse them have become a serious problem [6,7]. The use of iron tailings as a substitute for some aggregates in cement concrete has been widely investigated [8]. Researchers have found that the addition of iron tailings can decrease the cement concrete’s fluidity [9]. However, the corresponding mechanical strengths and durability are increased by the added iron tailings. Aluminum ash is an industrial waste generated during the electrolysis or melting of aluminum [10]. Researchers have used the aluminum ash for manufacturing cement-based materials and have found that aluminum ash with appropriate dosage can improve the mechanical strengths of cement concrete during the early curing ages (not exceeding 7 days) [11]. However, the corresponding cement concrete’s mechanical strengths at late curing ages are decreased by the added aluminum ash.

The reactive powder concrete is a high-performance concrete material with high strength and durability [12]. The preparation cost of ordinary high-performance concrete is high, so the goal of reducing high-performance concrete can be achieved by reducing the cost of raw materials [13]. The waste fly ash, the secondary aluminum dross and the electrolytic manganese residue have been reported to be applied in the high-performance concrete [14]. These materials at suitable dosages can increase the high-performance concrete’s flexural and compressive strengths at rates of 11.1%~26.7% and 13.4%~31.8% [15,16]. Moreover, the appropriate dosages of the solid waste ash can improve the durability of concrete. Although some solid waste ashes have been applied to concrete materials, little research has been performed on the use of assembly units of several types of waste ashes for manufacturing cement concrete.

CO₂ is a kind of industrial waste gas. Excessive CO₂ emissions can easily cause harm like environmental warming. Several methods have been applied in dealing with excess CO₂ emissions. The use of CO₂ in building materials can consume some excess CO₂ emissions while improving the performance of concrete [17]. Researchers have found that CO₂ curing of secondary aluminum ash, manganese slag and waste fly ash can increase the cement concrete’s fluidity, mechanical strengths and durability. The assembly units of CO2-cured secondary aluminum ash and iron tailings may be advantageous to the mechanical properties of high-performance concrete [18]. However, little attention has been paid to this.

Carbon fibers, carbon nanotubes and steel fibers are commonly used to enhance the strength of high-performance concrete. However, the cost of carbon fibers and carbon nanotubes are extremely high [19]. The steel fibers are easy to corrode, especially when the high-performance concrete is prepared in extreme environments [20]. Due to these reasons, basalt fibers are chosen as reinforced fibers in high-performance concrete. The fibers’ volume ratio and the aspect ratio may be significant to high-performance concrete’s various macro-performance indicators [21]. However, few researching achievements about these have been reported.

In this study, the fresh high-performance concrete’s (HPC’s) slump flow and the setting time are measured. The effect of the assembly units of CO2-cured iron tailings (IOT) and CO2-cured secondary aluminum ash (SAA) is considered. The mechanical performances, the dry shrinkage rate and the amount of leached Cr and Zn are measured. The electron microscopy energy spectrum analysis is obtained to analyze the mechanism of performance changes by adding the CO2-cured IOT and CO2-cured SAA. The innovation of this article is the use of mixed fibers to improve the various properties of HPC. Moreover, the CO2-cured IOT and the CO2-cured SAA applied to increase the HPC’s mechanical strengths and the leakage of toxic substances are another innovation. The assembly units of the CO2-cured IOT and the CO2-cured SAA used for obtaining HPC with excellent properties are the third research highlight. The research will provide new methods for reusing the solid waste.

2. Materials and Methods

2.1. Raw Materials

Binding materials including the ordinary Portland cement, silica fume (SF), iron tailings powder (ITP) and secondary aluminum ash (SAA) are used in this study. The quartz sand with a fineness modulus of 2.67 is used as aggregates. The iron tailings powder and the secondary aluminum ash are provided by the Yantai Hexin Environmental Protection Equipment Co., Ltd., Yantai, China. The ordinary Portland cement, the silica fume and the river sand are purchased from the Hebei Huishun Mining Co., Ltd., Shijiazhuang, China. Weifang Chengqi Building Materials Co., Ltd., Weifang, China provided the efficient polycarboxylate superplasticizer for this study. The densities of the ordinary Portland cement, silica fume, iron tailings powder and secondary aluminum ash are 3.0 g/cm³, 2.2 g/cm³, 1.4 g/cm³ and 1.21 g/cm³. The quartz sand shows a density of 2.65 g/cm³.

Table 1. The particle size distribution of the raw materials (%).

Types	Particle Size/μm
	0.3	0.6	1	4	8	64	360
ITP	0.28	0.45	2.48	16.37	34.12	97.11	100
P·O cement	0.13	0.31	2.59	15.07	28.64	93.31	100
SAA	0.04	0.38	0.62	1.13	3.92	25.85	100
SF	31.24	58.38	82.24	99.8	99.8	99.8	100
Quartz sand	0	0	0	0	0.06	20.98	100

and

Table 2. The chemical composition of the raw materials (%).

Types	SiO2	Al2O3	FexOy	MgO	CaO	SO3	K2O	Na2O	Ti2O	CdO	Cr2O3	PbO	CuO	ZnO	Loss on Ignition
ITP	28.34	4.88	13.89	-	19.56	7.54	1.02	1.46	1.01	0.07	0.09	0.09	0.09	0.5	21.46
P·O cement	21.76	6.59	4.79	2.8	61.27	2.77	-	-	-	-	-	-	-	-	3.12
SAA	5.55	79.51	4.86	6.64	2.49	-	-	0.95	-	-	-	-	-	-	-
SF	90.81	0.21	0.63	0.24	0.43	0.2	7.49	-	-	-	-	-	-	-	-
Quartz sand	99.6	-	0.4	-	-	-	-	-	-	-	-	-	-	-	-

show the chemical composition and cumulative passing rates of the raw materials. Basalt fibers with a density of 2.61 g/cm³ and an average tensile strength of 4530 MPa are used in this study. The basalt fibers’ average lengths are 6 cm, 12 cm, 18 cm and 25 cm. The average diameters of the basalt fibers are 13 um, 15 um and 18 um. The physical photos of the secondary aluminum ash and the iron tailings powder are shown in Figure 1. The particle size distribution and the chemical composition of the raw materials provided by the manufacturers are shown in Table 1 and Table 2.

Based on the principles of composite materials mechanics [22], the mechanical strengths of the HPC are dominated by the mechanical strengths of the HPC matrix and the basalt fibers. The length and the diameter of the basalt fibers can affect the basalt fibers’ bonding stress between the basalt fiber and the matrix. Moreover, the tensile stress value is decided by the bottom area of basalt fibers. According to prior studies [23,24], the aspect ratio of basalt fibers can affect the basalt fibers’ dispersion and the ratio of basalt fibers’ adhesive force to the corresponding tensile force and thus show significant effect on the HPC’s cracking resistance. Therefore, it is necessary to study the influence of aspect ratio on HPC’s performance.

In this study, the aspect ratio (AR) of basalt fibers represents the ratio of basalt fibers’ average length (L) to the corresponding average diameter (d). The aspect ratio of basalt fibers can be calculated using Equation (1).

$$AR={\displaystyle \frac{L}{d}}$$

(1)

2.2. The Manufacturing Process of the Specimens

The specimens are prepared via the following steps. The binder materials and the river sand are added to the JJ-20H planetary cement mortar mixer provided by Hebei Xingsheng Instrument Equipment Co., Ltd., Cangzhou, China and mixed at a mixing speed of 140 ± 1 r/min for 1 min. Then, a solution of water and superplasticizer is added to the mixture and mixed for another 3 min. Finally, the fresh high-performance concrete is used for the measurement of slump flow and the initial setting time. Moreover, the residual fresh high-performance concrete is poured into the molds for the manufacturing of test specimens. The mixing proportions of the HPC is shown in

Table 3. The HPC’s mixing proportions (kg/m³).

Group	Water	P·O Cement	CO2-Cured Iron Tailings Powder	CO2-Cured Secondary Aluminum Dross	SF	Quartz Sand	Water-Reducer
A1	243.8	1108	0 (0%)	0 (0%)	111.1	1219.1	18.1
A2	243.8	1108	0 (0%)	122.2 (10%)	111.1	1219.1	18.1
A3	243.8	1108	0 (0%)	244.4 (20%)	111.1	1219.1	18.1
A4	243.8	1108	0 (0%)	366.6 (30%)	111.1	1219.1	18.1
A5	243.8	985.8	122.2 (10%)	0 (0%)	111.1	1219.1	18.1
A6	243.8	985.8	122.2 (10%)	122.2 (10%)	111.1	1219.1	18.1
A7	243.8	985.8	122.2 (10%)	244.4 (20%)	111.1	1219.1	18.1
A8	243.8	863.6	244.4 (20%)	0 (0%)	111.1	1219.1	18.1
A9	243.8	863.6	244.4 (20%)	122.2 (10%)	111.1	1219.1	18.1
A10	243.8	863.6	366.6 (30%)	0 (0%)	111.1	1219.1	18.1

.

2.3. Experimental Methodology

2.3.1. The Slump Flow and the Setting Time

The fresh HPC’s slump flow and the initial setting time are measured using an NLD-3 cement mortar flowability tester provided by Cangzhou Huawang Construction Equipment Co., Ltd., Cangzhou, China and a ZKS-100 mortar setting time tester provided by Xi’an Qinling Tiancheng Intelligent Technology Co., Ltd., Xi’an, China. NLD3 cement mortar flowability tester is used for measuring the slump flow of fresh HPC paste. Firstly, the fresh HPC paste is evenly added to half of the flowability tester’s height and compact 15 times. Then, the remaining part is filled with fresh HPC paste and the equivalent insert, and compaction is provided to the fresh HPC paste. After that, the NLD-3 electric jumping table is started and vibrated for 25 times. Finally, the slump flow is determined according to the Chinese standard GB/T 2419-2005 [25]. Luteng brand mortar setting time tester offered by the Xian County Luteng Highway Instrument Co., Ltd., Xi’san, China is used for measuring fresh HPC’s setting time according to Chinese standard GB/T1346-2001 [26].

2.3.2. The Mechanical Strengths

Specimens with size of 40 mm × 40 mm × 160 mm are used for the determinations of flexural and compressive strengths. Firstly, specimens are moved to the flexural strength fixture, then the flexural load with a 0.05 kN/s loading rate is exerted on the specimens until the specimens are destroyed. The specimens broken via flexural load are used for the measurement of compressive strengths. The specimens are moved to the compressive strength fixture, then the loading is applied to the specimens with a compressive loading rates of 2.4 kN/s until the specimens are destroyed. The experiment is conducted according to the Chinese standard GB/T17671-2021 [27].

2.3.3. The Dry Shrinkage Rate

The LVG20 Split LVDT displacement sensor is used for measuring the dry shrinkage rate. Firstly, the LVG20 Split LVDT displacement sensor is placed on both ends of the specimens. Then, the change in length of the specimens is tested using a displacement sensor during curing. The dry shrinkage rate (DR) can be calculated using Equation (2), where ∆L is the change in length of the specimens, and L₀ is the specimens’ initial length.

$$DR={\displaystyle \frac{\Delta L}{L_0}}$$

(2)

2.3.4. The Measurement of the Toxic Heavy Metals

The specimens are immersed in deionized water for different amounts of time after the specimens are cured in a standard environment for 28 days. The liquid is removed from the solution and used for the measurement of heavy metal ions with an inductively-coupled plasma atomic emission spectrometer manufactured by Shandong Lanjing Electronic Technology Co., Ltd., Weifang, China.

2.3.5. The Microscopic Experiments

For the scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) experiment, a sample with a flat, rice-sized area is taken. A Nova Nano SEM scanning electron microscope offered by Taijiu Technology Co., Ltd., Suzhou, China, with a resolution of 3.5 nm, an acceleration voltage of 500 V to 30,000 V, a magnification of 18–30,000, a sample stage diameter of 30 mm and a Brooke Quantum EDS spectrometer for micro analysis are used to observe the microstructure and determine the elements in the concrete. Figure 2 shows the sample preparation and testing flowchart. The specimens’ photos of each group are shown in Figure 3.

3. Results and Discussions

3.1. The Slump Flow and Setting Time

The slump flow of fresh HPC is shown in Figure 4. The fitting function of slump flow with placing time is shown in

Table 4. The fitting function of slump flow with placing time.

Equation	Types	a	b	c	d	R2
	A1	206.86	−1.06	0.009	−9.880 × 10−5	0.997
	A2	213.97	−0.53	0.001	−4.94 × 10−5	0.999
	A3	223.08	−0.38	−0.004	4.94 × 10−5	0.995
	A4	256.79	−0.22	−0.22	−2.22 × 10−4	0.993
S = a + bt + ct2+ dt3	A5	220.73	−0.81	−0.008	−7.41 × 10−5	0.975
	A6	235.84	−1.08	0.009	−2.47 × 10−5	0.992
	A7	248.01	−0.48	−0.009	7.41 × 10−5	0.999
	A8	217.83	−0.96	9.52	−4.68 × 10−19	0.995
	A9	221.81	−0.37	−0.007	2.47 × 10−5	0.992
	A10	220.07	−0.20	−0.005	−2.47 × 10−5	0.998

. As obtained from Figure 4 and Table 4, the slump flow decreases in the form of cubic function with the placing time. This is attributed to the fact that the free water evaporates with placing time, leading to the decreasing fresh HPC’s fluidity [28]. Additionally, the CO2 curing of the SAA and IOT shows positive effect on the HPC’s slump flow. This can be explained by the fact that the CO2 curing of the SAA and IOT can reduce the number of pores on the surface of the SAA and IOT [29]; therefore, the absorbed free water is decreased by the CO₂ curing. Consequently, the slump flow is increased by the increasing dosages of CO2-cured SAA and IOT. The fresh HPC with 10% CO2-cured IOT and 20% CO2-cured SAA shows the highest fluidity. The CO2 curing of the IOT and SAA increases the slump flow by rates of 12.3%~26.3%. It can be observed from Table 4 that the fitting degrees of Figure 4 are higher than 0.97, confirming the fitting rationality of the functions.

Figure 5 shows the initial and final setting time of the fresh HPC. The fresh HPC’s initial and final time increased with the increased dosages of the CO2-cured SAA and IOT. This is attributed to the fact that the CO2 curing of SAA and IOT can decrease the alkaline substances on the surface of SAA and IOT, leading to the reduced cement’s early alkaline activity of cement, resulting in an increase in setting time [30]. Fresh HPC with 30% CO2-cured SAA shows the highest setting time. The error bars of the setting time is lower than the 10% of the setting time, indicating the accuracy of the experimental data.

3.2. The Mechanical Strengths

The flexural and compressive strengths of the HPC are exhibited in Figure 6a,b. The flexural and compressive strengths increase in the form of cubic function with the increased curing ages. This can be attributed to the increased hydration degree of HPC caused by the increased curing age [31,32]. Meanwhile, as observed from Figure 6a,b, when the curing age is lower than 14 days, the flexural and compressive strengths are decreased by the increased dosages of CO2-cured IOT and CO2-cured SAA. This can be attributed to the fact that the CO2 curing of IOT and SAA can decrease the activity of aluminum, etc., thus reducing the early hydration degree of cement and leading to decreasing mechanical strengths during the early curing age [33]. However, when the curing ages are in the range of 28 days~90 days, the mechanical strengths are improved by the CO2 curing of IOT and SAA. This is ascribed to the fact that the CO2 curing of IOT and SAA can decrease the early hydration activity, therefore, explaining the cracks generated in the early stage of hydration [34]. Consequently, when the curing age is higher than 7 days, the mechanical strengths are increased by the added CO2-cured IOT and CO2-cured SAA.

The bonding strength of the HPC after repairing the damaged HPC is shown in Figure 6c. As illustrated in Figure 6c, the bonding strength is increased with increasing curing ages. When the curing age is lower than 14 days, the CO2 curing of IOT and SAA shows a decreasing effect on the HPC’s bonding strength, which is due to a decreased hydration degree [35,36]. However, when the curing age reaches 14 days, the bonding strength is increased by the CO2-cured IOT and CO2-cured SAA. HPC with 20% CO2-cured IOT and 10% CO2-cured SAA shows the highest bonding strength. CO2 curing of IOT and SAA can improve the HPC’s flexural, compressive and bonding strengths by rates of 10.6%~21.3%, 11.1%~24.6% and 8.9%~23.2%.

To research the influence of basalt fibers on the HPC’s performance, 10% CO2-cured IOT and 20% CO2-cured SAA are added to the blank HPC. The flexural, compressive and bonding strengths of the HPC with different volume ratio of basalt fibers are shown in Figure 7. The fitting function of mechanical strengths with basalt fibers’ volume is shown in

Table 5. The fitting function of mechanical strengths with basalt fibers’ volume.

Equation	Types	a	b	c	d	R2
ft = a + bV + cV2+ dV3	0.5 d	17.55	−0.33	1.24	−0.35	0.959
	1 d	7.82	−0.54	1.30	−0.37	0.950
	3 d	11.70	−0.04	0.95	−0.29	0.999
	7 d	13.63	−0.71	1.60	−0.43	0.983
	28 d	17.55	−0.33	1.24	−0.35	0.935
	60 d	18.14	−0.75	1.83	−0.52	0.932
	90 d	19.58	−0.13	2.35	−0.76	0.996
fcu = a + bV + cV2+ dV3	0.5 d	60.23	−1.86	8.41	−2.46	0.978
	1 d	63.99	−4.73	12.39	−3.56	0.92
	3 d	70.61	−0.59	8.06	−2.52	0.994
	7 d	72.00	−1.02	10.10	−3.12	0.969
	28 d	83.24	−0.76	6.98	−2.14	0.962
	60 d	85.78	−2.33	8.57	−2.51	0.976
	90 d	88.62	−2.99	9.34	−2.78	0.908
fb = a + bV + cV2+ dV3	0.5 d	0.69	0.31	0.45	−0.17	0.991
	1 d	1.94	−0.19	1.48	−0.48	0.948
	3 d	2.93	−0.22	1.37	−0.43	0.957
	7 d	4.62	0.06	0.74	−0.20	0.967
	28 d	6.93	−0.40	1.29	−0.40	0.943
	60 d	7.11	−0.16	1.06	−0.36	0.981
	90 d	7.52	−0.08	1.29	−0.45	0.983

. As observed in Figure 7 and Table 5, the mechanical strengths increase with the curing ages, which is due to the increased hydration degree [37,38]. Meanwhile, the mechanical strengths vary in the cubic function with the volume ratios of the basalt fibers. HPC with 2% basalt fibers shows the highest mechanical strengths. This can be explained by the fact that the basalt fibers can connect the cracks in the HPC, resulting in increased mechanical strengths [39,40]. Therefore, when the fibers increase from 0% to 2%, the mechanical strengths increase. However, when the volume ratio of basalt fibers increases from 2% to 3%, the excessive aggregation of fibers in HPC leads to a decrease in defect strength of the HPC. Consequently, the mechanical strengths of HPC decrease when the basalt fibers increases from 2% to 3%. Also, 2% basalt fibers can increase the flexural, compressive and bonding strengths of HPC by rates of 8.1%~31.7%, 1.9%~15.3% and 5.1%~19.6%, respectively. It is depicted from Table 5; the fitting degrees of Figure 7 are higher than 0.91, confirming the fitting accuracy of the functions.

The flexural, compressive and bonding strengths of the HPC with different basalt fibers’ aspect ratios are shown in Figure 8.

Table 6. The fitting function of mechanical strengths with basalt fibers’ aspect ratio.

Equation	Types	a	b	c	d	R2
ft = a + bAR + cAR2+ dAR3	0.5 d	4.99	4.91 × 10−3	−3.48 × 10−6	7.42 × 10−10	0.866
	1 d	7.18	2.27 × 10−3	−9.92 × 10−7	2.62 × 10−11	0.910
	3 d	10.78	0.01	−2.95 × 10−6	5.67 × 10−10	0.839
	7 d	12.25	0.01	−4.51 × 10−6	9.63 × 10−10	0.888
	28 d	16.01	0.01	−4.93 × 10−6	1.10 × 10−9	0.837
	60 d	16.51	0.01	−5.80 × 10−6	1.28 × 10−9	0.933
	90 d	15.27	0.02	−1.43 × 10−5	3.29 × 10−9	0.910
fcu = a + bAR + cAR2+ dAR3	0.5 d	4.99	4.91 × 10−3	−3.48 × 10−6	7.42 × 10−10	0.866
	1 d	7.18	2.27 × 10−3	−9.92 × 10−7	2.62 × 10−11	0.910
	3 d	10.78	0.01	−2.95 × 10−6	5.67 × 10−10	0.839
	7 d	12.25	0.01	−4.51 × 10−6	9.63 × 10−10	0.888
	28 d	16.01	0.01	−4.93 × 10−6	1.10 × 10−9	0.837
	60 d	16.51	0.01	−5.80 × 10−6	1.28 × 10−9	0.933
	90 d	15.27	0.02	−1.43 × 10−5	3.29 × 10−9	0.910
fb = a + bAR + cAR2+ dAR3	0.5 d	−0.11	3.41 × 10−3	−2.25 × 10−6	4.01 × 10−10	0.888
	1 d	0.17	0.01	−5.92 × 10−6	1.32 × 10−9	0.947
	3 d	1.70	0.01	−5.73 × 10−6	1.33 × 10−9	0.893
	7 d	3.40	6.02 × 10−3	−4.76 × 10−6	1.03 × 10−9	0.902
	28 d	5.73	4.96 × 10−3	−3.75 × 10−6	7.99 × 10−10	0.857
	60 d	5.57	0.01	−4.94 × 10−6	1.12 × 10−9	0.897
	90 d	5.93	0.01	−5.53 × 10−6	1.24 × 10−9	0.917

is the fitting function of mechanical strengths with basalt fibers’ aspect ratio. The volume ratio of the basalt fibers is kept at 2.0%. As depicted in Figure 8 and Table 6, the mechanical strengths increase when the aspect ratio increases from 333.3 to 1000. HPC with basalt fibers’ aspect ratio of 1000 shows the highest mechanical strengths. Based on prior studies, the basalt fibers’ aspect ratio demonstrates significant effects on HPC’s mechanical strengths [41]. This can be explained by the fact that higher aspect ratios can increase the basalt fibers’ dispersion, thus increasing the corresponding mechanical strengths. Moreover, the ratio of basalt fibers’ adhesive force to the corresponding tensile force affects the HPC’s mechanical strengths. The HPC’s mechanical strength increases with the increasing ratio of basalt fibers’ adhesive force to the corresponding tensile force [42]. Therefore, the HPC’s mechanical strengths increase with the basalt fibers’ aspect ratio, increasing from 333.3 to 1000. However, when the aspect ratio is higher than 1000, some basalt fibers are fractured by the tensile force due to the fact that the bonding strength between the basalt fibers and the HPC matrix is higher than the basalt fibers’ tensile strength. Therefore, basalt fibers with an aspect ratio of 1000 demonstrate the highest improvement to HPC’s mechanical strengths. Compared with other basalt fibers’ aspect ratio, the flexural, compressive and bonding strengths of the HPC with the aspect ratio of 1000 are increased by rates of 3.1%~9.6%, 1.4%~8.7% and 2.3%~9.1%. As observed from Table 6, the fitting degrees of Figure 7 are higher than 0.85, confirming the fitting accuracy of the functions.

3.3. The Dry Shrinkage Rate

The dry shrinkage rate of HPC is shown in Figure 9, and the fitting function of dry shrinkage rate with different curing age is shown in

Table 7. The fitting function of dry shrinkage rate with different curing age.

Equation	Types	a	b	c	d	R2
	A1	0.383	0.02	−3.267	2.004 × 10−6	0.889
	A2	0.359	0.015	−3.054	1.915 × 10−6	0.862
	A3	0.337	0.018	−3.487	2.113 × 10−6	0.898
	A4	0.295	0.02	−3.911	2.334 × 10−6	0.937
	A5	0.300	0.018	−3.148	1.733 × 10−6	0.964
	A6	0.305	0.019	−3.146	1.717 × 10−6	0.948
	A7	0.315	0.021	−3.805	2.209 × 10−6	0.931
	A8	0.342	0.02	−3.515	1.987 × 10−6	0.959
	A9	0.315	0.021	−3.805	2.209 × 10−6	0.931
	A10	0.260	0.019	−3.392	1.964 × 10−6	0.922
SR = a + bt + ct2+ dt3	0%-Fiber	0.34	0.02	−3.52 × 10−4	1.99 × 10−6	0.959
	0.5%-Fibers	0.33	0.02	−3.12 × 10−4	1.72 × 10−6	0.949
	1%-Fibers	0.31	0.02	−2.80 × 10−4	1.57 × 10−6	0.952
	1.5%-Fibers	0.28	0.01	−1.94 × 10−4	1.02 × 10−6	0.990
	2%-Fibers	0.26	0.01	−2.04 × 10−4	1.18 × 10−6	0.958
	2.5%-Fibers	0.26	0.01	−2.10 × 10−4	1.71 × 10−6	0.972
	0.5 d	0.37	−2.11 × 10−4	1.76 × 10−7	−4.2 × 10−11	0.896
	1 d	0.42	−2.71 × 10−4	1.89 × 10−7	−3.4 × 10−11	0.820
	3 d	0.51	−4.22 × 10−4	3.26 × 10−7	−7.26 × 10−11	0.810
	7 d	0.59	−4.72 × 10−4	3.13 × 10−7	−5.48 × 10−11	0.881
	14 d	0.78	−8.38 × 10−4	6.66 × 10−7	−1.5 × 10−10	0.888
	28 d	0.80	−7.55 × 10−4	5.94 × 10−7	−1.34 × 10−10	0.832
	60 d	0.86	−6.58 × 10−4	4.58 × 10−7	−8.54 × 10−11	0.855
	90 d	0.92	−8.20 × 10−4	6.12 × 10−7	−1.28 × 10−10	0.832

. The dry shrinkage rate of HPC with different mass ratios of CO2-cured IOT and CO2-cured SAA is shown in Figure 9a. The HPC’s dry shrinkage rate increases in the form of cubic function with curing age. This is ascribed to the fact that the hydration degree is increased due to the increasing curing age [43,44]. Therefore, the free water in the HPC has been consumed due to hydration, resulting in the increasing HPC’s dry shrinkage rate [45,46]. However, the CO2 curing of IOT and SAA demonstrates a negative effect on the dry shrinkage rate when the curing age is less than 3 days, which is due to the fact that the CaO in the IOT and SAA react with the CO₂ forming CaCO₃ on the surface of the IOT and SAA, leading to the blocking of the hydration process [47,48]. Therefore, the early activity of HPC is decreased. Meanwhile, when the curing age reaches 3 days, the CO2-cured IOT and CO2-cured SAA show positive effect on the HPC’s dry shrinkage rate.

Figure 9b shows the dry shrinkage rate of HPC with different volume ratio of basalt fibers. As observed from Figure 9b, the HPC’s dry shrinkage rate increases in the form of cubic function with increasing curing time. However, the increased volume ratio of basalt fibers at dosage of 0%~2% shows reducing effect on the HPC’s dry shrinkage rate. This can be ascribed to the fact that the basalt fibers can limit the shrinkage of HPC [49]. Therefore, the HPC’s dry shrinkage rate is decreased. However, when the basalt fibers increase from 2% to 2.5%, the basalt fibers exhibit an increasing effect on the HPC’s dry shrinkage rate, which is due to the basalt fibers’ uneven dispersion [50].

The dry shrinkage rate of HPC with different basalt fibers’ aspect ratios is shown in Figure 9c. The volume ratio of the basalt fibers is 2.0%. As observed in Figure 9c, the HPC’s dry shrinkage rate varies in the form of cubic function with the basalt fibers’ aspect ratio. When the basalt fibers’ aspect ratio is 1000, the HPC demonstrates the lowest dry shrinkage rate. The basalt fibers’ aspect ratio of 1000 shows the higher fibers’ dispersion compared to the other aspect ratio. Comparing the HPC’s dry shrinkage rate with the corresponding mechanical strengths, objective phenomena have been discovered. The CO2-cured SAA and CO2-cured IOT reduce the dry shrinkage rate and the mechanical strengths during the early curing age (less than 14 days). However, the mechanical strengths at later curing ages (curing age after 14 days) are improved. This can be attributed to the fact that the CO2-cured SAA and CO2-cured IOT can decrease the cement hydration degree, leading to the decreasing HPC’s mechanical strengths during the early curing age [51]. However, the early hydration heat is decreased by the CO2-cured SAA and CO2-cured IOT, leading to the decreasing HPC’s dry shrinkage rate and formation of inner cracks by the early hydration heat [52]. Therefore, the mechanical strengths at later curing ages are increased by the CO2-cured SAA and CO2-cured IOT. As found in Table 7, the fitting degrees are higher than 0.83, which confirms the fitting rationality.

3.4. The Amount of Leached Cr and Zn

Figure 10 shows the amount of leached Cr and Zn with the immersing time ranging from 1 month to 6 months. As demonstrated in Figure 10, the amount of leached Cr and Zn demonstrate an increasing trend with immersing time. Moreover, the CO2 curing of IOT and SAA can decrease the amount of leached Cr and Zn. This can be ascribed to the fact that the CO2 curing of IOT and SAA can increase the reaction between CO₂ and CaO, forming the CaCO₃, which increases the IOT and SAA’s compactness [53,54]. Consequently, the amount of leached Cr and Zn is decreased. Additionally, as observed from Figure 11, the leached Zn is higher than the leached Cr.

The amount of leached Cr and Zn from HPC vary with the basalt fibers’ volume ratio and is shown in Figure 11. It can be observed in Figure 11 that the amount of leached Cr and Zn shows an increasing trend with immersing time. Moreover, the added basalt fibers with a volume ratio of 0%~1.5% show a negative effect on the amount of leached Cr and Zn. When the basalt fibers’ volume ratio ranges from 1.5% to 2.5%, the amount of leached Cr and Zn increase. This can be explained by the fact that the basalt fibers can limit the cracking of HPC due to the cement hydration caused by heat and drying shrinkage [55]. Therefore, the amount of leached Cr and Zn from HPC with basalt fibers increasing from 0% to 1.5% is decreased by the added basalt fibers. However, basalt fibers at high dosages can agglomerate, forming weak areas, leading to higher concentration of leached Cr and Zn [56]. Consequently, basalt fibers with a volume ratio range of 1.5%~2.5% in HPC demonstrate an increasing effect on the amount of leached Cr and Zn from HPC.

Figure 12 shows the amount of leached Cr and Zn from HPC with different basalt fibers’ aspect ratio. As observed from Figure 12, the amount of leached Cr and Zn from HPC firstly decreases and then increases with increasing basalt fibers’ aspect ratio. When the basalt fibers’ aspect ratio is 1000, the amount of leached Cr and Zn is the lowest. This can be ascribed to the fact that the increased basalt fibers’ aspect ratio can increase the fibers’ dispersion, leading to a higher effect on the resistance to HPC’s cracking. Hence, the amount of leached Cr and Zn is decreased by the added basalt fibers at an aspect ratio of 333.3–1000. However, when the basalt fibers is at an aspect ratio higher than 1000, more basalt fibers are destroyed by the tensile stress, inducing more cracks in the HPC [57]. Therefore, the amount of leached Cr and Zn is increased by the added basalt fibers at an aspect ratio higher than 1000.

The peak strength values obtained from the experimental tests have been added in the Attached

Table A1. Relationship between curing age (days) and maximum strength (MPa).

Maximum flexural strength	20.7	Curing age	90
Maximum compressive strength	88.5	Curing age	90
Maximum bonding strength	7.5	Curing age	90

,

Table A2. Relationship between fiber volume ratio (%) and maximum strength (MPa).

Maximum flexural strength	23.1	Fibers volume ratios	2
Maximum compressive strength	97.3	Fibers volume ratios	2
Maximum bonding strength	8.9	Fibers volume ratios	2

and

Table A3. Relationship between aspect ratio and maximum strength (MPa).

Maximum flexural strength	22.6	aspect ratios	1000
Maximum compressive strength	96.8	aspect ratio	1000
Maximum bonding strength	8.8	aspect ratios	1000

. While, the results of basalt fibers’ length and diameter on HPC’s mechanical strengths and dry shrinkage rate have been added in Attached

Table A4. The flexural strength with different basalt fibers’ aspect ratio.

Diameter (mm)	Length (mm)	Aspect Ratio (%)	Flexural Strength (Mpa)
			0.5 Days	1 Days	3 Days	7 Days	28 Days	60 Days	90 Days
13	6000	461.54	6.6	8	12.3	14.2	17.9	18.9	6.6
13	12,000	923.08	7.1	8.5	12.7	14.6	18.4	19.4	7.1
13	18,000	1384.62	6.9	8.5	12.5	14.4	18.1	19.1	6.9
13	25,000	1923.08	6.5	8.1	12.1	13.8	17.5	18.4	6.5
15	6000	400.00	6.4	7.9	12	14	17.8	18.7	6.4
15	12,000	800.00	6.9	8.3	12.5	14.5	18.3	19.3	6.9
15	18,000	1200.00	7	8.6	12.7	14.6	18.3	19.3	7
15	25,000	1666.67	6.7	8.3	12.3	13.9	17.7	18.6	6.7
18	6000	333.33	6.3	7.9	11.9	13.7	17.6	18.3	6.3
18	12,000	666.67	6.8	8.2	12.3	14.3	18.1	19.1	6.8
18	18,000	1000.00	7.3	8.6	12.8	14.7	18.6	19.5	7.3
18	25,000	1388.89	6.8	8.4	12.4	14.2	17.8	18.9	20.9

,

Table A5. The compressive strength with different basalt fibers’ aspect ratio.

Diameter (mm)	Length (mm)	Aspect Ratio (%)	Compressive Strength (Mpa)
			0.5 Days	1 Days	3 Days	7 Days	28 Days	60 Days	90 Days
13	6000	461.54	63.4	68.1	74.3	78.1	88.9	90.3	63.4
13	12,000	923.08	66.5	71.4	77.8	82.4	90.6	93.4	66.5
13	18,000	1384.62	66.5	71.8	76.1	81.1	90.3	92.4	66.5
13	25,000	1923.08	62.6	67.6	73.5	77.4	85.8	87.6	88.6
15	6000	400.00	62.2	67.3	72.5	77.6	86.7	87.5	62.2
15	12,000	800.00	65.1	70.5	76.7	81.3	90.1	92.3	65.1
15	18,000	1200.00	67.1	72.6	77.5	81.7	91.1	92.8	67.1
15	25,000	1666.67	63.7	69.8	74.3	78.1	87.2	88.9	63.7
18	6000	333.33	61.3	66.8	71.8	74.6	85.1	86.9	61.3
18	12,000	666.67	63.8	69.2	75.9	79.5	89.3	91.2	63.8
18	18,000	1000.00	68.7	73.9	79.7	83.7	91.3	93.6	68.7
18	25,000	1388.89	65.3	70.3	75.4	80.2	88.7	91.7	93.4

,

Table A6. The amount of leached Cr from HPC with different basalt.

Diameter (mm)	Length (mm)	Aspect Ratio (%)	Cr (mg/mL)
			1-Month	2-Months	3-Months	4-Months	5-Months	6-Months
13	6000	461.54	0.0000742	0.000131	0.000261	0.000589	0.00702	0.00861
13	12,000	923.08	0.0000631	0.000103	0.000167	0.000476	0.00632	0.00752
13	18,000	1384.62	0.0000607	0.000117	0.000136	0.000513	0.00683	0.00776
13	25,000	1923.08	0.0000607	0.000148	0.000198	0.000601	0.00752	0.00847
15	6000	400.00	0.0000751	0.000143	0.000287	0.000601	0.00743	0.00882
15	12,000	800.00	0.0000689	0.000114	0.000203	0.000501	0.00653	0.00795
15	18,000	1200.00	0.0000658	0.000106	0.000121	0.000482	0.00651	0.00734
15	25,000	1666.67	0.0000607	0.000134	0.000179	0.000569	0.00724	0.00809
18	6000	333.33	0.0000768	0.000157	0.000302	0.000632	0.00781	0.00901
18	12,000	666.67	0.0000721	0.000121	0.000235	0.000532	0.00687	0.00801
18	18,000	1000.00	0.0000607	0.000095	0.000102	0.000415	0.00613	0.00671
18	25,000	1388.89	0.0000607	0.000128	0.000158	0.000538	0.00696	0.00792
13	6000	461.54	0.0000742	0.000131	0.000261	0.000589	0.00702	0.00861

and

Table A7. The amount of leached Zn from HPC with different basalt.

Diameter (mm)	Length (mm)	Aspect Ratio (%)	Zn (mg/mL)
			1-Month	2-Months	3-Months	4-Months	5-Months	6-Months
13	6000	461.54	0.0000782	0.000151	0.000349	0.000732	0.00835	0.00786
13	12,000	923.08	0.0000732	0.000131	0.000319	0.000698	0.00775	0.00695
13	18,000	1384.62	0.0000779	0.000161	0.000342	0.000714	0.00791	0.00734
13	25,000	1923.08	0.0000826	0.000178	0.000376	0.000755	0.00856	0.00804
15	6000	400.00	0.0000805	0.000163	0.000361	0.000751	0.00851	0.00813
15	12,000	800.00	0.0000745	0.000138	0.000328	0.000706	0.00786	0.00726
15	18,000	1200.00	0.0000761	0.000136	0.000327	0.000701	0.00768	0.00702
15	25,000	1666.67	0.0000812	0.000171	0.000366	0.000736	0.00832	0.00779
18	6000	333.33	0.0000831	0.000175	0.000372	0.000768	0.00862	0.00829
18	12,000	666.67	0.0000751	0.000149	0.000337	0.000721	0.00811	0.00753
18	18,000	1000.00	0.0000713	0.000124	0.000312	0.000687	0.00741	0.00763
18	25,000	1388.89	0.0000793	0.000166	0.000358	0.000728	0.00815	0.00758

.

3.5. The Microscopic Analysis

The SEM and SEM-EDS photos of the HPC without any CO₂-cured solid waste, the HPC with the highest content of CO₂-cured solid waste, the assembly units of CO₂-cured iron tailings powder and the CO₂-cured secondary aluminum ash are shown in Figure 13 and Figure 14. It can be observed from Figure 14 that the fibrous hydration products, the compact hydration products and the flocculent parts are found in the SEM photos. As illustrated in Figure 13, the CO₂-cured secondary aluminum ash and the CO₂-cured iron tailings powder decrease the fibrous hydration products and the flocculent hydration products. Moreover, the compact products are increased by the CO₂ curing of iron tailings powder and secondary aluminum ash. The HPC with 10% CO₂-cured iron tailings powder and 20% CO₂-cured secondary aluminum ash shows the highest compactness hydrate products.

The SEM-EDS photos of the HPC with different dosages of CO2-cured IOT and CO2-cured SAA are shown in Figure 14. As depicted in Figure 14, the elements of C, O, Al, S, Si, Ca and Cl are found. The CO2 curing of SAA and IOT can decrease the elements of K and Ca. This can be ascribed to the fact that the CO2 curing of IOT and SAA can promote the reaction of CO₂ with Ca(OH)₂ and KOH, leading to fewer ionic K and Ca. Therefore, the measured Ca and K via EDS are decreased by the CO2 curing of IOT and SAA. Moreover, the elements of C and O are increased by the added CO2-cured IOT and CO2-cured SAA due to the fact that the reaction of alkaline substances with CO₂ increases the carbonate content. Consequently, the C element is increased by the added CO2-cured IOT and CO2-cured SAA.

4. Conclusions

HPC’s properties with the assembly unit of CO2-cured SAA and CO2-cured IOT are investigated. The influence of basalt fibers’ volume ratio and aspect ratio is considered. The conclusions are obtained as follows:

The fresh HPC’s slump flow decreases in the form of cubic function with corresponding placing time. The slump flow is increased by the CO2 curing of SAA and IOT. The fluidity of fresh HPC with the assembly units of 10% CO2-cured IOT and 20% CO2-cured SAA is the highest. At this dosage, the slump flow is increased by the increasing rate of 26.3%. The CO2 curing of SAA and IOT leads to the increasing fresh HPC’s setting time. Fresh HPC with 30% CO2-cured SAA shows the highest setting time.

The mechanical strengths including the flexural strength, the compressive strength and the bonding strength during the early curing age (not exceeding 7 days) are decreased by the CO2-cured SAA and IOT. When the curing age is higher than 7 days, the CO2 curing of the SAA and IOT can improve HPC’s mechanical strengths. At this moment, the HPC with CO2-cured IOT and 10% CO2-cured SAA shows the highest mechanical strengths. The HPC’s flexural, compressive and bonding strengths are increased by CO2 curing of IOT and SAA by rates of 10.6%~21.3%, 11.1%~24.6% and 8.9%~23.2%. The added basalt fibers are effective to improve the HPC’s mechanical strengths. HPC with the basalt fibers’ volume ratio of 2% and an aspect ratio of 1000 shows the highest mechanical strengths. The basalt fibers can increase the HPC’s flexural, compressive and bonding strengths by rates of 8.1%~31.7%, 1.9%~15.3% and 5.1%~19.6%, respectively.

The amount of leached Cr and Zn is decreased by the CO2-cured SAA and CO2-cured IOT. HPC with the assembly unit of 10% CO2-cured IOT and 20% CO2-cured SAA shows the lowest amount of leached Cr and Zn. The basalt fibers with the volume ratio of 1.5% and the aspect ratio of 1000 demonstrate the highest decreasing effect on the HPC’s leached Cr and Zn.

The CO2 curing of SAA and IOT can decrease the fibrous hydration products and the flocculent hydration products. The compact hydration products are increased by the CO2-cured IOT and CO2-cured SAA. HPC with the assembly unit of 10% CO2-cured IOT and 20% CO2-cured SAA shows the most compact hydration products. The amount of K and Ca is decreased by the CO2 curing of IOT and SAA. Moreover, the CO2-cured IOT and CO2-cured SAA increase the amount of C and O.