The Influence of CO₂ Curing on the Mechanical Performance and the Corresponding Chloride Ion Resistance of Alkali-Activated Compound Mineral Admixtures

Abstract

In this paper, the mechanical properties (the flexural strength, compressive strength and the drying shrinkage rate) of CO₂-cured alkali-activated compound mineral admixtures (blast furnace slag powder (BFS) and fly ash (FA)) are investigated. In addition, the corresponding chloride ion mobility coefficient is measured. Additionally, the freeze–thaw cycles with an NaCl concentration of 3% is studied. Thermogravimetric analysis and scanning electron microscopy are applied in analyzing the mechanical properties. The curing ages of the alkali-activated compound mineral admixtures are 1 day, 3 days and 28 days. Results show that the mechanical strengths are decreased by the addition of FA and increased by the increasing curing age and CO₂ curing. The maximum reducing rates of flexural and compressive strengths by FA are 47.6% and 42.3%. Meanwhile, the corresponding increasing rates by CO₂ curing are 26.5% and 23.1%, respectively. The improving effect of alkali-activated BFS by CO₂ curing is higher than that of FA. Furthermore, the drying shrinkage rate is increased by the increasing dosages of BFS, the increasing curing ages and CO₂ curing. Additionally, CO₂ curing and the increasing dosage of BFS leads to decreasing the chloride ion mobility coefficient. Finally, CO₂ curing and the addition of BFS can effectively improve the resistance of NaCl freeze–thaw cycles. The compactness of the hydration products is improved by the addition of BFS and the roughness of hydration products is increased by CO₂ curing.

1. Introduction

Cement-based materials are the most widely used building materials, having been invented hundreds years ago [1,2]. Housing construction, bridge construction, highway engineering, etc. have adopted cement-based materials for construction [3,4]. Cement-based materials show good mechanical properties, high durability, low cost and a wide application range [5,6]. However, a large amount of harmful gases are produced and a great quantity of energy is consumed in the production of cement. Therefore, more cementitious materials need to be developed to replace cement.

Blast furnace slag powder (BFS), fly ash (FA), silica fume, volcanic ash, etc. are solid wastes generated from industrial production and nature [7,8]. As reported in prior research, they possess a composition similar to cement. Therefore, these mineral admixtures have become excellent choices to replace cement. However, the mineral admixtures do not possess enough mechanical strength when cured in conventional curing processes [9,10]. Research points out that the addition of mineral admixtures can decrease the mechanical strength of cement-based materials when they are cured in a standard curing environment. Meanwhile, when the materials are cured in a high temperature curing environment, mechanical strengths are improved by the mineral admixtures.

Alkali-activated cementitious materials have the characteristics of high early strength, fast setting and hardening, excellent corrosion resistance, etc. [9]. BFS and FA are common industrial wastes in our daily life, showing high production and wide distribution [11]. A large amount of BFS and FA have polluted the atmosphere, causing smog. Moreover, long-term stacking will pollute rivers, lakes and soil and infiltrate into groundwater. Furthermore, massive accumulation will also seriously occupy land [12]. Alkali-activated mineral admixtures are manufactured by mixing mineral admixtures, sodium silicate, alkali, water, etc. [12,13,14,15,16]. The alkali-activated mineral admixtures present excellent mechanical strengths. In the 1950s Gluk hovsky et al. [17] used NaOH or sodium silicate as an activator to excite the mixture of crushed stone, boiler slag or blast furnace slag powder, quicklime, blast furnace slag and Portland cement, and prepared a cementitious material with strength up to 120 MPa [18,19]. Additionally, the compressive strengths of alkali-activated BFS and FA can reach values higher than 80 MPa and 100 MPa, respectively. Feng et al. found that the assembly unit of CaO and MgO can improve the mechanical strengths, high temperature resistance and compactness of the microstructure [20]. The compressive strength of ordinary concrete is less than 60 MPa while high performance concrete shows a compressive strength of 60 MPa–100 MPa; therefore, the compressive strength of alkali-activated BFS is close to the compressive strength of ultra-high performance concrete.

CO₂ is one of the main gases which causes the greenhouse effect. A large amount of CO₂ gas is produced during the production of cement. The cementitious materials cured with CO₂ can not only consume it, but also enhance the strength of materials, which is of great significance to alleviate greenhouse gases in the air [21,22]. CO₂ has been used for curing cement-based materials for several years [23,24]. As reported in prior research, CO₂ curing can improve the mechanical properties and the corrosion resistance of the reinforced cement matrix [25]. Additionally, CO₂ curing can effectively increase the mechanical strengths of reactive powder concrete by 30.1%–72.6%. The chloride ion mobility coefficient is decreased by 11.2%–41.6% with CO₂ curing. However, the effect of CO₂ curing on the mechanical strengths and chloride ion mobility coefficient of alkali-activated mineral admixtures is unknown. The increased use of mineral admixtures may reduce CO₂′s emissions [26].

This paper aims to study the influence of CO₂ curing on the mechanical performance and the resistance to chloride ion permeability and freeze–thaw resistance of alkali-activated compound mineral admixtures (blast furnace slag powder and fly ash). The corresponding mechanism of the mechanical performance is revealed by thermogravimetric (TG) analysis and scanning electron microscopy (SEM). This research will provide a new idea for developing diverse cementitious materials and consuming CO₂ in the future.

2. Experimental

2.1. Raw Materials

The sodium silicate is produced by Tongxiang Hengli Chemical Co., Ltd., Tongxiang, China. The melting point of the sodium silicate ranges from 40 °C to 48 °C. S95 grade mineral powder is manufactured by Lingshou Anda mineral powder factory, Shijiazhuang City, China. The corresponding density, the specific surface area and the loss on ignition of the BFS are 2.88 g/cm³, 435.8 m²/g and 1.98%, respectively. The FA is made by the Shijiazhuang Shunli mineral products Co., Ltd., Shijiazhuang City, China. The density of FA is 2.4 g/cm³. The sodium hydroxide is manufactured by Shanghai Dongmiao Chemical Technology Co., Ltd., Shanghai, China. The sodium hydroxide shows a density of 2.13 g/cm³, a boiling point of 1390 °C and a purity of 99.9%. The particle passing percentage and the chemical composition of raw materials are shown in

Table 1. Particle passing percentage of raw materials (%).

Types	0.3 μm	1 μm	4 μm	8 μm	64 μm	360 μm
BFS	0.03	3.5	19.6	35.0	97.9	100
FA	12.3	66.2	100	100	100	100

and

Table 2. The chemical composition of cement (%).

Types	SiO2	Al2O3	FexOy	MgO	CaO	SO3	K2O	Na2O	Ti2O	LI
BFS	34.1	14.7	0.2	9.7	35.9	0.2	3.5	-	-	-
FA	55.00	20.00	6.00	10.20	4.50	0.11	1.26	2.13	0.06	0.74

, respectively.

2.2. Specimen Preparation

The specimens are prepared following these steps:

Table 3. The mixing proportions of alkali-activated cementitious material (kg/m³).

Number	Water	P·O	SAC	SF	GGBS	Quartz Sand	Water-Reducer	Li2SO4	Calcium Formate	Tartaric Acid	Defoamer	Steel Fibers
1	244.4	370.5	370.5	370.5	111.1	978	20.3	0.6	2.6	1.9	0.6	78.5
2	244.4	370.5	370.5	370.5	111.1	978	24.4	0.6	2.6	1.9	0.6	117.75
3	244.4	370.5	370.5	370.5	111.1	978	12.2	0.6	2.6	1.9	0.6	157
4	244.4	370.5	370.5	370.5	111.1	978	16.3	0.6	2.6	1.9	0.6	196.25
5	244.4	370.5	370.5	370.5	111.1	978	20.3	0.6	2.6	1.9	0.6	235.5

indicates the mixing proportions of alkali-activated cementitious material. The NJ-160A cement paste mixer is applied in the mixing of the potash water glass and sodium hydroxide. The stirring speeds of the cement paste mixer are 140 r/min and 285 r/min. A 2 min stirring speed of 140 r/min and 2 min of 285 r/min are provided, respectively. Specimens are kept in the standard curing room (temperature of 20 ± 2 °C and relative humility of 98.6%) and TH-B concrete carbonization test box (CO₂ concentration of 8%) provided by Tianjin Gangyuan test instrument factory, Tianjin, China.

2.3. Measurement

Specimens with size of 40 × 40 × 160 mm³ are used for the measurement of the flexural and compressive strengths with the loading speeds of 2.4 kN/s and 0.05 kN/s, respectively. HYE-300B cement flexural and compressive constant stress testing machine is used for providing the loading. In this study, three specimens are used for the measurement of flexural strength, while six specimens are applied in the determination of compressive strength.

The Mitutoyo Japan Mitutoyo high precision handheld digital display produced by Shanghai Shoufeng Precision Instrument Co., Ltd., Shanghai, China is applied in the measurement of the dry shrinkage rate. Before the measurement, specimens are fixed in the BC-160 type length comparator. The measuring details of the dry shrinkage rate can be found in JGJT70-2009.

The dry shrinkage rate is calculated by Equation (1).

$$\epsilon =\frac{L_1-L_t}{L_t}$$

(1)

where ε is the dry shrinkage rate and L₁ and L_t are the lengths of specimen cured for 1 day and 28 days, respectively.

The steps of the measurement of the chloride ion permeability coefficient (CMC) are described as follows.

The specimens with a size of Φ100 × 50 mm³ are saturated in a vacuum saturator for 2 days. After that, specimens are moved for the CMC test with the NELD-CCM550 concrete chloride ion diffusion coefficient tester provided by Beijing Neerde Intelligent Technology Co., Ltd., Beijing, China.

Specimens with size of 100 × 100 × 400 mm³ are applied in the measurement of NaCl freeze–thaw cycles. Before the experiment, all samples are immersed in an NaCl solution with a concentration of 3%. After that, all specimens are moved to the DR-10A cement automatic quick freezing and thawing machine for the freeze–thaw measurement. The measuring details of determinations of CMC and NaCl freeze–thaw cycles are followed by the Chinese Standard GB/T 50082-2009. Three specimens of each group are used for the measurements of dry shrinkage rate, the CMC and the NaCl freeze–thaw cycles.

The central parts of the specimens are taken out and used for the measurements of the TG and SEM. The specimens with mung bean size are sprayed with gold under a vacuum environment. After the gold spraying is finished, the measurement of the scanning electron microscope photos is carried out. The powdered sample after crushing and screening is used for the thermal analysis. The temperatures on the TG curves are determined by the temperature sensor for automatic temperature recording. The accuracy of determining these values is ±0.8 °C, these descriptions have been added in the revised manuscript. The measuring details can be found in Ref. [27].

3. Results and Discussions

3.1. Influence of Steel Fibers on Working Performance

Figure 1 illustrates the slump flow of the fresh alkali-activated cementitious material. As shown in Figure 1, the slump flow decreases with increasing dosages of FA. The fact that the specific surface area of FA is larger than that of BFS leads to a decrease in the slump flow of fresh alkali-activated cementitious material. The values of all error bars are lower than 0.1, indicating the accuracy of the experimental results.

3.2. The Mechanical Strengths

Figure 2 shows the mechanical strengths of alkali-activated compound mineral admixtures. It is depicted in Figure 2 that the flexural and compressive strengths increase with increasing curing age and decrease with the increasing dosage of FA. Meanwhile, the reduction rate of flexural strength has a positive correlation with the dosages of FA and has a negative correlation with the curing age. Moreover, the addition of FA leads to increasing the reduction rate of compressive strength; furthermore, an increasing curing age results in a decrease of the reduction rate. This is ascribed to the fact that the FA shows a high degree of [SiO₄]⁴⁻ polymerization in a vitreous structure network, resulting in low activity; therefore, FA is difficult to excite by alkali [10]. Consequently, the mechanical strengths of alkali-activated mineral admixtures are decreased by the addition of FA. The decreasing rates of flexural and compressive strengths of alkali-activated compound mineral admixtures induced by the addition of FA are 32.6%–47.6% and 35.4%–42.3%.

The flexural and compressive strengths of CO₂-cured alkali-activated compound mineral admixtures are shown in Figure 3. As illustrated in Figure 3, the change rule of the flexural and compressive strengths of CO₂-cured alkali-activated compound mineral admixtures are similar to that of the standard cured alkali-activated compound mineral admixtures. Comparing Figure 2 and Figure 3, CO₂ curing can improve the mechanical strengths of the alkali-activated compound mineral admixtures. Meanwhile, when more FA is added, the improvement effect is worse. This is ascribed to the fact that BFS possesses a higher content of CaO, which can react with more CO₂ forming a higher content of calcium carbonate leading to more compact products [28]. Comparing Figure 2 and Figure 3, the increasing rates of flexural and compressive strengths by CO₂ curing are 16.7%–26.5% and 15.4%–23.1%, respectively.

Figure 4 shows the dry shrinkage rate of alkali-activated compound mineral admixtures.

Table 4. The fitting results between the dry shrinkage rate and the mass ratio of FA.

Equation	FA Dosage (%)	a	b	c	R2
$\frac{\Delta L}{L}=a{v}^2+bv+c$	0	−2.14 × 10−5	0.0056	0.45	0.99
	20	−1.50 × 10−5	0.0036	0.33	0.93
	40	−4.20 × 10−5	0.0025	0.10	0.99
	60	4.28 × 10−5	0.0014	0.06	0.99
	80	8.57 × 10−6	0.0004	0.04	0.98
	100	−7.14 × 10−8	1.20 × 10−4	0.02	0.98

shows the fitting results between the dry shrinkage rate and the mass ratio of FA. As depicted in Figure 4 and Table 4, the dry shrinkage rate increases in the form of a quadratic function with the mass ratio of BFS. Moreover, the dry shrinkage rate of alkali-activated compound mineral admixtures increases with the increasing curing age and CO₂ curing. This is ascribed to the fact that the hydration degree is increased with curing age, leading to a decrease in the free water inner specimens and increasing the dry shrinkage rate [29]. Moreover, CO₂ can react with Ca(OH)₂ forming CaCO₃; the increased amount of CaCO₃ and the porosity in the specimen is higher, resulting in an increase in the drying shrinkage rate [30]. Additionally, as depicted in Figure 4, the drying shrinkage rate increases with the increasing BFS dosage, due to higher alkalinity of BFS, thus forming more CaCO₃ and increasing the drying shrinkage rate. When the dosages of BFS increase from 0% to 100%, the drying shrinkage rate is decreased by 23.5%–32.1%. Whereas, with CO₂ curing, the drying shrinkage rate is increased by 36.5%–63.1%.

The CMC of the alkali-activated compound mineral admixtures. When the specimens are cured in the standard environment, the CMC increases with the increasing dosage of FA. As depicted in Figure 5, CO₂ curing leads to decreasing the CMC. When CO₂ curing is provided, the CMC first increases and then decreases with the increasing dosage of FA. This is due to the fact that the alkali excitation of BFS is higher than the FA, therefore, the compactness of alkali-activated compound mineral admixtures is improved by BFS [31,32]. Consequently, the CMC is increased by the increasing dosages of FA.

The mass loss rate (△m/m) of alkali-activated compound mineral admixtures is illustrated in Figure 6. The △m/m increases with the increasing dosages of FA and the increasing NaCl freeze–thaw cycles. This is attributed to the fact the alkali-activated activity of FA is lower than that of BFS [33,34]. Therefore, alkali-activated FA shows lower compactness, leading to more serious NaCl freeze–thaw damage. Meanwhile, the NaCl freeze–thaw cycles can accelerate peeling of the outer surface of alkali-activated compound mineral admixtures, thus increasing the △m/m. Additionally, CO₂ curing can improve the compactness of alkali-activated compound mineral admixtures, resulting in reducing the surface peeling.

The relative dynamic modulus of elasticity (RDME) of the alkali-activated mineral admixtures is illustrated in Figure 7. As observed in Figure 7, the RDME decreases with the increasing NaCl freeze–thaw cycles and the decreasing dosage of BFS. This is attributed to the fact that the NaCl freeze–thaw cycles induce inner cracks in the specimens. Moreover, as depicted in Figure 7, CO₂ curing leads to an increase in the RDME; when the addition of BFS dosage is increased, the improving effect is higher. CO₂ curing can accelerate the reaction between alkaline substances and CO₂, thus improving the compactness of alkali-activated mineral admixtures and increasing the corresponding RDME [35]. Moreover, the alkali-activated activity of BFS is higher than that of FA. As a consequence, the RDME is increased by an increasing dosage of BFS.

The TG and DTG curves of alkali-activated compound mineral admixtures are shown in Figure 8. As demonstrated in Figure 8, the TG values of alkali-activated compound mineral admixtures decreases in three steps. In the first step, the temperature varies from 30 °C to 156.7 °C, the TG values decrease due to the evaporation of free water. In addition, as illustrated in Figure 8, the second step can be observed, which is the temperature of 368.2 °C. In this step, the TG values decrease, which is ascribed to the decomposition of calcium silicate hydrate, the hydrated single sulfur calcium sulphoaluminate and hydrated polysulfur calcium sulphoaluminate [20,36]. As illustrated in Figure 8, the TG values are decreased by CO₂ curing, due to the fact that CO₂-cured alkali-activated compound mineral admixtures show less bound water than the alkali-activated compound mineral with standard curing, thus decreasing the reduction of TG values. Finally, when the temperature varies from 368.2 °C to 900 °C, the TG value decreases, which can be ascribed to the decomposition of hydrated calcium silicate and calcium carbonate. As observed in Figure 8, the addition of FA and CO₂ curing decreases the reduction of TG values. This is attributed to the lower alkali activity of FA, the increased calcium carbonate and the decreased of bound water by CO₂ curing [37].

The scanning electron microscope (SEM) photos of the of the alkali-activated mineral admixtures are shown in Figure 9. The samples include the alkali-activated FA, alkali-activated BFS and CO₂-cured alkali-activated BFS. As observed in Figure 9, the alkali-activated BFS shows more compact hydration products than the alkali-activated FA. Moreover, as illustrated in Figure 9, the hydration products of CO₂-cured alkali-activated BFS are more compact than the alkali-activated FA. This is attributed to the fact that the alkali excitation degree of BFS is higher than that of FA. Meanwhile, as depicted in Figure 9, more flocculent and granular hydration products exist on the surface of the CO₂-cured alkali-activated BFS. This may be ascribed to the increased CaCO₃ content formed by the reaction of CO₂ and CaO [27]. Therefore, it is further confirmed that alkali-activated BFS shows higher mechanical strength than that of alkali-activated FA, and CO₂ curing can improve the mechanical strength.

4. Conclusions

In this paper, the effect of CO₂ curing on the mechanical properties of alkali-activated compound mineral admixtures is investigated. The conclusions are obtained as follows.

(1)

The mechanical strengths of alkali-activated compound mineral admixtures are decreased by the addition of FA. Moreover, CO₂ curing is effective in improving the mechanical strength. The addition of FA can decrease the flexural and compressive strengths of alkali-activated compound mineral admixtures by 32.6%–47.6% and 35.4%–42.3%, respectively. Meanwhile, CO₂ curing can improve the flexural and compressive strengths of alkali-activated compound mineral admixtures by 16.7%–26.5% and 15.4%–23.1%, respectively. When, the BFS content is higher, the improving effect of CO₂ curing is higher.

(2)

The addition of BFS and CO₂ curing demonstrate an increasing effect on the drying shrinkage rate of alkali-activated compound mineral admixtures. Due to the addition of BFS, the drying shrinkage rate is decreased by 23.5%–32.1%, while, with CO₂ curing, the drying shrinkage rate is increased by 36.5%–63.1%.

(3)

The chloride ion impermeability and resistance of NaCl freeze–thaw cycles of alkali-activated compound mineral admixtures are improved by the increasing dosages of BFS and CO₂ curing.

(4)

CO₂ curing is able to increase the production of CaCO₃, while when the dosage of BFS is higher, the CaCO₃ product is higher. The addition of BFS can improve the compactness of the hydration products and CO₂ curing can increase the amount of rough hydration products.