Experimental Investigation of CO2-Induced Silica Gel as the Water Blocking Grout Effect of Aquifer Ions.

This study aimed to prevent water flow in microcracks and simultaneously achieve CO2 capture during grouting (CCG). Using sodium silicate (SS) as the primary material, the microcracks were grouted by a two-step approach. The low-initial-viscosity (5 mPa s) SS was first saturated within the microcracks followed by CO2 injection at 2 MPa. Through CO2 dissolution, silica gel was developed and tolerated a hydraulic pressure of up to 5.5 MPa. The effects of aquifer ions (Na+, Ca2+, Mg2+, HCO3 -, and SO4 2-) were equally evaluated at harsh conditions, and it was found that the strength of the silica gel was reduced, which was caused by salting out, low CO2 solubility, and precipitation. As a result, the hydraulic pressure was reduced to as low as 3 MPa. After 210 days, 16% of the silica gels (without ion inclusion) were reversible to the liquid phase, where a similar effect was found in the cases of Na+ and Mg2+ ions. The degradation increased with more Ca2+ ions (up to 55%) and decreased with more HCO3 - and SO4 2- ions. Microcracks grouted with CCG extended the CO2 utilization in grouting application. Combined with the effect of dissolved ions, the proposed approach is feasible in the field implementation for underground engineering under water bodies.

Introduction

China’s energy consumption mainly relies on coal production.[1] As “green development” becomes a social norm and national strategy, ecological considerations in coal mining have become a hot topic of concern.[2] From 2013, additional attention has been paid to the ecology of the coal mine with the implementation of “The Belt and Road Initiative”.[3] Water protection is among the most important measures to conduct coal mining in arid and semiarid areas. In underground coal mining, after the coal seam has been mined, the roof and floor rock of the mine area lose their equilibrium, resulting in deformation, destruction, and fracture of the surrounding rock strata.[4] This leads to water resource losses if left untreated. Many researchers[5−7] addressed this issue by three main approaches, namely, water conservation mining, underground reservoir storage, and simultaneous exploitation of coal and groundwater.[8] However, these methods could not perfectly prevent the flow of water down the mine goaf, unless they are cotreated with a grouting material.[9−11]

Currently, the available grouting materials could not effectively grout the more complex hydrogeological conditions for groundwater control.[12] Traditional cement grouting with standard methods could not seal the fissures to an acceptable level in certain advanced grouting requirements. Microcracks are also a flowing path that needs to be treated during coal mining in water bodies.[13−15] The minimum fissure aperture that traditional suspensions (such as cement slurry and ultrafine cement slurry) can penetrate is approximately 50 to 100 μm, and the permeability coefficient of injection parts is 10–1 to 10–2 cm/s. The microfissures and low permeability strata cannot be effectively grouted, which leads to poor sealing of rock fissures and large residual water inflow after grouting.[16−18] Many researchers[19−21] found that microfractures are preferential pathways for fluid flow. Furthermore, the fracture is subjected to expansion through a leaching process by the water–rock interaction.[22,23] Hence, microcracks should be sealed to achieve water protection.

Sodium silicate (SS) is a common grouting material that is catalyzed by various agents. Zullo et al.[24] achieved low-pressure grouting in historical sites using SS activated by NaHCO3. Li et al.[25] later investigated a mixture of polyurethane and SS to reinforce subsea tunnels. Seawater was found to reduce the peak strength of the grout by 10.9%. Furthermore, SS was also implemented in the drilling application. Coupled with glycerol triacetate or 1,4-butyrolactone, SS was feasible to achieve grouting while drilling (GWD) in a loose coal bed for the application of methane drainage.[26] In the oil and gas industry, SS is widely implemented as the plugging agent in naturally fractured formation.[27,28] Liu and Ott[29] reviewed the use of SS to isolate CO2-rich formation. An amorphous silica gel was developed by lowering the pH of SS with CO2 dissolution, through the reaction between SS and CO2 (eq ).

Regarding the efforts mentioned above by various researchers, SS is a potential material for CO2 capture, and the developed silica gel can then be used to decrease the hydraulic conductivity. However, its stability with respect to dissolved ions within the formation of groundwater has not been well-investigated. The degradation of the developed silica gel also remained unclear when the ions are involved. Therefore, this study aims at investigating the grouting efficiency of silica gel developed by the reaction between CO2 and SS. The study began with the rheological measurements of the silica gel considering the effect of ions. Then, grouting was conducted by a novel two-step approach. The degradation of silica gel was equally evaluated to ensure its application for the reduction of the hydraulic conductivity of coal mining under water bodies. The success of this study enables a novel means to achieve CO2 capture while grouting (CCG), therefore standing as a potential criterion to achieve carbon neutrality in the China schedule in 2060.[30]

Materials and Methods

Materials

Aquifer water (AW) was acquired from a coal mine located in Gaoping City, Shanxi Province, China. The aquifer water was analyzed for its constituents using inductively coupled plasma-mass spectrometry (ICP-MS, model 7900, Agilent Technology, United States). The dissolved ions are given in Table . Ionic water was then synthesized in-house from sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), sodium hydrogen carbonate (NaHCO3), and sodium sulfate (Na2SO4), all supplied by Sigma-Aldrich.

Table 1

Composition of Aquifer Water (in ppm)

Na+	Ca2+	Mg2+	Cl–	HCO3–	SO42–	pH
347.77	130.45	5.06	57.79	967.40	126.42	8.2

Sodium silicate (SS) was purchased from Tianjin Hengxing Chemical Reagents Co., Ltd., 99.99% pure. The fundamental properties of SS are given in Table .

Table 2

Fundamental Properties of SS

characteristics	value
total Na2O, % by mass	19.3
Na2O in silicate, % by mass	22.8
total SiO2, % by mass	23.5
Na2O/SiO2 ratio	1.03
viscosity, mPa s (in 1% solution, 20 °C)	4.5
acidity, pH unit (in 1% solution)	8.3
density, g/cm3	2.4

Fractured rock was tested for its oxide composition that was determined by X-ray fluorescence spectroscopy (XRF, model S8 Tiger spectrometer, Bruker, Germany) as given in Table .

Table 3

Oxide Composition of the Rock Sample

SiO2	Al2O3	Na2O	Fe2O3	K2O	MgO	CaO	other
68.85	17.17	4.55	2.41	3.16	0.73	1.51	1.62

Preparation

The SS solution was formulated by dissolving 10 g of SS in 100 mL of deionized water (DW). Ten aliquots of the SS solution were diluted from the base solution in a range from 1 to 10% by weight.

The effects of ions were investigated with respect to the compositions of aquifer water. For instance, Na+ ranged from 200 to 1000 ppm, Ca2+ ranged from 50 to 1000 ppm, Mg2+ ranged from 50 to 1000 ppm, HCO3– ranged from 200 to 2000 ppm, and SO42– ranged from 50 to 1000 ppm. The defined concentration of ions was dissolved in DW before adding SS.

The core sample was first cleaned in a 40 kHz sonication bath for 30 min and then dried in an oven at a constant temperature of 105 °C for 8 h to remove the residue water, after which the porosity (voids and fractures) of the sample was determined by the fluid saturation method.

Methods

A series of laboratory experiments were carried out to investigate the reduction behaviors in the hydraulic conductivity of CO2-induced silica gel in the fractured sample considering the effect of dissolved ions on the water of the aquifer. The experiment procedure is demonstrated in Figure .

Figure 1

Experimental flow chart.

Dynamic Rheological Test

Dynamic rheological properties were measured using the apparatus shown in Figure . Prior to testing, 30 mL of a defined concentration of SS solution was transferred to the reaction tank. After that, CO2 gas was bubbled within the tank at a constant rate of 1 L/min, while the dynamic viscosity and pH of the mixture were monitored using an NDJ-1 viscometer (shear rate of 100 s–1) and a pH meter connected at the outflow of the reaction tank. Data were recorded every minute. Once the silica gel formed, as visually observed, the shear rate was reduced to 50, 20, and 10 s–1 to investigate the fluid behavior.

Figure 2

Dynamic rheological properties and pH monitoring during the reaction.

The effects of water ions from the aquifer were investigated on the viscosity of the silica gel. This included the required reaction time (gel time), pH changes, and concentration of the ions.

Grouting Test

The core sample was mounted in a Hassler core holder as shown in Figure .

Figure 3

Schematic representation of the water blocking apparatus; (1) a Hassler core holder, (2) an injecting end, (3) a producing end, (4) a CO2 gas tank, (5) a jacketed cell containing injected fluids, (6) a pump, (7) a pressure gauge, (8) a fluid collector, and (9) a valve.

Prior to the grouting experiment, the initial permeability of the core was determined. Deionized water was injected at constant rates of 0.5, 1, and 1.5 mL/min to flow through the fractured samples. The absolute permeability was then calculated following Darcy’s law:where Q is the flow rate (m3/s), Kabs is the absolute permeability (m2), μ is the water viscosity (N s/m2), A is the cross section of the core (m2), ΔP is the pressure change across the column (MPa), and L is the length of the core (m).

A total of seven grout tests were performed to evaluate the efficiency of silica gel as a hydraulic conductivity reduction agent formulated from a reaction between CO2 and the SS solution. Dissolved anions and cations were equally considered in this work. In the base case, DW was first imbibed into the sample at a flow rate of 0.5 mL/min until saturated, observed without air bubbles forming in the fluid collector. After that, the injection was shifted to the SS solution while the injection rate remained unchanged. To ensure that the in situ SS reached the designed concentration, the SS concentration in the fluid collector was monitored. Then, the production valve was shut in while the injection was shifted to CO2 gas. The injection was carried out until the column pressure reached 2 MPa. The CO2 gas and the in situ SS in the system were allowed to react until equilibrium, observed by a pressure change below 5%.

The effects of dissolved ions and AW on the current approach were evaluated using ionic water (Na+, Ca2+, Mg2+, HCO3–, and SO42–) at their maximum concentration in this study and the synthesized AW, where the injection sequence is given in Figure . The core was first saturated with various ionic water or AW (0 to 10 min). After which, the SS solution was injected. The producing end was shut in at 40 min while CO2 gas was fed into the column. It took approximately 5 min to reach 2 MPa. Then, the column was left for the reaction, and the pressure of the column was monitored. After equilibrium, the producing end was opened, and deionized water was injected to determine the permeability of the core after grouting and the maximum injecting pressure that the grouted column could tolerate.

Figure 4

Illustration of the injection scenarios.

Permeability Test

The permeability test was conducted in a sense to measure the hydraulic conductivity of the core sample after the grouting test. Upon equilibrium pressure, DW was injected into the core sample to determine permeability after grouting. The water injection rate was increased stepwise to measure the maximum injection pressure that the grouted core sample could tolerate. Furthermore, the change in the sample permeability was calculated following Darcy’s law given in eq .

CO2 Solubility Test

Prior to the grouting test, the total porosity of the sample was determined. Thus, the mole of CO2 injected into the sample, , was calculated by the Peng–Robinson cubic equation of state. The equilibrium pressure, Pe, was measured when there was no observed pressure change. The partial pressure of CO2 in the vapor phase was determined by assuming that the phase obeyed Dalton’s law and was therefore expressed as the difference between the total equilibrium pressure and the pressure of the aqueous solution free of CO2.where Pi is the initial pressure.

The amount of CO2, , remaining in the gas phase was calculated using the Peng–Robinson cubic equation of state with the known , temperature, and volume of the gas phase. The amount of dissolved CO2 in the liquid phase was calculated as follows:

The solubility of CO2 within SS solution was thereby calculated using eq as follows:where is the solubility of CO2 within the solution (mol/cm3) and Vvoid is the volume of the porosity and fracture of the rock sample (cm3).

Degradation of Silica Gel

The degradation of silica gel was investigated by measuring the concentration of silica in its initial stage and upon stepwise reduction after 7, 30, 90, and 210 days in gelled samples using ICP-MS.

Results and Discussion

Rheological Properties

Silica gel was formulated by the dissolution of CO2 in sodium silicate solution. The rheological properties of the reacted mixture were measured. The results are shown in Figure .

Figure 5

Steady shear flow of silica gel with different concentrations of SS.

It is observed that after the CO2 reaction, the viscosity of the SS solution at the concentration from 1 to 4 wt % remained unchanged by increasing the shear rate from 10 to 100 s–1, which could thus be described as the Newtonian fluid.[31] At a higher concentration of SS from 5 to 10 wt %, the experimental measurements were found to better agree with the power law model, where the minimum R2 was 0.998 (Table ). Therefore, the power law could be utilized to represent the silica gel and is expressed as follows.where μa is the apparent viscosity, K is the consistency index, γ is the shear rate, and n is a flow behavior index. When n < 1, the mixture has the characteristic of shear thinning and can be defined further as a pseudoplastic fluid, when n = 1, the mixture has the characteristic of a Newtonian fluid, and when n > 1, the mixture has the characteristic of shear thickening and can be described as a dilatant fluid.

Table 4

Rheological Parameters of the Reacted SS Solution

			K (mPa sn)		n		R2
conc. (wt %)	model	fitting result	model	exp.	model	exp.
1	Newtonian	τ = 4.35γ	4.35	4.28	1	1	1
2		τ = 4.50γ	4.50	4.51	1	1	1
3		τ = 4.53γ	4.53	4.54	1	1	1
4		τ = 4.65γ	4.65	4.67	1	1	1
5	power law	τ = 133,224.11γ–1.035	133,224.11	134,035.75	–0.035	–0.032	0.998
6		τ = 146,928.92γ–1.012	146,928.92	145,529.12	–0.012	–0.013	0.998
7	pseudoplastic	τ = 465,179.10γ–0.932	465,179.10	466,854.63	0.068	0.070	0.999
8		τ = 528,128.26γ–0.909	528,128.26	527,714.52	0.091	0.088	0.999
9		τ = 530,818.33γ–0.894	530,818.33	531,768.39	0.106	0.107	0.999
10		τ = 607,078.86γ–0.884	607,078.86	606,948.46	0.116	0.115	0.999

As shown in Table , the consistency index K value of the silica gel increased from 4.35 to 4.65 mPa s by increasing the SS concentration from 1 to 4 wt %, respectively. The K value was significantly lower than that of other cases where the concentration was beyond 5 wt %. This was due to the fact that in this SS concentration range, no silica gel was formed, as shown in Figure . Basically, the aqueous form of sodium silicate possesses dissolved silica (SiO2), dissolved alkali (Na2O), and water (H2O). Once the silica molecule contacts with the acid, neutralization of stabilizing alkali occurs and silicate anions form polymeric silica connected by the Si–O–Si bond with expulsion of water molecules (Figure a). This action creates a silica sol (or colloidal silicate particles), and consequently, these sol particles aggregate to form gels[32] (Figure b). In the same sense, as CO2 dissolves in water, the developed carbonic acid (H2CO3) reduces the pH of the system, and polymerization then occurs. At a concentration over 5 wt %, the K value was 133,224.11 mPa s and increased to 607,078.86 mPa s at 10 wt %. Undoubtedly, this indicates the formation of a silica gel. This implies that the concentrated sodium silicate in the suspension increases its overall density. So, more sol is developed after the CO2 reaction and later agglomerated into gels.

Figure 6

Formation of silica gel after a reaction between CO2 and SS.

Figure 7

Silica gel formation once pH reduces: (a) polymeric silica forms by the Si–O–Si bond and (b) silica sol aggregates to become gels.

The flow behavior index, n, of the silica gel could be classified into three groups. In the ungelled group (1 to 4 wt %), viscosity was not affected by increasing the shear rate. The gelled group at a concentration between 5 and 6 wt % possessed the shear thinning characteristic, but the value was negative. This was plausibly due to the molecular degradation of the sample, which is well-reported in the literature.[33,34] The gelled group at a concentration of 7 to 10 wt % also had shear thinning characteristics, where the n value increased by including a higher SS content, indicating a greater gel strength. In this regard, a higher concentration of SS is more suitable to be applied as a grouting gel to reduce hydraulic conductivity in microcracks.

Further investigation of the effect of SS concentration on gel time is shown in Figure . The gel time was observed to be 15 min for concentrations between 5 and 7 wt %. It was reduced to 13 min by increasing the concentration to 8 and 9 wt %. However, at the highest concentration in this study, the gel time increased again to 15 min. There was a greater resistance of the CO2 gas to dissolve within the solution at a higher concentration closer to its saturation point. The same mechanism was explained by the use of salted water.[35]

Figure 8

Effect of SS concentration on gel time.

As shown in Figure a, the dissolution of CO2 within the mixture is a continuous process that results in a gradual decrease in pH. It was observed that the viscosity of the SS solution remained unchanged in the concentration range from 1 to 4 wt %, while there was a rapid increase in viscosity at a certain pH in the concentration range from 5 to 10 wt %. At a lower concentration of SS, the dissolved silicate is not able to agglomerate during the neutralization, so the silica gel did not form. In other words, higher-concentration cases possessed more silicate, which facilitates the agglomeration of the silica gel.

Figure 9

(a,b) pH changes during the reaction.

When the SS content was above the gelling concentration (5 to 10 wt %), the phase change from SS solution to silica gel is a rapid process when CO2 dissolution within the solution becomes saturated.[36] This was reflected by the observation in the pH change in Figure a. The same observation was found in the literature.[37,38] HCl and H2SO4 were used to lower the pH of the sodium silicate solution, and silica gel was found to form in the pH range between 6 and 8. In our study, we further inspected that the gelling pH could be higher with increasing concentration of sodium silicate. For instance, when the concentration rose above 8 wt %, the gelling pH was approximately 9 and higher. In this regard, the pH of the gel should be better presented by the pH difference (Figure b). In this sense, the required pH difference was 2.8 to turn the 5 wt % SS solution into silica gel. The pH difference gradually decreased as the SS concentration increased, where only a 1.4 pH was needed in 10 wt %. Therefore, CO2 neutralization is faster at higher SS concentrations. The main reason was that the amount of silica sol formed per unit of reduction in pH in a higher-SS concentration sample was greater, so the agglomeration of sol to develop gels was faster; thus, a lower pH difference was required and vice versa.

Effect of Ions on Silica Gel

The effect of ions and AW on the viscosity of the silica gel was investigated. The results are shown in Figure .

Figure 10

Effect of ions on the viscosity of the silica gel.

As shown, the efficiency of gelling by the reaction between SS and CO2 was reduced by including various ions. The viscosity of the silica gel after the reaction gradually decreased from 8960 to 6266 mPa s, from 9487 to 5933 mPa s, from 9267 to 1000 mPa s, and from 8593 to 5070 mPa s for the concentration of Na+ from 200 to 1000 ppm, the concentration of Ca2+ from 50 to 1000 ppm, the concentration of Mg2+ from 50 to 1000 ppm, and the concentration of SO42– from 50 to 1000 ppm, respectively. On the other hand, the impact of HCO3– on gelling was less pronounced. The viscosity increased from 9240 to 9463 mPa s when including the HCO3– concentration from 200 to 2000 ppm, respectively. In the case of AW, the viscosity reduced from 10,000 to 8540 mPa s compared to DW.

As greater Na+ ions were included in the deionized water prior to formulation of the SS solution, Na+ then reacted with SS first and developed the precipitate. The solubility of SS was then decreased at a higher Na+ content due to the salting-out effect. Hence, lower silicate was available in the solution. Therefore, there was less silica gel formation after reacting with CO2, resulting in a lower apparent viscosity. This was also reported in the literature.[39]

There was a debate about the inclusion of divalent ions of Ca2+ and Mg2+ within SS solution on gelling purposes. Hamouda and Amiri[40] addressed that divalent ions accelerated SS gelling, and the product was found to have a higher strength. This was in contrast to what we found during the test; the SS solution became cloudy with increasing concentrations of Ca2+ and Mg2+, as shown in Figure . The cloudiness was plausibly due to metal silicate precipitates that are less soluble.[41] The effect of Ca2+ and Mg2+ on silica gel polymerization by CO2 dissolution is similar. As presented in eqs and 8, the added CaCl2 and MgCl2 react with SS to produce calcium silicate hydrate (C–S–H) and magnesium silicate hydrate (M–S–H), respectively. As a result, the formation of silica gels by the CO2 reaction decreases with increasing Ca2+ and Mg2+. Moreover, it is plausible that due to the lower molecular weight of Mg2+, more MgCl2 than CaCl2 was required to formulate the same concentration of Ca2+ and Mg2+ ionic water. More silicate is then consumed from the solution to produce a hydrate or a precipitate rather than silica gel, which resulted in a lower apparent viscosity. It should be noted that no silica gel was developed after the CO2 reaction at 1000 ppm Mg2+. This implies that silicate is not available in the system when the cation concentration increases above its saturation point.

Figure 11

Ungelled SS solution with 1000 ppm Mg2+.

In the case of anions, Na2SO4 is dissolved in deionized water to produce Na+ and SO42–, and the solution is neutral. Although their reaction to SS is minimal, the solution thickened at a higher concentration. As a result, the dissolution of CO2 becomes lower, which restricts the formation of silica gel. On the other hand, NaHCO3 dissolves in deionized water to produce Na+ and HCO3–. HCO3– further breaks into CO32– and H+. As addressed by Ren et al.,[42] when contacting with SS, the gelling occurs via the absorption of the H+ in NaHCO3 solution by SS, thus producing SiO2 and Na2CO3 (eq ). Therefore, HCO3– works as the cogelling agent during the CO2 reaction.

To verify this, the effect of ions on gel time was monitored. The results are given in Figure .

Figure 12

Effect of ions on gel time.

As a result, HCO3– was found to accelerate the reaction rate, and the gel time was reduced from 16 to 13 min thanks to H+ released by dissolving NaHCO3 within the solution. The Na+ ion constantly reduced the reaction time to 13 min regardless of its concentration. Ca2+, Mg2+, and SO42– agreed well with their corresponding apparent viscosity. The gel time increased from 15 to 17 min, 14 to 21 min, and 18 to 22 min for Ca2+, Mg2+, and SO42–, respectively. For AW in this case, the gel time was 14 min. The results agreed with the literature,[40,43] where a shorter gelation time was observed in a low concentration of dissolved salts.

The effect of ions on the change in pH during gelation was investigated. The results are given in Figure .

Figure 13

Effect of ions on pH changes.

The pH changes in the gelling process are in a relationship with apparent viscosity. For instance, pH changes of Na+, Ca2+, Mg2+, and SO42– were reduced in the concentrated ion. This agreed well with the reduction in apparent viscosity, where the formation of silica gel was disturbed by various mechanisms given in the above section. In this sense, the availability of silica in the solution was reduced, which then required a smaller amount of CO2 to induce silica gel agglomeration. In contrast, the initial pH was higher as a result of the formation of NaOH from the higher HCO3– concentration. Therefore, the pH change required to develop a silica gel increased with concentration. The pH changes by AW also increased in comparison to DW as a reference case.

To further validate the discussion provided, we investigated the gel formulated under the effect of various ions for its morphology and elemental composition under SEM-EDS analysis. The results are presented in Figure . The gel structure was more consistent when Na+ and HCO3– were included, where the Si concentration was 28.31 and 40.22%, respectively. The results were well in agreement with the measured apparent viscosity, where more silicate was available in the solution, thus creating more silica gel after the CO2 reaction. On the contrary, a loose gel structure was observed in Ca2+, Mg2+, and SO42– with Si concentrations of 23.48, 21.23, and 22.93%, respectively. Various ions consumed silicate from SS solution with different extents. Therefore, the silica gel developed by the CO2 reaction with the SS solution depends on the silicate available in the solution.

Figure 14

SEM images and elemental analysis of silica gel formulated with ions.

Grouting Test

The grouting test by a reaction between CO2 and SS solution was studied considering dissolved ions and AW. The results are given in Figure .

Figure 15

Grouting test.

Grouting could be divided into three main sequences. In the first sequence, SS was injected from 0 to 30 min; the pressure in this step was relatively more stable, indicating good injectivity,[44] where the pressure change was approximately 0.3 MPa on average. The initial viscosity of the SS could travel without obstruction within the pores and fractures of the rock. Only in the cases of presaturation with Ca2+ and Mg2+, the pressure slightly increased at the later stage of SS injection (approximately at 28 to 30 min). This was caused by the precipitation of SS after the divalent ion interaction, as explained earlier and illustrated in Figure , which narrowed the flow path of the injecting fluid. CO2 was injected in the second sequence; thus, the pressure increased rapidly to approximately 2 MPa. From this stage, one could observe that the pressure change followed two patterns. The pressure changed exponentially in the early stage followed by a steady drop until equilibrium at 130 min for the presaturation of DW, AW, Na+, and HCO3–. Among them, Na+ and HCO3– possessed the most significant pressure changes in the early stage. Ca2+, Mg2+, and SO42– rather followed a linear pressure change until equilibrium. However, the pressure reduction in the Mg2+ case was the least, indicating low CO2 dissolution. As a result, the reduction in hydraulic conductivity when presaturated with Mg2+ water was low, around 0.4 MPa. As expected, the DW case achieved hydraulic pressures of 5.5 and 4 MPa for AW. The presaturation with ionic water induced hydraulic pressures of 5.2, 3.6, 3.1, and 3.0 MPa for HCO3–, Na+, Ca2+, and SO42–, respectively.

Permeability Test

The permeability of the rock was determined at its initial stage and after grouting with various kinds of presaturated water. The summary of the results is shown in Figure and Table .

Figure 16

Permeability of the rock sample before and after grouting.

Table 5

Properties of the Rock Sample

no.	D (10–2 m)	L (10–2 m)	V (m3)	ϕ (%)	ki (10–15 m2)	kg (10–15 m2)
DW	2.53	100.21	503.53	12.3	697.35	5.70
AW	2.55	100.20	511.47	12.3	697.59	7.74
Na+	2.52	100.25	499.75	12.1	668.72	8.77
Ca2+	2.51	100.23	495.70	11.9	639.23	10.00
Mg2+	2.55	100.20	511.47	11.6	613.66	74.76
HCO3–	2.53	100.24	503.68	12.5	705.49	6.19
SO42–	2.54	100.22	507.56	12.0	659.23	10.65

The results agreed with the grouting experiment, where up to approximately 99% of the permeability of the rock was reduced regardless of the presaturated fluids. However, it failed in the case of Mg2+, even though the permeability reduction was 87.8%. As suggested by the gelling test, Mg2+ at 1000 ppm caused precipitation rather than developing the gel during CO2 dissolution. Precipitates narrowed flowing paths within the rock but could not effectively stop the water flow. Therefore, the reduction in hydraulic conductivity was relatively weak. The same observation was found for other ions. For instance, up to 98.7% of permeability was reduced by presaturation of Na+ water, but the hydraulic pressure was 3.6 MPa. On the other hand, the hydraulic pressure in the case of presaturation with AW was 4 MPa, while its permeability reduction was 98.9%.

Solubility

The amount of CO2 trapped within the rock voids was determined and expressed in terms of solubility, as shown in Figure .

Figure 17

Effect of ions on CO2 solubility within the SS solution.

In an ideal case, the solubility of CO2 in silica gel was 3.5 × 10–4 mol/cm3. It was found that the effect of AW and HCO3– on CO2 solubility was minimal, where the solubility was 3.51 × 10–4 and 3.6 × 10–4 mol/cm3, respectively, indicating that the current approach is suitable for the target mining site. On the other hand, the CO2 solubility was reduced by 11.6, 21.9, 52.6, and 22.2% for the presaturation of Na+, Ca2+, Mg2+, and SO42– ions.

Overall, the solubility of CO2 within the SS solution was effective in both the ideal case (DW) and the average mining environment (AW). However, the trapping capability of CO2 using SS solution was lowered in the harsh environment where the concentration of certain ions was elevated.[45]

Degradation of Silica Gel

The available silica gel was investigated under ambient conditions. The results are given in Figures and 19.

Figure 18

(a–f) Change in silica concentration against time.

Figure 19

Degradation of silica gel.

The degradation of silica gel was dependent on the concentration of SS. At lower concentrations of SS, the silica gel (Sii) was reversible to a liquid form starting from 7 days and continuing until 210 days. For example, only 35 and 42% silica gel (Sif/Sii) was observed at 210 days using SS concentrations of 5 and 6 wt %, respectively. On the contrary, more silica gel was observed at a higher SS concentration. The degradation was ended at day 7 using 10 wt % SS, where up to 84% of silica gel was still available. Regardless of the initial concentration of SS, the attachment of the silica gel to the surface of the tube was better when it was formulated without dissolved ions (Figure ).

There was no significant effect on silica gel degradation when Na+ and Mg2+ ions were included, where the degradation rate was similar to that of DW. On the other hand, the effect of the Ca2+ ion on the stability of the silica gel increased linearly with its concentration, where up to 55% of silica gel was reversible to a liquid phase after 210 days.

In the case of anions, more silica gel was observed at a higher concentration of HCO3– and SO42–. Degradation decreased from 45 to 20% and from 78 to 68% for the HCO3– and SO42– ions, respectively.

Summary and Limitations

On the basis of the above experimental results, the effect of SS concentration and ions on grouting parameters is summarized in Table .

Table 6

Summary of the Effect of SS and Ions on Grouting Parametersa

	viscosity	gel time	hydraulic conductivity reduction	CO2 solubility	permeability reduction	stability
SS	ΔΔ	ΔΔ	Δ	Δ	Δ	ΔΔ
Na+	Δ∇	Δ	∇	∇	∇	-
Ca2+	Δ∇	Δ∇	∇	∇	∇	Δ∇
Mg2+	Δ∇	Δ∇	∇	∇	∇	-
HCO3–	-	ΔΔ	Δ	Δ	Δ	ΔΔ
SO42–	Δ∇	Δ∇	∇	∇	∇	ΔΔ

ΔΔ defines that the higher the concentration, the better; Δ∇ defines that the higher the concentration, the worse; Δ defines good; ∇ defines worse; - defines neutral.

The conclusion of the scanning criteria could be drawn on each parameter. To increase the viscosity or strength of the silica gel, the initial concentration of SS in the liquid phase should be higher, but the dissolved ions reduced the viscosity of the silica gel except HCO3–. The gel time was shortened by the increase of SS, Na+, and HCO3– concentration. Hydraulic conductivity reduction and CO2 solubility possessed the same pattern, where ions lowered the hydraulic conductivity reduction efficiency and CO2 dissolution caused by the decrease in gel strength. The stability of the developed silica gel was found to be more stable at higher concentrations of SS, HCO3–, and SO42–, while Ca2+ possessed a reverse effect. However, Na+ and Mg2+ did not show a significant effect on the stability of the silica gel.

Conclusions

The present study resolved the grouting issue of microcracks, enabling hydraulic conductivity reduction, simultaneously enhancing CO2 utilization in underground construction, achieving CO2 capture while grouting (CCG). Systematic experiments considering the effects of dissolved ions of aquifer water were conducted. Based on the above results, the following conclusions could be drawn.

Stable and high-strength silica gel was formed by CO2 reacting with SS with a concentration greater than 5 wt %. The developed silica gel could be described as a power law fluid with a pseudoplastic behavior.

Na+, Ca2+, Mg2+, and SO42– ions were found to decrease gel strength observed by a reduction in viscosity and precipitation. Nevertheless, the effect on gel strength was not pronounced by HCO3– and aquifer water.

The silica gel was capable to achieve a hydraulic pressure of up to 5.5 MPa. Presaturation of ionic and aquifer waters was found to lower the hydraulic pressure but still achieved the lowest pressure of 3 MPa. However, grouting is not suitable to use when the Mg2+ ion concentration is above 1000 ppm.

At a 2 MPa grouting pressure, CO2 was found to solubilize up to 3.51 × 10–4 mol/cm3 within the silica gel with a presaturation of aquifer water.

The silica gel was found to be more stable at higher concentrations of SS (9 and 10 wt %). The harsh concentrations of Na+, HCO3–, and SO42– did not affect the stability of the silica gel except Ca2+ and Mg2+.