1 Introduction
Cement grouting, well known as a ground reinforcing method and effective water control approach, has been widely applied in tunnel excavation, mining, dams building, and other fields [
1,
2]. However, with the continuous depletion of shallow mineral resources and the increasing depth of mining, cement grouting in deep rock formations encounters some challenges due to elevated ground temperatures. Currently, the maximum depth of coal mining in China has exceeded 1500 m, and accordingly the ground temperature is approximately between 40 and 45 °C [
3,
4]. Similarly, tunnel excavation in the high-altitude mountain regions of western China also faces high ground temperatures, which significantly affect the grouting process.
Elevated ground temperatures poses negatively impact on the rheological properties of cement grout in its fresh state [
5,
6], as well as the physically mechanical behaviors of hardened cement grouts [
7]. Huang et al. [
8] found that when the temperature increased from 20 to 50 °C, the fluidity of a cement slurry with a water−cement ratio of 1.0 decreased from 361 to 342 mm, and further dropped to 252 mm at 95 °C. Liu et al. [
9] reported the rheological parameters of cement grouts increase under the same shear rate as the temperature increases. Mohammed et al. [
10] observed that increasing the temperature from 25 to 50 °C led to at least a 107% increase in slurry shear stress. Fan et al. [
11] reported that while the early strength of the hardened cement grout increases with temperature, its later strength declines once the temperature exceeds 40 °C. Lothenbach et al. [
12] further indicated that elevated temperatures increase the porosity, thereby reducing its mechanical strength. In summary, as temperature rises, the cement grout’s fluidity decreases, making it difficult for the grout to flow in rock fractures and voids, leading to an unfavorable injectability and poor grouting quality. Traditional cement water-reducing agent, such as Polycarboxylate superplasticizer, can effectively enhance the flowability of cement grout at high temperatures, yet they also notably decrease the strength [
13]. Therefore, developing a novel additive that not only enhances the rheological properties of fresh cement grout at elevated temperatures but also improves the strength of the hardened grout is of significant importance for improving the grouting quality under high-temperature conditions.
Nanomaterials have attracted considerable attentions for their ability to enhance the performance of cement-based materials, owing to their nanoscale effects and exceptional physicochemical properties. Graphene oxide (GO), an oxidized derivative of graphene, has emerged as one of the most extensively studied nanomaterials in cement composites due to its low cost compared to graphene and its strong interfacial interactions with the cement matrix [
14]. The two-dimensional single-atom-layer structure enables the GO to bridge cracks and efficiently transfer mechanical loads. Furthermore, the abundant oxygen-containing functional groups-such as hydroxyl, carboxyl, and epoxy groups-can interact with active components or hydration products of the cement [
15], formatting strong bonding between GO and the hydrations of cement matrix, thereby reducing crack initiation and development. A lot of literature indicate that incorporating GO significantly enhances the mechanical properties of cement-based materials, such as compressive and flexural strength [
14,
16,
17]. While GO imparts many benefits to cement-based materials, it also presents a significant drawback: it significantly reduces the flowability of cement grout, due to its large specific surface area. This limitation has considerably restricted the use of GO in cement grouting materials. Unlike other cement-based materials, a high-quality grout must exhibit good flowability. However, due to its nanoscale particle size, GO not only aggregates and thereby encapsulates some of the free water but also adsorbs a significant amount of free water, which substantially reduces the rheology of cement grout [
18]. In the alkaline environment of hydrated cement slurry, the complexation with Ca
2+ further promotes the aggregation of GO [
19–
21]. The high pH of the pore fluid (pH > 12) causes dissociation of H
+ ions from the carboxyl groups on GO, leading to formation of COO
−-Ca
2+-
−OOC complexes with Ca
2+ present in the slurry. The interaction bridges the GO layers, causing stacking and aggregation [
22]. Other divalent or trivalent cations have similar interaction process [
23,
24]. These aggregations trap some of the free water, reducing its ability to properly wet the cement particles. If the inherent drawbacks of GO in cement grouts, including its tendency to adsorb free water and aggregate, can be significantly minimized, GO has the potential to enhance the rheological performance of cement grouting materials.
Currently, technical approaches such as ultrasonic dispersion, surfactants, and chemical modification are commonly employed to achieve uniform dispersion of GO in cement slurry, preventing its aggregation and the adsorption of free water. Ultrasonic dispersion can effectively disperse GO in aqueous solutions. However, with the high-Ca
2+ content generated by cement hydration, GO tends to re-aggregate [
25]. Additionally, ultrasonic equipment is costly, energy-intensive, and complex to operate, making it impractical for large-scale use in grouting applications. Surfactants can adsorb onto the surface of GO, improving its uniform dispersion. For instance, polycarboxylate superplasticizers (PCE), as typical surfactants, can adsorb onto the GO sheets [
26,
27], creating steric hindrance and electrostatic repulsion to facilitate the uniform dispersion of GO nanoparticles [
28]. However, Ghazizadeh et al. [
29] suggested that when PCE is mixed with cement grout, most of the PCE molecules will be removed from the pore solution, and thus adsorb onto the cement particles, or form precipitates with certain cement hydration products. It significantly reduces the number of PCE molecules available for effective dispersion. Additionally, due to the negative impact of PCE on cement setting and strength, the amount of PCE added is strictly limited to a specific range. As a result, it is challenging to use limited PCE directly to achieve uniform dispersion of GO. Chemical modification is the most widely used strategy to improve the dispersion of GO at present. The surface of GO is rich in oxygen-containing functional groups, which can serve as cores for modifying its molecular structure. Based on esterification and amidation, researchers have skillfully grafted organic groups with electrostatic repulsion or steric hindrance effects onto the GO surface, thereby preparing modified graphene oxide (MGO) with special functions. Wang et al. [
30] grafted polyetheramine onto GO through an amide reaction between amine groups and the carboxyl groups on the surface of GO. It was found that the modified GO can improve the flowability of fresh cement paste and toughness of hardened paste. Based on a free radical polymerization reaction, Li et al. [
31] polymerized methyl allyl polyoxyethylene ether and acrylic acid (AA) monomers, linking them to GO via ester bonds to prepare a modified GO. The modified GO could enhance the flowability of cement slurry to some extent, while also accelerating cement hydration and refining the cement crystal structure. Both the properties of GO and the performance of cement grouts may experience significant changes with variations in temperature. However, the effect of GO and modified GO in the performance evolution of cement grouting materials at elevated temperatures (e.g. for deep rock grouting) still lacks in-depth investigation.
Previous research has found that the elevated temperature (up to 35 °C) could considerably deteriorate the time-dependent rheology of thick cement grout (water to cement ratio ≤ 0.6) at fresh state, increase the porosity of hardened cement grouts and reduce their strength, resulting in a poor injectability of thick cement grout in deep rock grouting [
32]. To improve the performance of cement grouts in deep formations with a ground temperature of up to 45 °C, in this study a novel modified MGO was developed using free radical copolymerization, based on the cost-effective industrial-grade GO. Experimental investigations were conducted on the effects of the MGO on both rheological properties of fresh cement grouts and compressive strength of hardened grouts. The mechanism by which MGO enhances the performance of cement grouts was also discussed. The findings are believed to contribute to advancing cement grouting materials development and high-quality of rock grouting in deep underground engineering.
2 Materials and methods
2.1 Materials
The MGO employed in this study is self-synthesized using industrial-grade GO, Vinyltrimethoxysilane (VTMS), Methylallyl Alcohol Polyoxyethylene Ether (MAPE, Molecular weight 2400), Acrylic Acid (AA), ammonium persulfate (APS), ascorbic acid (AscA), Mercaptopropionic acid (MPA). The industrial-grade GO is produced by Suzhou Tanfeng Technology Co., Ltd. in China using the modified Hummers method. It is brownish-black in form of powder with a purity of 95%, a thickness of 1 nm and sheet diameters ranging from 10 to 50 μm, consisting of 1–2 layers. The microstructure of industrial-grade GO is shown in Fig. 1, and the Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) spectra are indicated in Figs. 2 and 3, respectively. VTMS from Bide Pharmatech Co., Ltd is a transparent liquid, with a purity of 98.56%. MAPE provided by Qingdao Usolf Chemical Technology Co., Ltd is in a state of white powder, with a purity of 99%. AA supplied by Shanghai Macklin Biochemical Co., Ltd is transparent liquid, with a purity of more than 97%.
The cement grout was prepared using 42.5R Ordinary Portland cement produced by Esheng Cement Co. Ltd. in China, sodium chloride (NaCl) which is in the form of small particles with a purity of 99.5%, and triethanolamine (TEA) from the Dow Chemical company (Midland, MI, USA), with a purity of 99%. The mix design of cement grouts is the same as that used in grouting practices, which could be found in previous study [
32].
2.2 Sample preparation
2.2.1 Synthesis of modified graphene oxide
The synthesis of MGO primarily involves two steps: the preparation of the precursor, i.e., the silanized graphene oxide (SGO), followed by the one-pot synthesis of MGO. First, GO is dispersed in ethanol solution using ultrasonic treatment and then VTMS is added into the mixture which was stirred for 3 h at 60 °C. Unreacted VTMS in the mixture are removed using an ethanol solution, and the product is then pulverized and finely ground to yield the precursor, SGO. One-pot method is then employed to synthesize the MGO. Under ultrasonic-assisted dispersion, SGO, MAPE, and APS are added to water with a mass ratio of 1:2:0.1, at a temperature of about 80 °C. Over a period of 1.5 to 2 h, two solutions are added into the mixture. The solution A consists of 2wt% AscA and 5wt% MPA in aqueous solution, whereas solution B is formulated with AA at 25% by mass relative to SGO. After that, the reaction of the mixture is allowed to proceed for an additional hour. The mixture is then filtered, dried at approximately 60 °C. The residue is pulverized and ground to obtain the final product, i.e., the MGO. The synthesis of the MGO is illustrated in Fig. 4.
2.2.2 Preparation of grouts
The cement grout used in this study is prepared with cement, water, NaCl, TEA and GO or MGO powder, as shown in Table 1. The mix ratios are as the same as that used in many grouting engineering in Chinese coal mining [
32]. In pure cement grouts, the incorporation of NaCl and TEA as accelerators can promote the cement hydration and enhance the setting [
33,
34]. To facilitate comparison, the pure cement grouts with only NaCl and TEA added are served as the control group. Mass ratio of NaCl and TEA was 0.5% and 0.05% of the cement mass, respectively. The dosage of GO or MGO added into the grout is consistently 0.05 wt% of the cement mass. Three water−cement mass ratios (
W/C) involved in this investigation are 0.5, 0.6, and 0.75. The grout temperature is set to 25, 35, and 45 °C. To minimize the impact of raw material temperatures on the grout, all materials are pre-conditioned to the designed grout temperature. The grout is mixed at 800 rpm for 5 min, and it is maintained at the designated temperature during both mixing and testing processes. For detailed information on temperature control for grout preparation, please refer to previous work [
32].
2.3 Testing procedure
To systematically evaluate the performance of cement grouts modified by MGO, a series of testing procedures were designed, covering MGO characterization, fresh-state properties, hardened-state strength, and microstructure analysis. The overall framework of these tests is illustrated in Fig. 5.
2.3.1 Characterization of modified graphene oxide
The successful synthesis of MGO can be determined by the changes of functional groups and interplanar spacing variation using the FTIR and XRD, and the grafting rate of polymer chains can be estimated by the mass loss during the thermal decomposition of the MGO via Thermogravimetric analysis (TGA). The universal attenuated total reflectance FTIR is conducted with a resolution of 4 cm−1 and 32 scanning times and wavenumber ranges from 500 to 4000 cm−1. XRD is performed using Cu Kα radiation (λ = 1.5406 Å), with a scanning range of 2θ from 5° to 55°. TGA is performed at a temperature range from 25 to 600 °C, and a heating rate of 10 K/min.
To evaluate the dispersion degree of MGO and GO, the optical microscopy and UV-Vis spectrophotometry are utilized to examine the microstructure and absorbance of GO and MGO in the cement grout. Direct observation of the microscopic morphology of GO and MGO in the grout is challenging, and the cement grout’s absorbance exceeds the maximum range of the spectrophotometer. Therefore, a saturated Ca(OH)2 solution (pH = 12–13) is prepared to simulate the high-alkaline, high-calcium environment of the cement grout. Both the dispersion of MGO and GO are observed in the Ca(OH)2 solution. The optical microscope used is produced by Ningbo Sunny Instruments Co., Ltd. (Ningbo, China), providing a magnification range from 40-fold to 1000-fold. The UV-Vis spectrophotometer is provided by Unico (Shanghai) Instrument Co., Ltd. (Shanghai, China), with a wavelength range of 190–1100 nm and a quartz sample cuvette.
2.3.2 Rheological behavior
The rheological curves of the cement grout are measured using an NXS-11B rotary viscometer with a thermostatic beaker. During testing, the cement grout is consistently kept at pre-defined temperature (i.e., 25, 35, and 45 °C), and the shear rate ranges from 0 to 996 s−1. Prior to testing, the grout is pre-sheared at high speed for approximately 20 s to reduce shear history effects. During testing, the shear rate is gradually increased from 0 to 996 s−1, then gradually decreases back to 0. At each level of shear rate, data are recorded immediately once the shear force stabilized. The initial rheological properties are measured within 5 min after mixing the grout, while the time-dependent rheological properties are tested within 4 h of mixing. The apparent viscosity η of the grout at the given shear rate can be calculated using Eq. (1).
where is the measured shear stress at the given shear rate .
A thermostatic water bath is used to precisely control the temperature of the cement grout, while the digital stirrer continuously stirs the cement paste to simulate the stirring conditions in the field, detailed temperature control please see previous works [
32,
35]. During the time-dependent rheological evaluation, the rheological curves are measured every 0.5 h using the NXS-11B rotary viscometer.
2.3.3 Bleeding
The bleeding test is conducted according to the Chinese standard SL/T 62-2020 [
36]. One hundred milliliters of the grout are poured into a graduated cylinder, and then the cylinder is covered with plastic film to prevent humidity changes between the grout and the surrounding environment. The cylinder is subsequently placed in a climatic chamber set to the predefined temperature to determine the bleeding rate at a specific grout temperature. The bleeding rate at rest time of
t is the ratio of the bleeding height of water to the total volume of grout in cylinder, as shown in Eq. (2), and the bleeding height is observed every 0.5 h.
where BHt is the height of bleeding water measured at specific rest time t.
2.3.4 Setting time
The setting time of the cement grout is measured using a Vicat apparatus, in accordance with the national standard GB/T 1346 (2011) [
37]. The samples for setting time measurement are cured in a climate chamber at 25, 35, and 45 °C with 90% humidity. Due to bleeding of water from the cement grout, two stacked molds are used to prepare the testing samples. Once no more water is bled from the grout, the top layer can be removed before initial setting, leaving the lower layer to form a fully prepared sample for the setting time test. Both sample preparation and curing are carried out in a temperature-controlled curing chamber.
2.3.5 Unconfined compressive strength
Grout specimens with 9 different mix ratios shown in Table 1 are prepared for un-confined compressive strength (UCS) testing. Standard cylindrical samples with a diameter of 50 mm and a height of 100 mm are utilized for the UCS. Samples are cured for 28 d in a climatic chamber at 25, 35, and 45 °C under 90% relative humidity. During the tests, a constant loading rate of 0.5 MPa/s was adopted according to suggestion for the International Society for Rock Mechanics. At each curing age, at least three specimens were tested for compressive strength; the average value was taken as the compressive strength of that age if the relative error of individual test results did not exceed 15% of the average.
2.3.6 Microstructure and porosity estimation
Microstructure of the hardened grout is characterized using a SEM at an accelerating voltage of 15 kV. The samples for SEM observation are prepared from fragments of the UCS test specimens. On the basis of SEM images, it allows not only for the observation of the microstructure of the cement grout, but the surface porosity of the samples can also be estimated using the gray scale method with digital image processing software [
38]. Porosity can be calculated using Eq. (3).
where is the surface porosity, is the area of pore pixels, and is total area of image pixels.
3 Results and discussion
3.1 Characterizations of modified graphene oxide
The FTIR spectra of GO, SGO, and MGO are shown in Fig. 6. The typical functional groups of GO include the O-H stretching vibration of hydroxyl groups at 3000–3500 cm
−1, the C=O stretching vibration of carbonyl groups at 1720 cm
−1, the C-O-C stretching vibration of epoxide groups at 1060 cm
−1, and the C=C skeletal vibration of unoxidized sp
2 carbon bonds at 1616 cm
−1 [
31]. Compared with GO, the characteristic absorption peak at 880 cm
−1 observed in SGO can be attributed to the stretching vibration of Si-OH groups from hydrolyzed VTMS [
39], confirming the successful preparation of the precursor, SGO. In contrast, MGO, when compared to GO and SGO, presents a new peak at a wavenumber of 2850–2920 cm
−1, which is primarily attributed to the C-H stretching vibration of -CH
2- and -CH
3 groups on the chains of MAPE. This suggests that the MAPE polymer chains were successfully grafted onto the surface of GO, to form a novel modified GO [
40,
41]. Additionally, the FTIR spectrum of MGO indicates significant increases of a C-O-C absorption peak at 1060 cm
−1 and a C=O absorption peak at 1720 cm
−1. The enhanced peaks on MGO can be attributed to the introduction of MAPE and AA functional groups, which further confirm the successful grafting of the polymer chains onto the GO surface.
The increase in interplanar spacing indicates the potential incorporation of new substances into the crystal structure, if all other conditions remain unchanged [
42]. This variation in spacing can be analyzed using XRD results. Figure 7 shows the XRD spectra of GO, SGO, and MGO, with interplanar spacing calculated based on the diffraction peak positions using Bragg’s law. The main diffraction peaks for GO, SGO, and MGO are at 2
θ = 12°, 2
θ = 11°, and 2
θ = 8.8°, respectively. Accordingly, the interplanar spacing of GO, SGO and MGO are 0.74, 0.8, and 1 nm. The increased interplanar spacing observed in both SGO and MGO can be attributed to the insertion of VTMS and polymer chains, which effectively reduces the interlayer interactions between GO sheets. Additionally, a weak broad peak appeared at 2
θ = 17.6° was observed in the XRD spectrum of MGO, which may be attributed to the introduction of polymer chains from MAPE and AA. These polymer chains could interact with each other or aggregate to form local ordered structures due to hydrogen bonding or van der Waals forces, resulting in a relatively broad peak in the 10°–20° range [
43]. The increasing of interplanar spacing and appearance of weak broad peak at 2
θ = 17.6° confirms again that the polymer chains have been successfully grafted onto the GO surface, resulting in the formation of a novel MGO.
The grafting rate of the polymer chains grafted onto the oxygen-containing functional groups on MGO is evaluated by TGA and derivative analysis (DTG) results. Figure 8 shows the TGA and DTG curves of GO, SGO, and MGO. It could be found that the thermal decomposition of GO and SGO occurs in two stages: the first stage involves the evaporation of free water, which mainly takes place at temperatures below 100 °C; the second stage is the decomposition of the oxygen-containing functional groups on GO. In contrast, the thermal decomposition of MGO includes a third stage, corresponding to the breakdown of the polymer chains grafted onto the GO surface within the temperature range of 240–450 °C. Thus, the grafting rate of MGO can be estimated by analyzing the mass loss rate at the third stage [
44,
45], and it is around 29%.
3.2 Effect of modified graphene oxide on initial rheology of cement grouts
Figures 9–10 show the initial rheological curves and apparent viscosity variations of cement grouts at 25, 35, and 45 °C. The cement grouts without GO or MGO are identified as Control group, to comparably analyze the effects of MGO on initial rheological properties of cement grouts at different temperatures. It is found that all grouts showed similar rheological behaviors at different temperatures. Regardless of whether GO or MGO is added, the shear stress of the grout increases with the shear rate, while the apparent viscosity decreases as the shear rate increases, indicating typical shear-thinning behavior. It suggests that the addition of either MGO or GO does not affect the initial rheological characteristics of the cement grouts under different temperatures. Furthermore, compared to the control group without GO or MGO, the addition of GO increases the apparent viscosity of the cement grout under all three temperature conditions, while the addition of MGO reduces it. This suggests that the MGO developed in this study has overcome the drawback of traditional nanomaterials, which typically decreased the flowability of the cement grout, and may even improve the initial flowability to some extent.
Yield stress and plastic viscosity are two important rheological parameters commonly used to determine the flow behavior of suspension-based cement grout. Yield stress represents the minimum shear stress required to initiate the grout flow, impacting its initial flow behavior. Plastic viscosity, on the other hand, measures the resistance caused by internal friction between grout components during flow, significantly affecting the pressure loss during the grout flowing. Based on the rheological curves, the rheological parameters are determined using the Bingham model as given by Eq. (4) and using the Herschel–Bulkley (H–B) model as shown in Eq. (5), respectively. Figures 11 and 12 show the results of the fitted rheological parameters, and the coefficients of determination (
R2) values are listed in Tables 2 and 3. The
R2 values of the H–B models are greater than 0.95, which are higher than those of the Bingham model. The rheology of cement grouts with low water−cement ratios might be more closely to an H–B fluid [
46].
where τ is the shear stress; γ is the shear rate; τ0 is the yield stress; μ is the plastic viscosity of Bingham fluid; K is the consistency index of H–B fluid, characterizing the cement grouts viscosity-related thickening property; and n denotes the flow behavior index, which reflects the nonlinearity of the cement grouts flow (n = 1 for ideal plastic fluid, n < 1 for shear-thinning behavior, n > 1 for shear-thickening behavior).
Compared to the pure cement grouts with a water−cement ratio of 0.5, the incorporation of GO increased the plastic viscosity by approximately 10% to 25%, and enhanced the yield stress by 5% to 13%. In contrast, the incorporation of MGO developed in this work decreased the plastic viscosity by 5% to 18%, and reduced the yield stress by 12% to 24%. The rheological parameters obtained from the H–B model fitting also exhibit a similar variation trend. For cement grouts with GO (W/C = 0.5), the yield stresses are increased by 5%–13%, while the K values are improved by 9%–20%. However, the incorporation of MGO can reduce the yield stress by 1%–11% and decrease the K values by 18%–45%. For H–B fluids, an increase in K results in higher viscosity. The addition of GO elevates viscosity, whereas MGO reduces it. This suggests that MGO developed in this work can mitigate the viscosity increase typically caused by traditional nanoparticles in cement grouts, exhibiting potential water-reducing properties. The same trend is observed when the W/C ratios are 0.6 and 0.75. It is evident that as the temperature rises from 25 to 45 °C, the MGO developed in this work can effectively reduce the rheological parameters of the thick cement grout, thus avoiding the common issue where the introduction of nanomaterials results in a reduction of grout flowability.
The ability of MGO to reduce the initial rheological parameters of thick cement grouts at different temperatures can be attributed to the polymer chains copolymer-grafted onto its surface and its excellent dispersion. These features enable the MGO to exert a spatial hindrance effect similar to that of a superplasticizer. As a result, the MGO can effectively prevents the aggregation of cement particles and their hydration products, while facilitating the release of encapsulated free water, thereby improving the initial flowability of the cement grouts. Figure 13 shows the microstructure of the suspension with MGO and GO, respectively. It is clearly evident that a significant amount of aggregation has occurred in the GO suspension, where GO sheets interact and bridge with each other, encapsulating some of the free water. In contrast, the MGO suspension indicates excellent dispersion, with almost no observable flocculation between the MGO particles. The differences in dispersion between the two suspensions can primarily be attributed to the steric hindrance from the polymer chains on the MGO surface and the electrostatic repulsion. Once the cement is mixed with water, cement particles start form flocculated structures, with some free water being trapped and unable to flow freely or lubricate the particles [
47]. Due to the copolymer grafting, a large number of high-molecular-weight polymer chains are attached to the oxygen-containing functional groups on the MGO surface. These long chains cannot only prevent MGO particles from aggregating but also effectively separate the cement particles, reducing the formation of flocculation structures of cement particles [
17,
48]. Additionally, the -COOH, -OH and other active functional groups on the MGO surface can ionize under alkaline conditions, imparting a characteristic negative charge to MGO [
49]. This charged nature not only makes MGO particles resistant to aggregation due to electrostatic repulsion, but also allows them to adsorb onto cement particles [
50], imparting a similar charge to the cement particles. As a result, the electrostatic repulsion further helps break up the cement flocculation structures and then releases free water. Consequently, the cement paste with added MGO exhibits significantly enhanced the flowability of cement grout.
The change in absorbance can also serve as an indicator of the dispersion quality of the suspension. According to the Reylength Equation as shown in Eq. (6), higher absorbance typically indicates better dispersion [
51].
where
A is the absorbance,
K is the light absorption constant,
v is the particles amount per unit volume. Figure 14 shows the absorbance curves of GO and MGO in the simulated suspension. The absorption peak at 230 nm in GO suspension is attributed to the π–π* transition of the C=C bond [
26]. In contrast, the π–π* transition peak in MGO suspension is shifted to 220 nm, indicating a blue shift (i.e., hypsochromic shift). The blue shift in the UV spectrum can be attributed to changes in the MGO molecular structure caused by polymer grafting, which shifts the spectral band toward shorter wavelengths, resulting in an increase in the energy required for electronic transitions [
52]. The blue shift provides further confirmation of the successful synthesis of MGO. Additionally, the absorbance intensity of suspensions with MGO and GO at their respective characteristic wavelengths also indicates the better dispersion of MGO. Furthermore, it can be found that the initial absorbance of the GO suspension is 0.322 at its absorption peak, which decreased to 0.2512 after 5 min, a relative decrease of approximately 22%. For the MGO suspension, the initial absorbance is 0.3924 at its absorption peak, which decreased to 0.3325 after 5 min, representing a relative decrease of about 15.27%. The absorbance value is closely linked to the dispersion of nanomaterials. The more uniformly the nanoparticles are dispersed and the less aggregation occurs, the higher the absorbance. Both the initial absorbance and the absorbance after 5 min are much higher for the MGO suspension than the GO suspension, indicating better dispersion of the MGO suspension. Consequently, the cement grout with added MGO exhibits superior flowability and lower rheological parameters.
3.3 Effect of modified graphene oxide on time-dependent rheology of cement grouts
The rheological properties of the cement grouts exhibit significant time-dependent behavior due to cement hydration. Excessive changes in the rheological parameters over time can significantly affect the flowability and penetration distance of the grout, thereby influencing its injectability and grouting quality. Therefore, it is essential to control not only the initial rheological properties of the cement grout but also to carefully regulate the time-dependent changes in its rheological parameters. The rheological parameters of both pure cement grout and cement grout with GO gradually increase over time [
53,
54]. This trend becomes more pronounced at higher temperatures, causing a significant deterioration in the injectability of high-temperature cement grout [
32]. When grouting in deep formations with high ground temperature, controlling the time-dependence in the rheology of thick cement grout is of great practical importance for improving the quality of grouting.
Figures 15–16 show the time-dependence of rheological parameters of the grouts with or without MGO or GO at different temperatures. It could be found that for the cement grout with W/C = 0.5, at 25 °C, within 4 h, the plastic viscosity of the control group increases from 63.26 to 97.5 mPa·s, while that of the GO-added grout increases from 79.05 to 112.71 mPa·s, and the MGO-added grout increases from 65.41 to 92.23 mPa·s. Correspondingly, the yield stress of the control group increases from 6.24 to 10.28 Pa, from 7.07 to 11.17 Pa for the GO-added grout, and from 5.46 to 9.23 Pa for the MGO-added grout. If the temperature is elevated up to 45 °C, the plastic viscosity of the control group with W/C = 0.5 increases from 61.97 to 183.78 mPa·s, while the GO-added grout increases from 60.62 to 213.96 mPa·s, and the MGO-added grout increases from 58.91 to 173.61 mPa·s. Similarly, the yield stress for the control group increases from 6.95 to 16.74 Pa, from 6.54 to 15.2 Pa for the GO-added grout, and from 5.59 to 15.5 Pa for the MGO-added grout. The variation in time-dependent rheology of cement grouts with other water−cement ratios also follows a similar trend, although the rate of change is slower compared to the grouts with W/C = 0.5. This suggests that, under different temperature conditions, the changes in the rheological parameters of cement grouts with MGO over time are less pronounced than those with GO. What’s more, the time-dependent variability of the MGO-added cement grout is even lower than that of the control group. Specifically, when the temperature increases from 25 to 45 °C, both the plastic viscosity and yield stress of the MGO-added cement grouts increase significantly less over time compared to the grouts with GO, and even less than that of the control group. Generally, the impact of time-dependence on plastic viscosity is much more noticeable than on yield stress, especially for the grouts with GO and the control group. The MGO developed in this study effectively reduces the time-dependence increase in cement grouts caused by the addition of GO.
The primary cause of time-dependence in the rheological parameters of cement grouts is the strengthening of the paste structure by cement hydration products [
55], and the ability of MGO to control the time-dependence can be attributed to its polymer chains. Once in contact with water, the active components of cement, namely the gypsum and C
3A and C
4AF, will dissolve and react rapidly, with the process being further accelerated by high temperatures. The initial products, i.e., AFm phase and Ettringite, will adsorb large amounts of free water, leading to thickening and an increase in rheological parameters. Additionally, the presence of TEA and NaCl in the grout accelerates the reaction rate of C
3A and C
4AF. NaCl can promote the hydration of C
3A, and TEA, as a catalyst, will enhance this effect, further accelerating the thickening of the grout at high temperature. When the MGO is introduced into the cement grout, the polymer chains on the surface of MGO exert spatial steric hindrance and electrostatic repulsion, which can help to separate the cement particles and their hydration products, extend the induction period of hydration and reduce the hydration rate [
56,
57], and prevent the formation of flocculates. This lowers the paste strength, releases some of the encapsulated free water, and slows the increase of rheological parameters. However, due to the grafting rate of approximately 29% for the MGO developed in this study, the number of polymer chains capable of exerting steric hindrance and electrostatic repulsion is relatively limited, causing the cement paste strength to still increase significantly over time. Additionally, as hydration progresses, the alkalinity of the cement grout further increases, resulting in degradation of polymer chains grafted on the MGO surface. To improve MGO’s ability to regulate the time-dependence in rheological properties of cement grout, future research should focus on increasing the grafting rate of MGO, selecting appropriate polymer chains for grafting, and understanding their degradation rates under different alkaline conditions created by the hydration of cement. An ideal MGO for cement grout would have a sufficiently high grafting rate, with polymer chains remain stable during the grouting operation period but degrade rapidly after a certain period, thus avoiding any negative impact on the setting of the cement grout.
3.4 Effect of modified graphene oxide on grout stability
The bleeding reflects the degree to which water separates from the grout. The higher the bleeding rate, the harder it is for the grout to completely fill fractures or voids in the formation to be injected. It not only affects the grouting quality but also influences the design of the volume of grouting materials. Under the same conditions, a lower bleeding rate always results in better grouting performance, less volume shrinkage after solidification. The bleeding behavior of the cement grout primarily occurs within the first two hours, so the 2-h bleeding rate is used to assess grout stability at different temperatures. As shown in Fig. 17, it could be found that the 2-h bleeding rate of all three kind of cement grouts decreases with increasing temperature and increases with increasing
W/C. The higher the water−cement ratio, the more pronounced the change in the 2-h bleeding rate with temperature. Taking a 5% as the threshold for stable grout [
58], all of these cement grouts are considered unstable. The incorporation of either GO or MGO does not significantly affect the stability of the cement grouts. The decrease in bleeding rate at elevated temperature can be attributed to the reduction in surface tension of the grout, which decreases with increasing temperature [
59]. With decreasing surface tension at elevated temperatures and the effect of TEA which can be considered a gas-producing agent, small air bubbles may be introduced into the mixture during agitation. The presence of these bubbles reduces the settling of cement components, leading to a decrease in bleeding rate at elevated temperatures [
34]. In grouts with higher
W/C ratios, more free water is available to be affected by these factors, resulting in a more pronounced reduction in bleeding rate.
3.5 Effect of modified graphene oxide on setting of cement grouts
The setting time is a crucial parameter for grouting materials. If it is too long, the early strength development will be too slow, and the grout may spread beyond the intended area. On the other hand, if the setting time is too short, the grout will thicken rapidly, leading to a shorter penetration distance and making it difficult to effectively grout the targeted region. Figure 18 shows the setting time of the grouts at different temperatures. The results indicate that the elevated temperature significantly reduces both the initial setting time and the final setting time. Under the same conditions, compared to the control group, the cement grout with GO shows a slight reduction in setting time, while the cement grout with MGO experiences a extension in setting time. Moreover, compared to the magnitude at 25 °C, the setting time of cement grouts with the same mix ratios at 35 °C is reduced by 40%–50%, and by 50%–60% at 45 °C. This pattern is consistent across cement grouts with all water−cement ratios. This suggests that although MGO can slightly delay the setting of the cement paste, the primary factor influencing the setting rate of the cement paste remains the temperature.
As the temperature rises, the rate of cement hydration accelerates, generating more flocculent products, which in turn enhances the mechanical strength of the grout and promotes cement grout setting. When MGO is introduced, the steric hindrance and electrostatic repulsion of the polymer chains grafted on the surface of MGO can partially reduce the mutual contact between cement particles and decrease the formation of flocs from hydration products, thereby extending the setting time of the grout to some extent. However, due to the limited number of polymer chains and the faster hydration of cement at higher temperatures. The accelerated hydration of cement increases the alkalinity of the grout, resulting in a higher degradation rate of the polymer chains. Consequently, the period during which MGO can exert its retarding effect is shortened. Nevertheless, we have demonstrated that grafting polymer chains onto GO to create MGO can effectively address the typical issue of flowability reduction if nanomaterials are introduced into the cement grout mixture, while also partially delaying the setting of cement grouts at high-temperature. This offers a highly promising approach for enhancing high-temperature cement grouts with nanomaterials. To further extend the setting time under high-temperature conditions, it may be necessary to introduce additional additives, such as borax, into the mixture.
3.6 Effect of modified graphene oxide on un-confined compressive strength and microstructure of hardened cement grouts
The strength performance of grout materials after setting directly impacts the quality of ground reinforcement using grouting. At elevated temperatures, the early strength of cement grout will be increased by accelerating the hydration rate [
60,
61], while the later compressive strength of hardened cement grouts tends to decrease [
60,
62], due to the density of the hardened grouts becomes lower and porosity increases at high temperatures [
32,
63]. Traditional methods of improving the rheological properties of cement grouts, such as adding water-reducing agents or increasing the water−cement ratio, inevitably lead to a reduction in the strength of the hardened grouts. In addition to improving the rheological properties of the grout at fresh state, it remains to be seen whether the MGO developed in this study can also enhance the compressive strength of the hardened grout. Figure 19 indicates the results of UCS at curing period of 7 d and 28 d. It could be found that for the control group with
W/C = 0.5, the 28-d UCS of pure cement grouts decreased by about 6% at 35 °C and by approximately 19% at 45 °C, while the 7-d UCS increased by 0.3% at 35 °C and 0.6% at 45 °C, compared to that at 25 °C curing condition. This can be attributed to the accelerated early hydration rate induced by high temperatures. The incorporation of MGO or GO cannot only mitigate the strength loss caused by high temperature but also further enhance the early strength. Specifically, compared to the control group with
W/C = 0.5, the 7-d and 28-d UCS of grouts with GO increased by 22% and 12.6% at 25 °C, 1.3% and 6.3% at 35 °C, and 10% and 4.3% at 45 °C, respectively. In contrast, the 7-d and 28-d UCS of grouts with MGO increased by 37.9% and 11.2% at 25 °C, 15.6% and 9% at 35 °C, and 11.1% and 12.2% at 45 °C, respectively. Similar trends are also observed for the grouts with MGO and
W/C = 0.6 or 0.75. The results confirms that the MGO developed in this study is capable of simultaneously improving both the rheological properties of the cement grout at fresh state and the strength development at hardened state, especially at elevated temperatures.
Microstructural investigation is often a key method for uncovering the changes of the macroscopic strength in hardened cement grout. Figure 20 shows the scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) results of the cement grouts with a water-cement ratio of 0.5 at 45 °C. Compared to Fig. 20(a), the carbon content in Figs. 20(b) and 20(c) is much higher in specific area highlighted with yellow square box, which can be attributed to the incorporation of MGO/GO [
64]. It is also observed that a large amount of cement hydration products, including C-S-H gel and CH platelets, have formed at the edges of MGO and GO. In contrast, the control group (without GO or MGO) has a smaller quantity of hydration products, with a substantial amount of un-hydrated cement particles remaining. The Ca/Si ratio obtained from EDS analysis can reflect the hydration rate. Yan et al. [
65] found that the Ca/Si ratio can be utilized to assess the degree of GO-promoted cement hydration, where a decrease in the Ca/Si ratio leads to an increased hydration. As shown in Fig. 20, after adding MGO or GO, the Ca/Si ratio of cement grouts decreased from 5.05 of the control group to 2.9 of grout with MGO and to 2.7 of grout with GO, respectively. This indicates that both MGO and GO can significantly promote the cement hydration. Silva et al. [
66] also confirmed the fact that the Ca/Si ratio of cement mortar samples decreases with the incorporation of GO, thereby it promoted the formation of C-S-H gel.
This is primarily because the nanoparticles serve as nucleation sites for cement hydration products like C-S-H, promoting its growth [
67]. The nanoparticles can also fill the voids between the cement hydration products, resulting in a denser cement grout matrix and consequently higher strength upon the addition of MGO. It is important to note that uniform dispersion is a prerequisite for nanomaterials to exert their performance-enhancing effects in materials. The extent to which GO enhances the mechanical properties of cement-based materials is significantly influenced by the size of floccules or the degree of aggregation. The larger the flocs and the greater the aggregation, the weaker the enhancement of the strength of cement-based material by GO [
68]. Furthermore, it is evident that the MGO disperses more uniformly in the cement grout compared to GO, whereas the GO tends to aggregate or stack together to a certain extent. This behavior is due to the fact that GO lacks the long polymer chains grafted on MGO, resulting in little steric hindrance or electrostatic repulsion. As a result, under the significant surface energy of nanoparticles, GO tends to stack and aggregate, which negatively affects its ability to enhance the strength of the cement grout. It is known that the more uniformly GO is dispersed in the cement paste, the stronger the interfacial connection between GO and the hydration product, C-S-H, ultimately leading to higher strength of the cement paste [
69]. In contrast to GO, MGO disperses more evenly in the cement grout, and the interfacial connection between MGO and the C-S-H is much stronger, resulting in cement grout with higher compressive strength after incorporation of MGO.
Figure 21 shows the SEM images of three types of cement grouts with a water−cement ratio of 0.5, exposed to curing conditions of 45 °C. It can be observed that the control group samples contain a large number of micro-cracks and pores, resulting in a comparatively loose structure. In contrast, the hardened cement grouts containing GO or MGO demonstrate a significantly denser structure, with a noticeable reduction in the number of micro-cracks and pores. Specifically, the cement grout with MGO appears to be the densest. Based on the SEM images, the estimated surface porosity of the samples using the grayscale method are 3.834% for the control group, 1.618% for the samples with GO, and 0.557% for grout with MGO, respectively. Based on the results of the 7-d and 28-d samples, it can be observed that the incorporation of MGO accelerates the formation of cement hydration products and optimizes the microstructure of the cementitious matrix, which significantly reduces the porosity and increases the strength of the samples.
4 Conclusions
To address the issue of deteriorated rheological behavior and reduced compressive strength of cement grouting at varying temperatures, this study develops a novel nanomaterial namely MGO. Through extensive experimental investigations, the effects of MGO on various aspects of cement grouts under different temperatures were evaluated, including its initial rheology, time-dependent rheological behavior, grout stability, setting time, compressive strength, and microstructure. The underlying reasons for the improvements in these properties brought about by MGO are also discussed. The results indicate that grafting polymer chains onto the oxygen-containing active functional groups on the surface of GO to form MGO can simultaneously enhance the rheological properties and strength of cement grout, improve its injectability at high temperatures, and facilitate better grout quality in deep underground grouting. The main conclusions are as follows:
1) For improving properties of cement grouts, a novel nanomaterial, MGO, is synthesized by grafting polymer chains from MAPE onto the surface of GO using a free-radical copolymerization method. The polymer chains are primarily grafted onto the -COOH and -OH functional groups on the MGO surface, with a grafting rate of approximately 29%. These long polymer chains alter some of the defects associated with the large specific surface area of GO, thus enhancing the potential for improving the properties of cement grouts using the MGO.
2) The MGO effectively enhances the initial rheological properties of cement grouts at varying temperatures. The lower the water−cement ratio, the thicker the grout, and the more significant the improvement in the initial flowability of the grout with MGO. Microstructural observations and UV-Vis spectroscopy show that, compared to GO, MGO contains a substantial number of long polymer chains, which could provide steric hindrance and electrostatic repulsion, resulting in improved dispersion of the material in cement grout.
3) The time-dependent rheological behavior of cement grouts at various temperatures can be virtually reduced by the MGO developed. The changes in plastic viscosity and yield stress over time in MGO-modified cement grouts are lower than those in the control group (pure cement grouts), and significantly lower than in GO-modified cement grouts, owing to the steric hindrance and electrostatic repulsion. Although cement hydration accelerates under high temperatures, the long polymer chains grafted onto MGO decompose more rapidly, which limits the extent to which MGO can modify the time-dependent rheology of cement grout at high temperatures. Nevertheless, the results still indicate that MGO with grafted polymer chains provides a promising potential in improving the properties of cement grouts.
4) The MGO has minimal impact on the stability of cement grouts but can delay the setting time to a certain extent. Based on the 2-h bleeding rate, the cement grout, whether modified with MGO or GO, remains unstable at varying temperatures. The long polymer chains of MGO can partially prevent the contact between cement particles and the formation of hydration product flocculates. However, the accelerated decomposition of the chains at high temperatures limits the extent to which MGO can delay the setting of cement grout.
5) The compressive strength of hardened cement grouts with MGO increases at various temperatures. The smaller the water−cement ratio, the more pronounced the increase in strength. Microstructural observations reveal that MGO is evenly dispersed throughout the cement paste, with the hydration product C-S-H growing closely around MGO. MGO not only fills the pores but also promotes the growth of cement hydration products, resulting in a denser structure and, consequently, improving the UCS of the hardened cement grout.
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