The cement used was a superfine cement (#42.5) that complied with Chinese standard GB 175–2007. Bentonite (sodium) was obtained from a mining processing plant that conforms to the Chinese standard of bentonite (GB/T 20,973–2007). Emulsified asphalt (cationic type) was supplied by Chemical Co., Ltd. Some properties of the materials are listed in Table 1.

A schematic of the CACG fabrication process is shown in Figure 1. Firstly, appropriate amount of bentonite was initially mixed with the required water by a mixer at 1,000 rpm for 5 min to form fresh bentonite grout. Next, appropriate amount of superfine cement was added, and the fresh suspensions were mechanically mixed at 1,000 rpm for 5 min. Finally, the proper proportion of emulsified asphalt was added to the fresh suspensions and mixed for 5 min by vigorous stirring (1,000 rpm) to form CACG. All the processes were performed under a standard temperature (20 ± 2°C) and humidity (≥50%). CACG performance was altered by changing the number of components, and the testing scheme is shown in Table 2.

In this work, the gel time of CACG was measured by the inverted cup method (tilted at 45°) as described in Zhang et al. (2018a). The fluidity was determined according to GBT-2419–2005. Bleeding was measured by conducting sedimentation tests according to DL/T5728-2016. The bulk shrinkage rate test was measured following the suggestions provided by Zhang et al. (2018b). The 28-day unconfined compressive strength (UCS) tests were conducted following the instructions of GB50107-2010 on cube specimens with a diameter ratio of 70.7 mm.

Gel time is a key parameter to characterize the pumpability of CACG. Figure 2 shows the effects of the bentonite dose and emulsified asphalt dose on the properties of CACG. As shown in Figure 2, with an increasing bentonite dose (from 5 to 15%), the gel time decreases from 150 to 87 min, and the decreasing amplitude is reduced when the bentonite dose reaches 11%. This is because bentonite particles have an excellent absorption capability, which can immobilize water and promote cement hydration (Manić et al., 1997; Pusch and Yong, 2003). With an increasing emulsified asphalt dose, the gel time slightly increases (Figure 2). When the emulsified asphalt dose is 20%, the gel time increases by 10.1%. Fluidity is a key parameter to determine the diffusion capacity of CACG. Figure 3 shows the effects of the bentonite dose and emulsified asphalt dose on the fluidity. With an increasing bentonite dose, the fluidity of CACG decreases, and a larger decrease occurs at the beginning (Figure 3); this result agrees with the results presented by Zhang et al. (2020). Figure 3 shows that with an increasing emulsified asphalt dose, the fluidity of CACG decreases. When the emulsified asphalt dose is 20%, the fluidity decreases by 23.8%. This is probably because the emulsified asphalt particles with a certain viscosity fill the space created by the cement-bentonite particles (Tregger et al., 2010).

The stability of CACG can be assessed by the bleeding and bulk shrinkage rates, and the lower bleeding and bulk shrinkage rates of CACG means it is more stable. Figure 4 and Figure 4 show the bleeding rate of CACG in the tests. With an increasing bentonite dose, the bleeding rate decreases, and the decreasing and increasing amplitudes decrease when the bentonite dose is 11%. Compared with the bentonite dose, the emulsified asphalt dose demonstrates little influence. With an increasing emulsified asphalt dose, the bleeding rate slightly decreases. As shown in Figure 5, it can be inferred that with an increasing bentonite dose, the bulk shrinkage rate of CACG decreases greatly. Figure 5 shows that the emulsified asphalt dose does not have an obvious effect on the bulk shrinkage rate. In conclusion, the bentonite dose has a positive effect on the bleeding and bulk shrinkage rates, namely, bentonite substantially decreases the bleeding and bulk shrinkage rates of CACG. Similar experimental phenomena have also been reported in the literature (e.g., Vipulanandan and Sunder, 2012).

Figure 6 shows the influence of the bentonite dose and emulsified asphalt dose on the unconfined compressive strength of 28-day CACG. Figure 6 shows that with an increasing bentonite dose, the 28-day unconfined compressive strength decreases. When the bentonite dose is 15%, the 28-day unconfined compressive strength decreases by 14.5%. Figure 6 shows that with an increasing emulsified asphalt dose, the 28-day unconfined compressive strength significantly decreases. When the emulsified asphalt dose is 20%, the 28-day unconfined compressive strength decreases by 31.5%. The bentonite in CACG adsorbs a large amount of free water and slows the hydration process of the cement (Horpibulsuk et al., 2012). Hence, the addition of excess bentonite will lead to a decreased reinforcement effect of the grouting.

The superior mechanical performance of CACG can be closely related to its microscopic structure. Figure 7A show the cross-sectional SEM images of CACG and cement grouts, respectively. As shown in Figure 7A, the cement gel in CACG were interconnected with each other to form a network by bentonite particles, and some asphalt film were uniformly distributed between networks to form an integrated whole. The above SEM observation shows that the structure of CACG is favorable because of its mechanical performance. The crystalline structure and composition of CACG were identified by XRD data, and the identified the typically overlapping by MDI Jade. As seen in Figure 4, the relatively strong and narrow diffraction peaks at 2θ = 5.20, 12.67, and 32.21 are generally considered as the characteristic peaks of SiO₂; the diffraction peaks at 2θ = 4.89, 7.58, 34.513 and 36.46 are considered as the characteristic peaks of Ca(OH)₂; the diffraction peaks at 2θ = 19.79, 27.54, 42.616 and 53.66 are considered as the characteristic peaks of Aft; the diffraction peaks at 2θ = 29.893, 47.534, and 52.116 are considered as the characteristic peaks of CaCO_3.

The components of CACG are superfine cement, bentonite, emulsified asphalt and water, each of which plays its own role. The possible formation mechanism of CACG is proposed: 1) First, the superfine cement particles are uniformly dispersed in the bentonite slurry, and there is a charge attraction between the superfine cement particles and bentonite particles to form cement–bentonite balls ^[20-21]. Then, a number of hydration products are formed that fill the pores that form in the cement–bentonite balls. Finally, stable cement–bentonite pellet structures are created. The macroscopic manifestations are that the stability of CACG is improved, and the bleeding and bulk shrinkage rates decrease. 2) After the emulsified asphalt is added to the cement–bentonite grout, a large amount of asphalt particles will adsorb on the surface of the cement–bentonite pellet structures to form an asphalt film, which wraps around the cement hydration products. As a result, some three-dimensional network structures are formed in CACG. Moreover, the asphalt particles adsorb on the surface of bentonite particles to form a water film, which decreases the friction among the bentonite particles. Moreover, the water film causes the asphalt particles to have the same charge properties; thus, they will not agglomerate into large particles. This is the reason why CACG is both uniformly dispersed and relatively stable.

Based on the excellent synergistic interactions among the superfine cement, bentonite and emulsified asphalt, CACG is a new grouting material with excellent performance and an adjustable gel time, good fluidity, high stability and satisfactory compressive strength.

Generally, grouting materials in water-bearing sand layers need very high fluidity, an adjustable gel time, lower bleeding and bulk shrinkage rates, and sufficient compressive strength. In this study, based on the requirements of the project participants, the recommended parameters for the grouting material include a gel time of more than 30 min to meet the pumping requirements, a bleeding rate of less than 5% and a bulk shrinkage rate of more than 80% to meet the grouting stability requirements, and a 28 days compressive strength of more than 2 MPa to meet the reinforcement requirements. The tests show that CACG is an excellent material for grouting in water-bearing sand layers with the recommended optimal percentage ratio of 1:1:0.065:0.2 for water, superfine cement, bentonite, and emulsified asphalt.