Investigation and Utilization of Alkali-Activated Grouting Materials Incorporating Engineering Waste Soil and Fly Ash/Slag

: The alkali-activated composites technique is a promising method for the in situ preparation of cavity filling/grouting materials from engineering waste soil. To investigate the feasibility of engineering waste soil utilization by the alkali activation process, the macroscopic and microscopic properties of the fly ash/slag-based alkali-activated composites, after solidification/stabilization (S/S) with sandy clay excavated at Baishitang Station of Shenzhen Metro, were studied. The uncon-fined compressive strength (UCS) test was conducted to evaluate the S/S effect of alkali-activated composites. The results show that the optimum quality ratio of slag and fly ash correspond to 7:3, the modulus of alkaline activator to 1.3, and the alkalinity of alkaline activator to 10%. The alkali-activated composite’s strength under these parameters can reach 45.25 MPa at 3 days, 49.85 MPa at 7 days, and 62.33 MPa at 28 days. A maximum 3-day UCS of 21.71 MPa, 75% of the 28-day UCS, was achieved by an engineering waste soil and alkali-activated composites mass ratio of 5:5, slaked lime content of 4.5%, and a water-to-solid ratio of 0.26, and it can also meet the required fluidity and setting time for construction well. Fluidity is primarily affected by the soil-to-binder ratio, which decreases as the ratio decreases, while the water-to-solid ratio increases fluidity. Slaked lime has minimal impact on fluidity. The setting time is mainly influenced by the soil-to-binder ratio, followed by slaked lime content and water-to-solid ratio, with setting time shortening as the soil-to-binder ratio and slaked lime content increase, and lengthening as the water-to-solid ratio increases. Through Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) tests, microscopic analysis showed that loose granular units are firmly cemented by alkali-activated composites. Based on the results of on-site grouting tests in karst caves, the alkali-activated grout materials reached a strength of 5.2 MPa 28 days after filling, which is 162.5% of the strength of cement grouting material, satisfying most of the requirements for cavity filling in Shenzhen.


Introduction
As urbanization in China progresses, the development and utilization of underground spaces have become a key aspect of urban planning.However, some cities in southern China encounter the problem of crossing complex karst development zones during underground engineering construction.Therefore, it is crucial to strengthen reinforcement measures for cavern structures [1,2].Currently, cement-based grouting materials are commonly used for the reinforcement and filling the caves.However, their production and application can have negative impacts on the environment.It is essential to develop a new type of grouting material that is environmentally friendly, low in carbon, and cost-effective.
In recent years, the landfill disposal of industrial wastes, such as slag and fly ash, which are continuously generated in the process of industrialization, has become a significant issue.The issue not only encroaches on precious land resources, but also contributes to grouting tests.This study combines indoor experiments, microscopic tests, and field tests to elucidate the strength mechanism of alkali-activated grout materials and verify their engineering practicality.

Proportioning of Alkali-Activated Composites' Cementitious Materials
2.1.Test Materials 2.1.1.Slag and Fly Ash The alkali-activated composites utilized in this test were primarily composed of slag and fly ash, sourced from Longze Water Purification Materials Co., Ltd., and Borun Refractory Co., Ltd., both located in Gongyi City, China.The slag was grayish white and the fly ash was dark gray.Their primary components were silicon dioxide (SiO 2 ) and aluminum oxide (Al 2 O 3 ), with surface areas of 2254 m 2 /kg and 34.15 m 2 /kg, respectively (shown in Tables 1 and 2).The particle size distributions of the slag and fly ash were determined by a laser particle size analyzer, and the particle size gradation curves are shown in Figure 1.

Alkaline Activator
In this study, water glass (Na 2 SiO 3 ) and sodium hydroxide (NaOH) were selected as activators for the slag fly ash-based alkali-activated composites; they were sourced from Jiashan County Youreinai Refractory Materials Co., Ltd., Jiaxing, China, and the Guangdong Province Engineering Technology Research Center for High-Purity Chemical Reagents, Guangzhou, China respectively.The purity of NaOH was analytically pure, with a solid content of 96%, while the modulus of composition of Na 2 SiO 3 was 2.4, with its main components detailed in Table 3.The production process of an alkaline activator involves several meticulous steps.Initially, the Na 2 SiO 3 and NaOH required for different moduli were calculated according to Formulas (1) and (2), then the corresponding masses of Na 2 SiO 3 and NaOH were weighed.The next step involved the gradual incorporation of NaOH into Na 2 SiO 3 , ensuring constant stirring until the solution became free of solid particles.After achieving the desired modulus, the Na 2 SiO 3 solution was coated and left to cool to room temperature.To ensure complete heat dissipation, the solution is typically allowed to stand for no less than 12 h.Finally, the prepared alkaline activator solution was coated, placed and labeled for spare parts.
The modulus of the water glass is as follows: The alkalinity of the water glass is as follows: In the formula, n(SiO 2 ) represents the molar quantity of silicon dioxide; n(Na 2 O) denotes the molar quantity of sodium oxide; m(Na 2 O) refers to the mass of sodium oxide; and m(SLG + FA) indicates the combined mass of slag and fly ash.

Alkali-Activated Composites' Parameter Test Plan
This test employs a single-factor control trial to investigate the influence of various factors on the strength of alkali-activated composites.Slag and fly ash are used as raw materials for alkali-activated composites.Although their component contents and primary particle size ranges differ, they complement each other effectively.Alkalinity, defined as the mass ratio of Na 2 O to the slag-fly ash combination, plays a crucial role.By adjusting the alkalinity, one can regulate the amount of alkali activator added, thereby affecting the reaction efficacy of geological polymerization.Modulus is a key parameter for measuring the performance and quality of alkali exciters, specifically refering to the molar ratio of SiO 2 to Na 2 O.
Owing to the diversity of materials, preliminary experiments and literature reviews have identified that precisely defining the optimal proportion range between slag and fly ash presents a significant challenge [36][37][38].Consequently, this study designed a number of different combinations of slag and fly ash ratios and the key parameters, such as alkaline activator modulus, alkalinity, and water-to-solid ratio, were kept constant.According to existing literature, the study has established the optimal range of alkalinity for the alkali activator to be between 5.0% and 12.5%, setting four level values and holding other factors constant [39,40].The alkaline activator modulus was found to be optimum at 0.9-1.5, with 4 modulus levels set and other factors fixed [41,42].As there are no specific guidelines for geological consolidation, this study follows the Chinese national standard "Test Method for Strength of Cement Mortar" (GB/T17671-2021) [43] to determine the curing periods.The chosen curing periods are 3 days, 7 days, and 28 days.The specific test program is presented in Table 4. Figure 2 shows the impact of varying slag-to-fly ash ratios on the UCS of alkaliactivated composites at different ages.Under standard curing conditions, with a constant water glass modulus and alkalinity, an initial increase followed by a decrease in the UCS was observed as the slag content increased and the fly ash content decreased.This trend in strength variation over different ages was generally consistent.The maximum UCS values observed at a slag-to-fly ash ratio of 7:3 across all curing periods were 45.25 MPa, 49.85 MPa, and 62.33 MPa.When the age of maintenance is 28 days, it is 133% and 712% of the strength of pure slag and pure fly ash alkali-activated composites, respectively.These findings indicate that the optimum ratio of slag and fly ash in alkali-activated composites is 7:3, and the effect of compound mixing is much better than that of the single mixing of slag or fly ash.
When comparing curing periods of 3 days and 7 days, the average strength of specimens increased by 30.1%.For slag-to-fly ash ratios of 10:0 and 9:1, the strength increments were minimal, at 0.04% and 0.77%, respectively.However, with a 5:5 ratio, the strength increased most significantly by 88.13%.Extending the curing period from 7 days to 28 days resulted in an average strength increase of 67.93%.
Figure 2 shows that alkali-activated composites cured for 3 days achieve 51.5% of their 28-day strength, while those cured for 7 days reach 65.0%.Between 3 and 7 days, strength growth is low with a higher slag content, but increases as the fly ash content rises.From 7 to 28 days, the strength growth rate initially decreases, and then increases with a higher slag content and lower fly ash content.This indicates that slag contributes most to early strength, while fly ash enhances strength at later stages.The smaller particle size and larger specific surface area of slag enable a more complete reaction with the alkali activator.Thus, a higher slag content is generally preferred, but adding some fly ash can also improve the strength due to its ongoing reaction.However, higher fly ash proportions result in less complete alkali activation reactions, making reduced additions of fly ash beneficial for enhancing strength.When comparing curing periods of 3 days and 7 days, the average stre mens increased by 30.1%.For slag-to-fly ash ratios of 10:0 and 9:1, the streng were minimal, at 0.04% and 0.77%, respectively.However, with a 5:5 ratio increased most significantly by 88.13%.Extending the curing period from days resulted in an average strength increase of 67.93%.
Figure 2 shows that alkali-activated composites cured for 3 days ach their 28-day strength, while those cured for 7 days reach 65.0%.Between strength growth is low with a higher slag content, but increases as the fly ash From 7 to 28 days, the strength growth rate initially decreases, and then inc higher slag content and lower fly ash content.This indicates that slag contri early strength, while fly ash enhances strength at later stages.The smaller pa larger specific surface area of slag enable a more complete reaction with the tor.Thus, a higher slag content is generally preferred, but adding some fly improve the strength due to its ongoing reaction.However, higher fly ash result in less complete alkali activation reactions, making reduced additio beneficial for enhancing strength.

The Influence of Alkali Activator Alkalinity on the UCS Behavior of A vated Composites
Figure 3 illustrates the effect of varying alkalinity on the unconfined strength (UCS) of alkali-activated composites at different ages.As alkalinity UCS of specimens cured for 3, 7, and 28 days initially rises and then falls; th increased by 12.8%, 5.3%, and 42.3%, respectively, for each group relative to one.At 10% alkalinity, the highest UCS values for 3, 7, and 28 days were reco B at 43.32 MPa, 45.22 MPa, and 56.33 MPa, respectively.At 5% and 12.5% alkalinity, UCS showed an initial decrease followed b over the curing period.At 7.5% and 10.0% alkalinity, UCS consistently in time.Specifically, the 7-day UCS of group B specimens varied by −2.02%,At 5% and 12.5% alkalinity, UCS showed an initial decrease followed by an increase over the curing period.At 7.5% and 10.0% alkalinity, UCS consistently increased over time.Specifically, the 7-day UCS of group B specimens varied by −2.02%, 7.42%, 4.39%, and −7.20% compared to their 3-day UCS values.After 3 days of curing, the UCS of alkali-activated composites reached at least 75% of their 28-day strength.By 7 days, their strength generally met or exceeded 75% of the 28-day strength, specifically 77.84%, 79.56%, 80.28%, and 88.60%, although the strength increase from 3 to 7 days was minimal.
The phenomenon observed can be attributed to the fact that, within a certain range, an appropriate increase in alkalinity is characterized by an increase in the alkali components within the reaction system.It facilitates a more thorough dissolution of silicoaluminate raw materials.Consequently, an optimal level of alkalinity not only accelerates the alkali activation reaction rate between alkali activators, slag, and fly ash, but also serves as a crucial prerequisite for ensuring the cohesive effectiveness of alkali-activated composites.An appropriate concentration of hydroxide ions (OH − ) accelerates the depolymerization and polymerization of the silicoaluminate components in slag and fly ash, thereby increasing the strength of the alkali-activated composites.However, excessive alkalinity leads to an increase in OH − within the alkali-activated composites system, resulting in the premature deposition of silicate products and inhibiting the formation and reaction rate of SiO 4 and AlO 4 tetrahedra.In addition, the presence of unreacted Na + bound to the hydrated layers of slag and fly ash prevents the further dissolution and release of reactive SiO 2 and Al 2 O 3 , inhibiting the activation reaction between slag and fly ash and the formation of alkaliactivated composites, thereby reducing their strength.On the other hand, if the alkalinity is too low, the alkali-activated composites' reaction suffers from insufficient alkalinity to break the bonds between the metal and oxygen elements, resulting in a lower dissolution rate of Si and Al elements.This incomplete reaction process within the alkali-activated composite binder results in reduced strength.It can therefore be concluded that an alkalinity of 10.0% is optimal to achieve the best cementing effect with slag and fly ash.

The Influence of the Alkali Activator Modulus on the UCS Behavior of Alkali-Activated Composites
Figure 4 shows the effect of various alkali activators on the unconfined compressive strength of alkali-activated composites over different ages.The trend indicates that, as the modulus of the alkali activator increases, the UCS of the alkali-activated composites initially increases and then decreases after 3, 7, and 28 days of curing.The peak strength across all ages groups is achieved when the alkali activator modulus is 1.3, resulting in UCS values of 43.32 MPa, 45.22 MPa, and 56.33 MPa, respectively.As the alkaline activator modulus increases, the 28-day strength of each alkali-activated composite group increases by 4.96% and 9.21%, before decreasing by −8.60%.It is noteworthy that, at a curing age of 3 days, the UCS reaches over 55% of the 28-day-cured body strength.By 7 days, the strength generally surpasses 75% of the 28-day-cured body strength, specifically 71.95%, 76.10%, 80.28%, and 77.21%.
The reasons for the above situation are: in the case of alkalinity in the appropriate range, the modulus is low, Na + is easy to enrich and adsorbed on the surface of slag and fly ash particles; this phenomenon will interfere with the process of their effective hydrolysis reaction.In the case of a higher modulus, the SiO 2 content is higher, resulting in a significant shortening of the alkali-activated composites' coagulation time, which will affect the dissolution of the slag and fly ash in the alkali-activated composites, and ultimately weaken it.A suitable modulus ensures an equilibrium concentration between Na + and active SiO 2 .Specifically, a moderate amount of active SiO 2 can effectively polymerize with the Al and Si elements released from the slag and fly ash while controlling the coagulation rate from being too fast, and also adsorb the free Na + in the solution.In addition, an appropriate amount of Na + can reduce its excessive adsorption on the surface of the hydration film of slag and fly ash, thus facilitating the further activation and corrosion of the internal structure of slag and fly ash.At the same time, the appropriate amount of Na + can also be used as a cementing component to fill the internal pores of the material and enhance the mechanical properties of the alkali-activated composites matrix.Therefore, within the study range of this experiment, the gelling effect of alkali-activated composites is most enhanced when the modulus of the alkali exciter is 1.3.modulus of the alkali activator increases, the UCS of the alkali-activated compos tially increases and then decreases after 3, 7, and 28 days of curing.The peak s across all ages groups is achieved when the alkali activator modulus is 1.3, resu UCS values of 43.32 MPa, 45.22 MPa, and 56.33 MPa, respectively.As the alkaline a modulus increases, the 28-day strength of each alkali-activated composite group in by 4.96% and 9.21%, before decreasing by −8.60%.It is noteworthy that, at a curin 3 days, the UCS reaches over 55% of the 28-day-cured body strength.By 7 da strength generally surpasses 75% of the 28-day-cured body strength, specifically 76.10%, 80.28%, and 77.21%.The reasons for the above situation are: in the case of alkalinity in the appr range, the modulus is low, Na + is easy to enrich and adsorbed on the surface of s fly ash particles; this phenomenon will interfere with the process of their effective ysis reaction.In the case of a higher modulus, the SiO2 content is higher, resulti significant shortening of the alkali-activated composites' coagulation time, which fect the dissolution of the slag and fly ash in the alkali-activated composites, a mately weaken it.A suitable modulus ensures an equilibrium concentration betw and active SiO2.Specifically, a moderate amount of active SiO2 can effectively poly with the Al and Si elements released from the slag and fly ash while controlling th ulation rate from being too fast, and also adsorb the free Na + in the solution.In ad an appropriate amount of Na + can reduce its excessive adsorption on the surfac hydration film of slag and fly ash, thus facilitating the further activation and corro the internal structure of slag and fly ash.At the same time, the appropriate amoun can also be used as a cementing component to fill the internal pores of the mater enhance the mechanical properties of the alkali-activated composites matrix.Th within the study range of this experiment, the gelling effect of alkali-activated com is most enhanced when the modulus of the alkali exciter is 1.3.

Test Materials
The experiment conducted in this chapter used consistent specifications and preparation processes for the slag, fly ash, and alkaline activator, as in 2.1.Therefore, we will not repeat them here.Instead, we will focus on the introduction of new test materials, specifically in situ soils from the Shenzhen area and slaked lime.

In Situ Soil from Shenzhen
The soil samples collected in situ from the construction site of Shenzhen Metro Line 3 (shown in Figure 5) exhibit a yellowish-brown hue and are predominantly in a hard plastic physical state.The soil has relatively weak adhesion and average textural properties, is roughly cut, and contains a gravel component approximately in the range of 10-30%.Furthermore, the sample contains weathered mineral particles of larger sizes in certain areas.These particles are mainly composed of quartz and exhibit heterogeneous hardness properties.The soil can be classified as a residual soil that originates from the granite parent rock.The specific parameters are detailed in Table 5, while the particle size distribution is illustrated in Figure 6.

Slaked Lime
In the process of reinforcing the in situ soil of Shenzhen using alkali-activated composites, it was observed that the alkaline conditions within the alkali-activated grout materials system were relatively mild compared to the pure alkali-activated composites cementitious system.This observation could limit the strength and speed of the alkali-activated composites' reaction to some extent.In light of this, to enhance the alkaline reaction environment and promote the reinforcement effect, slaked lime (Ca(OH) 2 ) was added to alkali-activated grout materials to reduce the acidic influence of sandy clay and provide calcium ions while ensuring alkalinity, thereby improving strength.The slaked lime, with a purity of 95%, was procured by Shanghai Mayer Biochemical Technology Co., Ltd.(Shanghai, China).

Alkali-Activated Composite Grouting Material UCS and Flowability Test Plan
This study delves into the impact of various parameters on the strength and fluidity of grouting materials, with a focus on the alkali-activated composite solidification characteristics of in situ soil in the Shenzhen area.It systematically examines the influence of three critical variables: the soil-to-binder ratio, slaked lime content, and water-to-solid ratio on the UCS and fluidity of alkali-activated grout materials.To reveal the interactions between the factors, a three-factor, four-level orthogonal test protocol is designed (as shown in Table 6).Three representative specimens from each set of test samples were selected for UCS tests after a standard 28-day curing age.The test results will aid in determining the optimal proportioning and mechanical properties of alkali-activated grout materials.Note: The soil-to-binder ratio in the table is the mass ratio of the in situ soil to the ground aggregate; the slaked lime content is the mass ratio of the slaked lime to the ground aggregate; and the water-to-solid ratio is the ratio of the sum of the masses of the water in the mixing water and the alkali stimulant to the total mass of the solids.

Results and Analysis of Alkali-Activated Composites Grouting Material Fluidity Test
Fluidity is a crucial indicator for grouting materials used in karst cave treatments, directly affecting their pumpability, injection efficiency, and quality.High fluidity materials can pass through pumping systems more smoothly, filling voids and cracks, enhancing reinforcement effects, and reducing resistance and wear during the pumping process, thereby extending equipment lifespan.Therefore, when selecting and preparing grouting materials, their fluidity should be thoroughly considered.This can be achieved by adjusting the water-cement ratio and additives to meet project requirements, and by conducting comprehensive indoor tests to evaluate their fluidity.
To ensure that the fluidity of the alkali-activated composite-stabilized soil in this experiment meets grouting requirements, the "Test Method for Fluidity of Cement Mortar" (GB/T 2419-2005) [44] was referenced to test the fluidity of the designed alkali-activated composite-stabilized soil.According to the "Construction Plan for Karst Treatment in the Low-White Section of Work Area 2 of the Phase IV Project of Shenzhen Urban Rail Transit Line 3", the fluidity of grouting materials is classified as either qualified or unqualified based on the fluidity test.The qualified fluidity range is 190-220 mm, while unqualified fluidity is either greater than 220 mm or less than 190 mm.The results of the fluidity test for the alkali-activated composite grouting material are shown in Table 7.According to the results of the fluidity test, within the scope of this experimental study, when the soil-to-binder ratio is less than 8:2, the fluidity of the alkali-activated composites grouting material meets the specific construction requirements.Table 8 and Figure 7 present the range analysis and mean analysis results for the fluidity test of alkali-activated composite grouting material, respectively.The analyses of range and mean indicate that the primary factor affecting the fluidity of alkali-activated composite grouting material is the soil-to-binder ratio.As the soil-to-binder ratio decreases from 8:2 to 5:5, the fluidity also decreases.The most significant change in fluidity occurs when the soil-to-binder ratio increases from 7:3 to 8:2.The second factor influencing fluidity is the water-to-solid ratio, with fluidity increasing as the water-to-solid ratio rises within the scope of this experiment.The addition of slaked lime, used as an alkaline supplement, has minimal impact on the fluidity due to its low content.

UCS Test Results and Analysis of Alkali-Activated Composite Grouting Material
When the curing periods were 3, 7, and 28 days, UCS tests were conducted on alkaliactivated composite grouting material samples to quantitatively evaluate the S/S effects of slag-fly ash alkali-activated composites under different influencing factors, as shown in Figure 8.The figure indicates that the strength of alkali-activated grout materials increases with the curing period.The primary growth period for strength is within the first 0-7 days.When cured for 3 and 7 days, the strength averages 56.2% and 86.1% of the 28-day strength, respectively.
During the UCS testing of alkali-activated composite grouting material samples, failure occurred.When the pressure reached the load limit of the sample, numerous microcracks appeared in the compression zone.With increasing pressure, these microcracks expanded and extended, eventually leading to the disintegration and fragmentation of the

UCS Test Results and Analysis of Alkali-Activated Composite Grouting Material
When the curing periods were 3, 7, and 28 days, UCS tests were conducted on alkaliactivated composite grouting material samples to quantitatively evaluate the S/S effects of slag-fly ash alkali-activated composites under different influencing factors, as shown in Figure 8.The figure indicates that the strength of alkali-activated grout materials increases with the curing period.The primary growth period for strength is within the first 0-7 days.When cured for 3 and 7 days, the strength averages 56.2% and 86.1% of the 28-day strength, respectively.strength, respectively.
During the UCS testing of alkali-activated composite grouting material samples, failure occurred.When the pressure reached the load limit of the sample, numerous microcracks appeared in the compression zone.With increasing pressure, these microcracks expanded and extended, eventually leading to the disintegration and fragmentation of the material.The failure mode of the specimens is shown in Figure 9.During the UCS testing of alkali-activated composite grouting material samples, failure occurred.When the pressure reached the load limit of the sample, numerous microcracks appeared in the compression zone.With increasing pressure, these microcracks expanded and extended, eventually leading to the disintegration and fragmentation of the material.The failure mode of the specimens is shown in Figure 9.  9 shows the extreme difference analysis of each factor on the activated grout materials at the maintenance ages of 3 days, 7 days, and 28 d table, it can be seen that the influence of each factor on the UCS of alkali-a materials at 3 days is arranged in ascending order as soil-to-binder ratio content > water-to-solid ratio, and the influence of alkali-activated grout days and 28 days on the UCS is arranged in ascending order as soil-to-binde to-solid ratio > slaked lime content.The alkali-activated composite grou reaches the highest UCS at the maintenance age of 28 days, while the soil-t corresponds to 5:5, the slaked lime content to 4.5%, and water-to-solid r comprehensive comparison can conclude that the influence of each factor activated grout materials of the UCS is as follows: the soil-to-binder ratio is t affecting the UCS of alkali-activated grout materials.At any maintenance decrease in the soil-to-binder ratio, its strength increases persistently.In the soil-to-binder ratio from 7:3 to 6:4, the 28-day UCS has the most obvious tre maintenance age is 3 days, the second factor that affects the UCS is slake Table 9 shows the extreme difference analysis of each factor on the UCS of alkaliactivated grout materials at the maintenance ages of 3 days, 7 days, and 28 days.From the table, it can be seen that the influence of each factor on the UCS of alkali-activated grout materials at 3 days is arranged in ascending order as soil-to-binder ratio > slaked lime content > water-to-solid ratio, and the influence of alkali-activated grout materials at 7 days and 28 days on the UCS is arranged in ascending order as soil-to-binder ratio > water-to-solid ratio > slaked lime content.The alkali-activated composite grouting material reaches the highest UCS at the maintenance age of 28 days, while the soil-to-binder ratio corresponds to 5:5, the slaked lime content to 4.5%, and water-to-solid ratio to 0.26.A comprehensive comparison can conclude that the influence of each factor on the alkaliactivated grout materials of the UCS is as follows: the soil-to-binder ratio is the main factor affecting the UCS of alkali-activated grout materials.At any maintenance age, with the decrease in the soil-to-binder ratio, its strength increases persistently.In the interval of the soil-to-binder ratio from 7:3 to 6:4, the 28-day UCS has the most obvious trend.When the maintenance age is 3 days, the second factor that affects the UCS is slaked lime, while, with the increase in the maintenance age to 7 days and 28 days, the second factor of UCS becomes the water-to-solid ratio.The UCS increased with the increase in the slaked lime content and increased with the decrease in the water-to-solid ratio at all ages.The reasons for the above situation are as follows.When alkaline activators and slaked lime act together on slag and fly ash, they can trigger a series of alkali activation reaction processes, which consists of two core phases: firstly, in the alkaline environment, the key components inside slag and fly ash, such as SiO 2 , Al 2 O 3 , etc., go through an initial depolymerization reaction phase.During this phase, the original chemical bonds are broken and rearranged in combinations, releasing elements such as reactive silicon, aluminum, and possibly sodium and calcium.The parameter soil-to-binder ratio has a decisive influence on the effective content of reactive metal oxides, which in turn affects the mechanical properties of the final alkali-activated composites.Therefore, the soil-to-binder ratio is the first factor affecting the UCS of alkali-activated grout materials at all maintenance ages.
As the reaction process advances, the released reactive elements continue to participate in the subsequent steps of the polymerization reaction through the formation of basic structural units, such as silica-oxygen tetrahedra and aluminum-oxygen tetrahedra.At this point, the presence of free Ca 2+ and Na + ions in the system under alkaline conditions promotes the connection and fusion between these units.The amount of slaked lime incorporated enhances the strength of the alkaline environment of the reaction system, which is essential for the polymerization reaction.Ultimately, these chemical reactions led to the generation of two types of gel substances: N-A-S-H type and C-A-S-H type.These gel substances not only fill into the microscopic pore structure of the composite soil material, but also act as an effective bonding medium to firmly bond the soil particles together, thus optimizing the structural integrity and density in the composite soil.Therefore, within the scope of this experimental study, an increase in slaked lime content resulted in an increase in the UCS of alkali-activated grout materials.However, as the reaction proceeds, the water in the alkali-activated grout materials will evaporate from the system, forming small cracks inside the alkali-activated grout materials.Hence, the increase in the water-to-solid ratio will adversely affect the UCS.With the prolongation of the maintenance time, more and more water in the system escapes from the alkali-activated grout materials system, making the cracks continue to increase in size.When the maintenance runs to 7 days and 28 days, the degree of influence of the water-to-solid ratio on the UCS is greater than the slaked lime ratio.The UCS values in all the periods of time decrease with the increase in the water-to-solid ratio.

Alkali-Activated Composite Grouting Material Setting Test Plan
This section examines the setting time of alkali-activated composite-stabilized soil.Using single-factor control experiments, variables such as the soil-to-binder ratio, slaked lime content, and water-to-solid ratio were set, with the optimal strength ratio obtained from orthogonal experiments serving as the control group.The soil-to-binder ratios were set at 8:2, 7:3, 6:4, and 5:5; slaked lime contents at 0.0%, 1.5%, 3.0%, and 4.5%; and water-tosolid ratios at 0.26, 0.28, 3.0, and 3.2.The aim was to explore the influence of these variables on the setting time of alkali-activated composite-stabilized soil.The specific experimental scheme is shown in Table 10.Currently, there are no specific standards for the setting time of alkali-activated composite-stabilized soil.However, alkali-activated composite-stabilized soil is similar to cement mortar, with the stabilizer playing a role similar to that of cement.The in situ soil in Shenzhen contains a higher sand content, akin to sand in mortar.Therefore, the setting time can be tested by referring to the method for testing the setting time of mortar mixtures as outlined in the "Standard for Test Methods of Basic Properties of Building Mortar" JGJ/T70-2009 [45].According to the "Construction Plan for Karst Treatment in the Low-White Section of Work Area 2 of the Phase IV Project of Shenzhen Urban Rail Transit Line 3", it is important to consider that some cavities may have fractures or channels connecting them to other cavities.The grouting material is prepared at the mixing station and transported to the construction site within 15 min.Therefore, for the application of alkali-activated composite grouting material, its setting time should be no less than 30 min and no more than 240 min.The setting time test for alkali-activated grout materials is shown in Figure 10.Currently, there are no specific standards for the setting time of alkali-activated composite-stabilized soil.However, alkali-activated composite-stabilized soil is similar to cement mortar, with the stabilizer playing a role similar to that of cement.The in situ soil in Shenzhen contains a higher sand content, akin to sand in mortar.Therefore, the setting time can be tested by referring to the method for testing the setting time of mortar mixtures as outlined in the "Standard for Test Methods of Basic Properties of Building Mortar" JGJ/T70-2009 [45].According to the "Construction Plan for Karst Treatment in the Low-White Section of Work Area 2 of the Phase IV Project of Shenzhen Urban Rail Transit Line 3", it is important to consider that some cavities may have fractures or channels connecting them to other cavities.The grouting material is prepared at the mixing station and transported to the construction site within 15 min.Therefore, for the application of alkaliactivated composite grouting material, its setting time should be no less than 30 min and no more than 240 min.The setting time test for alkali-activated grout materials is shown in Figure 10. Figure 11 shows the setting-time test results of alkali-activated grout materials under different soil-to-binder ratios.When the soil-to-binder ratio is 8:2, the setting time of the alkali-activated composite grouting material is the longest at 1098 min.As the ratio de-

Test Results and Analysis of Alkali-Activated Composite Grouting Material Setting Time
Figure 11 shows the setting-time test results of alkali-activated grout materials under different soil-to-binder ratios.When the soil-to-binder ratio is 8:2, the setting time of the alkali-activated composite grouting material is the longest at 1098 min.As the ratio decreases to 5:5, the setting time shortens to 43 min.When the soil-to-binder ratio of the alkali-activated composite grouting material is 5:5, it meets the construction requirements for the setting time of the alkali-activated composite grouting material.When the proportion of the curing agent increases from 20% to 50%, the setting rate improves by 36.07%,51.28%, and 87.43%.Therefore, as the soil-to-binder ratio decreases, the setting time of the alkali-activated composite grouting material shortens, and the setting rate increases.The more curing agent added, the greater the increase in the setting rate.Within the scope of this experiment, the shortest setting time is 43 min.This is because the curing agent plays a primary role in adhesion and strength in the material.Increasing its amount enhances its adhesive effect and accelerates the alkali-activated composites' reaction, thereby shortening the setting time.
Appl.Sci.2024, 14, x FOR PEER REVIEW 16 of 26 this experiment, the shortest setting time is 43 min.This is because the curing agent plays a primary role in adhesion and strength in the material.Increasing its amount enhances its adhesive effect and accelerates the alkali-activated composites' reaction, thereby shortening the setting time.Figure 12 shows the setting-time test results of alkali-activated grout materials with different slaked lime contents.When the slaked lime content is 4.5%, the setting time of the alkali-activated composite grouting material is the shortest; at 0.0%, the setting time is the longest.As the slaked lime content increases from 0.0% to 4.5%, the setting times are 198 min, 108 min, 76 min, and 43 min, with the setting rates increasing by 45.45%, 29.63%, and 43.42%.Therefore, with the increase in slaked lime content, the setting time decreases, ranging from a minimum of 43 min to a maximum of 198 min.This is because slaked lime enhances the alkaline environment of the system, promoting the alkali-activated composites' reaction.Figure 12 shows the setting-time test results of alkali-activated grout materials with different slaked lime contents.When the slaked lime content is 4.5%, the setting time of the alkali-activated composite grouting material is the shortest; at 0.0%, the setting time is the longest.As the slaked lime content increases from 0.0% to 4.5%, the setting times are 198 min, 108 min, 76 min, and 43 min, with the setting rates increasing by 45.45%, 29.63%, and 43.42%.Therefore, with the increase in slaked lime content, the setting time decreases, ranging from a minimum of 43 min to a maximum of 198 min.This is because slaked lime enhances the alkaline environment of the system, promoting the alkali-activated composites' reaction.
Figure 13 shows the test results of the setting time of alkali-activated composite grouting material under different water-to-solid ratios.When the water-to-solid ratio is 0.26, the setting time of the alkali-activated composite grouting material is the shortest; when it is 0.32, the setting time is the longest.As the water-to-solid ratio increases from 0.26 to 0.32, the setting times are 43 min, 87 min, 132 min, and 215 min, with setting rates increasing by 50.57%, 34.09%, and 38.60%.Therefore, within the range of this experiment, as the water-to-solid ratio increases, the setting time lengthens.This is because increasing the water-to-solid ratio weakens the alkaline environment of the system, thereby extending the setting time.
198 min, 108 min, 76 min, and 43 min, with the setting rates increasing by 45.45%, 29.63%, and 43.42%.Therefore, with the increase in slaked lime content, the setting time decreases, ranging from a minimum of 43 min to a maximum of 198 min.This is because slaked lime enhances the alkaline environment of the system, promoting the alkali-activated composites' reaction.Figure 13 shows the test results of the setting time of alkali-activated composite grouting material under different water-to-solid ratios.When the water-to-solid ratio is 0.26, the setting time of the alkali-activated composite grouting material is the shortest; when it is 0.32, the setting time is the longest.As the water-to-solid ratio increases from  In summary, the soil-to-binder ratio is the main factor affecting the setting time; the higher the content of the binder, the shorter the setting time.If the setting time of the alkali-activated composite grouting material is to meet the construction requirements for karst treatment in Shenzhen Metro, the soil-to-binder ratio should be set to 5:5.When the soil-to-binder ratio of the alkali-activated composite grouting material is 5:5, within the scope of this experimental study, altering the slaked lime content and the water-to-solid ratio can adjust the setting time to fall within the appropriate range.Next, the slaked lime content and the water-to-solid ratio also play significant roles: the higher the slaked lime content, the shorter the setting time; the higher the water-to-solid ratio, the longer the setting time.

FTIR Analysis
The FTIR tests conducted on Shenzhen's in situ soil, alkali-activated composites, and alkali-activated grout materials focus on phase transformation analysis.The obtained FTIR spectra, covering the range of 400 cm −1 to 4000 cm −1 , are illustrated in Figure 14.In summary, the soil-to-binder ratio is the main factor affecting the setting time; the higher the content of the binder, the shorter the setting time.If the setting time of the alkali-activated composite grouting material is to meet the construction requirements for karst treatment in Shenzhen Metro, the soil-to-binder ratio should be set to 5:5.When the soil-to-binder ratio of the alkali-activated composite grouting material is 5:5, within the scope of this experimental study, altering the slaked lime content and the water-to-solid ratio can adjust the setting time to fall within the appropriate range.Next, the slaked lime content and the water-to-solid ratio also play significant roles: the higher the slaked lime content, the shorter the setting time; the higher the water-to-solid ratio, the longer the setting time.

FTIR Analysis
The FTIR tests conducted on Shenzhen's in situ soil, alkali-activated composites, and alkali-activated grout materials focus on phase transformation analysis.The obtained FTIR spectra, covering the range of 400 cm −1 to 4000 cm −1 , are illustrated in Figure 14. Figure 14 shows that the absorption peak at 3620 cm −1 corresponds to the stre and bending vibrations of the H-O bond in molecular water, indicating the prese chemically bound water in the alkali-activated grout materials [46,47].Additionall distinct absorption peaks are observed at approximately 1480 cm −1 and 850 cm −1 , are attributed to the out-of-plane bending and antisymmetric stretching of the bond in CO3 2− [48].Furthermore, the presence of a prominent absorption peak be 950 cm −1 and 1050 cm −1 , which is mainly attributed to the asymmetric stretching vibr of the Si-O-X bond (where X can be Si or Al), indicates that polymerization reaction occurred within the mixture.This finding is consistent with previous research.
The study found that the inclusion of slag and fly ash in the reaction enhanc absorption peaks, indicating that they accelerated the polymerization reaction rat acceleration is attributed to the smaller bond angle and force constant of Al-O bon the substitution of Al by Si increases, the proportion of Si-O-Si bonds rises, leadin higher wave number.The absorption peaks between 450 cm −1 and 490 cm −1 are due in-plane bending vibration of Si-O-Si in the silicate tetrahedra of the hardener.In mary, the absorption peaks suggest that the final composition of the samples in both unreacted slag and fly ash powder (crystalline phase) and reaction products (s N-A-S-H and C-A-S-H), which consist of SiO4 and AlO4 tetrahedra [49].

Microstructural Properties
The properties of alkali-activated grout materials at a macroscopic level are mined by its microstructural characteristics.These characteristics comprise variat particle size, shape, micropore features, distribution patterns, as well as the modes ticle contact, compositional makeup, and interconnectivity methods.However, the structural evolution and features of alkali-activated grout materials are not discern the naked eye.Therefore, SEM is necessary to investigate them.The aim of this ap is to uncover the mechanisms of strength formation and S/S principles based on structural attributes.
Figure 15 shows the SEM and EDS test results for in situ soil in Shenzhen, activated composites, and alkali-activated grout materials.Alkali-activated comp consist of oxygen, silicon, and aluminum as the three elements with the highest co Figure 14 shows that the absorption peak at 3620 cm −1 corresponds to the stretching and bending vibrations of the H-O bond in molecular water, indicating the presence of chemically bound water in the alkali-activated grout materials [46,47].Additionally, two distinct absorption peaks are observed at approximately 1480 cm −1 and 850 cm −1 , which are attributed to the out-of-plane bending and antisymmetric stretching of the O-C-O bond in CO 3 2− [48].Furthermore, the presence of a prominent absorption peak between 950 cm −1 and 1050 cm −1 , which is mainly attributed to the asymmetric stretching vibrations of the Si-O-X bond (where X can be Si or Al), indicates that polymerization reactions have occurred within the mixture.This finding is consistent with previous research.
The study found that the inclusion of slag and fly ash in the reaction enhanced the absorption peaks, indicating that they accelerated the polymerization reaction rate.This acceleration is attributed to the smaller bond angle and force constant of Al-O bonds.As the substitution of Al by Si increases, the proportion of Si-O-Si bonds rises, leading to a higher wave number.The absorption peaks between 450 cm −1 and 490 cm −1 are due to the in-plane bending vibration of Si-O-Si in the silicate tetrahedra of the hardener.In summary, the absorption peaks suggest that the final composition of the samples includes both unreacted slag and fly ash powder (crystalline phase) and reaction products (such as N-A-S-H and C-A-S-H), which consist of SiO 4 and AlO 4 tetrahedra [49].

Microstructural Properties
The properties of alkali-activated grout materials at a macroscopic level are determined by its microstructural characteristics.These characteristics comprise variations in particle size, shape, micropore features, distribution patterns, as well as the modes of particle contact, compositional makeup, and interconnectivity methods.However, the microstructural evolution and features of alkali-activated grout materials are not discernible to the naked eye.Therefore, SEM is necessary to investigate them.The aim of this approach is to uncover the mechanisms of strength formation and S/S principles based on microstructural attributes.
Figure 15 shows the SEM and EDS test results for in situ soil in Shenzhen, alkaliactivated composites, and alkali-activated grout materials.Alkali-activated composites consist of oxygen, silicon, and aluminum as the three elements with the highest content, with mass fractions of 41.5%, 33.9%, and 14.3%, respectively.The addition of slaked lime as an alkaline additive in alkali-activated grout materials increases the calcium content.As a result, the four elements with the highest content are oxygen, calcium, silicon, and alu-minum, with mass fractions of 40.2%, 27.9%, 15.3%, and 14.3%, respectively.Observations made using SEM and EDS of both in situ soil and alkali-activated grout materials reveal that the in situ soil mainly consists of disordered and loosely arranged particle units.These particles are interconnected through agglomeration, contact cementation, and micropores between particles, resulting in a loose soil structure with limited strength.In contrast, soil treated with alkali-activated composite S/S undergoes a significant transformation.The previously loose particle units are encapsulated and firmly cemented by alkali-activated composites, establishing a robust network structure among fine soil particles, granules, and flocculants, forming larger-scale agglomerates.The relationship between these agglomerates and the cementitious structure becomes tighter and more uniform, significantly enhancing the microstructure and markedly reducing the voids between soil particles.limited strength.In contrast, soil treated with alkali-activated composite S/S undergoes a significant transformation.The previously loose particle units are encapsulated and firmly cemented by alkali-activated composites, establishing a robust network structure among fine soil particles, granules, and flocculants, forming larger-scale agglomerates.The relationship between these agglomerates and the cementitious structure becomes tighter and more uniform, significantly enhancing the microstructure and markedly reducing the voids between soil particles.
The observation of alkali-activated grout materials under a 250× microscope reveals a smooth cross-section interspersed with larger particles (shown in Figure 16).These particles, originating from the in situ soil's sand and gravel, do not participate in the chemical reaction.However, the alkali-activated composites immobilizes them, incorporating them into the structure as a coarse aggregate framework, which in turn contributes to the strength of the grouting materials.However, cracks are visible on the cross-sections of both the solidified soil and the alkali-activated composites (shown in Figure 15b,c).This is due to the introduction of excess water to maintain the fluidity of the grouting materials as a grouting material.After the alkali activation reaction has completed in the alkali-activated composites system, any remaining water that did not participate in the reaction or evaporate will remain within the specimen.As the system loses moisture, the previously water-filled pores become visible, resulting in cracks.The observation of alkali-activated grout materials under a 250× microscope reveals a smooth cross-section interspersed with larger particles (shown in Figure 16).These particles, originating from the in situ soil's sand and gravel, do not participate in the chemical reaction.However, the alkali-activated composites immobilizes them, incorporating them into the structure as a coarse aggregate framework, which in turn contributes to the strength of the grouting materials.However, cracks are visible on the cross-sections of both the solidified soil and the alkali-activated composites (shown in Figure 15b,c).This is due to the introduction of excess water to maintain the fluidity of the grouting materials as a grouting material.After the alkali activation reaction has completed in the alkali-activated composites system, any remaining water that did not participate in the reaction or evaporate will remain within the specimen.As the system loses moisture, the previously water-filled pores become visible, resulting in cracks.
(c) Alkali-activated grout materials.It is important to note that the EDS test provides qualitative evidence o and production of substances in polymer-solidified soils, rather than quan tors.In untreated soils, the atomic percentage of silicon (Si) elements is relat to the abundance of SiO2 in the soil.The presence of calcium (Ca) in alkali-a materials is attributed to the addition of slaked lime, which retains calcium introduction of alkali-activated composites material further increases the co silicon and aluminum elements in the soil.Additionally, aluminum (Al) a are also present in high concentrations in polymer-solidified soil samples d content of Al2O3 and SiO2 in the reaction materials.The EDS figure of the a grout materials reveals that it primarily comprises oxygen (O), calcium (C silicon (Si), and aluminum (Al).The atomic ratio at the scanning points ind main gel products in the sample are calcium aluminum silicate hydrate ( cium silicate hydrate (C-S-H), and calcium aluminate hydrate (C-A-H), alon amount of sodium aluminum silicate hydrate (N-A-S-H).The sample's st negatively impacted by varying degrees of carbonation, indicated by the p bon (C) elements.Additionally, the analysis shows a generally high concen suggesting that the calcium from the slaked lime reacts with the active SiO the system, forming various mixed gel clusters.It is important to note that the EDS test provides qualitative evidence of the formation and production of substances in polymer-solidified soils, rather than quantitative indicators.In untreated soils, the atomic percentage of silicon (Si) elements is relatively high due to the abundance of SiO 2 in the soil.The presence of calcium (Ca) in alkali-activated grout materials is attributed to the addition of slaked lime, which retains calcium in the soil.The introduction of alkali-activated composites material further increases the concentration of silicon and aluminum elements in the soil.Additionally, aluminum (Al) and silicon (Si) are also present in high concentrations in polymer-solidified soil samples due to the high content of Al 2 O 3 and SiO 2 in the reaction materials.The EDS figure of the alkali-activated grout materials reveals that it primarily comprises oxygen (O), calcium (Ca), carbon (C), silicon (Si), and aluminum (Al).The atomic ratio at the scanning points indicates that the main gel products in the sample are calcium aluminum silicate hydrate (C-A-S-H), calcium silicate hydrate (C-S-H), and calcium aluminate hydrate (C-A-H), along with a small amount of sodium aluminum silicate hydrate (N-A-S-H).The sample's strength may be negatively impacted by varying degrees of carbonation, indicated by the presence of carbon (C) elements.Additionally, the analysis shows a generally high concentration of Ca 2+ , suggesting that the calcium from the slaked lime reacts with the active SiO 4 and AlO 4 in the system, forming various mixed gel clusters.

Test Material
The main test materials in this section are slag, fly ash, alkaline activator, Shenzhen in situ soil, and slaked lime, whose main parameters and fabrication process are the same as those in the previous section and will not be repeated here.

Field Test Program for Alkali-Activated Grout Materials
Combined with the test section of karst treatment during the construction of the Shenzhen Metro Line 3 Phase IV Project, the treatment method of grouting with alkaliactivated grout materials was adopted.According to the test results of alkali-activated grout materials, the optimal soil-to-binder ratio of 5:5, slaked lime dosage of 4.5%, and water-to-solid ratio of 0.26 were used to produce the grouting materials, in which the slag and fly ash mass ratio of alkali-activated composites was 7:3, the alkaline activator modulus was 1.3, and the alkalinity was 10%.A suitable cave was selected and its basic information was measured and recorded, and the details of the cave are shown in Table 11.A drilling machine was used for drilling, the grouting hole adopted a drill bit with a diameter specification of 130 mm, and the exhaust hole had a drill bit with a diameter specification of 75 mm.Three grouting holes and one exhaust hole were laid out in a triangle shape, and a PVC pipe with diameters of 110 mm and 55 mm and a wall thickness of 3 mm was selected to be put into the grouting holes and exhaust holes; bentonite and cement slurry were used to fill in the gaps between the hole wall and the PVC pipe.Use mixing equipment to mix the alkali-activated grout material slurry, inject it into the hole through the grouting hole, and stop grouting when the vent hole overflows with alkali-activated grout materials of the same consistency as the grouting hole.

Test Results and Analysis
After 28 days of grouting, a combination of drilling and physical inspection was used to evaluate and analyze the grouting effect.The core samples of 28-day grouting were sent to Shenzhen Gangjia Engineering Inspection Co., Ltd.(Shenzhen, China).The results of the UCS test reveal that the test section of the cavern was effectively filled and reinforced, which verified the superiority of the performance of the alkali-activated grout materials and the applicability of the project.Sampling results and test reports are shown in Figure 17 and Table 12, respectively.

Test Material
The main test materials in this section are slag, fly ash, alkaline activator, Shenzhen in situ soil, and slaked lime, whose main parameters and fabrication process are the same as those in the previous section and will not be repeated here.

Field Test Program for Alkali-Activated Grout Materials
Combined with the test section of karst treatment during the construction of the Shenzhen Metro Line 3 Phase IV Project, the treatment method of grouting with alkaliactivated grout materials was adopted.According to the test results of alkali-activated grout materials, the optimal soil-to-binder ratio of 5:5, slaked lime dosage of 4.5%, and water-to-solid ratio of 0.26 were used to produce the grouting materials, in which the slag and fly ash mass ratio of alkali-activated composites was 7:3, the alkaline activator modulus was 1.3, and the alkalinity was 10%.A suitable cave was selected and its basic information was measured and recorded, and the details of the cave are shown in Table 11.A drilling machine was used for drilling, the grouting hole adopted a drill bit with a diameter specification of 130 mm, and the exhaust hole had a drill bit with a diameter specification of 75 mm.Three grouting holes and one exhaust hole were laid out in a triangle shape, and a PVC pipe with diameters of 110 mm and 55 mm and a wall thickness of 3 mm was selected to be put into the grouting holes and exhaust holes; bentonite and cement slurry were used to fill in the gaps between the hole wall and the PVC pipe.Use mixing equipment to mix the alkali-activated grout material slurry, inject it into the hole through the grouting hole, and stop grouting when the vent hole overflows with alkaliactivated grout materials of the same consistency as the grouting hole.

Test Results and Analysis
After 28 days of grouting, a combination of drilling and physical inspection was used to evaluate and analyze the grouting effect.The core samples of 28-day grouting were sent to Shenzhen Gangjia Engineering Inspection Co., Ltd.(Shenzhen, China).The results of the UCS test reveal that the test section of the cavern was effectively filled and reinforced, which verified the superiority of the performance of the alkali-activated grout materials and the applicability of the project.Sampling results and test reports are shown in Figure 17 and Table 12, respectively.The China Railway 14th Bureau selected the same construction section of the cavern TDB16 on site for the comparison test.By using P-O42.5-gradesilicate cement, according to the water-to-solid ratio of 0.45, the hole was filled and treated.The 28-day sampling test results are shown in Table 13.The field verification test provides a theoretical basis and reference for the optimization of the performance of the new grouting material used for karst treatment.Its UCS is 162.5% of the UCS of cement slurry grouting materials, which proves that the alkali-activated grout materials are suitable for the construction requirements of the engineering waste soil and karst treatment of the Shenzhen Metro Line 3 Phase IV Project, and also verifies the feasibility of alkali-activated grout materials as grouting materials for the treatment of the subway underneath the karst cave.

Conclusions
Grouting and filling are important ways of treating cavities.The aim of this study is to explore a new type of green, environmentally friendly, and economical grouting material to minimize the environmental impact caused by existing engineering grouting materials.The following conclusions were obtained by studying the optimal ratio, macroscopic and microscopic properties, and field test of alkali-activated grout materials: (1) The feasibility of slag, fly ash, and alkaline activators (water glass and sodium hydroxide) as alkali-activated composites was verified by indoor tests.Through the UCS tests, the influence of each factor on strength was obtained: within the scope of this test, the UCS of alkali-activated composites showed a tendency of increasing and then decreasing with the increase in the slag-fly ash ratio, modulus of alkaline activator, and alkalinity.The ratio of slag:fly ash = 7:3, alkaline activator alkalinity = 10%, and alkaline activator modulus = 1.3 were found to be optimal for the gelling effect of alkali-activated composites.(2) The alkali-activated grout materials were formed by mixing the alkali-activated composites with Shenzhen sandy clay.Indoor orthogonal tests show that the highest UCS of 30.18MPa is achieved at 28 days, with the main strength growth occurring from 0 to 7 days.The UCS increases with a lower soil-to-binder ratio, higher slaked lime content, and lower water-to-solid ratio.The optimal mix is a soil-to-binder ratio of 5:5, slaked lime content of 4.5%, and water-to-solid ratio of 0.26.When the soil-to-binder ratio is less than 8:2, the fluidity of the alkali-activated composite grouting material meets the specific construction requirements.The primary factor affecting fluidity is the soil-to-binder ratio, which decreases as the ratio decreases.The water-to-solid ratio also influences fluidity, increasing as this ratio rises.The addition of slaked lime has a minimal impact on fluidity.(3) The setting time of alkali-activated grout materials is primarily influenced by the soilto-binder ratio, slaked lime content, and water-to-solid ratio.The soil-to-binder ratio is the main factor affecting the setting time, followed by the slaked lime content and the water-to-solid ratio.The setting time of alkali-activated grout materials shortens with a decrease in the soil-to-binder ratio, decreases with an increase in the slaked lime content, and increases with an increase in the water-to-solid ratio.To meet the setting time requirements for karst treatment in Shenzhen Metro, the soil-to-binder ratio of the alkali-activated composite grouting material should be set to 5:5.With a soil-to-binder ratio of 5:5, adjusting the slaked lime content and water-to-solid ratio within the scope of this study can achieve an appropriate setting time.(4) The curing mechanism of alkali-activated composites were derived by FTIR, SEM, and EDS.The UCS of alkali-activated grout materials mainly occurs by applying an alkaline activator to slag and fly ash.The alkali-activated composites' polymerization reaction generates gelling substances, such as N-A-S-H and C-A-S-H, which encapsulate and cement the soil particles in situ.This results in the formation of cohesive bodies with larger scales in the alkali-activated grout materials, thus providing strength to the alkali-activated grout materials.(5) The engineering practicability of alkali-activated grout materials is validated through on-site verification tests.The soil-to-binder ratio was set at 5:5, the slaked lime ratio at 4.5%, and the water-to-solid ratio at 0.26.These parameters were employed to treat the cavern, resulting in a UCS of 5.2 MPa, which represents a 162.5% increase in strength compared to cement slurry grouting materials.This outcome validates the feasibility of the alkali-activated grout materials used for cavern treatment in Shenzhen, China.
Based on the findings, practice engineers should adopt the optimal ratios of slag to fly ash at 7:3, with an alkaline activator alkalinity of 10% and a modulus of 1.3.For alkali-activated composites grouting, maintain a soil-to-binder ratio of 5:5, slaked lime content of 4.5%, and a water-to-solid ratio of 0.26.If the experimental materials differ slightly from those in this study, consider exploring the optimal parameters for different materials around the parameter values of this study.Emphasize the 0-7-day curing period for significant strength development and use FTIR, SEM, and EDS to understand the curing mechanisms, focusing on N-A-S-H and C-A-S-H gels.

Figure 2 .
Figure 2. Group A specimen's unconfined compressive strength at various ages.

Figure 2 .Figure 3 .
Figure 2. Group A specimen's unconfined compressive strength at various ages.2.3.2.The Influence of Alkali Activator Alkalinity on the UCS Behavior of Alkali-Activated Composites Figure 3 illustrates the effect of varying alkalinity on the unconfined compressive strength (UCS) of alkali-activated composites at different ages.As alkalinity increases, the UCS of specimens cured for 3, 7, and 28 days initially rises and then falls; the 28-day UCS increased by 12.8%, 5.3%, and 42.3%, respectively, for each group relative to the previous one.At 10% alkalinity, the highest UCS values for 3, 7, and 28 days were recorded in group B at 43.32 MPa, 45.22 MPa, and 56.33 MPa, respectively.ppl.Sci.2024, 14, x FOR PEER REVIEW

Figure 3 .
Figure 3. Group B specimens' unconfined compressive strength at various ages.

Figure 4 .
Figure 4. Group C specimens' unconfined compressive strength at various ages.

Figure 4 .
Figure 4. Group C specimens' unconfined compressive strength at various ages.

Figure 6 .
Figure 6.Particle size distribution of in situ soil from Shenzhen.

Figure 7 .
Figure 7. Analysis of the average fluidity of alkali-activated grout materials.

Figure 7 .
Figure 7. Analysis of the average fluidity of alkali-activated grout materials.

Figure 8 .
Figure 8. Group D specimens' unconfined compressive strength at various ages.Figure 8. Group D specimens' unconfined compressive strength at various ages.

Figure 8 .
Figure 8. Group D specimens' unconfined compressive strength at various ages.Figure 8. Group D specimens' unconfined compressive strength at various ages.

Figure 9 .
Figure 9. Forms of damage in alkali-activated composite grouting material samples

Figure 9 .
Figure 9. Forms of damage in alkali-activated composite grouting material samples.

Figure 11 .
Figure 11.Setting time of gopolymer grouting materials with different soil-to-binder ratios.

Figure 11 .
Figure 11.Setting time of gopolymer grouting materials with different soil-to-binder ratios.

Figure 12 .
Figure 12.Setting time of gopolymer grouting materials with different slaked lime contents.

Figure 12 .
Figure 12.Setting time of gopolymer grouting materials with different slaked lime contents.

Figure 13 .
Figure 13.Setting time of gopolymer grouting materials with different water-to-solid ratios.

Figure 13 .
Figure 13.Setting time of gopolymer grouting materials with different water-to-solid ratios.

Figure 15 .
Figure 15.SEM test results at 1000-times magnification and EDS test results for different materials.

Figure 15 .
Figure 15.SEM test results at 1000-times magnification and EDS test results for different materials.

Figure 15 .Figure 16 .
Figure 15.SEM test results at 1000-times magnification and EDS test results for diff

Figure 17 .
Figure 17.The 28-day sample from the cave filling site.Figure 17.The 28-day sample from the cave filling site.

Figure 17 .
Figure 17.The 28-day sample from the cave filling site.Figure 17.The 28-day sample from the cave filling site.

Table 1 .
Oxide content in slag.

Table 2 .
Oxide content in fly ash.

Table 4 .
Investigation test on the ratio of slag to fly ash.
Note: slag:fly ash in the table for the mass ratio of the two (later collectively referred to as slag:fly ash); water does not participate in the alkali activation reaction, taking into account the fluidity of the alkali-activated composites' cementitious materials and the results of the previous pre-experimentation; the water-solid ratio is fixed at 0.38.2.3.Test Results and Analysis2.3.1.The Influence of the Slag-to-Fly Ash Ratio on the Strength of Alkali-Activated Composites

Table 5 .
Parameters of in situ soil from Shenzhen.

Table 5 .
Parameters of in situ soil from Shenzhen.

Table 7 .
Results of the alkali-activated composite grouting material fluidity test.

Table 8 .
Analysis of poor flowability in alkali-activated composite grouting material.

to-Binder Ratio Slaked Lime Content Water-to-Solid Ratio
Appl.Sci.2024, 14, x FOR PEER REVIEW 12 of 26

Table 9 .
Analysis table of strength variability of geogel grouting materials at different ages.

Table 12 .
The 28-day unconfined compressive strength test report for geomaterial grouting material core samples.

Table 13 .
The 28-day unconfined compressive strength test report for cement-based grouting material core samples.