1. Introduction
In China, road maintenance mileage accounts for 99% of the total road mileage. Every year, under the action of traffic loads and natural factors, highways have many diseases such as subgrade debonding (
Figure 1a), subgrade weakness (
Figure 1b) and road subsidence (
Figure 1c). Aiming at road diseases, the commonly repaired grouting materials are mostly cement-based grouting materials, which have good durability and stability while having high carbon emissions and environmental pollution. In addition, cement-based grouting materials cannot react with the activation inert minerals of road subgrade, causing bad interface bonding and eventually subgrade diseases [
1,
2]. A relevant study shows that the production of cement requires the use of a large number of natural resources, and for every 1 ton of cement produced, 2.8 tons of raw materials are consumed [
3]. During the production of cement, a large amount of CO
2 is emitted into the atmosphere, and CO
2 emissions from the cement industry have reached about 8% of global annual emissions [
4]. Since the European Union released the European Green Deal in 2019 to take the lead in putting forward the goal of achieving carbon neutrality and related policies, countries such as the UK, Sweden, China, South Korea and the US have also put forward the goal of carbon neutrality one after another. In the big international process of carbon neutrality that has attracted global attention, there is an urgent need for grouting materials that can deal with road diseases and are green and low-carbon.
Geopolymer is a new type of gelling material with the main components of inorganic silica-oxygen tetrahedra and aluminum-oxygen octahedra. It has a three-dimensional network structure in space, which is formed by the polymerization of natural minerals, solid wastes and artificial silica-aluminum compounds as raw materials through strong alkali action and lattice reconstruction, as shown in
Figure 2.
Due to its low density, low thermal conductivity, and low coefficient of temperature expansion, geopolymer has been widely used in aviation, civil engineering, ceramic arts, and nuclear waste containment engineering. With differences in raw materials and manufacturing processes, geopolymer has a wide type range and each has significant advantages and disadvantages, resulting in different suitable application fields. Research shows that fly ash, slag, construction waste and some other industrial wastes with low energy consumption can be used as raw materials for the development of geopolymer, which can save energy by more than 75% and reduce emissions by more than 95% [
5,
6]. So, the fly ash-slag-based geopolymer is the most widely used in civil engineering. The annual emission of slag in China’s iron and steel metallurgical industry is about 500 million tons, of which blast furnace slag accounts for as much as 50% [
7]. Compared with the blast furnace slag resource utilization rate of 100% abroad (e.g., Japan, USA, Germany, etc.) [
5], the Chinese resource utilization rate is slightly lower and most of the blast furnace slag is dumped, which is not only a waste of resources but also of great harm to the environment [
8]. In fact, the micro-aggregate effect and volcanic ash effect of blast furnace slag milled into powder can optimize the matrix activity of cementitious materials and enhance the adhesion of mortar and aggregate by combining with an alkali activator [
9]. Therefore it is a good choice to use slag as a precursor for geopolymer concrete.
As a common precursor of geopolymer material, the compressive strength of geopolymers formed from different types of fly ash is related to the variation of fly ash in particle size distribution, chemical composition, morphological properties and amorphous phase [
10]. It has been shown that the geopolymer made from ultrafine fly ash has a higher compressive strength than conventional fly ash, but the flexural strength is only one-twentieth of that of conventional fly ash geopolymer [
11]. In addition, calcium in fly ash can promote the formation of C-S-H gels, and fly ash with high calcium content is more conducive to improving the strength of geopolymer [
12], but the inadequacy of calcium fraction on the development of three-dimensional mesh structure of geopolymer restricts the application of high-calcium fly ash [
13]. Studies have shown that geopolymers with a high fly ash content tend to have better flowability but reduce the reaction rate and compressive strength [
14]. Fly ash geopolymer can significantly improve the overall strength of the soil in the application of cured soil [
15], but it has a slow reaction rate and low hardening strength, which make it difficult to achieve the bearing capacity requirements if it is directly applied in road subgrade repair. While slag is rich in calcium components, which have an extremely fast reaction rate under the effect of alkali activation, pure slag geopolymer has a very high hardening strength. Therefore, after alkali activation, the combination of fly ash and slag can not only ensure sufficient fluidity to fill the pores of the road base but also provide sufficient hardening strength to meet the bearing capacity requirements of roads. Studies have shown that the fly ash-slag geopolymer grouting materials can generate C-(A)-S-H, N-A-S-H and other gels to fill the pores of the subgrade to form a stable microstructure [
16]. This can also form wraps around the subgrade soil particles and react with the inert minerals therein to a certain degree of polymerization to form a dense matrix and improve the interfacial bond [
17], which is very beneficial to subgrade disease treatments.
For fly ash-slag-based geopolymer grouting materials, good mechanical properties such as flexural and compressive resistance are important to reinforce the subgrade and improve the overall bearing capacity of roads. However, a variety of factors affect the mechanical performance of the geopolymer, such as the modulus and concentration of alkali activator, Na
2O content, slag content and other factors. Among them, the alkali activator modulus (the ratio of the material amounts of SiO
2 and Na
2O in the liquid alkali activator, which can be converted into NaOH content) and concentration are the most important factors affecting the mechanical strength of geopolymer nodules [
18]. The variation in the alkali activator modulus can enhance the mechanical properties of the geopolymer, but higher or lower values than the optimal value will inhibit the strength rise instead [
18]. The influence of slag content on mechanical properties is the second influence of the alkali activator modulus. With the increase in slag content, more C-S-H and C-A-S-H gels will be generated, which can significantly improve the compressive strength of fly ash geopolymer grouting materials [
19], and the 28d strength of fly ash-slag geopolymer can reach 68 MPa with reasonable matching of all factors [
20]. Some scholars have summarized the composition of geopolymer grouting materials and the way of enhancing the mechanical properties, which are in terms of raw materials, matching ratios and admixtures [
21]. In order to further study the mechanism of influence of mechanical properties, many scholars have analyzed the microstructure of fly ash geopolymer grouting materials by means of XRD and SEM, showing that the incorporation of slag could improve the pore size distribution and generate a large number of sodium silica-aluminate and calcium silicate gels, which are conducive to the development of strength [
22]. The appropriate amount of fly ash content is also beneficial to reduce the cracks generated by geopolymer hardening, making the structure more uniform, and improving the compressive strength of the geopolymer [
23]. Some scholars have developed effective models that can predict the mechanical properties of geopolymers by Lasso regression and neural networks to clarify the variation pattern of the mechanical properties of geopolymer materials [
24,
25,
26,
27].
In addition, the fluidity of the geopolymer grouting materials determines whether the slurry can penetrate into the pores and cracks of the subgrade, therefore, determining the reinforcement effect of the subgrade. Many scholars’ studies have shown that the increase in slag and alkali activator content accelerates the polymerization reaction and generates a large amount of C-(A)-S-H gel in a short time and leads to poor flowability of the fly ash geopolymer slurry [
28,
29]. The synergistic effect of slag and alkali activator has also been considered, and the proportion of slag, alkali activator content and modulus has been investigated to ensure the mechanical properties of the fly ash-slag geopolymer grouting materials while enabling fluidity up to 23 s [
30].
In the above-mentioned research, scholars mostly use the “Two-step Method- liquid activator” (
Figure 3) to prepare geopolymers. Firstly, a liquid activator of the required modulus is prepared, then the raw silica-alumina material is reacted with the alkaline activator solution and the soil is cured. However, the “Two-step Method” geopolymer preparation requires an alkaline activator solution, which is difficult to transport and store, so the geopolymer cannot be applied in field construction. The “One-step Method-solid activator “ (
Figure 3) geopolymer preparation mainly mixes silica-aluminum raw materials with a solid alkaline activator, and then adds distilled water to prepare a geopolymer slurry for curing clay [
31]. This preparation process is suitable for the curing agent field construction technology, eliminating the environmental impact of the alkaline activator and reducing the transportation cost of the solution in field construction [
31]. However, when the “One-step Method” has been used to prepare the slag-fly ash-based geopolymer as the curing agent, the raw material ratio, solid alkaline activator content and other factors have not been reported on the mechanical properties of the geopolymer at present.
In view of this, in order to solve the problem that the liquid exciter of existing geopolymer grouting materials is not easy to store, and the mechanism of the influence of solid exciter dosing and slag dosing on the fluidity and mechanical properties of geopolymer is not clear, solid alkali (NaOH, Na2SiO3) was used as the activator in this study to investigate the mechanical properties and fluidity of fly ash-slag based geopolymer grouting materials. The changes in the mechanical properties and fluidity with the influencing factors such as slag and solid alkali content were researched, and the microscopic morphology of specimens with different slag content and different alkali content with a Scanning Electron Microscope (SEM) were studied to explore the related mechanism. According to the test results, the ridge regression model of slag content, solid alkali content and mechanical properties of fly ash-slag-based geopolymer grouting materials was established to predict the macroscopic mechanical properties of geopolymer.
5. Conclusions
In this paper, the mechanical properties and flowability of fly ash-slag-based geopolymer grouting materials are studied. Combined with the experimental data, a mechanical property prediction model is established based on the ridge regression theory to realize the prediction of the unconfined compressive strength of fly ash-slag-based geopolymer grouting materials at each age. The conclusions are as follows:
- (1)
The influence law and mechanism of the unconfined compressive strength of geopolymer grouting materials are following:
The increase in slag content can generate a large number of gels such as C-S-H and C-A-S-H to increase the disorder of the geopolymer system, thus improving the unconfined compressive strength of the geopolymer grouting material at all ages. With the increase in alkali activator (NaOH, Na2SiO3), the degree of polymerization increases first and then decreases, so the 28d unconfined compressive strength increases first and then decreases; the compressive strength of the geopolymer grouting materials at 1d, 3d and 7d did not change significantly with the increase in NaOH content but fluctuated with the increase in Na2SiO3 content. The microscopic structures based on SEM results verify the influence mechanism of mechanical properties. A regression prediction model of the unconfined compressive strength of different ages is established in the paper, which has good agreement with experimental data.
- (2)
The influence law and mechanism of the flexural strength of geopolymer grouting material are as follows:
With the increase in slag, the flexural strength of the geopolymer of 1d, 3d and 7d increases and reaches the maximum value on the seventh day. The flexural strength of 28d and steaming first increases and then decreases. The microscopic structures based on SEM results verify the influence mechanism of flexural strength. When the content of the alkali activator is low (NaOH ≤ 3%, Na2SiO3 ≤ 2%), the flexural strength increases and then decreases with the increases in curing age, and all of them reach the maximum value on the seventh curing day. When the content of alkali is high (3.5% ≤ NaOH ≤ 5%, 3% ≤ Na2SiO3 ≤ 6%), flexural strength tends to increase with the curing age.
- (3)
The influence law and mechanism of the fluidity of geopolymer grouting materials are following:
The fluidity of the slurry gradually decreases with the increase in slag content. This is because the increase in slag content not only weakens the “micro-bead effect” of fly ash but also introduces a large amount of calcium-rich phase materials, which promotes the generation of Ca(OH)2 precipitation and hydrated calcium silicate gel, increasing the consistency of the slurry. With the increase in alkali activator content, the fluidity of the slurry also gradually decreases. This is because the increase in alkali activator accelerates the reaction process of the geopolymer, which generates more hydration products at the same time and reduces the free water content, leading to the decrease in slurry fluidity.