1. Introduction
As industrialisation accelerates, the variety of construction materials has been increasing. In 2019, China’s construction industry was worth nearly 3.6 trillion dollars and employed around 55 million people. The share of construction output in the country’s GDP has been rising year on year, hiring 7% of the total employed population; it is clear that the construction industry is a pillar industry in China. As a major component of civil structures, experimental research is essential, and it is linked to the improvement of people’s living standards.
Research into cement-based building materials has focused on the durability, high strength, and rapid setting properties of cement-based materials. However, the frequent use of Ordinary Portland Cement (OPC) has resulted in significant greenhouse gas emissions and growing environmental concerns [
1]. OPC production consumes a large number of natural resources [
2], with heat and electricity consumption exceeding 2.72 GJ/ton and 65 kWh/ton, respectively [
2,
3]. Furthermore, despite some preventive techniques, the degradation of civil infrastructure made of cement or concrete and the shortening of structural age are inevitable durability issues worldwide [
4,
5,
6]. As a result, new materials are being sought that meet not only the strength requirements needed for basic building performance but also higher durability requirements. Geopolymer is a low carbon material first proposed by Davidovits in 1978 [
7,
8]. Geopolymeric cement can be produced by a reaction between industrial by-products (typically fly ash, slag, metakaolin, etc.) and alkaline solutions. The special structure of geopolymers makes them sustainable materials with many mechanical advantages [
9] and durability [
10]. Therefore, mineral geopolymers have been described as the most promising green cement material of the 21st century [
11].
In recent years, the replacement of cement and silica fume with metakaolin in concrete has become a hot topic [
12,
13,
14]. Metakaolin is classified as a reaction product of low calcium systems. Due to their highly cross-linked (mainly Q4) and zeolite-like structure, N-A-S-H gels have better mechanical properties than OPC [
15]. Unlike cement as a cementitious material alone, the ratio of metakaolin to activator has an influence on performance [
16], which means that the geopolymer content is highly important for this new system. The durability of the concrete is a crucial factor in determining the service life [
12,
17]. Recent studies on durability [
5,
18] have focused on the geopolymers of different materials and mechanisms of structural damage. Pasupathy et al. [
19] investigated the durability of fly ash-based geopolymer concrete exposed to external environments to study resistance to carbonation. Davidovits et al., [
20] highlighted that when fly ash geopolymers were placed in a 5% H
2SO
4 solution for a certain number of days, a mass loss of 7% was found for the metakaolin-based geopolymer. Hanrahan [
21] found that the durability of fly ash-based geopolymer concrete is largely governed by the internal configuration of the aluminosilicate gel composition in extreme environments (5% Na
2SO
4 solution). Due to the outstanding durability characteristics of geopolymers, geopolymer concrete has potential applications in pavement rehabilitation. Keyu Chen et al., [
22] mixed geopolymers with slag for pavement rehabilitation. Hawa et al. [
23] used geopolymers for road rehabilitation. X.Q. Peng et al., and B.B. Jindal [
24,
25] searched for the preparation of geopolymeric materials and their application to the rapid repair of cement concrete pavement.
However, the proportion of metakaolin and alkaline activators as remediation materials has not been determined to obtain more information about the durability of the repair geopolymerical cement, including resistance to permeability, sulphate corrosion, and freeze-thaw resistance. Furthermore, there has been relatively little research on the effect of the dosage of alkali activator on the durability of the geopolymer system and the effect of geopolymer prepared with partial replacement of cement by metakaolin as a remediation material. The metakaolin/cement ratio and the amount of alkali activator still need to be explored to understand the durability of geopolymer systems as rapid repair pavement materials in adverse weather conditions. At the same time,
In this paper, four possible factors that have a large influence on the geopolymer system, including metakaolin, alkali activator, βs, and modulus, were first analysed by orthogonal experiments prior to the durability tests. Orthogonal experiments were conducted to determine the main influencing factors that modify the performance of the geopolymeric cement. Then, corresponding durability tests were carried out based on the two sets of variables. Combining bond strength and micro-morphology, it was found that the possible causes of the variation in durability were metakaolin and alkali activator contents. By forming a geopolymerical cement with a suitable formulation, a road repair material with good repair function and durability is formed; this experiment also lays the foundation for the combination of mechanised intelligence and engineering use. In the future, the durability of metakaolin polymers can be more widely applied in practical engineering. The flow chart is shown below in
Figure 1.
4. Conclusions
In this research, metakaolin and activator contents, the two main factors, were involved in experiments to study the durability of repair metakaolin geopolymer under different factors. Macroscopic experiments were carried out to determine the visual impact. More importantly, microscopic experiments, including SEM and FT-IR justified and explained the macroscopic phenomena. The following conclusions can be drawn:
The experiments tested durability, including bond strength, resistance to penetration, resistance to sulphate corrosion and resistance to freezing and thawing. Indicators including flexural strength, maximum pressure value, weight loss, compressive strength, and corrosion resistance coefficient were used to measure the performance of the repair material; it can be found that a suitable ratio of metakaolin and activator content significantly improves the durability and repairability of the geopolymeric cement. When the ratio of metakaolin to cement is 1.5 and the ratio of activator to solids is 0.3, the geopolymer are the most durable and can be used as repair materials and pavements.
From the SEM and FTIR, it was possible to see more clearly the effect of different factors changes on the internal structure of the forming geopolymer, so that the reasons affecting the durability changes could be reasonably analyzed. When the ratios of metakaolin to cement and activator to solids are appropriate, the geopolymer formed had a good internal structure with low porosity; this effectively prevented the entry of harmful ions and reduced the effects of crystallisation due to temperature reduction and volume expansion during freeze-thaw which destroys the internal structure and therefore provides better durability properties.