1. Introduction
In recent years, on account of the rapid expansion of the automotive industry, the global annual production of waste tires has exceeded 200 million tons, of which China’s waste tire output accounts for more than 30% [
1]. Waste tires are non-biodegradable waste. If only landfilled, they can cause significant harm to the environment and natural resources. Additionally, these wastes are highly sensitive to temperature, so the accumulation of large quantities of waste tires heightens the risk of spontaneous combustion, seriously impacting the environmental safety and health, and facilitating the spread of pests and diseases. Therefore, the treatment and recycling of waste tires have become an important environmental protection industry, with the market value expected to exceed USD 9 billion by 2025 [
2,
3]. How to efficiently and economically apply these tire waste materials has become a major issue facing the world [
4,
5].
In response to the above issues, civil engineering practitioners have proposed an effective solution. Existing research indicated that rubberized cement-based materials can be formed by adding rubber aggregates from waste tires into cement-based materials as aggregates to replace all or part of the aggregates [
6]. This approach not only protects natural resources and reduces the environmental risks associated with waste tires, but also realizes the effective reuse of resources, offering good economic benefits. Meanwhile, due to the excellent elasticity, toughness, seismic resistance, and damping properties, the addition of rubber aggregates effectively enhances the deformability, impact resistance, fatigue resistance, and freeze resistance of cement-based materials [
7,
8,
9]. Moreover, rubber aggregates can absorb various stresses generated within the cement-based materials, inhibit concrete shrinkage deformation, and prevent or slow down cracking caused by microcracks or shrinkage. Of course, the addition of rubber aggregates also produces some negative impacts, such as the formation of relatively weak interfacial transition zones in the cement-based materials, thereby diminishing their strength [
10,
11,
12]. However, with ongoing research, measures such as surface modification or pretreatment of rubber particles have effectively addressed these negative impacts, expanding the potential application of rubberized cement-based materials in civil engineering [
13,
14,
15,
16].
At present, research on the rubberized cement-based materials strength has reached a generally consistent conclusion: as the rubber aggregates content increases, the concrete strength significantly decreases, but the toughness increases. However, conclusions on the influences of the rubber aggregates particle size on strength vary. Topcu [
17] concluded that coarse rubber particles decreased the mechanical strengths of concrete more than fine rubber particles, while Cao [
18] held the opposite view. Sukontasukku [
19] found that concrete with graded rubber particles had better compressive strength. In terms of deformation, Kaloush [
20] indicated that the temperature shrinkage coefficient of rubberized cement concrete decreased by 30% with a fourfold increase in rubber aggregates content. Zhou [
21] pointed out that rubber aggregates could obviously increase the shrinkage deformation of concrete. However, Raghvan [
22] clarified that a 5% rubber particle content could reduce the plastic shrinkage of mortar, thus decreasing the crack width from 0.9 mm in the control specimen to 0.14–0.6 mm. In addition, based on the existing research results, the researchers deeply analyzed the relationship between rubber aggregates’ content and material strength. Liu [
23] determined the correlations between the strengths of rubberized cement concrete and those of the control group through the analysis of experimental data. Kang [
24] and Long [
25] pointed out that the strength reduction coefficient of rubber roller-compacted concrete has a good linear relationship with the rubber aggregate content, which could be obtained by a linear fitting method. However, some researchers believed that this relationship should be fitted non-linearly. Khatib [
26] and Ghaly [
27] proposed that the compressive strength reduction coefficient of rubberized mortar (RM) should be a polynomial. Overall, the current research on rubberized cement-based materials mainly focuses on the service life performance, with less emphasis on early-age performance.
However, the early-age performance of cement-based materials cannot be ignored. The early age of cement-based materials is the key period for the formation of performance, which will influence the development trends of later performance [
28,
29,
30]. The early-age performance evolution of cement-based materials is closely related to the hydration process, but the development rates of various properties differ, which are primarily influenced by the mix proportion, water–cement ratio, age, etc. It was also reported that early-age mechanical properties were influenced by curing conditions and additives; air curing conditions impeded the strength gain of ordinary concrete, while elevated-temperature curing conditions improved the early-age strength, and the greater impeding effect was observed in fly ash concrete [
31]. Some researchers believed that in the early age, the tensile strength of concrete increased faster than that of compressive strength [
32]; conversely, other studies found that the tensile strength of concrete increased more slowly [
33]. In the early-age stage, cement-based materials undergo not only the development of mechanical properties but changes in volume, primarily including autogenous shrinkage, drying shrinkage, and thermal deformation. The autogenous shrinkage of traditional Portland cement concrete generally ranges from 40 × 10
−6 to 100 × 10
−6, equivalent to a temperature decrease of 4 to 10 °C. It is significantly impacted by the fineness and chemical properties of cement. The drying shrinkage of ordinary Portland cement concrete ranges from 200 × 10
−6 to 1000 × 10
−6, approximately 10 times that of autogenous shrinkage [
34,
35]. The amount of aggregate is a crucial factor influencing shrinkage potential. Increasing the amount of aggregate under the same water–cement ratio can reduce the shrinkage of concrete. Thermal deformation is the dimensional change caused by concrete temperature variations [
36,
37]. According to the thermodynamic properties of materials, the magnitude of concrete temperature deformation also depends on the thermal expansion coefficient of concrete. Zahabizadeh [
38] found that concrete had a high initial coefficient of thermal expansion, which then gradually declined to a local minimum. After the minimum value, the coefficient of thermal expansion remains basically unchanged or increased slightly.
Compared to ordinary cement-based materials, the presence of a large proportion of rubber aggregates alters the mix proportion, aggregate composition, and microstructure of cement-based materials, inevitably affecting its properties [
39,
40]. Especially for the early age, this effect is particularly obvious. However, the current research on rubber cement-based materials mainly focuses on the service period, and there are few studies on the evolution of properties in early age. The early-age performance of rubber cement-based materials not only affects the service life performance, but also serves as the primary basis for controlling non-load-bearing cracks during the early curing period, determining the technical parameters for the construction of rubber cement-based materials. Therefore, it is necessary to study the evolution patterns of the early-age mechanical performance and deformation behavior of RM and to conduct a thorough analysis of the impact of rubber aggregates on the reduction in early-age mechanical properties of mortar.
This paper investigated the impact of rubber aggregate content, particle size of rubber aggregates, and water–cement ratio on the early-age performance of RM through laboratory experiments. Meanwhile, based on the existing models, the relationships between the strength reduction coefficients and rubber aggregate content at different ages were established. Through this research, it can effectively reveal the development tends of the early-age behaviors of RM, and provide support for predicting the early-age mechanical properties of RM. At the same time, those findings can provide a basis for the application of RM and the further research of rubberized cement concrete.