Use of Waste Slag and Rubber Particles to Make Mortar for Filling the Joints of Snow-Melting Concrete Pavement

Abstract

Waste slag and rubber particles are commonly used to modify concrete, offering benefits such as reduced cement consumption and lower greenhouse gas emissions during cement production. In this study, these two environmentally friendly, sustainable waste materials were proposed for the preparation of mortar intended for snow-melting pavements. A series of experiments were conducted to evaluate the performance of the material and to determine whether its compressive and flexural strengths meet the requirements of pavement specifications. The mortar’s suitability for snow-melting pavements was assessed based on its thermal conductivity, impermeability, and freeze–thaw resistance. The results indicate that slag, when used in different volume fractions, can enhance the compressive and flexural strength of the mortar. Slag also provides excellent thermal conductivity, impermeability, and resistance to freeze–thaw cycles, contributing to the overall performance of snow-melting pavements. When the slag content was 20%, the performance was optimal, with the compressive strength and flexural strength reaching 58.5 MPa and 8.1 MPa, respectively. The strength loss rate under freeze–thaw cycles was 8.03%, the thermal conductivity reached 2.2895 W/(m * K), and the impermeability pressure value reached 0.5 MPa. Conversely, the addition of rubber particles was found to decrease the material’s mechanical and thermal properties. However, when used in small amounts, rubber particles improved the mortar’s impermeability and resistance to freeze–thaw cycles. When the rubber content was 5% by volume, the impermeability pressure value reached 0.5 MPa, which was 166.7% lower than that of ordinary cement mortar. Under freeze–thaw cycles, the strength loss rate of the test block with a rubber content of 25% volume fraction was 9.83% lower than that of ordinary cement mortar.

1. Introduction

Snow, ice, and rain–snow mixtures significantly reduce the friction coefficient between tires and airport road surfaces, leading to an increased risk of traffic accidents and potential harm to people and property [1]. Traditional methods for snow and ice removal require substantial manpower, large quantities of materials, and considerable time. Additionally, the use of chemical deicers can negatively impact road structures and the environment [2]. Recently, innovative snow-melting techniques have gained popularity, such as cable heating systems [3,4].

One approach involves embedding carbon fiber cables within the pavement layer [3], allowing for the effective melting of snow on road surfaces and bridge decks through electrothermal conversion, even under varying climatic conditions. To install carbon fiber heating wires in existing roads, the asphalt surface must first be removed. Grooves are then cut into the underlying concrete layer to embed the heating wires, after which the grooves are backfilled, and a new asphalt layer is applied. As illustrated in Figure 1, the objective of this study is to investigate the backfilling materials used in the joints of snow-melting pavements and to evaluate which materials are most suitable for specific applications.

Typical pavement repair materials can be categorized into inorganic, organic, and composite types. Among inorganic fillers, chemical grouting is highly effective, but it is expensive and contains components that can be harmful to both organisms and the environment. Cement mortar and modified cement concrete are widely used due to their high strength and affordability [5]. Common materials for joint filling include asphalt and modified asphalt, epoxy resins, polyamides, and alkenes. However, asphalt materials have limitations, such as weak bonding and poor resistance to aging. Not only can cement composite materials improve thermal performance, but many studies have shown that these materials, including graphene oxide (GO), graphene nanosheets, carbon nanotubes, and carbon nanofibers, can also improve thermal performance [6,7,8]. Graphene quantum dots (GQDs) with crystal sizes of less than 5–20 nm are an innovative carbon substrate material with great potential to impact material engineering [9]. Due to their nanoscale size and two-dimensional shape, GQDs exhibit excellent mechanical strength, thermal conductivity, electrical conductivity, and hydrophobicity [10].

Organic fillers are often preferred for their high strength, stable chemical properties, and low volume shrinkage. However, their mechanical properties differ significantly from those of concrete, making them prone to interface failure under load and temperature fluctuations [11]. Composite materials, primarily composed of polymer-modified concrete or mortar, offer a balanced alternative. Modified cement mortar, in particular, is more widely used as a filling material, especially when considering the long-term performance of pavement.

Cement mortar is widely used as a traditional backfill material, and its physical properties can be altered by incorporating different additives. To conserve natural resources, protect the environment, and produce green, high-performance cement, various low-quality raw materials, industrial wastes, and chemical byproducts are utilized. Additives, such as brick powder [12], waste incineration slag [13], iron powder [14], slag powder [15,16,17], rubber particles [18,19,20], and glass waste have been used to modify cement mortar [21,22].

To explore the impact of clay brick powder on cement mortar properties, Shao et al. [23] studied the compressive strength of samples mixed with waste clay brick powder as a supplementary cementitious material under different curing times. Their findings indicated that the microstructure became denser, leading to increased compressive strength. Lee et al. [24] used brick slag powder as an aggregate and found that particle size and additional water content significantly influenced mortar strength. Furthermore, recycled aggregates from crushed bricks (RCBA) created a relatively stable and dense interface transition zone with the cement paste, enhancing the mechanical properties of recycled mortar. Dang et al. [25] proposed partially replacing cement mortar with waste incineration bottom slag, which resulted in higher strength than ordinary Portland cement (OPC) mortar. Huang et al. [26] demonstrated that iron powder is a viable alternative aggregate for engineered cementitious composites (ECC), offering similar tensile and compressive strength compared to conventional micro-silica sand.

Slag additives can be incorporated at appropriate replacement rates to increase compressive strength and reduce drying shrinkage. These additives assist in generating additional hydrates, improving microstructural performance [27]. Shi et al. [28] investigated the use of slag powder as a cement replacement, studying the porosity of slag concrete, the freeze–thaw resistance of slag mortars in the presence of deicing materials, and the chloride binding and chemical properties of supplementary cementitious materials in mortar pore solutions. Fang et al. [29] found that slag reduced the workability and setting time of concrete, while the compressive strength increased with higher slag content. Liu et al. [30] demonstrated that adding rubber particles reduced the compressive strength of cement mortar to some extent but significantly enhanced its deformation capacity, especially the ultimate bending and tensile strain. Corinaldesi et al. [31] found that adding rubber particles reduced the material’s unit mass and thermal conductivity. The rough surface and waterproof properties of rubber contributed to increased gas content [32]. Zhang et al. [33] observed that the impact of rubber on concrete’s frost resistance is comparable to that of air-entraining agents. Similarly, Siddique et al. [34] reported that the addition of rubber particles significantly improved the overall frost resistance of concrete.

As a result of these studies, the sources and applications of cement mortar additives have expanded. However, there has been limited research on the use of filling materials for ice-melting pavements, particularly regarding the mechanical properties, freezing–thawing resistance, and thermal conductivity of cement mortar modified with slag micro-powder and rubber particles. The study adopted a cement-to-sand ratio of 1:3 and a water-to-cement ratio of 0.5, using slag powder instead of cement and rubber particles in partial replacement of sand, with equivalent volumes. The content of slag powder and rubber particles added to the cement mortar samples was 5%, 10%, 15%, 20%, and 25% of the volume fraction of the substitute as filling materials. This paper aims to enhance the performance of cement mortar by incorporating waste slag powder and rubber particles. The study investigates the effects of these materials on cement mortar performance and compares different cement ratios to identify the most suitable mix for snow- and ice-melting pavements.

2. Materials and Methods

2.1. Materials

This experiment is conducted in accordance with the Method for Testing the Strength of Cement Mortar (ISO Method) (GB/T17671-1999) [35], the Standard for Testing the Basic Properties of Building Mortar (JGJ/T70-2009) [36], and the Code for Design of Masonry Mortar Mix Proportion (JTJ-T98-2010) [37]. This study utilized standard sand produced by Xiamen Aisio Standard Sand Co., Ltd. ( Xiamen, China), and CEM 42.5R ordinary Portland cement produced by Huaxin Cement (Ezhou) Co., Ltd. (Nantong, China). The EPDM (ethylene propylene diene monomer) rubber particles used had a size range of 1–3 mm. The density of EPDM should be from 0.85–1.1 g/cm³, and the tensile strength requirement is usually from 5–20 MPa. The slag powder was laboratory-specific, S95-grade, high-performance granulated blast furnace slag powder.

Table 1. Chemical properties of slag powder (%).

Chemical Composition, %	Loss on Ignition	CaO	MgO	Al2O3	SiO2
Slag powder	4.09	35.58	11.32	16.32	36.10

presents the chemical composition of the slag powder, while

Table 2. Size distribution of ISO reference sand.

Hole size, mm	2.0	1.6	1.0	0.5	0.16	0.08
Rest, %	0	7 ± 5	33 ± 5	67 ± 5	87 ± 5	99 ± 1

shows the sand particle size distribution. The properties of the cement are detailed in

Table 3. Cement properties.

Type	Loss on Ignition	Setting Time (min)		Stability	Specific Surface Area (m2/kg)	Compressive Strength 28 d (MPa)	Tensile Strength 28 d (MPa)
		Initial	Final
CEM 42.5R	3.28%	222	227	Qualified	348.7	55.1	8.5

.

To conserve cement, reduce engineering costs, optimize the performance of the slag powder and rubber particles, and facilitate easier dispersion and mixing, the cement–sand–water mass ratio was set at 1:3:0.5. Slag powder was used to replace cement, and rubber particles were used to replace sand, both at equivalent volumes. In the experiment, cement was partially replaced with slag powder at 5%, 10%, 15%, 20%, and 25% proportions, while sand was partially replaced with rubber particles (1–3 mm) at 5%, 10%, 15%, 20%, and 25% proportions. The mix design for the samples is shown in

Table 4. Mixture proportions (kg/m³).

Mix ID	Cement	Slag Powder	Sand	Water	Rubber Particles
S	585.9	0	1757.8	293.0	0
SK1	556.6	29.3	1757.8	293.0	0
SK2	527.3	58.6	1757.8	293.0	0
SK3	498.0	87.9	1757.8	293.0	0
SK4	468.7	117.2	1757.8	293.0	0
SK5	439.4	146.5	1757.8	293.0	0
SX1	585.9	0	1669.9	293.0	87.9
SX2	585.9	0	1582.0	293.0	175.8
SX3	585.9	0	1494.1	293.0	263.7
SX4	585.9	0	1406.2	293.0	351.6
SX5	585.9	0	1318.3	293.0	439.5

.

2.2. Preparation and Curing of Specimens

Cement, sand, and additives were mixed in the specified proportions. The mixture was stirred slowly for 2 min using a JJ-5 mortar mixer (rotation: 140 ± 5 r/min, revolution: 62 ± 5 r/min) manufactured by Wuxi Jianyi Instrument & Machinery Co., Ltd. (Wuxi, China) while tap water was gradually added. Then, the mixture was stirred rapidly for 2 min (rotation: 285 ± 10 r/min, revolution: 125 ± 10 r/min). After mixing was complete, the mixture was poured into oiled trial molds of 40 mm × 40 mm × 160 mm and 70.7 mm × 70.7 mm × 70.7 mm, as well as Φ61.8 mm × 20 mm ring molds with a volume of approximately 60 cm³. The filled molds were then placed on a vibrating platform for 1 min to eliminate air bubbles. A plastic film was immediately applied to prevent water evaporation during hardening. After 24 h, the molds were removed, and the specimens were transferred to a standard curing room (20 ± 5 °C, 95% relative humidity) for 28 days.

2.3. Tests

2.3.1. Compressive Strength

A Meters Industrial Systems (China) Co., Ltd. (Shanghai China), tensile and compressive testing machine was used to conduct compressive tests on five groups of test blocks containing different amounts of aggregates. The dimensions of the test blocks were 70.7 mm × 70.7 mm × 70.7 mm. The test blocks were placed on the tester’s lower plate or pad plate. The pressure surface of the test piece should be perpendicular to the top surface of the molding, and the test piece center should be aligned with the test machine’s lower plate or pad center. The test machine was started. The ball seat was adjusted to balance the contact surface when the upper pressure plate was close to the specimen or the upper pad. The pressure test was charged continuously and uniformly with a charging speed of 1 kN/s. The lower limit was taken when the mortar strength was not greater than 2.5 MPa. If the specimen was rapidly deforming and close to damage, adjusting of the tester throttle was stopped until the specimen was damaged, and then the damage load result was recorded to obtain the compressive strength value of the modified mortar.

2.3.2. Tensile Strength

Tensile tests were conducted on test blocks measuring 40 mm × 40 mm × 160 mm. One side of the test piece was placed on the support cylinder of the testing machine, ensuring that the long axis of the test piece was perpendicular to the support cylinder. A load was applied vertically to the opposite side of the prism at a rate of 50 N/s ±10 N/s until the specimen broke, and the load at failure was recorded. The tensile strength of the mortar specimen was then determined using the following equation:

$$R_f={\displaystyle \frac{1.5F_{f}L}{b^3}}$$

(1)

R_f is the tensile strength, F_f is the load applied to the center of the prism when broken, L is the distance between the supporting cylinders, and b is the side length of the cube section. Among them, L = 100 mm, and b = 40 mm.

2.3.3. Thermal Conductivity Test

A DRE-III thermal conductivity tester, manufactured by Xiangtan Xiangyi Instrument Co., Ltd. (Xiangtan, China), was used to conduct thermal conductivity tests on each group of cake-shaped test blocks, with dimensions of Φ61.8 mm × 20 mm. The sensor was connected to the instrument and clamped between two round, cake-shaped test blocks. The power was then adjusted to zero once the power supply voltage was confirmed to be satisfactory. First, plexiglass and quartz glass were tested, and after ensuring the instrument’s results were normal, the parameters were adjusted accordingly. Each mortar block was then tested to determine its thermal conductivity.

2.3.4. Impermeability Pressure Value Test

Based on previous research [38,39], before conducting the water permeability test, the blocks were placed in a humid environment for three days. Afterward, the test blocks were removed from the curing room and dried in an oven. Once dried, the blocks were sealed with melted wax, ensuring the wax was completely dissolved before application. The blocks were then rolled in the melted wax for one week, leaving the ends uncoated. Finally, the blocks were placed into the test mold and tested for water penetration using an SS-15 mortar penetrometer manufactured by Hebei ZhongKe North Engineering Test Instrument Co., Ltd. (Cangzhou, China).

Each group was tested using six truncated conical impermeability test blocks with a top diameter of 70 mm, a bottom diameter of 80 mm, and a height of 30 mm. The initial pressure was set at 0.2 MPa for two hours, followed by 0.3 MPa for one hour, with subsequent increments of 0.1 MPa every hour. When there was water seepage on the end faces of 3 out of 6 specimens, the test could be stopped and the water pressure at that time recorded. If water was found to seep out from the periphery of the specimen during the test, the test was stopped and the specimen resealed. The maximum pressure when 4 out of 6 specimens in each group did not show water seepage was calculated according to the following formula to obtain the mortar impermeability pressure value:

$$P=H-0.1$$

(2)

where P is the impermeable pressure value and H is the water pressure when 3 out of 6 specimens are impervious.

2.3.5. Freezing and Thawing Resistance

In this test, an automatic concrete freeze–thaw apparatus was used, and the test blocks measured 70.7 mm × 70.7 mm × 70.7 mm. After curing for 28 days, the test blocks were placed in water (15 °C–20 °C) for 2 days and then removed, dried, numbered, and weighed. The blocks were then placed in the freeze–thaw apparatus (−15 °C to −20 °C) for four hours, followed by immersion in a water tank (15 °C–20 °C) for thawing. This freeze–thaw cycle was repeated 60 times, with each thawing process lasting at least four hours. After every five cycles, a visual inspection of the test blocks was conducted. When 2 out of 3 test blocks in the group showed significant damage, the freeze–thaw test of the test block was terminated.

According to the following formula, the strength loss rate after the mortar test block has been freezing and thawing is:

$$∆f_m={\displaystyle \frac{f_{m1}-f_{m2}}{f_{m1}}}\times 100\mathrm{\%}$$

(3)

In the formula, ∆f_m represents the mortar strength loss rate after n freeze–thaw cycles, where f_m1 denotes the average compressive strength of the control test blocks and f_m2 denotes the average compressive strength of the test blocks after n freeze–thaw cycles.

3. Results and Discussion

3.1. Mechanical Property

Figure 2a,b demonstrate the impact of varying slag powder and rubber particle contents on the compressive strength of the modified mortar test blocks. The addition of rubber particles leads to a reduction in compressive strength, with the strength decreasing as the rubber content increases. At a rubber content of 25%, the strength drops to just 8.6 MPa. Figure 2b shows that, for mortar test blocks containing slag powder, the compressive strength generally increases as the slag powder content rises from 0% to 20%. At 20% slag powder content, the compressive strength peaks at 58.5 MPa, which is 15.8% higher than the control sample without slag powder.

Figure 3a,b illustrate the effects of varying amounts of slag powder and rubber particles on the tensile strength of the mortar blocks. As the slag powder content increases from 0% to 20%, the flexural strength of the modified mortar test blocks rises from 7.2 MPa to 8.1 MPa, reaching 7.7 MPa at a 25% slag powder content. Conversely, the tensile strength decreases from 7.2 MPa to 2.6 MPa as the rubber particle content increases from 0% to 25%. The test results indicate that slag powder positively influences the flexural strength of the mortar test blocks, while rubber particles negatively affect it. The reason for the tensile strength test results of the mineral powder mortar test blocks is that the initial increase in strength is due to the occurrence of a volcanic ash reaction. The volcanic ash reaction consumes the hydration reaction product calcium hydroxide, promotes a hydration reaction, and improves the compactness and pore structure of the test block. The reason for the tensile strength test results of the rubber mortar test blocks is that the strength of rubber particles is lower than that of sand particles, and the addition of large particles reduces the compactness of the test blocks, resulting in a significant decrease in the flexural strength of the test blocks. This downward trend will continue with the increase of the rubber particle content.

Figure 4a,b show the tensile-to-compressive strength ratio for mortar test blocks containing slag powder and rubber particles. The ratio for blocks with slag powder remains relatively stable at 0.14, while the ratio for blocks with rubber particles increases from 0.143 to 0.302. These results suggest that slag powder has minimal impact on the tensile-to-compressive strength ratio, whereas rubber particles can enhance it.

Overall, the addition of slag powder enhances the compressive and flexural strength of the mortar test blocks. Test blocks containing slag powder exhibit strength values that are comparable to or higher than those of control blocks without slag powder. This suggests that replacing cement with slag powder is a promising approach, though further studies are needed to evaluate road durability. Due to the soft nature of rubber particles and their adhesive properties in the test blocks, the improvement effect of rubber particles on the toughness of the test blocks is very significant. The non-linear behavior of concrete with crumb rubber performs better in damping, and its elastic bending is higher [40]. In engineering practice, the results of this experiment provide valuable guidance for adding rubber particles to mortar to meet toughness requirements.

Conversely, the inclusion of rubber particles reduces the mechanical properties of the mortar test blocks. However, it is important to note that rubber particles can modify the tensile-to-compressive strength ratio of the material and increase its deformation capacity. The addition of rubber is beneficial for specific applications, such as environments with freeze–thaw cycles, where materials with low strength and high ductility are required.

3.2. Thermal Conductivity

Figure 5a,b illustrate the effects of varying amounts of slag powder and rubber particles on the thermal conductivity of mortar. Similar to its impact on mechanical properties, increasing the slag powder content from 0% to 20% raises the thermal conductivity of the mortar from 2.07 to 2.29, a 10.4% increase. However, when the slag powder content is further increased to 25%, the thermal conductivity slightly decreases to 2.2405. In contrast, the thermal conductivity of mortars containing rubber particles steadily declines from 2.0734 to 1.0313. This indicates that rubber particles negatively impact the thermal conductivity of mortar, making it more suitable as an insulating material. The main reason for the decrease in thermal conductivity of the test blocks after replacing sand with a certain amount of rubber particles is that the thermal conductivity of the rubber particles is much lower than that of the reference mortar (the thermal conductivity of rubber is about 0.2 W/(m * K)). After adding a low-thermal-conductivity substance, the insulation of the test block is improved, resulting in a significant decrease in thermal conductivity. The addition of rubber particles also leads to the appearance of many pores inside the test block, which is also an important factor in the decrease of the thermal conductivity of the test block.

3.3. Impermeability Pressure Value

Figure 6 shows the impermeability pressure values of mortars containing slag powder or rubber particles. When the slag powder content ranges from 0% to 20%, the mortar test blocks exhibit a steady increase in seepage pressure, rising from 0.35 MPa to 0.5 MPa, a 42.9% increase. However, at a 25% slag powder content, the pressure decreases slightly to 0.4 MPa. When rubber particles are added at a concentration of 5%, the impermeability pressure increases from 0.3 MPa to 0.5 MPa, but it then decreases from 0.5 MPa to 0.2 MPa as the rubber content increases from 5% to 25%. These results suggest that adding slag powder to the mortar test blocks enhances their impermeability. The positive effect of mineral powder on the impermeability of test blocks mainly lies in the volcanic ash reaction between mineral powder and cement hydration products, which promotes a cement hydration reaction and improves the compactness of the test blocks. The negative impact is that after replacing cement with more mineral powder, the amount of cement used decreases, leading to the inhibition of hydration reactions and a decrease in the compactness of the test blocks. The introduction of a small amount of rubber particles is beneficial for blocking connected pores, increasing capillary pressure, suppressing water seepage, and improving impermeability. However, as the rubber particles begin to connect and form permeable channels, the permeability of the mortar test blocks increases.

3.4. Cycles of Freezing and Thawing

Figure 7a,b illustrate the impact of different slag powder and rubber particle contents on the compressive strength of blocks after freeze–thaw cycles. Figure 8 shows the strength loss of mortar test blocks containing slag powder and rubber particles after these cycles. As the slag powder content increases from 0% to 25%, the strength loss decreases from 15.64% to 8.44%, while the strength loss of rubber particle mortar decreases from 15.64% to 5.81%. Whether replacing cement with slag powder or sand with rubber particles, both techniques have a significantly positive effect on the freeze–thaw resistance of the mortar test blocks. The influence of mineral powder on the frost resistance of test blocks is due to the improvement of the compactness of the test blocks, which relies on the reaction of volcanic ash to refine the pore size, resulting in good frost resistance of the test blocks. Rubber particles themselves have a certain degree of elasticity and plasticity. When mortar is subjected to freeze–thaw damage, rubber particles can alleviate the frost heave stress. As the rubber particle content increases, the improvement in frost resistance also becomes more pronounced.

4. Conclusions

This study focuses on the compressive strength, flexural strength, thermal conductivity, impermeability, and frost resistance of backfill materials used for road snow and ice melting. Using M30 cement mortar as a control, the effects of various additives and dosages on these aspects were analyzed, and more suitable additives and dosages for this type of project were proposed. Rubber test blocks have a significant negative impact on the mechanical strength and thermal conductivity of materials, but they have a good improvement effect on the frost resistance and impermeability of materials. They can be considered as insulation materials for future research.

• The use of micro-slag powder in appropriate amounts can enhance the mechanical properties of silicate cement mortar. At 20% slag powder content, the compressive strength peaks at 58.5 MPa, which is 15.8% higher than the control sample without slag powder. As the slag powder content increases from 0% to 20%, the flexural strength of the modified mortar test blocks rises from 7.2 MPa to 8.1 MPa. While rubber particles (1–3 mm in size) reduce the compressive and tensile strengths of the mortar, they increase the tensile-to-compressive strength ratio. At a rubber content of 25%, the strength drops to just 8.6 MPa. The tensile strength decreases from 7.2 MPa to 2.6 MPa as the rubber particle content increases from 0% to 25%.
• Adding slag powder also enhances the thermal conductivity of the mortar, with the thermal conductivity being closely related to the volume fraction of fine slag powder (optimal at 15–25%). Increasing the slag powder content from 0% to 20% raises the thermal conductivity of the mortar from 2.07 to 2.29, a 10.4% increase. In contrast, rubber particles (1–3 mm in diameter) reduce the thermal conductivity of the mortar, with greater amounts leading to lower conductivity. The thermal conductivity of mortars containing rubber particles steadily declines from 2.0734 to 1.0313.
• Adding more than 10% slag powder improves the impermeability of the mortar. When the slag powder content ranges from 0% to 20%, the mortar test blocks exhibit a steady increase in seepage pressure, rising from 0.35 MPa to 0.5 MPa, a 42.9% increase. A small amount of rubber particles can also enhance impermeability, but excessive rubber content reduces it. The impermeability pressure increases from 0.3 MPa to 0.5 MPa, but it then decreases from 0.5 MPa to 0.2 MPa as the rubber content increases from 5% to 25%.
• As the slag powder content increases from 0% to 25%, the strength loss decreases from 15.64% to 8.44%, while the strength loss of rubber particle mortar decreases from 15.64% to 5.81%. Both slag powder and rubber particles contribute positively to the freeze–thaw resistance of mortar.