2.2.1. Preparation
The CGRPC was prepared by utilizing coal gangue, which was designed to resist sulfate attack. Except when the height of the roadbed is over 6 m, the pressure stress of the protecting embankment materials should satisfy the design requirement of over 5.17 MPa [
34]. Therefore, the mix proportions were in accordance with the water-tight concrete preparation, whose fine aggregate occupied 35%–45% in total aggregate and whose water-to-binder ratio was 0.45–0.6 [
35].
Table 3 shows the CGRPC mix proportions, and their aggregates were different. CGRPC-G uniformly used the cleaned coal gangue in coarse and fine aggregate, CGRPC-S used the 0–4.75 mm river sand as the fine aggregate, and CGRPC-D used the dried coal gangue in both aggregates. The dried coal gangue’s internal natural water had been removed by the sun because, at the job site, the prepared coal gangue was normally exposed to the sun. Except for the aggregate, the cement was purchased from China United Cement Corporation (Located in Beijing, China), which was ASTM Type I cement. The chemical compositions of these materials are displayed in
Table 2.
The raw materials were firstly mixed without water for 2 min and then with water for an extra 2 min. A twin axles concrete mixer was used. Then, the mixtures were cast in the molds for 3 days. After that, the shaped mixtures (specimens) were demolded and transferred to a standard curing room (20 °C and 95% humidity) until being tested.
Next, for researching the CGRPC’s durability to resist sulfate attack, 40 × 40 × 160 mm
3 mortar specimens were made following a previous sulfate corrosion study [
36]. Although there were two different specimen types, their preparation was unified to reduce the disturbing factors. The water was provided from the local water plant, and it was different from the chemical wastewater. The two kinds of water were measured for their sulfate concentration by the method of spectrophotometric determination with barium chromate. Their sulfate concentrations were 13 and 164 mg/L, respectively, for the water and chemical wastewater. This difference meant that the resistance to sulfate attack needed be evaluated by the durability tests on CGRPC.
2.2.2. Properties
The 100 × 100 × 100 mm
3 CGRPC specimens were prepared for the concrete strength tests. Their 7 day, 28 day, and 60 day compressive strengths and split tensile strengths were tested in accordance with ASTM C39 [
37] and ASTM C496 [
38]. There were three specimens for each mix in each test, and the results are shown in
Figure 5. The CGRPC-S had over 10 MPa in the 28 day compressive strength test and over 1.5 MPa in the 28 day split tensile strength test. It can be illustrated that using coal gangue (coarse aggregate) and river sand (fine aggregate) in CGRPC could satisfy the strength requirement of the slope-protecting concrete [
34]. When dried coal gangue was used in CGRPC-D, its 28 day compressive strength reduced to 5 MPa, down by 50% (
Figure 5a). Although reducing the water will diminish the w/c (water-to-cement ratio), the flow and the workability of the specimen could be worse. Coal gangue presented with many distributed holes on its surface that could absorb some water [
39]. When the water mixed in the raw materials, a part of the water was used in coal gangue, and the original water that was used for cement hydration decreased. Then, the content of the rest of water decreased to the point that fresh concrete could not fill the molds well and had worse workability. This poor filling could lead to the appearance defects on the specimens (
Figure 6) and could result in a decline in strength afterward. However, the dried coal gangue did not cause noticeable deterioration to the split tensile strength (
Figure 5b). Using river sand as fine aggregate could effectively raise about 0.5 MPa in the split tensile strength for CGRPC.
By using Young’s modulus tester, the CGRPC concrete specimens 100 × 100 × 300 mm
3 in size were prepared to test the dynamic elastic modulus and dynamic shear modulus. After that, the relationship between elastic modulus and shear modulus can be expressed by Poisson’s ratio. Poisson’s’ ratio is calculated as Equation (1):
where
E is dynamic elastic modulus (GPa),
G is dynamic shear modulus (GPa), and
v represents Poisson’s ratio.
Two dynamic modulus results of CGRPCs are listed in
Table 4, followed by the respective Poisson’s ratio. Poisson’s ratio could reflect the horizontal elastic deformation of the CGRPC that was under pressure. With the curing time increasing from 28 days to 60 days, the Poisson’s ratio of CGRPC declined about 0.01–0.03. Additionally, replacing the river sand or dried coal gangue did not cause a noticeable difference in this ratio, which was frequently at 0.27 ± 0.03.
Next, to investigate the relationship between compressive strength and dynamic elastic modulus in CGRPC, this study referenced the power function models of low-strength coal gangue concrete [
1] and 0–20 MPa regular concrete [
40]. Meanwhile, considering CGRPC concrete was mixed with coal gangue, it could be regarded as lightweight concrete [
11]. The power function model of coral concrete (lightweight concrete) [
41] was also used for analyzing this relationship. By using the above power function models, the relationship between CGRPC’s elastic modulus and its compressive strength was built by the related Equations (2)–(4):
where
Ec (×10
3 MPa) represents elastic modulus, and
fc (MPa) represents compressive strength.
wc of Equation (2) is the unit weight (kg/m
3) of CGRPC and was calculated as 2350 kg/m
3 from
Table 3. Equations (2)–(4) are shown in
Figure 7, which displays the relationship between
Ec (×10
3 MPa) and
fc (MPa). Because of the three R-squared (
R2) values being over 0.7, the power functions could simultaneously fit the relationship well. So, the CGRPC showed similar mechanical characteristics with green low-strength concrete and lightweight aggregate concrete for predicting the compressive strength. Not only that, numerical investigation [
42] also helped to simulate elastic deformation and stimulate fracture propagation for coal gangue CGRPC by using these proposed equations.
This study designed drying–wetting cycles and sulfate attack conditions for the CGRPC equivalent mortar specimens to simulate real sulfate corrosion. Following ASTM C109 [
43], the mortar specimens were prepared with 40 × 40 × 100 mm
3 size for testing the compressive strength and the flexural strength. There were three samples for each mixture in each flexural strength test. Half of the samples in the flexural strength tests were also measured for the compressive strength test. The specimens were immersed in 5% Na
2SO
4 solution when they were in the wetting period. Additionally, the groups that experienced the drying–wetting cycles with water were also set as the comparison specimens. During the cycles, to obtain the mortar specimens’ standard strength, each mix had numerous specimens that were cured in a standard curing room to measure 28, 35, 42, 56, and 84 day strength tests. Every drying–wetting cycle lasted 24 h, including 16 h of wetting and 8 h of drying. During 8 h of drying, the specimens were moved to a drying oven at a temperature of 60 °C. After 7, 14, 28, and 56 cycles (days), the mass, compressive strength, and flexural strength of the specimens were tested. To determine which mix had excellent durability, their mass loss rate (%), compressive strength loss (%), and flexural strength loss (%) of each mix after experiencing different cycles were calculated by their average values. When the specimen experienced n drying–wetting cycles, the mass loss rate can be expressed by Equation (5):
where
represents its mass loss rate (%);
represents the weight of the specimen before the drying-wetting cycles; and
represents the weight of the specimen that experienced n drying-wetting cycles. Meanwhile, the compressive strength loss and the flexural strength loss can be expressed by using Equation (6):
where
represents the strength loss (%);
represents the strength of the specimen that experienced n drying-wetting cycles; and
represents the standard strength of the specimen cured in a standard room for n cycles.
The mass loss rates of the specimens under the drying-wetting cycles with sulfate are shown in
Figure 8. M-D could lose over 6% mass after 58 cycles, while these rates in M-G and M-D were only below 1.5%. The M-D specimens, being equivalent to CGRPC-D, were not cast in the same way because a lot of the mixing water was absorbed in the coal gangue. The emerging pores and seams were distributed from the surface and extended to the inside of the M-D specimens. Sulfate sodium precipitated and dissolved with the water evaporation and saturation during the drying-wetting cycles. When the sulfate precipitated at the specimens’ pores and seams, the crystallization pressure would crumble the surrounding cement paste [
44]. The specimens were gradually damaged and more cracked cement paste fragments peeled off.
The compressive strength loss and flexural strength loss of CGRPC mortar specimens under different conditions are shown in
Figure 9. Whether drying-wetting cycles included sulfate or not, the specimens’ flexural strength and compressive strength both declined. Sulfate salt could accelerate the deterioration under drying-wetting conditions. For the flexural strength, after 56 drying-wetting cycles without sulfate, the strength loss of M-S increased up to 35% (
Figure 9a). With sulfate addition under the same cycles, this percentage could grow to over 40% (
Figure 9b). Replacing river sand with coal gangue could enormously decrease the flexural strength. The flexural strength loss rate in M-D reached 78.3% under 56 sulfate cycles. However, the compressive strength shows a different declining tendency. It can be seen that the compressive strength loss rate grew moderately, notably in M-G and M-S (
Figure 9c,d). With the sulfate addition, the compressive strength loss rate of M-D increased to only 45%, and the percentages of M-S and M-G could be under 30%. When M-S experienced 56 cycles without sulfate, the compressive strength loss rate could be no more 10%. So, without sulfate mixed in, the drying-wetting cycles also deteriorated the mortar specimens’ properties to some extent. In this process, the specimens experienced a significant temperature change. Firstly, during the 60 °C drying phase, the specimen naturally thermally expanded, which originated from aggregate and cement. With the difference in the materials’ expansion coefficients, their volume changed differently. For the cement paste, its thermal expansion coefficient (α) was normally from 18 to 20 (10
−6/°C) [
45]; for the aggregate α, the river sand tested at 11.5 (10
−6/°C), and the coal gangue was equivalent to granite (α = 8 × 10
−6/°C) because their mineral components were similar to each other. Secondly, after the 60 °C drying progress, the specimens were cooled in water (20 °C). With water immersion, the body temperature of the specimen went down rapidly, and the volume changed corresponding to the respective cooling shrinkage coefficient (equal to α). During this transformation, the interface transition zone (ITZ) between aggregate and cement experienced inhomogeneous squeezing and slacking, which gradually led to failure due to the damaged seams. The ITZ gradually became more damaged with more drying–wetting (or thermal-cooling) cycles, so that the mortar became deteriorated. Not only that, but the ambient water could also penetrate inside the specimen along its damaged ITZ seams and soften the aggregate. Meanwhile, the bonding between cement and aggregate could be weakened by the water [
46], and the mechanical properties of the specimens were severely affected. Therefore, replacing coal gangue with river sand in fine aggregate could effectively improve the resistance to sulfate, as it could strengthen the ITZ of the CGRPC.