The Impact of Coarse Aggregate Parent Rock Types on the Freeze–Thaw Performance of Concrete: A Comparative Study

Abstract

Hydraulic engineering projects in high-altitude environments are subject to significant diurnal temperature variations, necessitating concrete with high freeze–thaw resistance. Aggregates play a crucial role in the freeze–thaw durability of concrete. However, the impact of different parent rock types on concrete’s freeze–thaw resistance remains underexplored. This study investigated the effect of five common coarse aggregate types—granite (Gr), tuff (Tu), sandstone (Sa), limestone (Li), and pebble (Pe)—on the freeze–thaw resistance of dam concrete subjected to freeze–thaw cycles. The relationship between the rock type’s properties and the degradation patterns of concrete with different aggregates under freeze–thaw conditions was analyzed. Additionally, the damage mechanisms at the paste–aggregate interface were explored using SEM-EDS, pore structure analysis, and nano-indentation, along with the characteristics of the hydration products in the transition zone. The results showed that the aggregate type significantly influences freeze–thaw resistance, with Gr performing best (Gr > Li > Pe > Tu > Sa), correlating with pore structure and pore spacing. Gr, due to its superior freeze–thaw resistance, was optimal for regions with stringent freeze–thaw conditions. Although the interface zone exhibited a lower elastic modulus and hardness compared to the paste region due to a lower total amount of hydration products, these differences did not substantially affect the freeze–thaw performance of the concrete. This study, contributing to the expansion of the existing knowledge base on the effects of aggregate types on freeze–thaw resistance, provided valuable engineering insights for the selection of coarse aggregates in hydraulic concrete applications in high-altitude regions.

1. Introduction

The durability of hydraulic concrete is influenced by various factors, many of which can lead to its performance degradation [1,2,3]. Numerous dams are situated in cold regions, exposed to extended freeze–thaw cycles, leading to a decline in freeze–thaw resistant performance [4,5]. Freeze–thaw resistance stands out as a pivotal indicator of hydraulic concrete quality. Although freeze–thaw-resistant performance can be assessed through direct testing, this method is time-consuming and labor-intensive. Currently, the impact of freeze–thaw cycles is typically simulated using indoor accelerated tests [6,7,8,9].

Researchers have studied the deterioration of concrete under a frost environment for many years [10]. As is evident from the literature, the damage inflicted on concrete increases with more freeze–thaw cycles. A well-designed mix, paired with appropriate cement, aggregates, and additives, can enhance concrete’s ability to withstand freeze–thaw cycles [11]. As the main component in concrete, aggregates play a crucial role in concrete’s freeze–thaw resistance. Studies have highlighted that incorporating aggregates not only addresses shrinkage concerns but also boosts the concrete’s compressive strength [12,13,14,15]. Moreover, aggregates contribute to improved packing density, reduced porosity, and permeability, thereby significantly enhancing the freeze–thaw resistance of concrete [16,17,18]. In the existing literature, the primary variables concerning aggregates in concrete are size and proportion ratio [19,20]. However, in addition to the proportion and size, the types of aggregates, mineralogical composition, and physical properties of the aggregates, such as water absorption, linear expansion coefficient, and porosity can also influence the freeze–thaw resistance of concrete [21,22,23]. Current research on concrete freeze–thaw-resistant performance with respect to aggregates mainly focuses on [24,25,26,27,28] the impact of various types of aggregate materials, such as recycled concrete and tailing, and the effects of aggregate mix proportions and aggregate sizes.

Current research has demonstrated that the intrinsic properties of aggregate minerals, such as quartz, feldspar, mica, and clay minerals, not only determine the mechanical strength of the aggregate itself but also govern the performance of the interfacial transition zone (ITZ) and the overall pore structure of the concrete matrix [29]. For example, granite aggregates [30], which are rich in quartz, generally exhibit favorable durability due to quartz’s low thermal expansion. However, the presence of feldspar and mica can induce microcracking at the mineral boundaries, compromising the ITZ integrity. Conversely, sandstone aggregates often contain significant amounts of clay minerals [31], which can absorb water and undergo expansion, thereby accelerating the freeze–thaw deterioration of concrete. Although existing research has explored the influence of aggregate types on the freeze–thaw resistance of concrete, a comprehensive and systematic comparative study involving several commonly used aggregate types remains limited. Many studies have focused on individual aggregate properties, but there is a lack of unified, side-by-side comparisons of how various aggregates perform in terms of their effects on concrete’s freeze–thaw durability. This represents a significant gap in the literature. Systematically comparing different commonly used aggregates and analyzing how their individual material properties influence the freeze–thaw resistance of the concrete they form can be a significant expansion of the existing studies in this field. By focusing on the relationship between aggregate properties and concrete performance, particularly through the performance of the ITZ and pore structure, a deeper understanding of the underlying factors that determine concrete’s freeze–thaw durability can be provided. Addressing this deficiency, our study systematically compares different aggregate types to elucidate how their mother rock types impact the microstructure and durability of hydraulic concrete under cyclic freeze–thaw conditions, ultimately offering critical insights for optimized aggregate selection in cold-region applications.

This research aimed to study the effect of common aggregate categories on hydraulic concrete freeze–thaw resistant performance. Concretes of five common aggregate types were investigated. Freeze–thaw cycles, pore tests, SEM-EDS, nano-indentation, uniaxial compressive strength tests, etc., were employed to investigate the dominant factors affecting the freeze–thaw resistant performance of aggregates. The damage procedures, pore parameters with five categories of aggregates, mechanical characteristics, and hydration product performance of the aggregate–cement ITZ were also studied. This study contributed to expanding the database on aggregate’s effects on freeze–thaw resistance, offering valuable insights for selecting coarse-aggregate parent rocks in hydraulic concrete for cold regions.

2. Experimental Details

2.1. Materials

2.1.1. Cement and Mixtures

The testing specimens were constructed with ordinary Portland cement, and the strength was 52.0 MPa (28 days). The chemical composition details of the cement are shown in

Table 1. Cement properties: chemical compositions.

Chemical	SiO2	Fe2O3	Al2O3	Na2O	CaO	SO3	MgO	K2O	Cl	Loss on Ignition
Composition	20.42	2.81	3.89	0.46	63.56	3.80	0.40	0.16	0.06	3.80

. The physical and mechanical properties can be found in

Table 2. Cement properties: physical and mechanical characteristics.

Fineness *	Density (kg/m3)	Standard Consistency	Setting Time (min)		Flexural Strength (MPa)			Compressive Strength (MPa)
			Initial	Final	3 d	7 d	28 d	3 d	7 d	28 d
0.5%	3125	24.8%	184	259	6.0	7.5	10.2	25.5	33.7	52.0

* Where fineness of 0.5% refers to 0.5% of the cement particles retained on the 0.08 mm sieve based on the Chinese Standard [34].

. Grade I fly ash [32] produced by Qujing in Southern China was selected. Natural sand provided by the Yinjiang Hydropower Station at Jinsha River Section in Panzhihua City, Sichuan Province, China is used as the fine aggregate in the concrete mixture. The fineness modulus is 2.78, with a 7.3% stone powder content with a particle size smaller than 0.16 m [33], and the apparent density is 2760 kg/m³.

2.1.2. Coarse Aggregates and Mix Proportion

Coarse aggregates of real hydraulic engineering were used. Five categories of aggregates from actual projects of well-known hydraulic dams in China, namely Jinping, Xiluodu, Wudongde, Yebatan, and Baihetan, were selected. The aggregates include varieties such as granite (Gr), limestone (Li), pebbles (Pe), tuff (Tu), and sandstone (Sa), as shown in Figure 1, and all the aggregates appear to be lighter than standard color [30]. Quality inspection results of different types of coarse aggregates can be found in

Table 3. Quality inspection results of different types of coarse aggregates.

Types	Saturated Surface-Dry Apparent Density	Saturated Surface-Dry Water Absorption Rate	Needle-Like Content [35]	Crushing Index
Gr	2673 kg/m3	1.0%	3.2%	8.0%
Li	2669 kg/m3	0.5%	3.6%	6.9%
Pe	2760 kg/m3	0.6%	1.8%	2.8%
Tu	2762 kg/m3	1.2%	2.8%	5.8%
Sa	2725 kg/m3	0.6%	8.6%	7.6%
Requirements [33]	≥2550 kg/m3	≤2.5%	≤15%	≤16%

. The ’needle-like content’ refers to the proportion of aggregate particles, whose maximum one-dimensional size (typically length) is greater than 2.4 times the average particle size of the corresponding size fraction. It is calculated as the mass of needle-like particles divided by the total sample mass, expressed as a percentage [35]. The requirements in the Chinese Standard [30] are also listed in the last row to show the comparison between the samples and the requirements. It can be observed that Tu (2762 kg/m³) and Pe (2760 kg/m³) exhibit the highest densities, while Gr (2673 kg/m³) and Li (2669 kg/m³) have relatively lower densities. This suggests that Tu and Pe possess denser mineral compositions, and aggregates with higher density typically exhibit greater strength and durability. Regarding saturated surface-dry water absorption rates, Tu has the highest absorption rate (1.2%), whereas Li has the lowest (0.5%). This may indicate that Tu has a higher porosity. In terms of needle-like content, Sa has the highest percentage (8.6%), while Pe has the lowest (1.8%). This implies that Pe’s particle shape is closer to the ideal cubic form, which is beneficial for the workability and strength development of concrete. Generally, a lower crushing index indicates that the aggregate has higher strength and durability. In our experiments, Pe has the lowest crushing index (2.8%), whereas Gr has the highest (8.0%). This suggests that Pe possesses a higher resistance to crushing, which may contribute to enhancing the aggregate’s strength or resistance to freeze–thaw damage.

The water–cement ratio for all five groups of specimens is 0.4, and the fly ash content is 30%. The admixtures X404 superplasticizer and Bote GYQ air-entraining agent are used. The five aggregates were classified into three particle size ranges: 5–10 mm, 10–20 mm, and 20–30 mm, with a ratio of 40:30:30 to meet the requirements of the Chinese standard SL352-2020 [36]. Note that the mix ratio is calculated using the volumetric method; the volume of the aggregates remains constant. Since the apparent densities of different types of stones vary, the resulting coarse aggregate mass differs accordingly in

Table 4. Different aggregate concrete mix proportions.

Types	Water (kg/m3)	Cement (kg/m3)	Fly Ash (kg/m3)	Sand (kg/m3)	Aggregate (kg/m3)			Mixture Performance
					5–10 mm	10–20 mm	20–30 mm	Slumps (mm)	Air Content (%)
Gr	92	161	69	722	409	409	547	51	5.5
Tu	92	161	69	722	409	409	547	48	4.8
Sa	92	161	69	722	411	411	548	50	5.5
Li	92	161	69	722	424	424	565	50	5.5
Pe	92	161	69	722	424	424	565	65	5.5

. Concrete mix proportions and the state of the mixtures for the five different aggregates are presented in Table 4.

2.2. Concrete Specimen Preparation

Standard specimens with the size of 100 mm × 100 mm × 400 mm were formed and subjected to freeze–thaw cyclic tests. All the testing steps follow the requirements of the Chinese Standard [36]. The mixing is carried out mechanically. Before mixing, the mixer should be thoroughly cleaned with water. Prior to molding, a pre-mix of mortar with the same water-to-cement ratio as the concrete is prepared and used to coat the inner walls of the mixer, ensuring that some paste adheres to the walls. After this, the remaining pre-mixed material is poured out. Then, the weighed aggregates, cement, fly ash, and water (with the admixtures dissolved in water) are sequentially added to the mixer. The mixer is then operated for 2–3 min. Finally, the mixed concrete is discharged, and the material sticking to the mixer walls is scraped off. The mixture is manually stirred 2–3 times to ensure uniformity. The concrete slump is adjusted to between 40 mm and 60 mm through the addition of a superplasticizer and air-entraining agent while ensuring the air content remains between 4.5% and 5.5%. The test procedure is depicted in Figure 2. Next, fill the mixture into design-sized molds and compact by a vibrating table. Before making the specimens, the molds should be wiped clean, and a layer of mineral oil should be evenly brushed on the inner walls of the molds. After the specimens are formed, the surface of the concrete should be smoothed and leveled along the mold edge. Simultaneously, 100 mm × 100 mm × 100 mm cubic specimens were molded for testing the concrete surface pore structure. After curing for 24 days, the specimens then were immersed in tap water, and the temperature of the water was kept at 20 °C ± 3 °C for an additional 4 days in order to achieve the near-saturation conditions. The 15 mm × 15 mm × 15 mm cubic specimens, with the same water–cement ratio as the standard freeze–thaw resistant ones, were also prepared. These smaller specimens, incorporating an approximately 10 mm diameter aggregate, are sliced for nano-indentation tests to study the impact of different aggregate categories on the mechanical behavior and thickness of the aggregate–mortar ITZ.

2.3. Test Equipment and Methods

2.3.1. Freeze–Thaw Cycle

The rapid freeze–thaw testing machine (TDS-300) made of Nanjing Anai Testing Equipment Co., Ltd., Nanjing, China was used to conduct rapid freeze–thaw tests on the concrete samples, following relevant standard procedures [36]. To minimize errors caused by random factors, the test specimens were divided into three groups per condition. After a 4-day immersion period, the specimens undergo a careful drying process. Subsequently, their mass and natural frequency before tests were recorded to serve as reference data for undamaged concrete before freeze–thaw influences, facilitating subsequent comparative analysis.

Place the specimens into the test box for the freeze–thaw test, ensuring that the water level covers the top surface of the specimens by 20 mm. The temperature inside the chamber was initially lowered to −18 ± 2 °C within 2 h and maintained for 4 h. Following this, the temperature was rapidly increased to 5 ± 2 °C and held for another 4 h. This sequence completed one freezing-thawing cycle, with each cycle lasting approximately 11 h. The mechanical properties and physical properties were tested after 25 freeze–thaw cycles and repeated in this procedure. According to Chinese Standard [36], the test concluded upon reaching 200 freeze–thaw cycles, a relative dynamic modulus decreases to the degree of 60% of its original value, or a mass loss rate achieving 5%. Figure 3 depicts the freeze–thaw cycling test chamber, while Figure 4 shows photographs of the concrete specimens before and after freeze–thaw tests.

2.3.2. Pore Test

To ensure the effectiveness of freeze–thaw testing, the water level was maintained at least 20 mm above the surface of the specimens [36]. The instrument used for concrete internal pore analysis was the Rapid Air 457 Concrete Air Void Analyzer made by Concrete Experts International (Vedbæk, Denmark), as shown in Figure 5. First, the specimen was cut perpendicular to the exterior surface, then sanded and polished to expose the surface to be tested. The surface was then painted black, and BaSO₄ powder was thoroughly coated, with the redundant powder carefully removed. Subsequently, these specimens were secured on the workbench, and parameters such as dimensions (100 mm × 100 mm), wire length (2413 mm), and observation area (80 mm × 80 mm) were set. After preparation (as shown in Figure 6), the pore analyzer was turned on to perform the test, and the pore parameters were directly output by the instrument. Due to the reliability of the analyzer’s data [37] and the minimal differences among the three sets of data, their average was taken as the final output.

2.3.3. SEM-EDS

For microscopic analysis, the ZEISS Sigma 300 scanning electron microscope (SEM) and the Oxford Xplore 50 energy dispersive spectrometer (EDS) system were employed to examine the specimens. Point scan, line scan, and surface scan were employed to analyze the elemental clustering analysis spectrum of the concrete ITZ. Since concrete is a non-conductive material, it can accumulate electrical charges during imaging, leading to distorted results. Therefore, a thin conductive layer needs to be applied to its surface. In this case, we used a sputtering instrument, model Quorum SC7620. Each specimen was treated with a 120 s sputtering process at a current of 110 mA to ensure clearer imaging results. The SEM was set to 0.02–30 kV, with adjustments available in 10 V increments. It provides a resolution of 1.0 nm at 15 kV acceleration voltage and offers magnification from 10× to 1,000,000×. Energy spectrum analysis was conducted on selected positions and materials within the microstructure, using a working distance of 8.5 mm.

2.3.4. Nano-Indentation Test

Nano-indentation tests require an impeccably smooth specimen surface. In this investigation, the ECOMET 250 phase polishing machine (Figure 7) was used for initial coarse grinding, followed by fine grinding, and ultimately, polishing post-fine grinding. The Hysitron Ti-950 nano-indenter was used for nanoindentation testing, as shown in Figure 8. Testing points were set every 5 μm, starting from the surface of the polished coarse aggregate. There were three test points on the surface of the coarse aggregate and ten test points in the mortar section in total, extending from the aggregate to the cement paste matrix.

2.4. Measurement

2.4.1. Saturated Surface-Dry Water Absorption Rate

The saturated surface-dry water absorption rate (SSDWAR) was assessed according to DL/T 5144-2015 [30] and is calculated using Equation (1).

$$\mathrm{SSDWAR}={\displaystyle \frac{W_{ssd}-W_d}{W_d}}\times 100.$$

(1)

where $W_{ssd}$ is the weight of the specimen when its surface is dry (g) and $W_d$ is the original dry weight (g).

2.4.2. Mass Loss Rate

The mass loss rate after n freeze–thaw cycles (%), $W_n$, is calculated by Equation (2),

$$W_n={\displaystyle \frac{G_0-G_n}{G_0}}\times 100.$$

(2)

where $G_0$ is the initial mass before freeze–thaw (g), and $G_n$ represents the final mass after n freeze–thaw cycles (g).

2.4.3. Relative Dynamic Modulus

Relative dynamic modulus after n freeze–thaw cycles (%), $P_n$, is calculated using Equation (3),

$$P_n={\displaystyle \frac{f_n^2}{f_0^2}}\times 100.$$

(3)

The unit of $f_0$ is Hz, which is the natural frequency of the testing specimen before the experiment. The unit of $f_n$ is g; it represents the natural frequency after freeze–thaw cycles.

2.4.4. Linear Expansion Coefficient

The linear expansion coefficient can be calculated using the following formula:

$$\mathrm{\alpha }={\displaystyle \frac{\Delta \mathrm{L}}{{\mathrm{L}}_0\Delta \mathrm{T}}}$$

(4)

where $\mathrm{\alpha }$ is linear expansion coefficient (µm/m·°C), ΔL is the change in length (mm), L₀ is the original length of the sample (mm), and ΔT is the change in temperature (°C).

2.4.5. Water Absorption Rate

The water absorption rate can be calculated using the following formula

$$\mathrm{A}={\displaystyle \frac{{\mathrm{W}}_2-{\mathrm{W}}_1}{{\mathrm{W}}_1}}\times 100$$

(5)

where A = water absorption rate (%), W₁ = dry weight of the sample (g), and W₂ = weight of the sample after immersion (g).

2.5. Research Design

The research flowchart of this manuscript is depicted in Figure 9.

3. Results and Discussion

3.1. Properties of the Rocks

Different aggregates exhibit significant variations in linear expansion coefficients and water absorption characteristics. Larger pores generally do not notably affect later water absorption by aggregates, whereas the absorption through fine capillary pores (<50 nm) is a prolonged process. A high concentration of fine capillary pores during aggregate formation leads to a higher water absorption rate, contributing to extended water absorption and increased concrete shrinkage. Water absorption rates (Figure 10) and linear expansion coefficients of different aggregates were tested [38,39]. The linear expansion coefficient can be calculated based on Equation (4) and the water absorption rate can be calculated using Equation (5). The results are shown in Figure 11. In the linear expansion coefficient tests, each group consists of two samples. The final curves presented in Figure 11a are the averages of the data from the two samples. In the water absorption test, two specimens were designed for each group, but one group each for Sa and Tu was invalid and excluded from the tests. All of the curves each represent only one set of data in Figure 11b.

It can be observed that, among the five groups, the linear expansion coefficient and the water absorption rate are highest in Sa, which may be due to its porous structure and relatively high moisture content. The high porosity of Sa allows water to easily penetrate into the rock, resulting in a higher water absorption rate and a larger linear expansion coefficient. In contrast, Li has the lowest linear expansion coefficient and water absorption rate, which may be attributed to its higher density and tighter pore structure, making it more difficult for water to penetrate and, thus, leading to lower water absorption and expansion. The other three rock types (Gr, Tu, and Pe) exhibit intermediate values for both linear expansion coefficient and water absorption rate, with slight variations in the ranking (including one Li specimen). This may be due to differences in internal defects, mineral composition, pore structure, and moisture content among the rocks, causing the relationship between water absorption and linear expansion coefficient to not always align. For example, some rocks may show a higher linear expansion coefficient due to micro-cracks or mineral composition differences but have lower water absorption, or vice versa.

3.2. Properties of Concrete with Different Rocks

The concrete freeze–thaw resistant ability is generally characterized by the mass loss rate and relative dynamic modulus [40]. The mass loss rates with different aggregate types are depicted in Figure 12, while Figure 13 indicates the relative dynamic modulus of the specimens. It reveals that concrete specimens formed with five distinct aggregates manifest discernible variations in freeze–thaw-resistant performance.

Generally, the concrete mass loss rate increases when the numbers of freeze–thaw cycles increase, while the relative dynamic modulus of the testing specimens decreases with the increasing number of freeze–thaw cycles. Therefore, the performance of concrete’s freeze–thaw resistance can be judged by the changes in these two indicators in the same freeze–thaw conditions. As shown in Figure 12, the mass loss rates, ranging from low to high, are as follows: Gr < Li < Pe < Tu < Sa. After 25 freeze–thaw cycles, there is a slight increase in the mass loss rate of the testing specimens, and when the number of the freeze–thaw cycles reached 50, a further small increase is observed. Upon reaching 100 freeze–thaw cycles, the mass loss of the freeze–thaw-resistant specimens accelerates, indicating a faster surface erosion. As can be seen in Figure 13, from high to low, the relative dynamic modulus ranges as follows: Gr > Li > Pe > Tu > Sa. On the other hand, after 25 freeze–thaw cycles, the dynamic modulus of the concrete freeze–thaw-resistant specimens remains relatively constant. At 50 cycles, there is a slight decrease, and the decline is slow between 50 and 150 cycles. Beyond 150 cycles, the dynamic modulus starts to decrease rapidly. After the number of freeze–thaw cycles reached 200, the relative loss rate of the concrete dynamic modulus is less than 60%. Thus the freeze–thaw-resistant performance of the five groups of mother rocks ranks from strong to weak as follows: Gr > Li > Pe > Tu> Sa.

Furthermore, upon a comparative analysis of the material properties of the rock itself studied above (Figure 11) and the freeze–thaw-resistant performance of concrete containing that type of rock, some correlation between the two can be observed. The coarse aggregate parent rock of the concrete, Sa, which exhibits the highest linear expansion coefficient and water absorption rate, also leads to the greatest mass loss and the lowest relative dynamic modulus of elasticity in the concrete. This can be attributed to the higher porosity and water absorption capacity of Sa, which facilitates the penetration of moisture into the concrete. During freeze–thaw cycles, the expansion and contraction of the absorbed water accelerate the deterioration of the concrete. Additionally, the relatively loose structure of sandstone may result in poor performance of the ITZ, further diminishing the concrete’s freeze–thaw resistance. In contrast, Li, with the smallest linear expansion coefficient, shows a smaller mass loss in the concrete. This can be explained by the denser structure of Li, which reduces its water absorption and limits moisture penetration, thus causing less expansion and contraction during freeze–thaw cycles and enhancing the concrete’s freeze–thaw resistance. Furthermore, the lower porosity of Li may improve the bond strength within the ITZ, contributing to better overall freeze–thaw durability of the concrete.

However, for the other rock types, the relationship between linear expansion coefficient, water absorption, and the concrete’s mass loss and relative dynamic modulus is not entirely consistent. What is more, Table 3 reveals that the crushing index, in descending order, is Gr, Sa, Li, Tu, and Pe. Although a lower crushing index indicates higher aggregate strength and durability, considering that concrete failure is generally not caused by internal crushing of the aggregate itself but rather by failure in the overall mixture’s performance, the correlation between this rock property and the physical properties of concrete under freeze–thaw conditions may be insignificant. Consequently, it can be concluded that the ranking of the crushing index does not correspond to the observed mass loss and relative dynamic modulus. The correlation between SSDWAR in Table 3 and the mass loss rate is also studied. Overall, it can be observed that a higher SSDWAR generally corresponds to a higher mass loss rate of concrete after freeze–thaw cycles. This is primarily because higher water absorption rates may indicate a higher internal porosity of the aggregates, which can negatively affect the durability of concrete. For instance, Tu has the highest SSDWAR, and its corresponding concrete exhibited the highest mass loss rate and the poorest frost resistance after freeze–thaw cycles. Conversely, Li has the lowest SSDWAR, resulting in a relatively lower mass loss rate. However, it is worth noting the exceptions of Gr and Sa. Despite the higher porosity indicated by Gr’s elevated absorption rate, its pores are relatively small, making it difficult for absorbed water to generate large expansive pressures upon freezing. Moreover, Gr’s stable crystalline structure resists internal lattice displacement or fracture under freeze–thaw stress, while the uniformly distributed pores help disperse internal stress, reducing the localized stress concentration. On the other hand, although Li has low porosity, its primary component, CaCO₃, results in a relatively brittle structure with low toughness. Under a temperature-induced stress concentration, this makes the ITZ more prone to micro-cracking, ultimately impairing its frost resistance.

The results confirm that the freeze–thaw resistance of concrete is not solely dependent upon the physical properties of the aggregates but is also influenced by the internal structure of the concrete, may be particularly the characteristics of the air void system and the performance of the ITZ. Therefore, the primary factors affecting the freeze–thaw resistance of concrete are likely the internal structural features of the material, including air void distribution, ITZ bonding, and moisture transport properties, which require further investigation.

3.3. Pore Characteristics

The normal distribution curve of pores with different aggregates is illustrated in Figure 14, while the distribution patterns of pore size are indicated in Figure 15. The characteristic parameters corresponding to different aggregates are presented in

Table 5. Pore parameters of concrete with different categories of aggregates.

Parameters		Number	Pore Content	Average Chord Length	Spacing Factor	Specific Surface Area	Slurry-to-Air Ratio
Gr	<0.5 mm	1623	5.53%	0.082 mm	0.101	48.65 mm−1	5.73
	<1.0 mm	1667	6.72%	0.097 mm	0.110	41.09 mm−1	4.72
	Total	1679	7.42%	1.107 mm	0.114	37.48 mm−1	4.27
Li	<0.5 mm	1190	4.69%	0.095 mm	0.136	42.08 mm−1	6.76
	<1.0 mm	1233	5.88%	0.115 mm	0.148	34.77 mm−1	5.39
	Total	1248	6.69%	1.129 mm	0.162	30.90 mm−1	4.74
Pe	<0.5 mm	1330	4.90%	0.129 mm	0.116	45.03 mm−1	6.47
	<1.0 mm	1356	5.63%	0.400 mm	0.123	39.90 mm−1	5.63
	Total	1371	6.43%	1.159 mm	0.130	35.35 mm−1	4.93
Tu	<0.5 mm	1141	4.38%	0.093 mm	0.157	43.18 mm−1	7.24
	<1.0 mm	1169	5.18%	0.107 mm	01.66	37.38 mm−1	6.12
	Total	1181	5.93%	1.121 mm	0.173	32.99 mm−1	5.35
Sa	<0.5 mm	1040	3.59%	0.083 mm	0.165	48.05 mm−1	8.83
	<1.0 mm	1064	4.27%	0.097 mm	0.174	41.25 mm−1	7.42
	Total	1077	5.01%	1.112 mm	0.196	35.65 mm−1	6.33

. Figure 16 indicates the analysis results of the pore parameters in concrete constructed with various aggregate categories. As can be concluded, the pore content is highest in Gr concrete specimens (7.42%) and lowest in Sa concrete specimens (5.01%). This is because the measured pores include a portion of the pores that are indeed inherent in concrete originally, leading to the fact that the tested pore content of hardened concrete specimens becomes higher than that of the concrete mixtures. The total pore number and content ranked as Gr > Li > Pe > Tu > Sa, which is completely the same as the freeze–thaw resistant ability of concrete with different aggregates. After analyzing the average chord length of the pores in the five groups of specimens, it is indicated that the Gr specimens have the smallest average chord length, while the Li specimens have the largest. Additionally, the differences in the average chord length of the pores among the five groups are relatively small. In general, a higher pore content and a smaller spacing coefficient between the pores are expected. The data in the table confirm that the experimental results follow this pattern.

In general, an increase in pore content leads to a reduction in denudation. However, when the pore content is excessively high, pores are prone to forming continuous pores, increasing water absorption during freeze–thaw cycles and subsequently causing an increase in spalling. Additionally, a smaller spacing coefficient between the pores is generally associated with better freeze–thaw resistance in concrete. The spacing coefficients, ranging from weak to strong, are Gr, Pe, Li, Tu, and Sa, which is almost the same as the freeze–thaw resistance performance of concrete specimens ranked from strong to weak. Notably, the Gr specimen exhibits the highest pore content, the smallest average chord length of pores, and the smallest spacing coefficient between pores, indicating superior freeze–thaw resistance. This underscores the reliability of using the spacing coefficient between pores as an indicator of concrete freeze–thaw resistance.

On the other hand, after analyzing the concrete mass loss rate values after 200 freeze–thaw cycles, as depicted in Figure 12, it is evident that the mass loss rate for Sa is 3.24%, while for Gr it is 2.10%, making the loss rate for Sa ${\displaystyle \frac{3.24\%}{2.10\%}}=1.543$ times that of Gr. From the analysis of the relative dynamic modulus in Figure 13, Gr’s is 77% and Sa’s is 66%, so the loss rate of dynamic modulus for Sa is ${\displaystyle \frac{1-66\%}{1-77\%}}=1.478\%$ times that of Gr. At the same time, when analyzing Table 5, it can be seen that Gr’s pore content is ${\displaystyle \frac{7.42\%}{5.01\%}}=1.481$ times that of Sa. The multiples calculated from the four sets of data are relatively close, all around 1.5. It can be concluded that, under the same conditions, the type of aggregate parent rock can have a substantial impact on the freeze–thaw-resistant performance of dam concrete in areas with significant temperature differences between day and night. The maximum difference among the five types of parent rocks cannot be ignored. Consequently, it is necessary to consider in the engineering design that Sa has a poorer freeze–thaw resistance, and appropriate measures should be taken to enhance concrete performance. Meanwhile, Gr, with its stronger freeze–thaw resistance, is suitable for projects in regions with more stringent freeze–thaw conditions.

3.4. Performance of the ITZ

3.4.1. Mechanical Behaviors

Nanoindentation test photos of the ITZ are shown in Figure 17, and the trends in changes of the elastic modulus and hardness along the interface of the test specimen are shown in Figure 18 and Figure 19. A distinct weakened region exists in the ITZ area between the boundaries of the aggregate and the cement paste area. However, there is no clear boundary between the ITZ and the cement paste. As for the region proximal to the aggregate, there is a sudden drop in both the elastic modulus and hardness compared to the aggregate area. Subsequently, they gradually increase along the direction of the paste. Upon reaching a relatively stable value of elastic modulus, the paste region is considered to be reached.

The minimum elastic modulus in the ITZ of the Gr specimen is 3.85 GPa. Judging from the trend of elastic modulus change, the estimated thickness of the ITZ in the Gr specimen is around 30 μm. The average elastic modulus of the ITZ and the paste region are approximately 9.92 GPa and 17.04 Gpa, respectively. Similarly, for the aggregates Li, Pe, Tu, and Sa, the minimum values of the elastic modulus in the ITZ of concrete are as follows: 11.17 GPa, 3.38 GPa, 1.96 GPa, and 6.00 GPa, respectively. Judging from the trend of elastic modulus change, the estimated thicknesses of the ITZ are approximately 10 μm, 35 μm, 30 μm, and 30 μm, respectively. The average elastic modulus of the ITZ are 13.23 GPa, 8.26 GPa, 8.49 GPa, and 11.32 GPa, respectively. The average elastic modulus of the paste region are approximately 20.30 GPa, 17.04 GPa, 17.84 GPa, and 18.52 GPa, respectively. Therefore, it can be concluded that the ITZ elastic modulus for the five test specimens, from largest to smallest, are Li > Sa > Gr > Tu > Pe. The thicknesses of the ITZ for the five samples, from largest to smallest, are Pe > Gr = Tu = Sa > Li. The Li showed the relatively smallest ITZ thickness and the largest elastic modulus, while the Pe aggregate indicated, relatively, the largest transition zone thickness and smallest elastic modulus, which are different from the freeze–thaw resistance of the concrete samples.

As shown in Section 3.2 the freeze–thaw resistant performances of the five groups of mother rocks ranks from strong to weak are Gr > Li > Pe > Tu> Sa. The correlation between ITZ elastic modulus and ITZ elastic modulus appears to be weak, which may be attributed to the fact that the formation mechanism of the ITZ is primarily governed by the interfacial properties between the cement paste and the aggregate. For instance, the chemical reaction between Gr and cement is relatively weak, which does not significantly alter the ITZ structure. The mineral orientation and compatibility within the ITZ are poor, leading to a reduction in its elastic modulus. In contrast, Li possesses a relatively dense crystalline structure and the highest intrinsic elastic modulus, which enhance interfacial bonding strength and increase the stiffness of the ITZ. Conversely, Pe is typically formed through natural water erosion, resulting in smooth surfaces and nearly spherical shapes, and the concrete with Pe has the thickest ITZ. This leads to weak bonding strength and a loose interfacial structure, thereby producing the lowest elastic modulus in the concrete ITZ.

A comparative study on the properties of the ITZ and air void characteristics in Table 5 is conducted to analyze their correlations. It is indicated that there is no definitive pattern among these properties. However, aggregates such as Gr and Li consistently rank higher, while Tu tends to rank lower. The relationship between these factors remains uncertain, suggesting the need for more extensive experimental research to thoroughly explore the specific mechanisms by which these parameters influence ITZ performance under freeze–thaw conditions.

3.4.2. Hydration Products Characteristics

The results of the aggregate–cement ITZ obtained from the nano-indentation tests and the previous sections in this study indicate that there is no direct correlation between the mechanical properties and thickness of the aggregate–cement ITZ and the corresponding freeze–thaw-resistant ability of the concrete. Therefore, only one type of aggregate (Gr) was selected here to investigate the hydration product characteristics of the aggregate–cement ITZ. This will provide insights from the perspective of hydration products into why the aggregate–cement paste ITZ is relatively weaker than the paste region.

The SEM images of the ITZ and the cement paste at a certain distance from the interface in the Gr aggregate concrete are presented in Figure 20. As observed in the figure, at a hydration age of 28 days, the morphology of the hydration products in the ITZ is generally similar to that in the paste region, but distinctions exist in terms of the quantity, size, and growth characteristics of these hydration products. The prominent hydration products comprise fibrous and needle-like C-S-H, needle-shaped AF_t, and flake-like Ca(OH)₂, with some well-crystallized Ca(OH)₂ exhibiting agglomerated clusters. The fly ash particles in the ITZ have undergone a certain degree of hydration, as evidenced by the surface being covered with hydration products. The hydration products in the cracks of the ITZ are relatively large, extending radially outward, and clearly showing the presence of AF_t, with a length of approximately 3 μm to 4 μm. This also confirms a relatively high water–cement ratio in the interface region. In the cement paste region, numerous foil-like hydration products are distinctly visible, distributed within the pores, and with sizes ranging from approximately 1.0 μm to 1.5 μm.

Figure 21 shows the distribution trajectories of Mg, Al, Si, and Ca in the ITZ of 28 d Gr concrete. Elements traverse from the aggregates through the interface to the cement matrix along specified paths. It is observed that the ITZ of the testing concrete measures approximately 60 μm to 65 μm. Meanwhile, the total amount of hydration products in the ITZ is lower than in the cement paste area. Furthermore, the distribution of various elements within the internal structure of concrete exhibits discontinuity and non-uniformity, aligning well with the outcomes of previous elemental clustering and semi-quantitative elemental analyses. Notably, there is a significant enrichment of the Ca elements within the ITZ.

4. Conclusions

This study aimed to investigate the impact of coarse aggregate parent rock types on the freeze–thaw performance of hydraulic concrete. Five common coarse aggregates–granite, tuff, sandstone, limestone, and pebble–were selected for testing. A series of experiments were conducted, including freeze–thaw cycle tests, pore analysis, SEM-EDS, and nano-indentation, to analyze both the degradation behavior of the concrete and the characteristics of the interfacial transition zone (ITZ) and hydration products. The study provided comprehensive data to evaluate the correlation between coarse aggregate rock types and the freeze–thaw durability of the concrete. The conclusion is summarized as:

(1)

Under identical conditions, the freeze–thaw resistance of hydraulic concrete specimens containing different types of aggregates followed the order of granite > limestone > pebble > tuff > sandstone. The type of aggregate parent rock had a significant impact on the freeze–thaw resistance of concrete. Specifically, sandstone exhibited the poorest freeze–thaw resistance, while granite showed superior freeze–thaw durability, making it particularly suitable for use in regions subjected to harsh freeze–thaw conditions. This highlighted the necessity for engineering designs to account for the varying freeze–thaw performance of aggregates, especially considering the inferior resistance of sandstone;

(2)

Sandstone, with the highest linear expansion coefficient and water absorption rate, led to the greatest mass loss and lowest relative dynamic modulus, likely due to its high porosity and moisture absorption. In contrast, limestone, with the lowest expansion coefficient, showed better freeze–thaw resistance, attributed to its denser structure and lower water absorption. For other rock types, the relationship between aggregate properties and concrete performance was less consistent, suggesting that freeze–thaw resistance was influenced not only by the rock’s physical properties but also by the internal structure of the concrete;

(3)

The highest pore content among the granite concrete specimens was 7.42%, while the sandstone concrete specimens exhibited the lowest pore content, at 5.01%. The ranking of pore content aligned perfectly with the ranking of the freeze–thaw resistant ability of concrete with various aggregates. Hence, utilizing the pore spacing factor for evaluating hydraulic concrete’s freeze–thaw-resistant ability proved reliable. Other factors, such as pore numbers and spacing factor, also demonstrated good alignment with the specimens’ freeze–thaw resistance;

(4)

A noticeable weak area existed within the ITZ between the aggregate and the cement paste. However, there was no distinct boundary between this ITZ and the cement paste. However, the performance of the ITZ did not directly correlate with the freeze–thaw-resistant ability of hydraulic concrete. At the 28-day hydration stage, the morphologies of the hydration products in both the ITZ and the paste region were fundamentally similar. However, there are variations in the quantity, size, and growth characteristics of these hydration products. In the ITZ, a noticeably lower quantity of hydration products was exhibited, and they were relatively coarser in size;

(5)

The internal pore structure played an important role in determining the freeze–thaw resistance of hydraulic concrete. While there were variations in the ITZ-influenced hydration product distribution, they did not significantly affect the concrete’s freeze–thaw resistance. To better understand the impact of the aggregate–cement paste ITZ on the freeze–thaw durability, further refinement of the experimental design was needed to reduce the variables and achieve more precise results.