Research on the Water Absorption and Release Characteristics of a Carbonized γ-C₂S Lightweight Aggregate in Lightweight and High-Strength Concrete

Abstract

Lightweight aggregate concrete, known for its light weight, thermal insulation, and excellent durability, has garnered significant attention and is considered an ideal material for lightweight ultra-high-performance concrete. Previous research has discovered that prewetting lightweight aggregates can continuously release water during the setting and hardening process of concrete, providing internal curing. However, the moisture release behavior of prewetted lightweight aggregates under different temperature and humidity conditions, as well as their internal curing mechanisms in low water–cement ratio mixtures, remains unclear and requires further investigation. In response to environmental sustainability, this study utilizes industrial waste γ-C₂S to produce a high-strength carbonized γ-C₂S lightweight aggregate (CC) and primarily compares the water absorption and release characteristics of three different types of lightweight aggregates, focusing on the influence of curing temperature and humidity on the water release behavior of the prewetted CC and establishing a water release model for the prewetted CC in cement-based materials. The experimental results indicate that the water absorption rates of the self-made high-performance lightweight aggregate (CC), magnesian lightweight aggregate (MC), and shale lightweight aggregate (SC) conform to the typical Boxlucas equation. In an air environment, the CC has the longest water release duration, followed by the MC, with the SC being the fastest. The water storage performance of the prewetted SC was poor, while the 100% prewetted CC exhibited better water storage during the mixing stage. When the CC is 100% prewetted, it can significantly increase the free water content in the interfacial transition zone, aiding in the hydration of the interfacial transition zone and enhancing the efficiency of shrinkage compensation by the expansive agent. This improvement contributes to the mechanical strength and volumetric stability of cement-based materials.

1. Introduction

With the advancement of marine engineering, the construction of large-scale marine projects such as long-span bridges has been increasing, making the lightweight and high-strength properties of precast concrete components key to the development of prefabricated assembly bridge technology [1]. Consequently, the preparation of lightweight and high-strength concrete has become a hot research topic in the field of high-performance concrete [2]. Traditional lightweight concrete is mainly divided into two categories: aerated concrete and lightweight aggregate concrete [3]. However, the mechanical strength of aerated concrete is low, limiting its application to non-load-bearing structures. In contrast, lightweight aggregate concrete, with its excellent thermal insulation, light weight, seismic resistance, and durability [4,5], can replace ordinary concrete for both non-load-bearing and load-bearing structural components [6]. It has already been applied to some elevated bridges. However, the current mechanical strength of lightweight aggregate concrete is relatively low, which is insufficient to ensure the long-term safe operation of large-span bridges and other large-scale marine structures.

Addressing the aforementioned issues, scholars such as D. Cusson [7] and S. Zhutovsky [8] have discovered that prewetting lightweight aggregates allows them to continuously release water during the setting and hardening process of concrete, thus forming internal curing [9]. K.S. Chia’s [10] research indicates that, compared with ordinary aggregates and non-pre-wetted lightweight aggregates, the addition of prewetted lightweight aggregates can enhance the density and microhardness of the interfacial transition zone, thereby improving the mechanical strength and chloride ion penetration resistance of concrete. Currently, the media for internal curing using prewetted lightweight aggregates mainly include superabsorbent polymers [11], artificial lightweight aggregates [12,13,14], and natural porous materials [15]. However, superabsorbent polymers undergo significant volume shrinkage after releasing water, which can create pores in the concrete; natural porous materials have drawbacks such as low strength and unstable water absorption and release performance. Therefore, developing an ultra-high-strength, highly absorbent, and slow water-releasing artificial lightweight aggregate is considered a potential internal curing material for lightweight ultra-high-performance concrete.

In recent years, both domestic and international scholars have conducted extensive research on lightweight aggregates and carbonation treatment technologies. For instance, M. Mastali [16] and colleagues found that carbonized slag aggregates achieve higher strength compared with ordinary lightweight aggregates, but the internal curing mechanism of these aggregates in concrete remains unclear. Meanwhile, S. Jamil [17] discovered that appropriate carbonation methods can enhance the density of aggregates by generating carbonates, significantly altering their microstructure to possess more pores and a larger specific surface area, thus greatly improving their water absorption capacity. However, systematic studies on the water release mechanisms during the internal curing phase are still relatively weak and require further exploration.

In response to the demands for environmental protection and resource utilization, using industrial byproducts such as steel slag and fly ash to produce lightweight aggregates can not only reduce costs but also promote resource recycling [18]. γ-Dicalcium silicate (γ-C₂S) is a common silicate mineral with potential hydration activity. By combining it with fly ash, refined steel slag, red mud, and pore-forming agents, it is possible to create a high-performance aggregate based on γ-C₂S [19]. However, natural γ-C₂S has low activity and is difficult to apply directly in construction materials. Through carbonation treatment, introducing carbon dioxide (CO₂) into the γ-C₂S structure significantly enhances its activity and pore structure. Researchers such as Ming Lei [20] have found that carbonation treatment significantly increases the porosity and specific surface area of γ-C₂S. These characteristics suggest that carbonized γ-C₂S lightweight aggregate could be a potential internal curing material for lightweight ultra-high-performance concrete.

It is well known that the internal curing effect of lightweight aggregates in lightweight ultra-high-performance concrete is primarily determined by their water absorption and release characteristics. Although scholar J. Castro [21] has conducted some research on the water release performance of prewetted lightweight aggregates, the water release behavior of prewetted lightweight aggregates under different temperature and humidity conditions and their internal curing mechanisms in low water–cement ratio lightweight ultra-high-performance concrete remain unclear. Therefore, this study aims to prepare an ultra-high-strength, high water absorption, and slow water-releasing carbonized γ-C₂S lightweight aggregate (CC) using γ-C₂S as the raw material and compare it with a magnesian lightweight aggregate (MC) and a shale lightweight aggregate (SC). This study investigates the water absorption and release characteristics of these three different types of lightweight aggregates, focusing on the influence of curing temperature and humidity on the water release behavior of prewetted high-performance lightweight aggregates. It elucidates the impact mechanisms of prewetting degree and curing temperature and humidity on the free water gradient distribution in the cement paste–lightweight aggregate interfacial transition zone. Additionally, it establishes a water release model for prewetted lightweight aggregates in cement-based materials, providing a theoretical foundation for the internal curing effect of prewetted lightweight aggregates in improving the related properties of lightweight ultra-high-performance concrete.

2. Raw Materials and Experimental Methods

2.1. Raw Materials

The cement used in the experiment was P.I 52.5 Portland cement produced by Hubei Huaxin Cement Co., Ltd. (Wuhan, China), with a specific surface area of 3690 cm²/g. The silica fume was sourced from Sichuan Langtian Co., Ltd. (Beijing, China), with a specific surface area of 215,000 cm²/g and a water demand ratio of 125%. The ultrafine fly ash microspheres were produced by Tianjin Zhucheng New Materials Co., Ltd. (Weifang, China), with a specific surface area of 13,000 cm²/g. Their main chemical compositions are shown in

Table 1. Main Chemical Compositions of Raw Materials (wt%).

Raw Materials	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	K2O	Na2O	Loss
cement	20.87	4.87	3.59	64.47	2.13	2.52	0.65	0.11	0.77
silica fume	96.34	0.61	0.16	0.54	0.25	0.13	0.21	0.08	1.68
fly ash microspheres	58.50	32.40	3.01	1.60	0.53	0.33	1.72	0.42	1.22

.

The lightweight aggregates used in the experiment included carbonized γ-C₂S lightweight aggregate (CC), magnesian lightweight aggregate (MC), and shale lightweight aggregate (SC), with diameters ranging from Φ0.15 to 4.75 mm. The CC aggregate is a self-produced high-performance lightweight aggregate, the MC spherical lightweight aggregate is produced by Qidong County Xiangyue Lightweight Ceramsite Factory (Qidong, China), and the SC spherical lightweight aggregate is produced by Yichang Langtian Company (Yichang, China), consisting of a reshaped, nearly spherical shale lightweight aggregate.

The components and their respective mass percentages for preparing CC lightweight aggregate are as follows: phosphogypsum 43%–68%, γ-C₂S 12%–28%, fly ash 5%–10%, mineral powder 5%–10%, red mud powder 3%–6%, sodium aluminate 1%–3%, and water 5%–7%. Initially, the measured phosphogypsum, γ-C₂S, mineral powder, fly ash, red mud powder, sodium aluminate, and water are mixed and stirred together. Subsequently, they undergo stepwise pressing, crushing, rounding, and sieving to obtain coarse aggregate. Then, they are cured in saturated lime water, combined with granulation, carbonation (firstly, put the aggregate after granulation into the reactor, adjust the carbonation pressure to 0.4 MPa, keep the relative humidity at 70%, the temperature at 20 °C, the water–gel ratio at 0.18, and then pass the concentration of 99% of CO₂ into the reactor, and the whole time of carbonation is maintained at 24 h so as to complete the carbonation process of lightweight aggregates), and drying processes to produce a fine interconnected pore aggregate with spherical shape, having a compressive strength of >20 MPa, compressive strength of >100 MPa, bulk density ≤900 kg/m³, water absorption of 8%–18%, and a size of 0.01~100 nm, with micro-inter-connected pores occupying more than 50% of the total porosity (see Figure 1). SEM microscopic testing (see Figure 2) indicates that the prepared spherical internal curing aggregate has a relatively dense internal pore structure with high micro-inter-connected pore characteristics. Through pore structure regulation, the internally cured aggregate, after prewetting, does not easily release water during the cement slurry mixing process, and exhibits a slow water release rate during the hardening stage, effectively maintaining the internal relative humidity of high-strength voluminous concrete.

Firstly, the CC, MC, and SC samples were dried and degassed to remove water, volatile substances, gases, and adsorbents; then, nitrogen adsorption experiments were carried out by cooling the samples down to the temperature of liquid nitrogen (about 77 K) to ensure that nitrogen can be adsorbed on the surface of the samples, and the amount of desorbed nitrogen was recorded during the gradual reduction in the pressure. Then, according to the BET theory, adsorption isotherms were linearly fitted to the adsorption isotherms in the appropriate pressure. The adsorption isotherms were then fitted linearly over the appropriate pressure range according to BET theory. Finally, the pore size distribution of the three aggregates was calculated based on the adsorption and desorption data using the BJH method. The pore size distribution of the CC, MC, and SC is shown in Figure 3. From Figure 3, it can be observed that CC and MC are predominantly composed of fine interconnected pores with sizes ranging from 0.01 to 100 nm, distributed uniformly and with small sizes. On the other hand, the SCs mainly consist of pores larger than 1 μm, with some even reaching sizes of about 1000 μm, existing in a quasi-spherical pore form. The distribution and size characteristics of pores within lightweight aggregates directly impact their water-releasing capacity and internal curing efficiency in concrete.

Table 2. Compactness, bulk density, and cylinder compressive strength of CC, MC, and SC.

Lightweight Aggregate	Degree of Compaction				Bulk Density (kg·m−3)	Cylinder Compressive Strength (MPa)
	Wa (%)	Pa (%)	Db (kg·m3)	Drel (%)
CC	17.12	35.90	1331	54.87	716	6.74
MC	12.41	24.27	1548	61.56	948	7.24
SC	8.16	15.76	1483	53.42	793	6.15

displays the compactness, bulk density, and cylinder compressive strength of the CC, MC, and SC. It can be observed from the table that all three types of lightweight aggregates have relatively low compactness and contain numerous pores internally. Specifically, CC and MC primarily consist of fine interconnected open pores, resulting in higher water absorption rates and stable water retention, while the SCs have more closed pores and fewer open pores, leading to water absorption rates of 17.12%, 12.41%, and 8.16% for the CC, MC, and SC, respectively. Additionally, the cylinder compressive strength of the three lightweight aggregates follows the order: MC > CC > SC. Compared with the MC and SC, CCs have advantages such as lower bulk density, higher water absorption rate, higher cylinder compressive strength, and predominance of fine interconnected pores, making them more suitable as internal curing materials for preparing lightweight high-strength concrete.

2.2. Design of Mix Proportions

To investigate the influence of prewetted lightweight aggregates on the internal humidity of cement paste and the gradient distribution of free water in the interface transition zone, this study chose CCs as the research object. Firstly, the dry lightweight aggregates were graded according to the mass ratios of Φ0.15~0.3 mm; Φ0.3 mm~0.6 mm; Φ0.6~1.18 mm; Φ1.18 mm~2.36 mm, and Φ2.36~4.75 mm as 0.05:0.1:0.18:0.27:0.4. Then, the continuously graded lightweight aggregates were subjected to non-pre-wetting, 50% prewetting, and 100% prewetting treatments. Subsequently, low water–cement ratio cement paste with prewetted lightweight aggregates was prepared according to the specific experimental mix proportions shown in

Table 3. Mix Proportions of Cement Paste (g/L).

Degree of Prewetting	Cement (g)	Silica Fume (g)	Fly Ash (g)	Water- Cement Ratio	Internal Curing Water (g)	Lightweight Aggregate (g)	Water Reducer (%)
non-pre-wetting	334	98	77	0.18	0	266	2.0
50% prewetting	334	98	77	0.18	22.7	266	2.0
100% prewetting	334	98	77	0.18	45.4	266	2.0

.

2.3. Experimental Method

2.3.1. Test of the Water Absorption Rate of Lightweight Aggregates within a Specific Time Range

Reference to the national standard GB/T25995-2010 ‘fine ceramics density and apparent porosity test method’ [22]. Weigh approximately 500 g of lightweight aggregate and place it in a 10 °C forced air-drying oven until it reaches a constant weight. After soaking the lightweight aggregate in deionized water for a certain period, quickly remove it and pour it onto a twisted wet towel. Hold both ends of the towel with your hands and lift it slightly to create a concave shape, allowing the lightweight aggregate to roll back and forth on the towel surface 8 to 10 times until the material surface is free of moisture. Then, transfer the lightweight aggregate to a porcelain dish and weigh it (accurate to 0.01 g). Calculate the water absorption rate of the lightweight aggregate during a specific time period based on the change in mass before and after water absorption, using the formula specified in Equation (1).

$$W_0={\displaystyle \frac{M_2-M_1}{M_1}}\times 100\%$$

(1)

where W₀ is the water absorption rate of the lightweight aggregate during a specific time period (%); M₁ is the absolute dry mass of the lightweight aggregate (g); and M₂ is the mass of the lightweight aggregate after soaking in water for a certain period (g).

2.3.2. Development of the Water Absorption Curve of Lightweight Aggregates

Accurately obtaining the water absorption curve of lightweight aggregates not only helps optimize the prewetting process but also provides theoretical guidance for subsequent curing and water replenishment. Combining the water absorption rate testing method mentioned earlier, the water absorption rate of lightweight aggregates is measured at intervals of 30 s within the time range of 0 to 300 s when immersed in deionized water. Based on the variation in water absorption rate with immersion time, the water absorption curve of lightweight aggregates is plotted.

2.3.3. Test of the Water Release Rate of Lightweight Aggregates Over a Specific Time Period

The change in external environmental temperature and humidity directly affects the internal curing ability of lightweight aggregates, making it necessary to accurately determine the water release performance of the CCs, MCs, and SCs. The experiment begins by weighing approximately 500 g of lightweight aggregates and placing them in a 105 °C forced air drying oven until a constant weight is achieved. The aggregates are then soaked in deionized water, removed after 24 h, squeezed with a damp towel to remove surface water, and weighed in a porcelain dish with known weight, accurate to 0.01 g. Next, the prewetted lightweight aggregates are placed in an environmental test chamber with constant temperature and humidity. At the designated testing time, the aggregates are quickly removed from the porcelain dish and sealed with plastic wrap to prevent moisture loss. After reaching room temperature, they are weighed again. The water release rate of the lightweight aggregates over a specific time period is calculated based on the loss of mass before and after placement in the environmental test chamber, as shown in Equations (2) to (3).

$${DW}_c=M_3-M_4$$

(2)

$${DW}_r={\displaystyle \frac{{DW}_c}{M_3}}\times 100\%$$

(3)

where DW_c is the amount of water released by the lightweight aggregates after a certain period of time (g); DW_r is the water release rate of the lightweight aggregates during a specific time period (%); M₃ is the mass of the lightweight aggregates after soaking in deionized water for 24 h (g); and M₄ is the mass of the lightweight aggregates after being placed in the environmental test chamber for a certain period of time (g).

2.3.4. Development of the Water Release Curve of Lightweight Aggregates

Accurately obtaining the water release curve of lightweight aggregates lays the foundation for exploring the water release internal curing mechanism of prewetted lightweight aggregates in lightweight ultra-high-performance concrete. Based on the aforementioned water release rate testing method, the water release rate of lightweight aggregates is tested at different temperature and humidity conditions after 6 h, 12 h, 18 h, 24 h, 30 h, 36 h, 48 h, and 60 h of placement. The water release curves of lightweight aggregates under different temperature and humidity conditions are plotted based on the variation in water release rate with placement time.

The environmental test chamber used in this study is the GDJS-408 high- and low-temperature alternating thermal test chamber manufactured by Wuxi Yierda Testing Equipment Manufacturing Co., Ltd. (Wuxi, China). It has a temperature range of 0 to 100 °C, with a temperature deviation of ≤±2 °C, and a humidity range of 30 to 98% RH, with a humidity deviation of ≤±3%RH.

2.3.5. Test of the Internal Humidity in Cementitious Slurry

The experiment used a 100 × 200 mm cylindrical mold. Firstly, the cementitious slurry prepared according to Table 3’s mixing proportions was poured into the mold up to a height of 100 mm. Then, a pipe sleeve was placed in the center of the mold, and slurry was poured until reaching a diameter of 2 mm from the top of the sleeve. A spacer was inserted into the sleeve to prevent direct contact between the probe and the slurry. Subsequently, the probe was inserted, and paraffin wax was filled up to the top of the mold. The outside of the mold was sealed with plastic wrap, and after demolding, it was further sealed with aluminum foil and film for curing. The changes in temperature and humidity inside the hardened slurry over curing time were recorded, as depicted in the schematic diagram of internal humidity testing of concrete specimens in Figure 4. The temperature and humidity test probe used is the HC2A-S type temperature and humidity probe produced by the Swiss Rotronic Company (Bassersdorf, Switzerland), with a temperature range of −50 to 100 °C, a temperature deviation of ≤±0.1 °C, a humidity range of 0 to 100% RH, and a humidity deviation of ≤±0.8% RH.

2.3.6. Test of the Free Water Content of Cement Paste Surrounding Aggregates

The experiment involved preparing cementitious slurry according to the mixing proportions in Table 3. After thorough mixing, the slurry was poured into 40 × 40 × 40 mm molds and placed in a curing box with the temperature maintained between 20 and 60 °C (±2 °C) and relative humidity between 50 and 70%RH (≤±3%RH). The slurry was cured for 10 min, 0.5 h, 1 h, 2 h, 4 h, 8 h, 1 day, 3 days, 7 days, 14 days, and 28 days. When the cementitious slurry was not fully hardened, samples were taken using a sampler from the 1 mm area around the lightweight aggregates and from areas far from the aggregate matrix. Once the slurry had completely hardened, a diamond cutting machine was used to slowly cut the cement paste. Samples were then taken from the 0.05 to 0.2 cm thickness area around the lightweight aggregates and from the cementitious matrix away from the aggregates. The samples were ground using 100 to 200 mesh diamond sandpaper, and the ground cementitious powder was collected. For each batch of samples, 5 to 7 areas were selected for sampling. After sampling, the samples were weighed, accurate to 0.001 g. The hydration of the slurry was terminated using anhydrous ethanol, and then the samples were dried to a constant weight in a forced air-drying oven at 105 °C. The free water content of the cement paste around the aggregates and in the matrix was calculated based on the mass loss before and after drying, as shown in Equation (4).

$$W_f={\displaystyle \frac{M_5-M_6}{M_6}}\times 100\%$$

(4)

where W_f is the content of free water in the cement paste around the aggregates or in the matrix (%); M₅ is the mass of the cementitious slurry around the aggregates or in the matrix before drying (g); and M₆ is the mass of the cementitious slurry around the aggregates or in the matrix after drying (g).

3. Results and Discussion

3.1. The Water Absorption Characteristics of Different Types of Lightweight Aggregates

3.1.1. The Water Absorption Curves of Different Types of Lightweight Aggregates

To understand and master the water absorption characteristics of lightweight aggregates, this section first studies CC, MC, and SC with diameters of Φ2.36–4.75 mm, comparing the water absorption patterns of different types of lightweight aggregates. The water absorption curves of three different types of lightweight aggregates are shown in Figure 5. It can be observed that with the increase in soaking time, the water absorption rate of all three types of lightweight aggregates initially rises rapidly and then gradually levels off. Among them, the water absorption rate of the SC is the highest, reaching 72% saturation after soaking for 10 s and nearly 96% after 60 s. Compared with the SC, the water absorption rates of CCs and MCs are slower, stabilizing after soaking for 210 s and 240 s, respectively. This is mainly due to two reasons: (1) the saturation water absorption rate of CCs and MCs is much higher than that of the SC; (2) CCs and MCs have more evenly distributed microconnected air voids inside, resulting in a slower rate of free water absorption, whereas the air voids in the SC are larger and visible to the naked eye, leading to a faster water absorption rate.

After soaking in deionized water for 300 s, the water absorption rates of the CCs, MCs, and SCs were 16.74%, 11.95%, and 8.08%, respectively, all lower than their respective saturation water absorption rates of 97.78%, 96.29%, and 96.31%. This is mainly because the water absorption process of lightweight aggregates is actually the process of water entering and filling the internal pores of the aggregates under the capillary action. With the increase in free water adsorption, the channels between the micropores inside the lightweight aggregates are gradually filled, slowing down the rate of gas discharge and external water adsorption inside the pores, resulting in a gradual decrease in the water absorption rate of the lightweight aggregates as the soaking time increases. When the channels between the micropores are completely filled, as the free water adsorption continues to increase, the gas inside the pores will gradually compress, and the pressure inside the pores will gradually increase. When the pressure inside the pores is equal to the external water pressure, the water absorption process of the lightweight aggregates reaches equilibrium. Additionally, the saturation water absorption rate of the lightweight aggregates is obtained by testing with a TXY type water absorption rate tester after vacuuming out the internal pores, which leads to the water absorption rates of the three types of lightweight aggregates after 300 s of soaking being lower than their saturation water absorption rates. It can be considered that, compared with the MCs and SCs, the CCs have the highest water absorption rate.

3.1.2. Establishment of Water Absorption Models for Various Types of Lightweight Aggregates

Based on the water absorption curves of the three different types of lightweight aggregates, it can be observed that with increasing time, the water absorption rates of all three aggregates gradually increase, while the rate of absorption slows down, conforming to the typical Box–Lucas curve pattern. Therefore, in this study, we utilized the Box–Lucas equation to fit and construct water absorption models for the CCs, MCs, and SCs, as shown in Equation (5).

$$W=a\times (1-e^{-\mathrm{bt}})$$

(5)

The equation is represented as follows: W represents the water absorption rate of the lightweight aggregate within time period t; t stands for the duration of water absorption; a denotes the saturated water absorption rate of the lightweight aggregate; and b is a parameter related to the size and distribution of internal pores within the lightweight aggregate, where 0 < b < 1.

Following the Box–Lucas equation, this study employed the method of least squares to fit the curves depicting the changes in water absorption rates over time for the CCs, MCs, and SCs, with results displayed in Figure 6. Based on these fitting outcomes, the study derived water absorption models for the three different types of lightweight aggregates, as presented in Equations (6)–(8).

$$\mathrm{CC}: y=1.628\times {10}^{-1}\times (1-e^{-0.0199t}), {\mathrm{R}}^2=0.9902$$

(6)

$$\mathrm{MC}: y=1.171\times {10}^{-1}\times (1-e^{-0.0238t}), {\mathrm{R}}^2=0.9923$$

(7)

$$\mathrm{SC}: y=7.89\times {10}^{-2}\times (1-e^{-0.1403t}), {\mathrm{R}}^2=0.9965$$

(8)

It can be observed that the water absorption models for the three types of lightweight aggregates are consistent with their respective trends in water absorption, and they exhibit a good fit with the actual measured data, with R² values exceeding 0.99. Therefore, it is considered that the water absorption behavior of lightweight aggregates can be characterized using the Box–Lucas equation model.

3.2. The Moisture Release Characteristics of High-Performance Lightweight Aggregates in Air

3.2.1. The Moisture Release Curves of Various Types of Prewetted Lightweight Aggregates

First, the three different types of lightweight aggregates were immersed in water for 300 s to achieve water saturation. Subsequently, the moisture release behavior of these prewetted lightweight aggregates in air was studied under conditions of 20 °C temperature and 70% RH humidity. The results are depicted in Figure 7.

From the graph, it can be observed that as time progresses, the free water content in all three spherical lightweight aggregates initially decreases rapidly and then gradually slows down, which is in direct contrast to their moisture release rate trends. Additionally, the moisture release processes for the CCs, MCs, and SCs primarily occur within 48 h, 30 h, and 18 h, respectively. This indicates that high-performance lightweight aggregates have the best water storage capacity and lowest moisture release rate, followed by magnesia lightweight aggregates, while shale lightweight aggregates exhibit the poorest water storage capacity and highest moisture release rate.

The moisture release process of prewetted lightweight aggregates is essentially the diffusion of adsorbed water within the internal pores of the aggregates under the driving force, which is the pressure difference between the saturated vapor pressure, actual partial pressure of water, and negative pressure within the material pores under certain temperature and humidity conditions. This is expressed specifically as Equation (9).

$$\Delta P=P_s-P_a-P_c$$

(9)

In the equation, ΔP represents the moisture release driving force of prewetted lightweight aggregates, measured in Pascals (Pa); P_s stands for the saturated vapor pressure of water, also measured in Pascals (Pa); P_a refers to the actual partial pressure of water, also measured in Pascals (Pa); and P_c denotes the negative pressure within the material pores, measured in Pascals (Pa).

Extensive research has revealed that the actual partial pressure of water, Pa, and the saturated vapor pressure of water, P_s, have the following relationship with the relative humidity, RH, of the external environment:

$$P_a=P_s\times RH$$

(10)

At the meantime, the negative pressure within the material pores can be calculated using Equation (11):

$$P_c={\displaystyle \frac{2\gamma \cos \theta }{r}}$$

(11)

where γ represents the surface tension of water, measured in Newtons per meter (N/m), and r denotes the radius of the internal micro-pores within the material, measured in meters (m).

$$\Delta P=P_s(1-RH)-{\displaystyle \frac{2\gamma \cos \theta }{r}}$$

(12)

When the relative humidity of the external environment is 70% RH, according to Equation (9), the actual partial pressure of water is 0.7 times the saturated vapor pressure, or 0.7P_s. Given that the saturated vapor pressure of water at 20 °C, P_s (20 °C), is a constant value of 2338.43 Pa, the moisture release driving force for prewetted lightweight aggregates under conditions of 20 °C temperature and 70% RH humidity can be calculated as follows:

$$\Delta P=701.529-{\displaystyle \frac{2\gamma \cos \theta }{r}}$$

(13)

From Equation (13), it is evident that the smaller the pore size within lightweight aggregates, the lower the moisture release driving force for prewetted aggregates. As analyzed earlier, SCs have larger pore sizes, resulting in higher moisture release driving forces, thus exhibiting characteristics of high moisture release rates and shorter moisture release times. On the other hand, CCs and MCs primarily consist of fine interconnected pores, leading to relatively lower moisture release driving forces. Consequently, they demonstrate lower moisture release rates and longer moisture release times. Additionally, due to the higher free water content in prewetted CCs, they require the longest moisture release time.

3.2.2. The Moisture Release Behavior of High-Performance Lightweight Aggregates under Varying Temperature and Humidity Conditions

The Moisture Release Curves of High-Performance Lightweight Aggregates at Different Temperatures

Previous research has revealed that compared with existing MCs and SCs, the prepared CCs exhibit higher mechanical strength, higher water absorption rates, and more excellent water storage and release properties. Therefore, this study mainly focuses on prewetted spherical CCs and thoroughly investigates the influence of environmental temperature and humidity on the moisture release behavior of prewetted lightweight aggregates in air.

At a relative humidity of 70% RH, the moisture release curve variations in prewetted lightweight aggregates at different temperatures are depicted in Figure 8. From the graph, it is apparent that environmental temperature has a significant impact on the moisture release behavior of lightweight aggregates. With increasing temperature, the moisture release rate of the aggregates gradually rises, and the moisture release time decreases. At a temperature of 20 °C, the moisture release of the lightweight aggregates rapidly increases within 0 to 24 h, then slows down, reaching a moisture release rate of approximately 94.39% at 60 h. As the temperature rises to 40 °C, the moisture release rate of the aggregates reaches 94.51% at 18 h and remains nearly constant thereafter. When the temperature further increases to 60 °C, the moisture release rate of the aggregates peaks at 95.51% at 12 h, with minimal changes afterward. This is mainly due to the direct correlation between the saturated vapor pressure of water and environmental temperature. As the temperature rises, the saturated vapor pressure of water increases. According to Equation (12), under constant conditions, the higher temperature increases the saturated vapor pressure of water, thereby enhancing the moisture release driving force of the lightweight aggregates. This acceleration of the moisture release process leads to an increase in the moisture release rate and a decrease in the moisture release time of prewetted lightweight aggregates.

The Moisture Release Curves of High-Performance Lightweight Aggregates at Varying Humidities

In addition to temperature, environmental humidity is also a crucial factor affecting the moisture release characteristics of lightweight aggregates. Therefore, this study also investigates the impact of relative humidity on the moisture release behavior of prewetted lightweight aggregates.

At a temperature of 20 °C, the variation patterns of moisture release curves for prewetted lightweight aggregates at different humidities are illustrated in Figure 9. As the relative humidity increases, the moisture release rate of the aggregates gradually slows down, and the moisture release time lengthens. Specifically, when the relative humidity is 50% RH, the moisture release process of the aggregates primarily occurs within 30 h. At 60% RH, the moisture release process takes place mainly within 0 to 36 h, while at 70% RH, the aggregates consistently release moisture steadily within 42 h. This phenomenon occurs because as the relative humidity rises, the actual partial pressure of water increases, leading to a decrease in the moisture release driving force of prewetted lightweight aggregates. Consequently, this slows down the moisture release rate and extends the moisture release time of prewetted lightweight aggregates.

The Moisture Release Process of High-Performance Lightweight Aggregates under Varying Temperature and Humidity Conditions

The moisture release rate over time for prewetted lightweight aggregates at different temperature and humidity conditions is depicted in Figure 10. It is evident from the graph that higher environmental temperatures lead to higher moisture release rates for prewetted lightweight aggregates. Conversely, higher environmental humidity results in slower moisture release rates for prewetted lightweight aggregates. Specifically, at a temperature of 20 °C and humidity of 70% RH, the moisture release rate of the aggregates is the slowest, and the moisture release time is the longest, with a small amount of moisture still being released after 60 h. On the other hand, at a temperature of 60 °C and humidity of 50% RH, the moisture release rate of the aggregates is the fastest, and the moisture release time is the shortest, with almost complete moisture release within 9 h.

Experimental data analysis reveals a strong linear relationship between the reciprocal of moisture release rate (1/Dw) and the reciprocal of moisture release time (1/T) for prewetted lightweight aggregates under varying temperature and humidity conditions, as illustrated in Figure 11.

Additionally, as time progresses, the moisture release rate of lightweight aggregates gradually slows down while the moisture release amount increases, consistent with the variation trend of the Langmuir model. Therefore, this study utilizes the Langmuir deformation equation to characterize the relationship between the moisture release rate of prewetted lightweight aggregates and time. The fitting equation is represented as Equation (14). Moreover, the corresponding values of parameters α, β and the fitting factor R² under different temperature and humidity conditions are presented in

Table 4. The parameters α, β and the fitting factor R² under different temperature and humidity conditions.

T	RH	α	β	R2
20 °C	50%	0.9568	5.5121	0.9899
	60%	0.8920	7.8542	0.9956
	70%	0.8148	13.6511	0.9989
40 °C	50%	0.8940	4.0359	0.9872
	60%	0.8578	4.2263	0.9901
	70%	0.8393	5.3776	0.9931
60 °C	50%	1.0001	0.3662	0.9885
	60%	1.0004	0.4342	0.9905
	70%	1.0005	0.5880	0.9960

.

$$D_w={\displaystyle \frac{\alpha t}{1+\beta t}}$$

(14)

3.3. The Moisture Release Characteristics of High-Performance Lightweight Aggregates in Low Water–Binder Ratio Mortar

3.3.1. Cement Paste Internal Humidity

From Figure 12, it is observed that within the age of 5 days, the relative humidity inside the cement paste decreases slowly, then rapidly decreases after 5 days, and after 15 days, the rate of decrease slows down and gradually stabilizes. Due to the influence of negative pressure inside the pores, prewetted lightweight aggregates can release water to the surroundings. Samples with 100% prewetted lightweight aggregates show significantly higher internal relative humidity compared with the other three groups, and at 28 days, the internal relative humidity of this sample remains around 90%. In comparison with the blank group without lightweight aggregates, the group with 50% prewetted lightweight aggregates absorbs some moisture through capillary action during the early hydration stage in the low water–binder ratio cementitious system, leading to a reduction in internal relative humidity. However, during the middle and late stages of hydration, prewetted lightweight aggregates gradually release water into the surroundings, causing an increase in the internal humidity of the cement paste. On the other hand, non-pre-wetted lightweight aggregates absorb more water during the mixing process, reducing the water–binder ratio of the system. Although they can release water outward during the middle and late hydration stages, the water supply is limited, resulting in lower internal relative humidity compared with the blank group throughout the curing period.

3.3.2. The Gradient Distribution of Free Water in the Transition Zone between Cement Paste and Lightweight Aggregates

Under the premise of curing temperature at 20 °C and humidity at 70%, the influence of prewetting on the gradient distribution of free water in the transition zone between cement paste and lightweight aggregates is illustrated in Figure 13. It can be observed that when non-pre-wetted lightweight aggregates are added, the free water content in the transition zone within the first 8 h of hydration is lower than that in the matrix further away from the aggregates. This is primarily because non-pre-wetted lightweight aggregates absorb some moisture from the surrounding cementitious slurry through capillary action during the mixing process, resulting in a reduction in free water content and a decrease in local water–binder ratio in the transition zone. As the hydration and hardening of the slurry progress, the relative humidity inside the concrete decreases significantly. The adsorbed water in the pores of lightweight aggregates starts to gradually release outward under pressure difference, leading to closer free water content in the transition zone around the lightweight aggregates and the matrix.

When 50% prewetted lightweight aggregates are added, they absorb moisture from the surrounding slurry during the mixing stage, leading to a decrease in free water content in the transition zone. In the middle and late hydration stages, these lightweight aggregates release moisture outward, causing an increase in free water content in the transition zone.

When 100% prewetted lightweight aggregates are added, the free water content in the transition zone is similar to that in the matrix within the first 8 h of hydration. However, after 8 h, the free water content in the transition zone becomes significantly higher than that in the matrix. This is mainly because saturated prewetted lightweight aggregates have excellent water storage properties during the mixing process. When the free water content in the slurry decreases and the internal relative humidity decreases during the hydration stage, saturated prewetted lightweight aggregates continuously release water to the surrounding slurry under the negative pressure difference in the pores. This leads to an increase in free water content and relative humidity around the lightweight aggregates, aiding in enhancing the hydration degree in the transition zone and the reaction efficiency of the expansive agent. Consequently, this improves various properties of lightweight ultra-high-performance concrete.

Subsequently, this study investigated the gradient distribution of free water in the transition zone between prewetted high-performance spherical cement paste and lightweight aggregates under different temperature and humidity conditions. The impact of curing humidity on the gradient distribution of free water in the transition zone is shown in Figure 14 under a temperature of 20 °C.

From the figure, it can be observed that when the curing humidity is between 50% and 70% RH, the variation in free water content in the transition zone within the initial 0–1 h of hydration is minimal, and the water–binder ratio remains around 0.18. This is mainly because during the initial hydration stage, the hydration rate of the cementitious slurry is slow, resulting in minimal water consumption and thus almost unchanged free water content in the transition zone. Between 1 and 24 h, the hydration of the cementitious slurry intensifies, leading to a rapid decrease in free water content in the transition zone. After 24 h, the hydration rate of the cement slows down, and the free water content in the transition zone begins to gradually stabilize. Additionally, with an increase in relative humidity, more moisture is provided by the external environment, which can partially compensate for the water consumed during the hydration process of the cementitious materials, leading to an increase in free water content in the transition zone.

Under a relative humidity (RH) of 70%, the effect of curing temperature on the gradient distribution of free water in the transition zone is illustrated in Figure 15. The graph indicates that as the temperature increases, the free water content in the transition zone of prewetted cement paste and lightweight aggregates gradually decreases. This phenomenon is attributed to the increased volatilization of internal moisture in the cementitious slurry at higher temperatures, leading to an accelerated internal water release rate and subsequently reducing the free water content in the transition zone. Furthermore, when the curing temperature reaches 60 °C, the free water content in the transition zone decreases rapidly with increasing curing time. This rapid decrease is primarily due to excessive evaporation of free water at 60 °C, where the replenishment of moisture from prewetted lightweight aggregates and the curing environment is insufficient.

From the above observations, it can be inferred that the incorporation of prewetted lightweight aggregates during the hydration process of cementitious slurry can increase the free water content in the transition zone between cement paste and lightweight aggregates. This not only accelerates the hydration in the transition zone but also enhances the efficiency of the expansion agent’s reaction, thereby improving the mechanical, workability, and durability properties of cement-based materials. Additionally, reducing curing humidity or increasing curing temperature will decrease the free water content in the transition zone, which could adversely affect its structure and properties. Therefore, selecting an appropriate curing regimen can effectively enhance the internal curing effect of prewetted lightweight aggregates in low water–binder ratio cementitious slurries.

3.3.3. The Water Release Model of Prewetted Lightweight Aggregates in Cementitious Materials

In low water–binder ratio cementitious materials, the water release driving force for internal curing through the release of moisture from prewetted lightweight aggregates into the surrounding mortar is influenced by the internal moisture difference between the prewetted lightweight aggregates and the cementitious mortar, as well as the negative pressure difference within the pores. According to Equation (9), the driving force for the outward diffusion of adsorbed water from the internal pores of prewetted lightweight aggregates is given by

$$\Delta P_1=P_{s1}-P_{a1}-P_{c1}$$

(15)

In the equation, ΔP₁ represents the driving force for outward diffusion of adsorbed water from the internal pores of prewetted lightweight aggregates, measured in Pa; P_s1 stands for the saturated vapor pressure of water, measured in Pa; P_a1 represents the actual vapor pressure of water in the pores of the lightweight aggregates, measured in Pa; and P_c1 represents the negative pressure within the pores of the lightweight aggregates, measured in Pa.

At the same time, the driving force for the diffusion of free water from the surrounding mortar into the prewetted lightweight aggregates is given by

$$\Delta P_2=P_{s2}-P_{a2}-P_{c2}$$

(16)

In the equation, ΔP₂ represents the driving force for the diffusion of free water from the surrounding mortar into the prewetted lightweight aggregates, measured in Pa; P_s2 stands for the saturated vapor pressure of water, measured in Pa; P_a2 represents the actual vapor pressure of water in the pores of the mortar, measured in Pa; and P_c2 represents the negative pressure within the pores of the mortar, measured in Pa.

Therefore, the driving force for internal curing through the release of moisture from prewetted lightweight aggregates into the surrounding mortar can be expressed as

$$\begin{gathered} \Delta P_0=\Delta P_1-\Delta P_2 \\ =\left(P_{s1}-P_{a1}-P_{c1}\right)-\left(P_{s2}-P_{a2}-P_{c2}\right) \\ =(P_{a2}-P_{a1})+(P_{c2}-P_{c1}) \end{gathered}$$

(17)

Based on Equation (17), we can conclude that prewetted lightweight aggregates can release moisture into the surrounding mortar only when ΔP₀ is greater than 0, meaning that the difference in humidity between the prewetted lightweight aggregates and the surrounding mortar, along with the negative pressure difference, is greater than zero.

Since the saturated vapor pressure of water is only influenced by the ambient temperature, and it remains constant when the curing temperature is constant, we can derive the following equation without considering the effects of heat release from cement hydration:

$$P_{s1}=P_{s2}=P_s$$

(18)

The actual vapor pressure of water and the saturated vapor pressure of water are related to the relative humidity of the surrounding environment; therefore,

$$P_{a1}=PS\times {RH}_2$$

(19)

$$P_{a2}=PS\times {RH}_1$$

(20)

where RH₁ denotes the internal relative humidity of the prewetted lightweight aggregates, and RH₂ refers to the internal relative humidity of the mortar surrounding the aggregates.

Therefore, combining the negative pressure relationship of material pores shown in Equation (11), it can be inferred that the driving force for prewet lightweight aggregates to release water to the surrounding slurry for internal curing is

$$\begin{gathered} \Delta P_0=\left(P_s\times {RH}_1-PS\times {RH}_2\right)+({\displaystyle \frac{2\gamma \cos {\theta }_2}{r_2}}-{\displaystyle \frac{2\gamma \cos {\theta }_1}{r_1}}) \\ =P_s({RH}_1-{RH}_2)+2\gamma ({\displaystyle \frac{\cos {\theta }_2}{r_2}}-{\displaystyle \frac{\cos {\theta }_1}{r_1}}) \end{gathered}$$

(21)

where γ represents the surface tension of water in N/m; r₁ is the radius of the internal pores of lightweight aggregate in meters (m); and r₂ is the radius of the internal pores of the cementitious matrix in meters (m). Combining with Dalton’s classic moisture evaporation model, the water release rate equation for prewetted lightweight aggregate providing internal curing to surrounding mortar is derived as follows:

$$V^{\prime }=C^{\prime }\left[P_s({RH}_1-{RH}_2)+2\gamma ({\displaystyle \frac{\cos {\theta }_2}{r_2}}-{\displaystyle \frac{\cos {\theta }_1}{r_1}})\right]r_1^2$$

(22)

where V′ represents the rate at which prewetted lightweight aggregate releases water to the surrounding mortar, and C′ is the model parameter.

Combining Figure 12, Figure 13, Figure 14 and Figure 15 with Figure 10, it can be observed that the water release process of the self-made prewetted high-performance lightweight aggregate in low water–cement ratio cementitious materials differs significantly from its behavior in ambient air. This is mainly because the water release process of prewetted lightweight aggregates in air is only influenced by the humidity difference, while in low water–cement ratio cementitious slurries, it is affected by both the humidity difference and the capillary pressure due to the negative pressure inside the pores (osmotic pressure). In low water–cement ratio cementitious materials, the water release process of prewetted lightweight aggregates can be divided into three stages, as follows:

Stage I—In the initial stage of hydration, the degree of hydration of the slurry is low, and the pore size is relatively large, mainly in the micrometer-sized pores, which are much larger than the fine connected pores with sizes of 0.01 to 100 nm inside the lightweight aggregates. The cementitious slurry is not capable of absorbing much water from the prewetted lightweight aggregates under the capillary pressure. As hydration progresses, the internal humidity of the cementitious slurry gradually decreases. Under the humidity difference, the prewetted lightweight aggregates begin to release water to the surrounding slurry, but the water release process is relatively slow.

Stage II—With the increase in hydration degree, the density of the hardened slurry improves continuously, and its internal pores mainly range from 0 to 80 nm, which is smaller than the internal fine pores of the lightweight aggregates. At this stage, prewetted lightweight aggregates can release water to the surrounding cementitious slurry under the capillary pressure. Meanwhile, there exists a certain humidity difference between the prewetted lightweight aggregates and the surrounding slurry. Under the combined action of humidity difference and capillary pressure, the water release rate of prewetted lightweight aggregates increases, manifested by a significant increase in the internal humidity of the hardened slurry and the free water content in the interface transition zone.

Stage III—In the late stage of hydration, the amount of water released by prewetted lightweight aggregates is relatively high, and the relative humidity inside their pores decreases significantly, approaching or even slightly lower than the internal humidity of the cementitious base material. Therefore, in this stage, the water release of prewetted lightweight aggregates mainly occurs through capillary pressure to the surrounding slurry. However, due to the small size of the internal pores of high-performance lightweight aggregates and the slurry, as well as their proximity, the capillary pressure is relatively low, resulting in a decreased water release rate of prewetted lightweight aggregates and a slower water release process.

4. Conclusions

(1)

The water absorption rates of self-made high-performance lightweight aggregates (CCs), magnesia lightweight aggregates (MCs), and shale lightweight aggregates (SCs) initially increase and then stabilize with increasing water absorption time, following the typical Boxlucas equation pattern.

(2)

Under ambient conditions, the water release time of prewetted lightweight aggregates is in the order of CC > MC > SC.

(3)

The water storage performance of prewetted SCs is relatively poor; non-pre-wetted or 50% prewetted CCs absorb a certain amount of water during the mixing process, resulting in a reduction in the local water–cement ratio of the slurry; and 100% prewetted CCs have good water storage performance during the mixing phase.

(4)

Adding 100% prewetted lightweight aggregates can significantly increase the free water content in the interface transition zone, which helps improve the degree of hydration in the transition zone and the compensatory shrinkage efficiency of the expansion agent, thereby improving the mechanical strength and volume stability of cement-based materials.

(5)

In low water–cement ratio cement-based materials, the water release process of prewetted lightweight aggregates can be simply divided into three stages: Stage I—initial hydration stage; Stage II—increase in hydration degree; and Stage III—late hydration stage.