A Comparative Study on the Impact of Different Antifreeze Agents on Composite Cement at −10 °C

Abstract

Construction in cold regions faces significant challenges due to delayed cement hydration and frost damage caused by sub-zero temperatures. This study investigated the effects of three antifreeze agents on the performance of composite cement under sub-zero temperatures. The setting time, compressive strength, freezing point, and hydration mechanisms were evaluated. The results revealed that CaCl₂ optimally accelerated hydration, while achieving the continuous development of compressive strengths through freezing-point depression and dense microstructure formation. NaNO₂ exhibited the lowest freezing point but delayed setting at high dosages, while Li₂CO₃ showed limited impact due to insufficient freezing point depression. Li₂CO₃ showed limited efficacy under continuous low temperatures but enabled strength recovery after the temperature transition from sub-zero to ambient conditions. This research provides a basis for the application of antifreeze agents in the composite cement system for construction in cold environments.

1. Introduction

Cement technologies for cold regions have garnered global attention, as construction under such conditions is inherently more challenging than in temperate environments due to the detrimental impacts of low ambient temperatures on both project timelines and structural quality [1,2,3]. During winter construction, there are two primary risks: first, extremely cold conditions delay cement hydration, leading to suppressed early compressive strength development; second, when the internal temperature of concrete falls below −5 °C, approximately 92% of capillary water freezes, damaging the microstructure, while only minimal liquid water remains for hydration [4].

To address frost heave damage, enhancing early-age cement properties, accelerating hydration, and minimizing freezable water content prior to freezing are essential [4,5,6]. Conventional approaches, including heating water and aggregates, employing thermal storage curing, and covering poured concrete with insulating materials, are widely implemented [7,8]. While these measures can effectively mitigate risks, the long-term investment of manpower and energy leads to excessively high construction costs and significantly increases CO₂ emissions, contributing to environmental pollution [7,8]. Therefore, there is an urgent need for a low-cost, environmentally friendly material that can be used at sub-zero temperatures, addressing the challenges posed by excessively cold weather conditions.

Portland cement, as the most widely used cement in engineering, offered advantages including substantial late-age strength growth, low price, and simple production processes. However, sub-zero temperatures during winter construction delay the setting and hardening of Portland cement and increase CO₂ emissions [9,10,11]. Sulfoaluminate cement, known for its low carbon footprint, rapid hydration, and high early strength, is typically employed in cold-region projects and rapid repairs [12]. However, its high production costs and limited raw material availability restrict widespread adoption [13,14]. Studies demonstrated that blending sulfoaluminate cement with Portland cement create a composite system that combines rapid early hydration, shortening setting time, and boosting early strength at sub-zero temperatures with Portland cement’s economic benefits [15,16]. Nevertheless, when the temperature decreases below −5 °C, the frost resistance of the composite cement is significantly compromised due to the freezing of 92% capillary water [6]. To address this limitation, antifreeze agents have been proposed as a viable strategy to reduce frost-induced damage and expand the applicability of cement [17,18,19]. Therefore, identifying appropriate antifreeze agents compatible with composite systems is of paramount importance.

Inorganic antifreeze agents have been widely utilized in winter construction due to their cost-effectiveness, accessibility, and operational simplicity. Their mechanisms of action primarily involve lowering the freezing point of pore solutions to delay ice formation and accelerating cement hydration through ionic nucleation effects, thereby enhancing early strength development [20,21,22]. Among these agents, chloride salts (e.g., CaCl₂, NaCl), nitrites (e.g., NaNO₂, Ca(NO₂)₂), and lithium carbonate (Li₂CO₃) are commonly employed, with their functional advantages and limitations varying significantly based on composition and application scenarios.

Calcium chloride (CaCl₂) is recognized as a highly effective antifreeze agent in cold climates. Studies demonstrate that Ca²⁺ ions improve particle dispersion and accelerate C₃S hydration, while interactions with C₃A generate insoluble chloroaluminates (e.g., Friedel’s salt), promoting aluminate phase reactions [22,23]. For instance, Wang et al. [23] reported that 3%–4% CaCl₂ reduced the initial setting time of Portland cement by 60% and increased its 28 d compressive strength by 25% under 3 °C curing. However, the corrosion risk of Cl⁻ on steel reinforcement restricts its application to non-reinforced concrete structures [24,25]. Sodium nitrite (NaNO₂) has emerged as a prominent non-chloride antifreeze agent, which is particularly valued for its dual functionality in corrosion inhibition and hydration acceleration. Studies have demonstrated that NaNO₂ effectively prevents steel reinforcement corrosion by forming a protective oxide layer on the steel surface, thereby neutralizing chloride-induced electrochemical reactions [26]. In Portland cement systems, NO₂⁻ ions preferentially react with C₃A to form nitrite-modified AFt (NO₂-AFt), which suppresses the conversion of AFt to SO₄²⁻-AFm phases, thereby increasing ettringite content and early strength [11]. Compared to chloride-based agents, NaNO₂ exhibits superior compatibility with reinforced concrete, achieving a freezing point reduction to −15 °C at an 8% dosage without corrosion risks [11]. Lithium carbonate (Li₂CO₃) exhibits exceptional performance in sulfoaluminate cement systems. The small ionic radius and high polarization capability of Li⁺ shortens the hydration induction period and promotes AFt nucleation, significantly improving early strength [27,28]. Shen et al. [28] observed that 1% Li₂CO₃ reduces the initial and final setting times of sulfoaluminate cement from 37 and 64 min to 8 and 13 min, respectively. Although the aforementioned antifreeze agents have demonstrated significant efficacy in individual cement systems, their effects in composite systems remain unclear.

In this study, three antifreeze agents (CaCl₂, NaNO₂, Li₂CO₃) are selected to evaluate their effects on the setting time, compressive strength, and freezing point of a Portland/sulfoaluminate composite cement under −10 °C curing. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are employed to analyze hydration products and microstructural evolution, elucidating the mechanisms by which these agents influence composite cement performance.

2. Materials and Methods

2.1. Materials

The chemical compositions of Portland cement (P.O 42.5) and sulfoaluminate cement (52.5) are shown in

Table 1. Chemical compositions of Portland cement (wt. %).

SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2Oeq	f-CaO	Loss
20.84	4.68	3.56	63.43	3.29	2.30	0.55	0.78	1.48

and

Table 2. Chemical compositions of sulfoaluminate cement (wt. %).

SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	K2O	P2O5	TiO2	MnO2	SrO	Cl−
8.24	23.27	2.20	45.57	1.96	15.70	0.42	0.09	1.48	0.046	0.13	0.08

. Silica sand, with a fineness modulus of 2.3–3.0, was employed as the fine aggregate. Analytical-grade CaCl₂, NaNO₂, and Li₂CO₃ were used as antifreeze agents, with purities exceeding 96.0%, 99.0%, and 99.0%, respectively.

2.2. Methods

2.2.1. Setting Time

The setting time test was conducted using cement paste with a Portland/sulfoaluminate cement ratio of 9:1 and a w/c ratio of 0.3. The 9:1 ratio was selected based on prior studies, which demonstrated that exceeding a 10% sulfoaluminate cement content in the composite system significantly altered hydration mechanisms, leading to impractical setting time reductions for field applications [6]. The w/c ratio of 0.3 was optimized to balance two critical factors: freezable water mitigation and workability-structure trade-off. Antifreeze agent dosages included CaCl₂ (0%, 0.5%, 1%, 1.5%, 2%), NaNO₂ (0%, 1%, 2%, 3%, 4%), and Li₂CO₃ (0%, 0.1%, 0.2%, 0.4%, 0.6%). The cement paste was prepared according to the mix proportions. Prior to mixing, the antifreeze admixture was dissolved in water to form a homogeneous solution, which was subsequently combined with cement. The time of complete cement/water mixing was recorded as the starting time (t₁). For the initial setting test, the test needle was vertically lowered onto the paste surface, tightened for 1 to 2 s, and released. The initial setting was defined when the needle penetrated to a depth of 4 ± 1 mm from the base plate, with the corresponding time recorded as t₂. The initial setting time (Δt₁) was calculated as t₂ − t₁. The final setting time (Δt₂ = t₃ − t₁) was determined when the annular attachment of the final setting needle failed to leave an indentation on the specimen surface, recorded as t₃.

2.2.2. Compressive Strength

The compressive strength test was performed on mortar specimens with a Portland/sulfoaluminate cement ratio of 9:1, a w/c ratio of 0.3, and a cement/sand ratio of 1:1. The mortar preparation process, including specimen dimensions and testing procedures, was based on the Chinese standard GB/T 17671-2021 “Method for Testing the Strength of Cement Mortar” [29], as follows: the composite cement was dry-mixed for 120 s; the antifreeze solution was added and mixed for 30 s; sand was incorporated with slow mixing for 30 s, followed by fast mixing for 30 s and an additional 60 s. Mortar was cast into steel molds (40 mm × 40 mm × 160 mm), vibrated 60 times, and immediately sealed with a polyethylene film. The specimens were immediately transferred to a −10 °C curing chamber for low-temperature hydration. Two curing regimes were applied: (1) continuous −10 °C curing; (2) −10 °C curing for 7 d followed by 21 d at 20 ± 2 °C and 95 ± 5% relative humidity. Compressive strength was measured at four designated ages, with six specimens tested per age to ensure statistical reliability.

2.2.3. Freezing Point

The temperature history curve test was conducted using a single mortar specimen (100 mm × 100 mm × 100 mm). A T-type thermocouple embedded in the specimen center was connected to a temperature recorder, which logged data to a computer system (Figure 1) [6]. The specimen was then transferred to a −10 °C curing chamber for monitoring. Upon test completion, the temperature history curve was plotted, and the freezing point was identified using the methodology depicted in Figure 2 [6].

2.2.4. X-Ray Diffraction

The hydration products of the composite system were analyzed using X-ray diffraction (XRD). At designated ages, core samples were extracted, ground into powder, and immersed in isopropanol to terminate hydration. The powder was then pressed into pellets and loaded into a Bruker AXS D8 X-ray diffractometer (Billerica, MA, USA) with CuKα radiation (40 kV, 40 mA). Scans were performed over 2θ angles from 5° to 90°at a rate of 10°/min.

2.2.5. Scanning Electron Microscopy

The microstructure of the composite system was analyzed using scanning electron microscopy (SEM). At designated ages, a core region was extracted from each specimen and processed into a 5 mm × 5 mm × 5 mm cube. Then, it was immersed in isopropanol for 48 h to terminate hydration, and dried for 72 h. Prior to imaging, a fresh, smooth cross-section was exposed by fracturing the cube. The sample was sputter-coated with gold prior to imaging. SEM observations were conducted using an FEI Quanta 250 instrument (Hillsboro, OR, USA).

3. Results

3.1. Setting Time

The setting time of the composite system with varying CaCl₂ dosages is shown in Figure 3a. Under a negative temperature, the hydration rate of the cement slows down, resulting in prolonged initial and final setting times (136 min and 188 min, respectively, for the 0% dosage). The addition of CaCl₂ significantly mitigated this issue, with higher dosages yielding shorter setting times. As seen in Figure 3a, adding CaCl₂ notably shortened both the initial and final setting time of the composite system. Additionally, a higher dosage resulted in shorter setting times. At 2.0% CaCl₂, initial and final setting times were reduced by 77.9% and 69.7%, respectively, compared to the control. This acceleration was attributed to CaCl₂ reacting with C₃A to form hydrated chloroaluminates [30]. Additionally, CaCl₂ reacted with CH, reducing the CH content in the system by forming calcium chlorate, which further accelerated the hydration of C₃S and C₂S [17,31,32].

In contrast, NaNO₂ exhibited opposing effects, as shown in Figure 3b. While the control system (0% NaNO₂) showed initial and final setting times of 136 min and 188 min, NaNO₂ incorporation progressively delayed both. Moreover, as the NaNO₂ dosage increased, the setting time increased slowly at first and then rapidly when the dosage exceeded 3%. At a 4% dosage, initial and final setting times increased by 88.2% (256 min) and 89.4% (356 min), respectively, compared to the control. Thus, excessive NaNO₂ may delay the hydration of the composite system [33].

From Figure 3c, Li₂CO₃, similar to CaCl₂, reduced setting times but with lesser efficacy. At a 0.6% dosage, initial and final setting times decreased by 65.4% and 29.3%, respectively, compared to the control. The reduction was mainly due to the fact that Li⁺ ions shorten the induction period, accelerating sulfoaluminate cement hydration [27]. However, its impact was weaker than CaCl₂, likely due to limited freezing-point depression and preferential interaction with sulfoaluminate phases rather than silicates.

3.2. Compressive Strength

When subjected to continuous negative temperature curing, as shown in Figure 4a, the incorporation of CaCl₂ significantly enhanced the compressive strength of the composite system at various ages. When the CaCl₂ content was 0%, the compressive strength of the composite system at all ages had been suppressed by the negative temperature. The compressive strength of the composite system at −3 d, −7 d, and −28 d was only 3.1 MPa, 6.2 MPa, and 4.9 MPa, respectively. As the CaCl₂ content increased, the compressive strength of the composite system also increased accordingly. When the CaCl₂ content reached 2%, the compressive strength of the composite system reached the maximum value, i.e., 22.0 MPa, 41.2 MPa, and 43.7 MPa at −3 d, −7 d, and −28 d, respectively, which had been five to eight times higher than that of the blank. CaCl₂ promoted the hydration of C₃S and C₃A in the composite system and accelerated the formation of hydration products. During the initial stage of continuous negative temperature curing, it had still rapidly constructed a microscopic skeleton with certain strength, thereby improving the compressive strength of the composite system.

From Figure 4b, it can be observed that the incorporation of NaNO₂ also improved the compressive strength of the composite system at various ages. During the early stages of negative temperature curing at −3 d and −7 d, as the NaNO₂ content increased from 1.0% to 4.0%, the compressive strength of the composite system at −3 d and −7 d had continued to increase. When the NaNO₂ content reached 4%, the compressive strength reached the maximum. During the later stage of −28 d, the compressive strength of the composite system reached the maximum when the NaNO₂ content was 3%, which was 47.2 MPa, more than eight times higher than that of the blank. The incorporation of NaNO₂ lowered the freezing point of the liquid phase, thereby reducing the content of freezable water. Under −10 °C curing, it provided a large amount of unfrozen water in the early stage to ensure the continuous hydration of the composite system.

As shown in Figure 4c, Li₂CO₃ improved the compressive strength of the composite system to a certain extent. However, as the age increased, the growth in compressive strength was not significant. The incorporation of Li₂CO₃ promoted the hydration of C₃A and C₄A₃$\stackrel{\mathrm{-}}{\mathrm{S}}$ in the paste, thereby accelerating the formation of the hydration product AFt. This rapidly constructed a relatively dense microscopic network structure in the early stage, resisting the freezing damage caused by negative temperature curing conditions to a certain extent. However, its effect on lowering the freezing point was limited. Under continuous negative temperature curing, the free water in the system froze, making it difficult for the composite system to undergo continuous hydration.

From Figure 5a, it can be seen that when the curing condition shifted from a negative temperature to positive temperature, the compressive strength of the composite system significantly increased at −7 + 21 d compared to continuous negative temperature curing for −28 d. After the curing temperature shifted from negative to positive, some amount of frozen water melted and was reintroduced into the hydration process [19]. Under this curing condition, CaCl₂ also enhanced the compressive strength of the composite system. When the amount of CaCl₂ was 0.5%, the compressive strength at −7 + 21 d reached a maximum of 89.8 MPa, representing a 68.3% increase compared to the blank. When shifting the temperature from −10 °C to 20 °C, the previously frozen free water thawed and again became available for cement hydration. In the composite system, sulfoaluminate cement initiated rapid hydration, generating AFt to enhance strength. Concurrently, CaCl₂ accelerated the hydration of silicate phases, facilitating the formation of a denser microstructure, thereby achieving enhanced mechanical performance.

As shown in Figure 5b, with increasing amounts of NaNO₂, the compressive strength of the composite system at −7 + 21 d exhibited a trend of first increasing and then decreasing. When the amount of NaNO₂ was 2%, the compressive strength at −7 + 21 d reached a maximum of 64.5 MPa. However, when the amount of NaNO₂ was as high as 4%, the compressive strength of the composite system was only 52.4 MPa. This indicated that excessive NaNO₂ was detrimental to the hydration of the composite system at room temperature.

From Figure 5c, it can be observed that an appropriate amount of Li₂CO₃ significantly improved the compressive strength of the composite system compared to continuous negative temperature curing. When the amount of Li₂CO₃ was 0.2%, the compressive strength at −7 + 21 d reached a maximum of 77.0 MPa, representing a 44.4% increase compared to the control group. However, excessive Li₂CO₃ was detrimental to the development of compressive strength. When the amount of Li₂CO₃ was 0.6%, the compressive strength of the composite system was only 48.7 MPa. Compared to NaNO₂, Li₂CO₃ exhibited a more significant strength enhancement after the temperature transition to room temperature conditions. This contrast revealed that the freezing of free water substantially impaired Li₂CO_3’s ability to promote hydration reactions.

3.3. Freezing Point

Table 3. Effects of antifreeze agent on the freezing point of composite cement.

Antifreeze Agent	Dosage	Freezing Point
Blank	/	−0.1
CaCl2	0.5	−1.7
	1.0	−1.2
	1.5	−2.5
	2.0	−3.2
NaNO2	1.0	−2.3
	2.0	−3.1
	3.0	−4.1
	4.0	−4.9
Li2CO3	0.1	−1.8
	0.2	−2
	0.4	−2.3
	0.6	−1.1

shows the internal liquid phase freezing point of the composite system. When without an antifreeze agent, the freezing point was −0.1 °C, indicating that the composite system was susceptible to frost damage at low temperatures of −10 °C. After adding CaCl₂, the freezing point of the composite system decreased significantly. As the amount of CaCl₂ increased, the freezing point of the composite system continued to drop. When the CaCl₂ content was 2%, the freezing point of the composite system decreased to −3.2 °C. As an inorganic salt, CaCl₂ not only lowered the freezing point of the liquid phase of the composite system but also accelerated the dissolution of C₃S and C₃A in the composite system, promoting the formation of hydration products and enabling the system to rapidly establish a microscopic network structure in the early stages [31]. As the pores inside the slurry were gradually filled with hydration products, the pore diameters decreased, and the freezing point of the composite system was reduced [32].

The effect of NaNO₂ in lowering the freezing point of the composite system was more significant. With increasing the NaNO₂ dosage, the freezing point of the composite system continued to decrease. When the NaNO₂ dosage was 1%, 2%, 3%, and 4%, the freezing point of the composite system decreased to −2.3 °C, −3.1 °C, −4.1 °C, and −4.9 °C, respectively. According to Raoult’s Law, the addition of NaNO₂ effectively increased the liquid phase concentration in the composite system, and the freezing point of the liquid phase decreased as the vapor pressure inside the pores decreased [33]. Therefore, the content of unfrozen water in the composite system increased, ensuring the adequate progression of early hydration reactions. The formed hydration products filled the pores in the composite system, reducing pore diameters and further lowering the liquid phase freezing point [6]. Thus, under continuous negative temperature curing conditions, the composite system had sufficient unfrozen water available for hydration reactions, enabling continuous strength growth.

The effect of incorporating Li₂CO₃ in lowering the freezing point of the composite system was relatively insignificant. When the Li₂CO₃ dosage ranged from 0.1% to 0.6%, the freezing point of the composite system was between −1.1 °C and −2.3 °C. CaCl₂ and NaNO₂ maintained high solubility at low temperatures, and their dissociation generated a greater number of ions, enabling effective freezing point depression. In contrast, Li₂CO₃ exhibited extremely low solubility, further reduced under sub-zero conditions, with insufficient total particle concentration post-dissociation to significantly alter the freezing point. Therefore, under continuous negative temperature curing conditions, a large amount of free water froze, and Li₂CO₃ had a non-significant promoting effect on the hydration of the composite system, with minimal strength growth observed. However, when the temperature transitioned to positive, a large amount of water was released. At this point, Li₂CO₃ effectively promoted the hydration of the composite system, leading to rapid strength growth.

3.4. XRD Analysis

The effect of CaCl₂ on the hydration products of the composite system is shown in Figure 6a, and the diffraction peak intensities of the phases were quantitatively extracted and plotted in Figure 7a for comparative analysis. Compared to the raw materials and the blank system, the addition of CaCl₂ resulted in a significant decrease in the diffraction peak intensity of the clinkers C₃S and C₂S in the composite system, while the diffraction peak intensity of the hydration product AFt and CH was notably enhanced. This indicates that the incorporation of CaCl₂ promoted the hydration of the silicate phases in the system, thereby accelerating the formation of CH crystals. With the increase in CaCl₂ content, the diffraction peak intensity of gypsum in the composite system significantly decreased, while the diffraction peak intensity of the hydration product AFt further increased. Studies [11] have demonstrated that Ca²⁺ ions enhance particle dispersion through electrical double-layer effects, while their increased concentration in pore solution elevate ionic activity coefficients and reaction kinetics, thereby accelerating C₃S hydration. Concurrently, Cl⁻ ions chemically interact with C₃A phases to form insoluble chloroaluminates, which preferentially drive aluminate reaction pathways.

The effect of NaNO₂ on the hydration products of the composite system is shown in Figure 6b, and the diffraction peak intensities of the phases were quantitatively extracted and plotted in Figure 7b for comparative analysis. Compared to the blank system, the incorporation of NaNO₂ decreased the diffraction peak intensity of C₃S and C₂S, while substantially increasing those of hydration products AFt and CH. When 1% NaNO₂ was added, the gypsum diffraction peak remained detectable after −7 d of curing but nearly disappeared by −28 d, indicating the progressive consumption of gypsum during hydration. At a higher NaNO₂ dosage (4%), gypsum was almost entirely consumed within −7 d of sub-zero temperature curing. The experimental results of the freezing point demonstrated that NaNO₂ significantly reduced the freezing point of cement paste. This phenomenon was attributed to the colligative properties of solutions, where the extent of freezing point depression correlated with the total dissolved particle concentration. Owing to its high solubility, NaNO₂ dissociated into abundant Na⁺ and NO₂⁻ ions in the pore solution, resulting in an effective freezing point depression through ionic interference with ice nucleation thermodynamics. In addition, Liu et al. [11] have indicated that NaNO₂ promotes the rapid nucleation and growth of hydration products by providing additional nucleation sites, which also accelerate early hydration reactions.

The effect of Li₂CO₃ on the hydration products of the composite system is illustrated in Figure 6c, and the diffraction peak intensities of the phases were quantitatively extracted and plotted in Figure 7c for comparative analysis. Compared to the blank system, the incorporation of Li₂CO₃ resulted in a reduction in the peak intensities of C₃S and C₂S in the composite system, while the diffraction peak of hydration products (AFt and CH crystals) gradually intensified with the increasing Li₂CO₃ content. When the Li₂CO₃ dosage increased from 0.2% to 0.6%, the diffraction peak intensity of gypsum decreased under continuous sub-zero temperature curing, whereas the peak intensity of CH crystals was progressively strengthened. Zhang et al. [27] have demonstrated that Li₂CO₃ reacts with AH₃ to produce LiOH, which increases the system’s pH and enhances the dissolution of calcium sulfoaluminate phases. This process releases additional Al³⁺ and SO₄²⁻ ions, accelerating AFt formation. Simultaneously, Li⁺ ions coordinate with hydroxyl groups to stabilize [Al(OH)₆]³⁻ clusters, reducing the energy barrier for AFt nucleation and promoting rapid crystallization. Therefore, the hydration of sulfoaluminate cement in the composite cement is accelerated. However, due to its limited ability to depress the freezing point, Li₂CO₃ exhibits inferior effectiveness in enhancing the strength of the composite system under sub-zero temperatures compared to CaCl₂ and NaNO₂.

3.5. SEM Analysis

The morphological effects of different admixtures on the hydration products of the composite cement at −7 d are illustrated in Figure 8. In the blank system, limited hydration products were observed on the surface of cement particles. Combined with XRD results, the hydration reaction was significantly inhibited by the sub-zero curing temperature, leaving a substantial amount of unhydrated cement particles at −7 d. With the addition of 2% CaCl₂, abundant gel-like hydration products covered the cement particle surfaces, accompanied by hexagonal plate-like CH crystals. Compared to the blank system, CaCl₂ accelerated the hydration reaction under sub-zero conditions, forming a denser microstructural network. For the system incorporating 4% NaNO₂, the microstructure of hydration products exhibited a high compactness, with extensive amorphous gel-like phases covering the surfaces. The addition of NaNO₂ depressed the freezing point of the pore solution, reduced the content of freezable water, effectively sustained hydration at sub-zero temperatures, and mitigated frost heave-induced damage. In the case of 0.6% Li₂CO₃, needle-shaped AFt crystals and amorphous gel-like hydration products were observed on cement particle surfaces. However, numerous pores were evident. Although Li₂CO₃ promoted the hydration of sulfoaluminate cement and AFt formation, its limited freezing point depression not only hindered hydration but also increased microporosity due to frost heave-induced structural damage.

4. Conclusions

This study investigated the effects of three antifreeze agents (CaCl₂, NaNO₂, and Li₂CO₃) on the performance of composite cement under sub-zero temperatures (−10 °C). The main conclusions are as follows:

(1) Three antifreeze agents enhanced the strength of the composite system under sub-zero conditions to different extents. Both CaCl₂ and NaNO₂ demonstrated sustained strength development under continuous sub-zero temperature curing (−10 °C), with additional strength gains observed after transitioning to room temperature. In contrast, Li₂CO₃ showed a limited early-stage improvement under persistent sub-zero conditions but achieved remarkable strength recovery following the temperature shift to room temperature.

(2) The freezing point depression efficacy in the composite system followed the order NaNO₂ > CaCl₂ > Li₂CO₃. Both CaCl₂ and NaNO₂ maintained substantial unfrozen water contents, enabling continuous hydration under continuous sub-zero temperature curing. In contrast, Li₂CO₃ permitted the amount of water frozen, fundamentally restricting its strength-enhancing effectiveness under sustained sub-zero conditions.

(3) The three antifreeze agents acted in the composite system through different pathways. CaCl₂ predominantly enhanced strength through accelerated silicate phase hydration, forming dense hydration products. NaNO₂ mainly provided unfrozen water to ensure the hydration of cement by lowering the freezing point, but excessive NaNO₂ inhibited hydration processes. Li₂CO₃ promoted the strength increase mainly by affecting the hydration rate of the sulfoaluminate phase. This only worked if unfrozen water was present, while it did not have the effect of lowering the freezing point.

(4) Practical implementation requires scenario-specific strategies: For sustained sub-zero conditions, 1.5%–2.0% CaCl₂ is recommended in composite cement due to its superior freezing point depression and hydration acceleration, though corrosion inhibitors must be incorporated for steel-reinforced applications. Under intermittent low-to-ambient temperature cycles, 0.2%–0.4% Li₂CO₃ offers advantages by enabling rapid strength recovery in chloride-sensitive environments. For cost-driven projects, 2%–3% NaNO₂ provides balanced ice suppression and strength enhancement without chloride-related durability risks. Future research should prioritize investigating the synergistic effects of hybrid antifreeze agents to advance sustainable cold-region construction practices.