Concrete industries in cold regions, such as Northeast and Northwestern China, are facing serious challenges in achieving targeted performance criteria of concrete during winter construction. The early freezing of concrete during winter construction affects the hydration process of cement, rendering porous structure and increased internal frozen water content, which is detrimental to the development of mechanical properties and durability of concrete. Thus, investigating the performance and microstructure of early-age frozen concrete and proposing protection techniques are of great importance to the design, preparation, and durability improvement of concrete infrastructures in cold areas.

There is a large amount of unhydrated free water in the newly mixed concrete, which can be transformed into ice once frozen at low temperatures. This process generates 9% increase in volume, leading to expansion stress on concrete. Since the cement paste at an early age has a low hydration degree, its frost resistance is relatively weak due to its low tensile strength. Once the expansion stress exceeds the ultimate tensile strength of the material, cracks will occur and accelerate the deterioration of concrete. In addition, when the temperature rises above the freezing point of the pore solution, the ice melts and leaves cavities in concrete, resulting in irreversible losses of strength and durability.

The properties and microstructure of early-age frozen concrete are affected by factors, such as mixture proportion, water-to-binder ratio, antifreeze admixture type and dosage, freezing onset time, freezing temperature, freezing duration, and pre-curing regime, etc. The water-to-binder ratio is the primary factor influencing the damage degree of early-age frozen concrete because it directly determines the content of freezable water and concrete strength. When the water-to-binder ratio decreases from 0.46 to 0.29, the liquid water contents in the total and capillary pores decrease by 34.0% and 56.8%, respectively. Under a given water-to-binder ratio, freezing onset time is the most significant factor affecting the properties and microstructure of concrete, followed by freezing duration and temperature. With the decrease in freezing temperature, the extension of freezing duration, and the advance of onset freezing time, the mechanical properties of concrete gradually decline. This results in increased total and large pore volumes and decreased gel pore volume. In addition, early freezing lead to decreased polymerization degree and amount of C-S-H.

To prevent early-age frost damage, concrete should obtain sufficient strength before it freezes, that is, the critical strength. In the technical standards of concrete for winter construction in some countries, the critical strength value is specified, generally ranging from 3 MPa to 8 MPa. Several techniques have been employed to ensure sufficient strength to alleviate the early-age frost damage and guarantee the safety of concrete structures during cold weather concreting. These include postponing the onset time of freezing with the aid of heating devices, enhancing the early-age strength of concrete using rapid-setting cement, nano-materials, alkali activator, etc., reducing the freeable water content through densifying the microstructure of concrete, and inhibiting the freezing of internal water using antifreeze admixtures. The purpose of postponing the onset time of early freezing is to keep concrete at appropriate temperatures to reach the critical strength as soon as possible. The use of nano-materials, early-strength components, and alkali activators can accelerate the early hydration of cement and improve the early strength of concrete. By reducing the water-to-binder ratio or extending the curing time, the internal structure is densified to reduce the amount of freezable water. In addition, antifreeze admixtures can inhibit the freezing of internal water by reducing the freezing point of the solution of fresh concrete mixture. At present, concrete mixed with antifreeze admixtures combined with heating devices are common means to prevent the early-age frozen damage of concrete.