Cement concrete materials are one of the most widely used building materials in modern infrastructure, but their production process is associated with high energy consumption and significant carbon emissions, imposing substantial environmental and resource pressures. While traditional mineral admixtures (such as fly ash and slag) can improve concrete workability, enhance mechanical properties, reduce hydration heat, and improve durability by partially replacing cement, they suffer from low early-stage activity. The application of advanced grinding or sorting technologies to refine mineral admixtures into ultrafine particles has proven effective. This process increases the specific surface area of the admixtures, thereby enhancing particle surface energy and reactivity, which compensates for the deficient early-stage activity of conventional mineral admixtures. Ultrafine mineral admixtures demonstrate remarkable potential in improving concrete workability, early-stage mechanical performance, and durability. In recent years, this approach has garnered widespread attention in academic and industrial research.

This paper first elucidates the characteristics of fly ash microsphere, ultrafine fly ash, and ultrafine slag powder. Subsequently, it systematically investigates the impacts of these three typical ultrafine mineral admixtures on various properties of cement and concrete, accompanied by comparative analyses of their performance differences compared with conventional mineral admixtures. Furthermore, the intrinsic properties, compatibility design principles, and comprehensive effects of ultrafine composite mineral admixtures on cement concrete performance are expounded. Finally, the current application status of ultrafine mineral admixtures in cement concrete is summarized. Ultrafine mineral admixtures exert multiple beneficial effects in cementitious systems, including filling effect, morphological effect, nucleation effect, pozzolanic effect, density effect, dispersion effect, specific surface area effect, and interfacial effect. Their dosage and fineness significantly influence critical parameters such as water demand for standard cement consistency, setting time, rheological properties, and hydration heat release. Appropriately formulated fly ash microsphere, ultrafine fly ash, or ultrafine slag powder with optimized fineness can enhance concrete workability, improve durability, inhibit shrinkage, and suppress alkali-aggregate reactions, though potentially compromising carbonation resistance. These materials also demonstrate pore structure refinement, microstructural optimization, and mechanical performance enhancement. The primary distinction between ultrafine and conventional mineral admixtures (e.g., fly ash and slag) lies in particle fineness, which yields differential performance outcomes despite sharing identical chemical reaction mechanisms. Conventional admixtures typically enhance concrete workability, long-term strength, and durability at the expense of early-age strength reduction. In contrast, ultrafine variants leverage superior pozzolanic reactivity and filling capability, where the increased specific surface area amplifies nucleation effects, leading to significant improvements in early-age strength development and workability. Their micro-aggregate effect and enhanced pozzolanic activity further contribute to more pronounced durability enhancement. Compared with single-type ultrafine admixtures, ultrafine composite mineral admixtures employ “gradient hydration” and “functional complementarity” mechanisms to synergistically accelerate hydration processes. This strategy effectively increases amorphous C-S-H gel formation, optimizes pore structure of hardened paste, and enhances matrix compactness through multi-scale interactions. In the concrete mix design, the dosage of ultrafine mineral admixtures is recommended to be controlled between 20% and 35%, which can significantly improve the workability and mechanical properties of concrete. However, excessive dosage may trigger a significant dilution effect, which is detrimental to the overall performance of concrete. Ultrafine mineral admixtures have shown great application potential in enhancing the workability of cement-based repair materials, manufacturing cement-based refractory materials, producing high-performance insulation materials, enabling steam-free curing of prefabricated components, and improving the comprehensive performance of ultra-high-performance concrete (UHPC). Currently, the application of ultrafine mineral admixtures mainly faces two major challenges: First, the relevant standard and specification system is still incomplete. Second, it is challenging to produce ultrafine powders that meet the standard requirements using diverse and complex raw materials. Due to the complex sources of mineral admixtures, the performance of ultrafine mineral admixtures can vary significantly, and improper dosage control may adversely affect the performance of concrete. Therefore, it is urgent to improve the standard specifications, enhance the preparation processes and equipment, reduce energy consumption and pollution, and further investigate their effects on the hydration mechanisms of cementitious materials to promote their wider application.

Compared with traditional mineral admixtures such as fly ash and slag, ultrafine mineral admixtures, characterized by higher specific surface area and pozzolanic reactivity, have shown significant advantages in improving the workability, mechanical properties, and durability of cementitious materials. Against the backdrop of green and low-carbon transformation in the cement and concrete industry, significant progress has been made in the application research of ultrafine mineral admixtures. By reducing the clinker factor and decreasing the cement content per unit of concrete, they provide an effective pathway for achieving sustainable development in building materials. Future research should focus on the following key areas: First, improving existing grinding equipment and processes to achieve rational composite grinding of mineral admixtures, thereby enhancing quality and reducing costs. Second, leveraging artificial intelligence technology to accurately predict the performance of ultrafine mineral admixtures, significantly improving design efficiency. Third, conducting in-depth studies on the hydration synergistic effects and microstructural evolution mechanisms of different ultrafine mineral admixtures. Fourth, refining technical standards and specifications to promote product quality improvement and the expansion of application fields. With technological advancements and increasing environmental demands, ultrafine mineral admixtures will play a more important role in enhancing the performance of cement concrete, reducing costs, and driving the development of green buildings.

Ettringite plays an important role in the early strength and volume stability of cement. In a high-concentration sodium sulfate solution, Na⁺ and SO₄^2– will be embedded between the structural layers of AFm to form a sodium-substituted AFm phase(U-phase). Therefore, the stability of ettringite and AFm under high temperatures and alkaline environments requires attention. Ettringite will not only transform into AFm but also form U-phase in a high-temperature and alkaline environment. However, the specific process of the mutual transformation of ettringite and AFm under the combined action of high temperature and alkalinity is still unclear. The phase transition of hydration products is bound to affect the performance of cement-based materials in high-temperature and high-alkalinity environments. Therefore, it is important and necessary to clarify the transformation process of Ettringite to AFm. Based on this, ettringite was first synthesized by adding a small amount of NaOH to the solution system, and the effect of NaOH on the synthesis and transformation of ettringite was explored, especially the effect on the evolution of the above-mentioned synthesized ettringite to AFm in an environment of 105 ℃. A detailed discussion was conducted, which provided new understanding of the stability of ettringite in high-alkali and high-temperature environments and also provided theoretical support for the performance degradation of cement-based materials under such conditions.

The raw materials Ca(OH)₂ and Al₂(SO₄)₃·18H₂O were used to synthesize the ettringite sample by solution method, and the alkaline environment was adjusted by NaOH. Samples were taken at high temperature at designed time intervals to explore the transformation process of ettringite and washed with deionized water and anhydrous ethanol in turn, dried in a vacuum drying oven at 40 ℃, and ground into powder. The phase composition of all samples was tested by the D8 ADVANCE X-ray diffractometer, and the structure of the samples was analyzed by TOPAS V6 software. ZEISS Gemini 360 FE-SEMs was used to observe the microscopic morphology of the samples. The thermal stability of the samples was tested by a NETZSCH STA449F3 synchronous thermal analyzer. Based on density functional theory (DFT), the Castep module in Materials Studio was used to optimize the crystal structure and calculate the electronic structure properties.

The addition of NaOH is beneficial to the formation of ettringite and has an inhibitory effect on the formation of dihydrate gypsum. Under certain alkalinity conditions, the formation of ettringite is favorable. With the increase of NaOH addition, the short rod-shaped ettringite decreases and tends to be fine needle-shaped, with a certain curvature parallel to the c-axis. The increase in OH^– ion concentration causes [Al(OH)₆]³ to form rapidly, resulting in an accelerated nucleation rate of ettringite and a smaller crystal size. In addition, the addition of NaOH reduces the crystal cell parameter c and the crystal cell volume of ettringite. The defect formation energy of Na⁺ replacing Ca1, Ca2, Al1 and Al2 sites in the ettringite crystal structure is Na@Al1>Na@Al2>Na@Ca1>Na@Ca2 from large to small, it can be concluded that Na@Ca2 is the most stable crystal configuration, which indicates that Na⁺ is more inclined to replace the Ca2 site. Compared with the N-0 sample without NaOH added, the decomposition temperature of the small-sized ettringite synthesized in the NaOH environment is lower, and the stability of the sample is relatively weak.

NaOH has a particularly significant effect on the transformation of ettringite to AFm. The intermediate phase U phase exists in the transformation of ettringite to AFm. The rod-shaped ettringite first split to varying degrees at both ends to form a flaky AFm phase. In addition, there are a small number of small needle-shaped rod-shaped ettringite crystals attached to the surface of AFm and transform from AFm into a thicker hexagonal flaky U phase. , and finally form a stable AFm phase. Na⁺ and SO₄^2– are co-embedded between the layers of the AFm structure, resulting in a larger interlayer spacing of U-phase compared to AFm.

The ettringite samples synthesized by adding a small amount of NaOH at room temperature have fewer impurities than pure ones, the unit cell parameters are reduced, and Na ions easily replace some Ca2 sites in ettringite. With the increase in NaOH addition, the morphology of the synthesized ettringite changes from thick rods to curved needle rods, and the thermal stability of the ettringite decreases. The ettringite synthesized by adding a small amount of NaOH has an intermediate phase U-phase during the transformation to AFm under a high temperature environment, and all of them are transformed into the AFm phase after being kept at a high temperature of 105 ℃ for 24 h; however, the ettringite samples without NaOH almost do not undergo transformation. The transformation process of ettringite to AFm under NaOH and a high-temperature environment can be divided into three stages. Stage 1 (within 3 h): In this stage, most of the calcium sulfoxide directly transforms into the AFm phase. Stage 2 (4–6 h): Two reactions occur in the system. One is that Na⁺ and SO₄^2– ions react with AFm to form the intermediate phase U-phase with a larger interlayer spacing. At the same time, Ca²⁺ and SO₄^2– ions in the liquid phase react with AFm again to form calcium sulfoxide, which then decomposes into the intermediate phase U-phase; Stage 3 (8–24 h): The U-phase begins to gradually transform into the larger AFm stable phase.