IntroductionEttringite 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.MethodsThe 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.Results and discussionThe 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.ConclusionsThe 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.