Understanding ettringite in cement-based materials and reasonable control

Ettringite (AFt) is a hydrous sulfoaluminate mineral, often written as 3CaO·Al2O3·3CaSO4·32H2O. Ettringite is the early hydration product of Portland cement and the main hydration product of sulfoaluminate cement. In the early hydration process of Portland cement, gypsum reacts with aluminate or ferric aluminate minerals to form ettringite, which delays the hydration of cement and can play an early strength role. In sulfoaluminate cement, calcium sulfoaluminate mineral reacts with gypsum under the condition of water to form ettringite, accounting for 50% to 60% of the total hydration products, which is the main factor determining the early strength development of sulfoaluminate cement. At the same time, ettringite is also the main factor for the expansion of some calcium sulfoaluminate expanders [1]. Ettringstone can be said to be everywhere, not only limited to the well-known hydration products of cement, but the broad scope of ettringstone exists in a wider range.

1. Basic characteristics of ettringstone:

Ettringite crystals belong to the tripartite system, and the cell parameters are c=2.15nm, a=b=1.125nm. The composition of ettringite can be expressed as {Ca6[Al(OH)6]2·2H2O}·(SO4)3·2H2O, and its basic structural unit is -- cylinder {Ca6[Al(OH)6]2·2H2O}6+, SO42- ions and other water molecules are connected with these positively charged cylinders. That is, the crystal structure of ettringite is composed of columns parallel to the C-crystal axis, and the remaining water molecules and sulfate ions occupy the pores between the columns [2-3]. Hartman et al. [4] used deuterium atom (D atom), the isotope of hydrogen, and combined neutron diffraction technology with full spectrum fitting and finishing technology to more accurately determine the positions of all atoms in ettringite crystals, especially the positions of O and H atoms, and gave a more intuitive crystal structure diagram

It is generally believed that ettringite in cement concrete is mainly needle-shaped, but these ettringite crystals are often clustered together in different forms, so radial, annular, spherical or massive ettringite clusters are often observed in the size range of 100~300μm. However, due to the different synthesis methods and conditions of ettringite, the morphologies of the synthesized ettringite are quite different, and some new morphologies are even found, such as tubular, cylindrical and hexagonal columns

2. Synthesis method and influencing factors of ettringite:

At present, the synthesis methods of ettringite are mainly single ore hydration and solution synthesis methods, single ore hydration method generally uses tricalcium aluminate and gypsum or calcium sulfoaluminate and gypsum for hydration synthesis of ettringite, solution synthesis method uses calcium hydroxide and aluminum sulfate reaction to synthesize ettringite.

Theoretically, as long as the relevant ions in the solution reach a certain degree of saturation, ettringite crystals will be formed and precipitated, so the concentration of ions in the reaction solution, temperature, pH and formation space and other factors have a great impact on the formation of ettringite. The effects of synthesis temperature and liquid pH value on ettringite are briefly introduced.

2.1 Temperature

At home and abroad, it is agreed that temperature has an important effect on the stability of ettringite, and the stable temperature of ettringite in cement is about 70℃. YukieShimada believes that ettringite will transform into AFm phase in alkaline solution at 80℃, and the morphology and size of ettringite will decrease with increasing temperature, whether in cement slurry or chemical reagents. However, these studies mainly focus on the stability of ettringite synthesis at different temperatures. The study of Zhang Wensheng et al. showed that the formation of ettringite could be promoted with the appropriate increase of temperature. At 80℃, the reaction basically directly formed ettringite with various morphologies. The reaction ettringite cannot exist stably at 100℃ and will be converted into AFm after a short time.

2.2 pH Value

The stable existence of ettringite in solution depends on the pH of liquid phase, and its stable existence pH range is 10.5 ~ 13.0. Studies have shown that the morphology and size of ettringites synthesized under different pH values are significantly different, but different scholars have different views, because they have different conditions for synthesizing ettringites. A large number of studies have shown that the increase of alkalinity or pH value has a certain inhibitory effect on the formation of ettringite. When the alkalinity of pore solution increases, the concentration of ions required for the stability of ettringite also increases correspondingly. Brown et al. studied the reaction of C3A and gypsum in KOH solution with a concentration of (0.5-2.0)mol/L, and found that the presence of KOH inhibited the formation of ettringite, and the inhibition was most obvious when the concentration of KOH was between (0.5-1.0)mol/L, and even the formation of C-S-H gel adsorbed with Al and SO42- was observed. The study of Zhang Wensheng et al. showed that with the increase of pH value in liquid phase, the length-diameter ratio of ettringite gradually decreased, and the morphology of ettringite changed from long rod to fine needle.