Understanding ettringite in cement-based materials and reasonable control

3. The role of ettringstone:

The role of ettringite in cement stone includes two aspects: favorable and unfavorable. Ettringite is often associated with destructive swelling, and some even call it "cement bacillus". Of course, the retarding effect of ettringite in Portland cement, and the active role of ettringite in promoting the early strength development of cement and compensating for shrinkage have also been recognized. Still, there is a lot of uncertainty about the role of ettringite. The effects of ettringite on cement setting behavior, strength and volume stability are briefly described below.

3.1 Influence of ettringite on cement setting behavior

The rapid hydration of C3A is the main reason for the rapid setting of Portland cement clinker, and the addition of gypsum can play a retarding role. Early literature and works generally assumed that the retarding effect of gypsum was due to the formation of ettringite crystals covering the surface of cement particles. As the reaction continued, the ettringite film became thicker. Due to the increase of the volume of the ettringite generated by C3A, cracks appeared in the film. Therefore, ions and water could pass through the film and continue to hydrate with C3A, and the retarding effect ended. Moreover, in the presence of Ca(OH)2, the effect of gypsum retarding is more pronounced because the resulting ettringite crystals are relatively small and better able to cover the surface of the irregular C3A mineral. However, some researchers believe that the retarding effect is not necessarily ettringite, but also may be hydration products such as AFm or aluminum glue. Some researchers even believe that the stick-like morphology of ettringite is unlikely to hinder ion migration, and believe that the slow coagulation is due to the adsorption of Ca2+ or (and)SO42- to the surface of C3A particles, occupying the active site of C3A dissolution, preventing further hydration of C3A. However, regardless of the explanation, it is an important fact that the addition of gypsum can play a better retarding effect on Portland cement clinker.

3.2 Influence of ettringite on cement strength

Whether it is Portland cement or sulfoaluminate cement, ettringite is a hydration product that appears earlier in the hydration of cement, so ettringite has a great impact on the strength development, especially the early strength development. It is generally believed that the formation of ettringite can promote the early strength development of cement. Some researchers have even added ettringite seed to improve the strength of sulfoaluminate cement.

From the perspective of the proportion of components required for the formation of ettringite, the amount of gypsum required for the formation of all C3A in Portland cement is usually much higher than the actual incorporation amount, so under normal circumstances, an appropriate increase in the amount of gypsum will improve the early strength of cement, indicating that ettringite has a promoting effect on the early strength development of Portland cement. Of course, the effect of ettringite on the early strength development of Portland cement is also related to other factors.

Ettringite is the main hydration product of sulfoaluminate cement, so it is very important for its strength development. Due to the high hydration activity and fast hydration rate of calcium sulfoaluminate mineral in sulfoaluminate cement, a large amount of ettringite and aluminum glue are generated in the early stage, and the generated ettringite will quickly crystallize to form a hard skeleton structure. The aluminum glue constantly fills the void to make the cement obtain a high early strength. When Garcia-Mate et al. studied the influence of gypsum types on the performance of sulfoaluminate cement, they found that the content of ettringite was the most important factor affecting the strength development of sulfoaluminate cement, and the early cement containing dihydrate gypsum had the largest amount of ettringite formation and the highest strength.

3.3 Influence on volume stability

The formation of ettringite is often accompanied by volume expansion, which is favorable for maintaining the volume stability of cement-based materials within a certain range, but excessive expansion will cause structural damage.

On the one hand, a lot of literature pays special attention to the damage caused by the expansion of ettringite. For large volume cement concrete projects or high temperature curing concrete products, due to the high internal temperature in the early stage, the formed ettringstone will decompose, and in the later stage of cement hardening, the ettringstone will form again, which is easy to cause cracking and damage of concrete. For sulfate attack, due to the high concentration of SO42- in the external environment, it will enter the concrete and react with AFm or aluminate in the cement stone to form ettringite, causing expansion, which may damage the concrete structure. On the other hand, even if the expansion of ettringite is very large, it can still be used under certain conditions. Ettringite generated by expansion agent or cement components in the hydration process is the most common expansion source. Therefore, using the volume expansion generated by the expansion components of ettringite in the hydration process to compensate for the shrinkage of cement-based materials can effectively restrain its shrinkage cracking, and the favorable expansion generated by ettringite has therefore been effectively applied. In addition, in the preparation of self-stressed concrete using self-stressed cement with ettringstone as the main expansion source, due to the self-expansion of the ettringstone is constrained and the stress is generated. Before the concrete structure or members bear the service load, the load bearing capacity of reinforced concrete products can be greatly increased by applying pressure to the concrete in the tension area. These all make full use of the favorable side of ettringite expansion.