Factors Affecting Slump Loss of Concrete-Cement
The mineral composition and mineral morphology of cement clinker directly affect the hydration and hardening process of cement and the adsorption of additives, so it has a great influence on the construction performance of concrete. The main cause of slump loss is that cement hydration consumes free water and produces hydration products, which increases the viscosity of fresh concrete. The four minerals of cement clinker are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium ferrialuminate. Among them, the hydration of tricalcium aluminate is the fastest. If there is no suitable setting component, tricalcium aluminate hydrates quickly to form sheet-like tetracalcium aluminate hydrate. These hydration products overlap with each other, resulting in the rapid loss of fluidity of fresh concrete. The hydration reaction of tricalcium silicate is very fast, and because tricalcium silicate is a mineral with high content in cement clinker, its hydration degree directly affects the setting and hardening of paste. Therefore, the cement with tricalcium aluminate and tricalcium silicate content in clinker, especially the cement with high tricalcium aluminate content, is easy to cause concrete slump loss because of its rapid initial hydration.
Gypsum in cement components also has a great impact on the slump of concrete. In the process of cement grinding, because of the high clinker temperature, the dihydrate gypsum used in cement will be dehydrated to form hemihydrate gypsum and anhydrous gypsum, which will increase the activity of sulfate. Because the solubility and dissolution rate of dihydrate gypsum are less than that of hemihydrate gypsum, but greater than anhydrous gypsum, gypsum can adjust the setting time of cement hardening. When a certain amount of gypsum is added, the hydration speed of cement will be slowed down, but when the amount of gypsum is insufficient or excessive, the hydration speed of cement will be increased and the fluidity of slurry will be lost. In the presence of gypsum, C3A reacts with gypsum to form ettringite. If the activity of gypsum is matched with C3A, gelatinous ettringite is formed, which covers C3A surface and inhibits its hydration. At this time, concrete works well. If the activity of gypsum is insufficient, needle-rod ettringite and hydrated calcium aluminate will be produced, resulting in the loss of fluidity of cement paste. If the content of gypsum is too high, strip secondary gypsum will be formed, which will lead to the loss of fluidity.
In order to improve the label of cement, the simplest way for cement manufacturers is to add appropriate grinding aids to improve the specific surface area of cement, change the gradation of cement particles, increase the void ratio of the accumulated weight of cement particles, increase the water demand, and accelerate the hydration reaction of cement, increase the slump loss and poor construction performance.
During the production of cement clinker, a small amount of alkali is brought into cement clinker by raw materials, some of which are melted into clinker minerals, and some of which are in the form of soluble alkali. Soluble alkali can promote the hydration of cement and affect the construction and strength of concrete. Some experts have studied the adaptability of super-alkali to concrete admixtures. The research points out that soluble alkali accelerates the hydration reaction of cement, which leads to the increase of water demand of cement, the decrease of slump and the increase of slump loss.