The Performance And Mechanism Of Liquid Alkali-free Accelerator

This paper mainly analyzed the influence of a kind of liquid alkali-free accelerator on the setting time and strength of cement, the adaptability of cement as well as the impact of temperature on the agent performance. In addition, the effect of vary content of sulfate salt on the setting time and strength were studied in this paper. By chemical analysis, the content of liquid calcium, water and calcium hydroxide in vary age cement mortar with setting-accelerator were measured. Using XRD, SEM and other means of test, the microstructure of hardened cement was analyzed and the mechanism of setting-accelerator was discussed.Experimental studies have shown that: alkali-free accelerator adaptability of different kinds of cement was acceptable. With the increase of accelerating agent content, the setting time decreased greatly, earlier strength grew and later strength loss smaller. To be exactly, 4% addition of liquid alkali-free accelerator was added to Hengwei cement. And 1d compressive strength of cement mortar increase about 15%; 28d compressive strength ratio is 96.9%. In the contrary, as to the standard cement, content of 5%,1d compressive strength can reach 17.2MPa and 28d compressive strength ratio is 109%. Beside of this, unlike traditional accelerator which possess disadvantages of high content and low early strength, this kind of setting-accelerate agent overcome the shortcomings of relatively higher 28d compressive strength loss of the concrete with the accelerator. This accelerator’performance was affected by the temperature. When temperature increase, this kind of accelerator could promote the hydration of cement. And the reduction of sulfate could reduce the setting time, promote early strength and improve the setting properties of cement.Chemical analysis showed that: the mixing of the accelerator has changed the process of hydration, also changed the content of hydration products. In this process, liquid calcium reduced; the content of bound water and calcium hydroxide increased. Because of the large amounts combination of calcium and water, the rapid loss of fluidity caused the reduction of setting time, increase of early strength and higher degree of hydration of cement.Structural analysis showed that: the mixing of this kind of liquid alkali-free setting-accelerator altered the mount and the morphology of calcium hydroxide crystals into ettringite crystals with the short column intertwined with fine-needle crystals. Unlike conventional ettringite crystals grow from the surface of cement particles, the intertwined ettringite crystals spread throughout the body of hardened cement paste and combined with other hydration filled the pores in the cement paste. Since a large number of ettringite production consumed amount of Ca²⁺, the content of CH became small. It can be seen that the addition of this agent did not alter the type of hydration, but alter the number, location and shape of the products.Setting-accelerate mechanism as follow: with the addition of accelerating agent, the plaster react with calcium hydroxide immediately, the mount of calcium reduce quickly. As a result, the period of the induction of C3S has been shorten or disappeared, which promote the hydration of C3S.In the process, liquid calcium combined with water form calcium silicate hydrate gel and ettringite, resulting in lower liquid calcium and higher mount of combined water and calcium hydration. The morphology of ettringite present short prismatic and fine needle-like, which grow freely without any tendency. This random distributed ettringite crystals can form a network structure with lots of calcium silicate gel filling the gaps, resulting in shortened setting time and increase early strength.