With the development of the concrete industry and the increasing requirements for different parts and structural components of concrete in construction, as well as construction methods, the control of concrete setting time directly affects the progress of construction and other issues. Early strength and delayed setting are important research topics for concrete. Under the influence of temperature, different types and contents of Retarder will have a significant impact on the setting time of concrete.
1.Types of retarder and their mechanisms of action on concrete
(1) Types
There are many types of retarder, which can be divided into two categories according to their chemical composition: inorganic Retarder and organic Retarder. Inorganic Retarder include phosphates, zinc, iron sulfates, copper sulfates, borates, fluorosilicates, etc.; organic Retarder include lignosulfonates, hydroxy carboxylic acids and their salts, polyols and their derivatives, sugars and carbohydrates, etc.
(2)Mechanisms of action
In general, most organic Retarder have surface activity. They produce adsorption on the solid-liquid interface, changing the surface properties of the solid particles; or by adsorbing a large amount of water molecules through the hydrophilic group in the molecule to form a thicker water film layer, causing crystals to be shielded from each other and changing the structure formation process; or by adsorbing insoluble calcium salts formed by certain functional groups in the molecule with free calcium on the surface of mineral particles, thus inhibiting the hydration process of cement and producing a retarding effect. Most inorganic Retarder can form complex salts (such as calcium aluminate) with cement, precipitate on the surface of cement mineral particles, and inhibit cement hydration. The mechanism of Retarder is complex and is usually the result of the combined action of various coagulant mechanisms mentioned above.
2.The impact of retarders on the performance of fresh concrete
(1) Effects on setting time
The impact of retarders on the setting time of concrete is related to factors such as the type and dosage of the retarder, the method of addition, the type of cement, the mix ratio of the concrete, the season of use, and the construction method. An ideal retarder should have a significant retarding effect at a low dosage, and the setting time should be adjustable within a certain dosage range without producing abnormal setting phenomena. In addition, it is particularly important for the retarder to significantly delay the initial setting time of the concrete, while keeping the interval between initial and final setting times short.
Different retarders have significant differences in their effects. At lower dosages, their effects can be classified into two types: one significantly prolongs the initial setting time but shortens the interval between initial and final setting times, indicating that they have the characteristic of inhibiting the initial hydration of cement and promoting early hydration; the other has little effect on the initial setting time but significantly prolongs the final setting time without affecting normal hydration in the later stage. The former is suitable for controlling fluidity, and the latter is suitable for controlling hydration heat. Therefore, only by correctly understanding the properties and variation rules of admixtures can they be used reasonably to achieve the best effect.
(2) Effects on strength
Generally speaking, the effect of retarder on concrete is mainly a physical effect. They do not participate in the hydration reaction of cement, nor do they produce new hydration products. Instead, they slow down (even stop) the reaction process to varying degrees, similar to the role of inert catalysts. Therefore, their influence on the strength of concrete mainly comes from the changes in the hardened structure. From the perspective of strength development, the early strength (around 7 days) of concrete with a proper amount of retarder is lower than that without, but it can generally catch up or even surpass the latter after about 7 days, and the 28-day strength is significantly improved compared to the latter. Data shows that the 90-day strength still maintains a trend higher than the latter. The impact on the flexural strength of concrete is similar to that of compressive strength, but not as significant.
As the dosage of retarders increases, the early strength of concrete decreases, the growth rate of strength slows down, and the time to reach the design strength is longer. If the retarder is selected improperly or overused, it will not only significantly reduce the early strength of concrete but also reduce the strength in the middle and later stages. The main reason is excessive retardation, which causes the concrete to not set and harden for a long time, leading to excessive evaporation and loss of moisture inside the concrete, resulting in slow or even halted hydration of cement, a low degree of hydration, and irreversible damage to the strength of the concrete.
3.Quickly determine whether the retarder is excessive
Excessive retarder can seriously affect the setting time of concrete, and increasing temperature will shorten the setting time. By conducting a slurry experiment with admixtures and raising the curing temperature of the experiment in a constant temperature box, it is possible to quickly determine whether the retarder is excessive.
At the same temperature, the initial setting time of the slurry increases with the addition of retarder, and the effect of sucrose is about twice that of sodium gluconate. At different temperatures, the effect of retarder on the initial setting time of the slurry decreases as the temperature increases, and the higher the temperature, the shorter the initial setting time. As the amount of retarder increases, the setting time lengthens, and the higher the temperature, the shorter the setting time.
Therefore, when the retarder is fixed in the admixture, the initial setting time can be quickly determined by controlling the temperature, and it can be judged whether the retarder in the admixture is excessive. According to the content of retarder in the admixture, 3%-5% in summer and a temperature of 70℃ should be selected, and the initial setting time of the slurry should be controlled within 3.5 hours; 0%-3% in winter and a temperature of 60℃ should be selected, and the initial setting time of the slurry should be controlled at around 3 hours. Beyond this range, the concrete is prone to problems such as prolonged setting time, even excessive retardation and failure to set.
