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The Adaptability of Polycarboxylate Admixtures to Other Concrete Raw Materials
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With the development of the concrete industry and the use of various raw materials, the requirements for different parts, structural components, and construction methods of concrete in construction are becoming increasingly high. Controlling the setting time of concrete admixtures directly affects construction progress and other issues.
Ultra-early strength and ultra-slow setting are important research topics for concrete. Rapid determination of whether the retarder in external additives exceeds the standard can better serve the concrete industry and provide a reference for concrete production.
There are many types of retarders, which can be divided into two categories based on their chemical composition: inorganic retarders and organic retarders. Inorganic retarders include phosphates, zinc, iron sulfate, copper sulfate, borates, fluorosilicates, etc; Organic retarders include lignosulfonates, hydroxycarboxylic acids and their salts, polyols and their derivatives, sugars and carbohydrates, etc.
Generally speaking, most organic retarders have surface activity and adsorb at the solid-liquid interface, altering the surface properties of solid particles; Or by adsorbing a large number of water molecules through hydrophilic groups in the molecule, a thicker water film layer can be formed, which changes the structure formation process of the crystal from mutual contact to shielding; Or through certain functional groups in its molecules and free calcium, insoluble calcium salts can be formed and adsorbed on the surface of mineral particles, thereby inhibiting the hydration process of cement and achieving a retarding effect.
Most inorganic retarders can form double salts (such as ettringite) with cement, precipitate on the surface of cement mineral particles, and inhibit cement hydration. The mechanism of retarder is relatively complex, usually the result of the combined action of multiple retarder mechanisms mentioned above.
Impact on Condensation Time
The influence of retarder on the setting time of concrete is related to the type, dosage, and method of retarder, as well as the cement type, concrete mix proportion, season of use, and construction method. The ideal retarder should have a significant retarding effect with a small dosage, and have strong adjustability of coagulation time within a certain dosage range, without producing abnormal coagulation phenomena. In addition, it is particularly important that the retarder should delay the initial setting time of the concrete for a long time, and the interval between initial and final setting should be short.
The effects of different retarders vary greatly. At lower dosages, their functional characteristics can be manifested in two ways: one is to prolong the initial setting time significantly, but shorten the interval between initial and final setting, indicating that they have the characteristics of inhibiting early cement hydration and promoting early hydration; Another approach is to have a relatively small impact on initial setting, significantly prolonging the final setting time, but not affecting normal hydration in the later stage.
The former is suitable for controlling fluidity, while the latter is suitable for controlling heat of hydration. Therefore, only by correctly understanding the properties and changes of additives can they be used reasonably and achieve the best results.
Impact on Strength
Generally speaking, the effect of retarders on concrete is mainly physical, that is, they do not participate in the hydration reaction of cement, nor do they produce new hydration products. They only slow down (or even stop) the reaction process to varying degrees, similar to the effect of inert catalysts. Therefore, their impact on concrete strength mainly comes from changes in the structure after hardening.
From the perspective of strength development, the early strength of concrete with moderate addition of retarder (around 7 days) is lower than that without admixture. Still, it generally catches up with or exceeds that without admixture after 7 days, and the strength at 28 days is significantly improved compared to that without admixture. The data show that the 90d intensity can still maintain a trend higher than the latter. The influence law on the flexural strength of concrete is similar to compressive strength, but not as significant as compressive strength.
With the increase of retarder dosage, 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 variety is selected improperly or an excessive dosage is used, it will not only seriously reduce the early strength of concrete but also lower the mid- to late-strength.
The main reason is excessive slow setting. If concrete does not set and harden for a long time, it will cause excessive evaporation and loss of moisture inside the concrete, resulting in slow or even stopped cement hydration reaction, low hydration degree, and insufficient hydration products, causing irreversible loss of concrete strength.
Excessive use of retarders can seriously affect the setting time of concrete, while an increase in temperature can shorten the setting time. Using additives for slurry experiments and increasing the curing temperature in a constant temperature chamber can quickly determine whether the retarder exceeds the standard. According to relevant information, the dosage of retarder in additives is 3% to 5% when the summer temperature is above 25 ℃, and less than 3% when the winter temperature is below 20 ℃.
Study the setting time of retarders at different temperatures and quickly determine whether the retarder of additives exceeds the standard by experimental method and experiment. Add different proportions of retarders to the admixture, conduct a slurry experiment, and measure the setting time.
Take 300g of cement, 87g of water, and 1.5% admixture for the experiment. The flowability of the slurry without retarder is 165mm.
At the same temperature, the initial setting time of the slurry increases with the increase of retarder, and the effect of white sugar is about twice that of sodium gluconate. At different temperatures, the effect of retarders on the initial setting time of the slurry decreases with increasing temperature, and the higher the temperature, the shorter the initial setting time.
As the retarder increases, the setting time prolongs, and the higher the temperature, the shorter the setting time. Therefore, when the retarder in the admixture is constant, the initial setting time can be quickly determined by controlling the temperature to determine whether the retarder in the admixture exceeds the standard.
According to the content of retarder in the admixture, it is recommended to use a temperature of 70 ℃ and control the initial setting time of the slurry within 3.5 hours in summer, with a range of 3% to 5%; In winter, a temperature of 60 ℃ should be selected for 0-3%, and the initial setting time of the slurry should be controlled at around 3 hours. Beyond this range, concrete is prone to longer setting times, and even problems of excessively slow setting without setting.
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