1. Definition and Characteristics of Retarders
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A retarder is an additive that can delay the hydration reaction of cement, thereby extending the setting time of concrete. The use of retarders can maintain the plasticity of fresh concrete for a long period of time, facilitate pouring, improve construction efficiency, and do not have adverse effects on the later performance of the concrete.
For commercial pumped concrete or concrete constructed in high temperature environments in summer, using retarders can also reduce slump loss, ensure normal transportation and pumping construction of concrete, improve work efficiency, and avoid material waste. For large volume concrete, adding retarders can also reduce the absolute temperature rise of the concrete, delay the occurrence of temperature peaks, and effectively avoid the occurrence of temperature stress cracks in the concrete.
2. Types of Retarders
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 mainly include phosphate, metaphosphate, zinc salt, iron sulfate, copper sulfate, fluorosilicate, and borax. In recent years, the widely used inorganic retarders are phosphate and metaphosphate type retarders.
Organic retarders mainly include the following types:
(1) . Hydroxycarboxylic acids, amino carboxylic acids and their salts, commonly including citric acid, gluconic acid, salicylic acid, and their salts. The dosage is generally 0.005%~0.02% of the cement mass;
(2) . Polyols and their derivatives. This type of retarder has a relatively stable retarding effect and is less affected by temperature. The dosage is generally 0.005%~0.02% of the cement mass;
(3) . Carbohydrates, such as glucose, sucrose, molasses, and their derivatives. Due to its wide range of raw materials, low price, and stable retarding effect, it is widely used, with a dosage generally ranging from 0.001% to 0.03% of the cement quality.
3. Retarders’ Mechanism of Action
The mechanism of action of retarders is quite complex and difficult to generalize with a single theory. The currently recognized theories include adsorption theory, formation complex salt theory, precipitation theory, and control calcium hydroxide crystal growth theory.
3.1 Adsorption Theory
The retarder adsorbs on the surface of cement particles to form a dense adsorption film layer, changing the double layer structure of the cement particle surface, and inhibiting the process of water adsorption and hydration reaction of cement particles. In addition, some retarder ions can adsorb onto the surface of cement hydration product crystals, inhibit crystal growth, and also play a role in delaying hydration. This theory is more applicable to sugars, polyols, and their derivatives.
3.2 Formation Complex Salt Theory
The saturation of Ca2+and the formation of Ca (OH) 2 crystals are important reasons for the end of the cement hydration induction period. Functional groups such as - OH and - COO - in the retarder molecule can form complex salts with Ca2+in the solution, inhibiting the crystallization of Ca (OH) 2 and effectively prolonging the induction period of cement hydration. This theory is more applicable to hydroxyl carboxylic acids, amino carboxylic acids, and their salts.
3.3 Precipitation Theory
The sedimentation theory suggests that retarders can form a insoluble precipitation layer on the surface of cement particles, prevent the contact between water and cement particles, inhibit the dissolution of surface components of cement particles, and thus delay the hydration reaction of cement. This theory is more applicable to molasses retarders.
3.4 Control Calcium Hydroxide Crystal Growth Theory
According to the theory of controlling the growth of calcium hydroxide crystal, the retarder blocks the crystallization of Ca (OH) 2, so that C3S cannot normally generate hydrated calcium silicate gel, thus inhibiting the hydration reaction of cement. This theory is more applicable to inorganic retarders.
4. Evaluation Method for the Retarding Effect of Retarders
At present, the evaluation methods for the retarding effect of retarders are still relatively limited, mainly focusing on the determination of setting time and the difference in setting time.
For the clean slurry, the standard method is the testing method specified in the "Test Method for Water Consumption, Setting Time, and Soundness of Cement Standard Consistency" (GBT -). The instrument used in this method is a standard Vicat apparatus. Before measuring the setting time, the water consumption for standard consistency needs to be determined in advance, and then the cement slurry is prepared according to the water consumption for standard consistency. The initial and final setting times of the cement slurry are measured. The advantages of this method are convenience and speed, while the disadvantage is that there is a significant human error, mainly reflected in the randomness of the operation process and readings, especially when determining the final setting time, the human error is more obvious.
For concrete, the standard method is the testing method specified in "Concrete Admixtures" (GB-). The instrument used in this method is a penetration resistance meter. Before measuring the setting time, the concrete mixture needs to be sieved out of mortar using a 5mm (circular hole sieve) vibrating screen and loaded into a standard metal cylinder to obtain the relationship curve between the penetration resistance value and time. Then, when the penetration resistance value reaches 3.5MPa, the corresponding time is calculated as the initial setting time; When the penetration resistance value reaches 28MPa, the corresponding time is used as the final setting time. The advantages of this method are accurate results and small errors, while the disadvantages are that the operation process is relatively complex and time-consuming.
With the advancement of technology, domestic and foreign scholars have also attempted to use some new methods to evaluate the retarding effect of retarders, such as hydration temperature rise, X-ray diffraction, low-field nuclear magnetic resonance, resistance method, etc. Although simple and effective, the accuracy still needs to be verified by a large amount of engineering practice, and no relevant standards have been formed. Therefore, traditional methods are still the main method in engineering at present.
The difference in setting time is the most intuitive manifestation of the effectiveness of a retarder, and the quality of a retarder is not solely determined by these two factors. An ideal retarder should have a significant retarding effect in a small dosage range, with strong adjustability of setting time within a certain dosage range, and no abnormal setting phenomenon. Furthermore, it is particularly important that the retarder can significantly delay the initial setting time of the cement slurry, while minimizing the interval between initial and final setting.
5. Conclusion
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In the rapid development of commercial concrete today, retarders play a very important role. With the continuous deepening of people's understanding of retarders, their application will also become more and more widespread, especially in the production of large volume concrete that needs to reduce the instantaneous hydration heat of cement and delay the peak hydration of cement, retarders are indispensable.
Posted On: 03/18/ | Posted by: DY Concrete Pumps
You want your concrete to be more effective and efficient. But how do you prevent concrete from setting immediately while providing a smooth and consistent finish?
Finding a suitable retarder is the best solution. Choosing one to use is a little more tricky.
Sugar is one of many concrete retarders. While it can help you get the concrete solution you want, it might also have some unfavorable effects. Whether sugar is effective depends on how much you mix in your concrete. Some people can get the consistency right.
Understanding how sugar works in concrete and its results can help determine its effectiveness. Comparing sugar to alternative retarders can help you know how it can impact your structure or work.
Sugar can prevent concrete from setting too quickly and becoming unusable. Concrete that sets too quickly can become stiff and be challenging to use.
For example, hardened concrete spreads less efficiently and can be ineffective in bonding or securing a structure. It also means people must use more concrete, increasing waste and expenses. Adding sugar can keep cement softer, making it more flexible and effective. Sugar is also an affordable and widely available resource.
Sugar acts as a retardant when you add it to concrete, slowing the hydration process down. Concrete sets through hydration or a chemical reaction between water and cement molecules. Hydration forms calcium silicate, which creates solid crystals. The crystals bind cement and water and help it set. Heat can speed up hydration and bonding, causing concrete to set faster in hot weather.
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Adding too much sugar to concrete can significantly decrease the formation of calcium silicate crystals, preventing cement from setting. When concrete doesn’t set, it can become weak and more susceptible to damage and corrosion. Here are some more negative effects of using sugar in concrete:
While slowing hydration can slow concrete setting time, it can cause it not to set at all. Cement that does set will not harden or cure properly, leaving it vulnerable to the elements and damage.
Sugars can absorb liquids, making concrete structures and surfaces more susceptible to water damage. If it rains or you spill liquid on concrete, it might retain moisture, leading to cracking, mold and discolorations.
Sugar impacts the setting and hardening process. It also can create air bubbles in the mixture and increase the air content. Too much air can make concrete less compact. If concrete isn’t dense enough, it can wear quickly when it experiences heavy impacts or high traffic, making it less durable. For example, concrete pavement with lots of foot traffic or heavy loads can become less rigid quickly, showing signs of wear like cracks and holes.
Mixing sugar in concrete can attract pests like ants and termites. Pests can decay concrete structures like walls and floors over time, compromising the structural integrity. Insects can also invade concrete structures, creating an unpleasant environment.
While sugar crystals are tiny, they can cause noticeable surface defects when combined with concrete. Sugars can cause white discoloration or efflorescence due to moisture seeping through cracks. It can also cause speckling, spotting or honeycombing, affecting the look of concrete surfaces.
Poorly set concrete can corrode easily, requiring frequent repairs, maintenance and replacements. Replacing concrete slabs or entire structures can be costly, especially if you have to install new concrete sooner or more often than expected.
Sugar can be unpredictable in setting times. Some sugar and concrete mixtures can have longer setting times, while others may cure or harden faster. Unexpected setting time can delay curing, hardening, painting and tiling. It can slow your productivity and completion time.
Putting sugar in concrete can impact its quality. Sugar concentrates can be challenging to control, leading to inconsistent mixtures. You might add too little sugar and have a minimal effect on slowing hydration. Conversely, you might add too much and stop hydration or cement setting altogether.
Sugar can make concrete mixture either too thin or sticky. You might have difficulty spreading and setting concrete on various structures, affecting the results. For example, when pouring concrete on a surface, the mixture might be thick and bumpy in one area and thin and smooth in the other, causing and uneven or uniform appearance.
Sugar in concrete can break down or decay over time, causing organic decomposition. Decomposition can lead to defects in concrete, like holes, gaps and poor structural integrity.
Chemical retarders are an excellent alternative to sugar. Professionals use these retarders because they slow down hydration more gradually. The liquid form typically provides better control of mixing quantities and concrete consistency. Chemical retarders also offer a more manageable and workable mixture, making it easier to place and spread. Here are some commonly used chemical retarders:
Retarding admixtures slows down the reaction between cement and water to reduce the setting time of concrete. Delaying the hydration process can improve the workability of concrete at high temperatures.
Water-reducing admixtures reduce the water ratio in concrete, increasing its compressive strength. Concrete treated with water-reducing admixtures is often more solid and durable under heavy loads.
Accelerating or set-controlling admixtures combine the benefits of retarding and water-reducing admixtures. When added to concrete or cement, these retarders increase hydration, reduce setting time and increase early strength development in concrete. Professionals can use accelerating admixtures to improve start-up and finishing processes and enhance the workability of cement in cold conditions.
Unlike retarding admixtures, superplasticizers increase concrete workability and strength without changing the water-to-cement ratio. These retarders can produce high-strength, cohesive and smoother concrete finishes. They can also reduce the concrete needed in projects and withstand freezing conditions. Professionals can use superplasticizers in pumped concrete for building high-rise budlings and bridges.
Getting the right concrete consistency is essential for your high-quality work. Unset concrete can impact structural durability, appearance and longevity. If your concrete sets too quickly, it can affect your ability to spread and use it.
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