Retarders are an essential component used in the construction industry to delay the initial setting time of concrete by up to an hour. They are particularly useful in hot weather conditions where high temperatures can cause rapid hardening, making it challenging to mix, transport, and place concrete. In addition to retarding the setting time, retarders also act as water reducers, further enhancing their usefulness.
The mechanism behind retardation involves the retarder molecules adsorbing onto the surface of the cement particles, preventing them from reacting with water and delaying the formation of hydration products. This process enables the concrete to remain workable for a more extended period, which is especially useful in large construction projects where time is of the essence.
There are various types of retarders available, including lignosulfonates, hydroxycarboxylic acids, and sugars. These retarders work by different mechanisms, but all aim to slow down the setting time of concrete, making it more workable for an extended period.
The use of retarders has several advantages. In addition to allowing more time for mixing, transporting, and placing concrete, they also improve the concrete’s workability, making it easier to finish and smooth. Additionally, by delaying the setting time, retarders can reduce the risk of plastic shrinkage cracks that can occur due to the rapid hardening of concrete in hot weather conditions.
Overall, retarders are an essential component used in the construction industry to delay the setting time of concrete, particularly in hot weather conditions. Their mechanism of action involves adsorbing onto the surface of cement particles to prevent them from reacting with water, and they come in various types, each with its unique mechanism. Retarders improve the workability of concrete, making it easier to finish and smooth, and they can reduce the risk of plastic shrinkage cracks.
Mechanisms of Retardation
The addition of a retarder to concrete can temporarily slow down the hydration reactions by extending the dormant period. This can be achieved by either dissolving the retarder in the mixing water or spraying it on the surface of the concrete. The specific mechanisms of retardation depend on the type of cement and retarder used in combination.
It is important to note that the retardation mechanisms are temporary and will eventually disappear after a predictable period of time. This means that the hydration process will eventually continue as normal.
There are four distinct ways in which the retardation process can occur between the retarder and cement. However, the specific mechanism at play will depend on the specific types of cement and retarder being used. It is crucial to carefully consider these factors in order to achieve the desired results when using retarders in concrete.
1. Adsorption
The surface of cement particles can be treated with a retarding admixture to slow down the process of hydration. When this admixture is applied, it creates a protective layer around the cement particles. This layer acts as a barrier to prevent water molecules from reaching the surface of the unhydrated cement particles. As a result, the hydration process is slowed down, and there is not a significant amount of hydration products to provide rigidity to the cement paste.
As a consequence of this, the paste remains in a plastic state for an extended period. This delay in the hardening process allows for more time to work with the cement paste without it hardening prematurely. It is essential to carefully control the amount and timing of the retarding admixture to achieve the desired results and avoid any unwanted delays in the hardening process. By using a retarding admixture, it is possible to manipulate the properties of the cement paste and achieve the desired consistency and strength.
2. Nucleation
The addition of water to cement causes the release of calcium ions and hydroxyl ions from the surface of cement particles. This release occurs when a certain concentration of these ions is reached. Once this critical point is reached, the hydration products C2S and CS begin to crystallize.
To delay this process, a retarding admixture is added to the cement. This admixture is designed to be absorbed by the calcium hydroxide nuclei, which then prevents the growth of these nuclei until a certain level of supersaturation is achieved. By doing so, the retarding admixture slows down the crystallization process of the hydration products, providing more time for the concrete to be poured and shaped. This is especially useful in situations where the concrete needs to be transported over long distances or in hot weather conditions where the rate of evaporation is high.
3. Complexation
During the initial stages of cement hydration, calcium ions released by the cement grains react with other compounds to form complexes. This reaction results in an increase in the cement’s solubility. The complexes that are formed play a crucial role in the subsequent stages of hydration.
As hydration continues, the presence of a retarding admixture causes the concentration of certain ions, including Ca2+, OH-, Si, Al, and Fe, to increase in the aqueous phase of the cement paste. The accumulation of calcium and hydroxyl ions in the solution prevents the precipitation of these ions into calcium hydroxide, which is a key component of the cement hydration process. This retardation of the reaction slows down the hydration process and alters the properties of the cement.
In summary, during cement hydration, the formation of complexes with calcium ions released by the cement grains causes an increase in solubility. In the presence of a retarding admixture, an increased concentration of certain ions occurs, which prevents the precipitation of calcium hydroxide and retards the hydration process. Understanding the various factors that affect cement hydration is critical to the development of high-performance concrete.
4. Precipitation
Precipitation and adsorption share similarities, but in the case of precipitation, insoluble derivatives of retarder are produced by reacting with a highly alkaline solution. This chemical reaction results in the pH of the solution increasing to over 12 within a few minutes of contact with water and cement.
The formation of protective coatings through the precipitation of these insoluble derivatives around the cement particles impedes the cement hydration process. This protective coating acts as a diffusion barrier that hinders the water molecules from making sufficient contact with the cement particles, leading to a reduction in cement hydration.
Overall, precipitation and adsorption may be similar, but the formation of insoluble derivatives and protective coatings during the precipitation process can have significant implications for cement hydration. The development of these protective coatings can hinder the interaction between water and cement, slowing down the cement hydration process.
Types of Retarders
Retarders are commonly categorized into two groups based on the nature of the retarders. The first category includes mechanical or friction retarders, which rely on mechanical forces to slow down a vehicle. These types of retarders are typically used in heavy-duty vehicles such as trucks and buses. They work by applying friction to the wheels or transmission system, converting the vehicle’s kinetic energy into heat energy, and dissipating it into the surrounding environment.
The second category of retarders includes electromagnetic or electric retarders, which use magnetic forces to slow down a vehicle. These types of retarders are often used in electric and hybrid vehicles because they do not rely on mechanical friction, which can cause wear and tear on the brakes. Electric retarders work by generating a magnetic field that opposes the motion of the vehicle’s wheels, creating a braking effect that helps to slow down the vehicle.
1. Organic Retarders
The first category of chemical compounds is known as lignosulphonates. These are substances that are derived from lignin, which is a complex organic polymer found in plant cell walls. Lignosulphonates are typically used as additives in concrete to improve its properties such as workability and strength.
The second category of compounds are hydroxycarboxylic acids and their salts. These compounds contain both a hydroxyl group (-OH) and a carboxylic acid group (-COOH) in their molecular structure. They can be found in a variety of natural sources such as fruits and milk, and are also used in many industrial processes including food and pharmaceutical production.
The third category of chemical compounds is known as phosphonates. These are organic compounds that contain a phosphonic acid functional group (-PO3H2). They are commonly used as chelating agents to remove metal ions from industrial water systems, and also as flame retardants in plastics.
The final category of compounds is sugars. Sugars are simple carbohydrates that occur naturally in many fruits and vegetables, and are also produced through industrial processes such as the hydrolysis of starch. They are widely used in the food industry as sweeteners, and also in the production of biofuels and pharmaceuticals.
2. Inorganic or Chemical Retarders
Phosphonates are a class of chemical compounds that contain a phosphorous atom attached to a carbon atom with a double bond, and two additional oxygen atoms attached to the phosphorous atom. They are commonly used as water treatment agents, detergent builders, and as additives in agricultural and industrial applications.
Borates are chemical compounds that contain boron and oxygen atoms. They are used in a wide range of industrial applications, such as the production of ceramics, glass, and detergents. Borates can also be used as a flame retardant and as a preservative in wood products.
Salts of Pb, Zn, Cu, As, and Sb are compounds that contain a metal ion bonded to a negatively charged ion, or anion. These salts can have a variety of uses, such as in the production of batteries, pigments, and as a component of alloys. However, some of these salts, such as lead and arsenic salts, can be toxic to humans and the environment, and therefore their use is strictly regulated.
Effects of Retarding Admixtures on Properties of Concrete
1. Strength
When retarding admixtures are added to concrete, the initial compressive strength of the concrete is reduced compared to a similar concrete mix that does not contain these admixtures. This means that the concrete will take longer to reach its full strength potential. The purpose of retarding admixtures is to slow down the setting time of the concrete, which can be useful in certain construction scenarios where a longer working time is required. However, it is important to note that the use of retarding admixtures will result in a lower early strength, which may impact the overall construction timeline. Therefore, the decision to use retarding admixtures in concrete should be made after careful consideration of the specific requirements of the construction project.
2. Workability and Rheological Values
Retarding admixtures are a type of chemical additive used in concrete to slow down the setting process. While their primary function is to increase the setting time of concrete, they can also have a small impact on the workability of the material. Specifically, these admixtures can cause an increase in the initial slump of the concrete.
The initial slump refers to the measurement of the concrete’s consistency immediately after it is mixed. It is determined by measuring the distance that the concrete slumps or settles when a cone-shaped mold is removed. The use of retarding admixtures can cause an increase in the initial slump of concrete by as much as 60 to 100 millimeters.
While this increase in slump may not seem significant, it can have an impact on the overall workability of the concrete. Workability refers to the ease with which the material can be mixed, transported, and placed in its final location. If the concrete is too stiff, it may be difficult to work with, and if it is too fluid, it may be prone to segregation and settling.
Therefore, while retarding admixtures are primarily used to slow down the setting time of concrete, it is important to note that they can also have an impact on the workability of the material. By increasing the initial slump, these admixtures can help to ensure that the concrete is easy to work with and achieves the desired properties when it sets.
3. Slump Loss
Retarding admixtures are known for their ability to effectively address the issue of slump loss in concrete, leading to increased initial workability. This has been a proven benefit of these types of admixtures. By incorporating retarding admixtures, the issue of slump loss can be minimized, resulting in concrete that retains its workability for a longer period of time. This, in turn, enables workers to manipulate and shape the concrete more easily during the initial stages of the construction process. Therefore, the use of retarding admixtures has become a popular solution for contractors and builders looking to improve the workability of their concrete.
4. Air Entrainment
Retarding admixtures are typically not designed to introduce air into concrete mixtures. However, there are certain types of retarders that can have this effect. In fact, retarders that are based on hydroxycarboxylic acid may even decrease the air content in concrete.
It is important to note that while some retarders may entrain air, this is not their primary function. Retarding admixtures are typically used to slow down the setting of concrete, which can be helpful in a variety of situations, such as when pouring large volumes of concrete or when working in hot weather conditions.
Despite the potential for air entrainment with some retarders, it is generally not recommended to rely on them solely for this purpose. If air entrainment is desired in a concrete mixture, it is usually best to use an admixture specifically designed for that purpose.
Overall, when working with retarders in concrete, it is important to carefully consider their effects on the properties of the mixture, including air content, setting time, and strength development, in order to achieve the desired results.
5. Freeze-Thaw Cycles
Air entraining admixtures are commonly utilized in the construction industry to enhance the durability of concrete against freeze-thaw damage. In freezing conditions, water within the concrete can cause cracking and deterioration due to the expansion that occurs upon freezing. However, when air entraining admixtures are added to the mix, they create tiny air pockets within the concrete that serve as expansion chambers for the freezing water. This process helps to alleviate the pressure on the concrete and minimizes the likelihood of damage occurring. By incorporating air entraining admixtures into concrete, it can greatly improve its ability to withstand harsh weather conditions and remain structurally sound over time.
6. Bleeding
Retarding admixtures are substances added to concrete to slow down the setting process. As a result of their use, the setting of the concrete is delayed. However, one of the drawbacks of using such admixtures is that they increase the likelihood of bleeding in the concrete.
Bleeding is a phenomenon that occurs when water from the concrete mixture rises to the surface, leaving behind a layer of cement particles. This phenomenon is more common in retarded concretes, which are those that have been treated with retarding admixtures. The delay in the setting process allows the water to move more freely through the concrete mixture, which increases the chances of bleeding.
Therefore, it can be concluded that the use of retarding admixtures in concrete can lead to increased bleeding. This is due to the delay in the setting process that these admixtures cause. It is important to be aware of this potential issue when using retarding admixtures in concrete and to take appropriate measures to minimize the risk of bleeding.
7. Heat of Hydration
Retarding admixtures are a type of chemical additive that can be used in concrete to slow down the setting time. While they can delay the hardening of concrete, they do not have any effect on the heat output of the material.
However, retarding admixtures can help to delay the rise of the peak temperature of concrete. This delay in temperature rise is typically proportional to the amount of time by which the admixture retards the setting time of the concrete.
In other words, if the retarding admixture delays the setting time of the concrete by a certain amount, it will also delay the point at which the peak temperature is reached by a similar amount. This can be beneficial in situations where excessive heat generation during the curing process can cause damage to the concrete, such as in large pours or in hot weather conditions.
8. Volume Deformation
According to the given context, the addition of retarding admixtures does not have a significant effect on creep and drying shrinkage. However, it may have a slight impact on plastic shrinkage, potentially increasing it.
Creep is the tendency of a material to deform over time when subjected to a constant load, while drying shrinkage refers to the reduction in volume of a material due to moisture loss during the drying process. The inclusion of retarding admixtures does not appear to significantly affect these two properties in concrete.
On the other hand, plastic shrinkage refers to the contraction of concrete during the early stages of hydration, caused by the rapid evaporation of water from the surface. The use of retarding admixtures may result in a slight increase in plastic shrinkage, which is something to be taken into consideration in concrete mix design.
9. Durability
The durability of retarded concrete is expected to be on par with that of plain concrete if it is cured properly. This means that the process of slowing down the setting time of the concrete through the use of retarders should not negatively affect its overall durability compared to plain concrete. As long as the curing process is done correctly, retarded concrete can be just as durable as its plain concrete counterpart.
Advantages of Retarder in Concrete
Complex concrete placement or grouting often requires careful planning and execution to achieve the desired result. One such special architectural surface finish is the exposed aggregate finish, which requires extra attention to detail during the placement and finishing process to achieve the desired aesthetic effect. This finish involves exposing the small stones or aggregates in the concrete surface, creating a textured and visually appealing appearance.
High temperatures can accelerate the setting process of concrete, which can be a concern when attempting to achieve the desired finish. Compensating for this effect is important to ensure that the concrete remains workable long enough to achieve the desired result. This may involve adjusting the mix design or using additives to slow down the setting process.
In addition to the challenges posed by high temperatures, preventing cold joint formation in successive lifts is also a consideration when placing complex concrete structures or grouting. Cold joints occur when fresh concrete is poured against partially cured concrete, which can result in a weakened bond between the two layers. Proper planning and execution, such as ensuring that the surface of the first lift is properly prepared before the next lift is placed, can help prevent the formation of cold joints and ensure a strong, durable finished product.