Superplasticizers are commonly utilized in the construction industry to enhance the fluidity of concrete on-site. These additives have the ability to maintain high slump values for a certain duration of time, usually between 30 to 45 minutes. However, certain types of Portland cement do not work effectively in conjunction with superplasticizers. A more in-depth examination of the mechanisms involved in the action of superplasticizers is required to fully comprehend this issue.
Cement and Superplasticizer Reactivity and Compatibility
High-performance concrete production faces several challenges, with the most prevalent being the slump-loss problem. When attempting to achieve a large slump value with a lower water/cement ratio, this issue arises. Typically, high-performance concrete requires 120 to 135 liters of mixing water per cubic meter, compared to 160 to 180 liters for ordinary concrete. However, this amount varies depending on various factors, such as the entrained air content, maximum coarse aggregate size, and nature.
The rheology of high-performance concrete is mainly influenced by the rate at which Portland cement mineral phases react with water molecules and the rate at which newly formed compounds trap superplasticizer molecules upon contact with water and Portland cement. Two phases that consume a significant amount of water and exhibit rapid hydration are Calcium hemihydrates (Plaster of Paris) and ettringite, which form in the interstitial phase. Previous research has shown that superplasticizer molecules are adsorbed on di and tricalcium silicates upon their addition to Portland cement. This is done to efficiently control their hydration or substantially retard it when necessary.
The rate at which the cement consumes water molecules during the first moment of mixing is referred to as the cement’s rheological reactivity. During hydration, a certain number of superplasticizers are also consumed. The initial consumption of superplasticizer molecules after mixing is referred to as cement/superplasticizer compatibility. It is challenging to produce high-performance concrete with a very low water-cement ratio that maintains a slump value of 50mm 30 minutes after achieving an initial slump value of 200mm. Over time, concrete surfaces become shiny, and adding more superplasticizer to increase the slump value is not recommended as it lacks water. In some cases, adding a small amount of water to recover the slump instantaneously is necessary, but the gained slump value is lost once the reaction occurs.
To optimize high-performance concrete for a particular location, it is necessary to investigate whether the cement has lower rheological activity and whether the superplasticizers used compete with ettringite crystals. Unfortunately, there is no theoretical method to determine this behavior, so a trial and error approach is necessary.
Rheopump to Study the Behavior of Superplasticizers and Cement
Researchers at the University of Sherbrook have developed an efficient and reliable method for testing cement, called the rheopump. This test can be used with a variety of cements that have varying levels of tri calcium aluminate, tetra calcium alumino ferrite, limestone filler, di and tri calcium silicates, and different fineness.
The test involves an arrangement of small modified pumps that recirculate a cement grout with a water cement ratio of 0.35. The grout includes a reference superplasticizer of naphthalene, which is dosed for four minutes based on the type of cement being used. After dosing, the flow time for 1L of the prepared grout through the Marsh Cone is measured to check its fluidity.
Once the initial measurement is complete, the grout is placed in a plastic container and continuously agitated until the next measurement is taken 40 minutes later. By using a single bag of cement, this method can determine the initial reactivity and compatibility of the chosen cement with the specified superplasticizer within an hour.
It’s worth noting that the rheopump has never failed, even when used to measure slump loss in concrete. Overall, this method provides a quick and effective way to test the compatibility of different types of cement with the specified superplasticizer.
Fig.1: Schematic Diagram Representing Rheopump
The rheopump is a versatile tool that can serve multiple purposes in the field of concrete technology. One of its applications is to determine the appropriate amount of retarder needed to slow down the slump loss. Slump loss refers to the decrease in the workability of concrete over time, which can be a concern in certain construction scenarios. By using the rheopump, one can accurately measure the effect of different retarder dosages on the slump loss and determine the optimal amount to use.
In addition, the rheopump can also be used to assess the compatibility of different types of superplasticizers with a given type of cement. Superplasticizers are additives that are commonly used in concrete mixtures to enhance their workability and flowability. However, not all superplasticizers are compatible with all types of cement, and the rheopump can help determine whether a particular combination will be effective. By measuring the rheological properties of the mixture, such as its viscosity and flow rate, one can assess the compatibility of the superplasticizer with the cement and make any necessary adjustments.
Overall, the rheopump is a valuable tool for concrete researchers and practitioners who need to fine-tune the properties of their mixtures. Its ability to measure rheological properties with high precision makes it an indispensable tool for optimizing concrete mix designs and ensuring that the final product meets the desired specifications.
Problems in Choosing Appropriate Superplasticizer of High-Performance Concrete
Concrete production requires careful selection of constituents to ensure high performance. However, concrete producers often lack complete knowledge in this area and face various challenges.
One of the major challenges is related to the physical nature of the materials used. Due to the limited number of bins and the need to provide regular service to customers, there is little room for flexibility in this area. This can lead to problems with the quality of the concrete produced.
Another challenge faced by concrete producers is related to the economical nature of the industry. Depending on the geographic location, locally available and economically viable resources are often prioritized over other options. This can limit the variety of constituents available for use in concrete production.
Concrete producers may also face challenges related to the social nature of the industry. For example, they may be limited in their selection of superplasticizers due to ties with known cement or quarry companies. Personal dealings with specific companies can further restrict the variety of options available for selection. These social constraints can make it difficult for concrete producers to choose the best constituents for high-performance concrete.
Selection of Superplasticizer for High-Performance Concrete
The following are some essential guidelines that every concrete producer must adhere to in order to achieve the desired outcome.
It is crucial for concrete producers to follow specific regulations to ensure that the end product meets the required standards. These guidelines are put in place to ensure the safety and durability of the concrete. Ignoring them can lead to weak and unstable concrete, which can be dangerous and costly.
Concrete producers must also pay close attention to the quality of the materials used in the production process. Poor quality materials can affect the strength and durability of the concrete, leading to potential safety hazards. It is important to use materials that meet the required standards and have been tested and approved for use in concrete production.
Furthermore, the mixing process must be done correctly to ensure that the concrete has the desired strength and consistency. The mixing process must be thorough and consistent, and the correct amount of water must be added to the mixture to achieve the desired consistency.
Finally, proper curing and finishing techniques must be employed to ensure that the concrete reaches its full potential. Curing involves keeping the concrete moist and at the correct temperature for a specific period of time, while finishing involves smoothing and leveling the surface of the concrete to achieve the desired appearance and functionality.
In conclusion, following the rules and guidelines set out for concrete production is essential for producing high-quality and durable concrete that meets the required standards. Ignoring these rules can lead to potentially hazardous and costly outcomes.
Type of Superplasticizer
The choice between naphthalene and melamine superplasticizers is not solely dependent on their solid content. Although naphthalene superplasticizers are recommended due to their higher solid content, ranging from 40 to 42 percent, compared to melamine superplasticizers with a solid content of 22 to 30 percent. The efficiency of superplasticizers depends on various factors, including the quantity and quality of solids present, molecular chain length, amount of impurities and residual sulphates.
Choosing a superplasticizer also involves considering economic efficiency, technical and commercial aspects. The cost of reaching the desired workability is a crucial factor. Other considerations include the quality of service provided by the admixture company, consistency in quality and regularity of delivery. Confidence in the admixture company is also essential.
While technical factors are essential, some precasters choose melamine superplasticizers as they provide surfaces free from bubbles. In North America, melamine was commonly used due to market status and commercial reasons. However, with the increased use of high-performance concrete worldwide, naphthalene superplasticizers have also gained popularity.
Brand of Superplasticizer
The production of superplasticizers, whether they are naphthalene-based or melamine-based, is dominated by a small number of manufacturers. As a result, there has been a proliferation of commercial superplasticizers with similar properties, all of which are produced using the same reactor.
To determine the origin of specific commercial superplasticizers, it is possible to analyze their infrared spectrum and subject the solid components to thermal analysis. These methods can be used to identify the specific manufacturer responsible for producing a particular superplasticizer. By conducting these analyses, it is possible to gain a better understanding of the production processes used by different manufacturers and to differentiate between commercially available superplasticizers with similar properties.
Type of Superplasticizer from a Brand
In the past, contractors had no choice but to use a single brand of superplasticizer due to limited availability. However, the situation has changed and now there are multiple brands offering different varieties of superplasticizers. This variety of options makes it challenging for contractors to select the best product from a range of similar products.
Portland cement is classified into various types, each showing distinct characteristics influenced by different factors. On the other hand, superplasticizers, although available in different types, show less variability in their properties when compared to Portland cement. This is because superplasticizers are manufactured using a straightforward process that involves a limited number of raw materials, which are relatively pure.
Type of Formulation – Liquid or Solid
Superplasticizers are used in construction to improve the workability of concrete. They are available in two forms, solid and liquid. However, for ease of use and limited mixing time, liquid superplasticizers are predominantly used. It is essential to keep in mind that ambient temperature changes can affect the efficiency of superplasticizers.
Naphthalene superplasticizers, in particular, can freeze at a temperature of -4 degrees Celsius. If the ambient temperature drops below 5 degrees Celsius, their viscosity will decrease, causing a change in their properties. This change can significantly impact the superplasticizer’s efficiency. If superplasticizers freeze, they must be stored at a maximum temperature of 35 degrees Celsius for 24 hours.
Certain superplasticizers are prone to developing bad odors, fungi, and bacteria at high temperatures, which affects their performance negatively. To get the maximum benefit from superplasticizers, it is crucial to store them at temperatures between 10 and 30 degrees Celsius.
Dosage of Superplasticizer
Concrete plants commonly measure the dosage of superplasticizers in liters of commercial solution per cubic meter of concrete. However, this method differs from the scientific practice of representing superplasticizer dosage in terms of solid content. In order to maintain control over the water cement ratio of high-performance concrete, it is crucial to consider the amount of water contributed by the superplasticizer.
It is imperative to understand the water contribution of superplasticizers, as this affects the overall water content of the concrete mixture. Superplasticizers are a key component in the production of high-performance concrete, and their dosage must be precisely controlled. Using a solid content representation would provide more accurate measurements, as it would not include the water content of the solution.
Concrete plants must balance the desired water cement ratio with the need to use superplasticizers to achieve the desired level of workability. By understanding the contribution of water from the superplasticizer, they can adjust the dosage accordingly to maintain the desired level of performance. Scientific representation of superplasticizer dosage in terms of solid content can provide a more accurate picture of the water content in the mixture, and aid in achieving optimal results in high-performance concrete production.
Utilization with a Water Reducer
It is a common practice for certain concrete producers to use lignosulphonate based water reducers as a cost-saving measure in place of superplasticizers. The typical dosing rate for these water reducers ranges from 0.5 to 1.5 liters per cubic meter of concrete. This approach has been adopted as a standard rule in Norway, as well as by producers in North America. The primary advantage of this method is that it can reduce the amount of superplasticizers needed, resulting in cost savings.
Utilization with a Retarder
The issue of slump loss during concrete mixing can be solved by incorporating a certain amount of retarding agent along with the superplasticizer. This method is particularly effective when using rheologically active types of cement. The retarding agent should be added in an amount equal to 5 to 10 percent of the weight of the superplasticizer. By doing so, the problem of slump loss can be resolved without retarding the setting of the concrete.
When choosing a retarding agent, it is best to use sodium gluconate rather than lignosulphonate retarding agents. Sodium gluconate is more effective in reducing the entrapping of air bubbles, making it more practical and easier to control than lignosulphonate. However, when using both superplasticizers and retarding agents simultaneously, the producer must determine the optimum dosage based on cost and compressive strength in the short term. Delaying the compressive strength to be gained within a single day can delay work, so the amount of superplasticizer and retarding agent must be optimized accordingly.
It is important to consider the climatic factors when determining the amount of superplasticizer and retarding agent to be used. This is because the temperature of the concrete greatly affects the reactivity of the cement. The use of superplasticizers and retarding agents is a tricky method that requires careful consideration of multiple factors.