Cement & Super Plasticizers Compatibility
With the increasing number of types and brands of cement, as well as variants of construction chemicals, there are issues that arise related to the interactions between these two essential ingredients of good quality concrete.
Modern concretes almost always possess additives, particularly chemical admixtures to enhance the properties of fresh and hardened concrete. Then, there are others chemicals for viscosity modification, shrinkage reduction, corrosion inhibition and/or alkali-silica reaction mitigation. Most users apply a trial-and-error approach to the use of chemical admixtures, often resulting in an unfortunate negative experience and/or low cost-effectiveness, which perhaps results in a bias against admixtures in general.
Common problems that arise as a result of incompatibility between cement and water reducers are: rapid loss of workability, excessive quickening / retardation of setting, and low rates of strength gain. Very often, there even exists incompatibility between a particular chemical and a certain batch of the same (otherwise compatible) cement, indicating that the nature of the problem is complex, and needs further understanding.
Moreover, high performance concretes, which are in wide use today, almost always incorporate a mineral admixture or filler, such as silica fume, fly ash and limestone powder. This further complicates the physico-chemical behaviour of the cement-based system since the mineral admixtures play an important role in the evolution of the hydration reactions and the availability of free water during the early ages of concrete.
Super Plasticizers (SP)
Super Plasticizers are high range water reducers and are incorporated in concrete either to maintain the same workability at reduced water cement ratio (w/c) or to increase the workability by maintaining constant w/c. After the introduction of the first generation of superplasticizers (SPs) in the 1960s, these products have become essential for the placement of concrete in areas with low accessibility or with high density of reinforcement, as well as to provide an increase in the pumpability of concrete.
The newly developed concretes, such as self compacting concrete, require the presence of a superplasticizer to achieve the desired properties. SPs are generally supplied as liquid formulations, with the active solids content in the range of 30–40%. Currently, the most widely used superplasticizers are the sulphonated formaldehyde condensates and the polycarboxylates.
Problems arising out of compatibility issues are often mistaken for problems with concrete mixture design because of the lack of information about the subject amongst practicing engineers.
The incompatibility Issues
The term incompatibility refers to the adverse effect on performance when a specific combination of cement and superplasticizer is used. Common problems include flash setting, delayed setting, rapid slump loss, improper strength gain, etc. These issues in turn could affect the hardened properties of concrete, primarily strength and durability.
The use of superplasticizers has become very common in India. It is very difficult to ensure that an admixture that produces all the desired effects with Cement A would do the same with Cement B. Incompatibility could also arise as a result of the use of additional mineral additives, or while using combinations of chemicals.
Users, who are unaware of compatibility issues, often suffer when the supply of cement and/or admixture is changed midway through a project. Admixture manufacturers try to overcome the problem by formulating project-specific chemicals. Obviously, this is only a short term solution. For a more comprehensive approach, a thorough understanding of the causes and remedies of incompatibility is necessary.
Factors Affecting Compatibility
- Effect of Chemical Structure of SP: The maximum adsorption of SP depends on the chemical composition of the cement, as well as the chemical structure of the super plasticizer (SP). When a higher amount of SP is adsorbed, there is better initial fluidity but the fluidity may not be maintained sufficiently over time. In such cases, the dosage of the SP has to be increased to provide an additional amount for maintaining the fluidity. The formation of air bubbles in the cement paste due to the incorporation of the SP can also help in its fluidification but could also reduce the strength and durability if the air content is high. Different types of admixtures have differing influences on air entertainment in concrete.
At low dosages, all fractions of the polymer are adsorbed whereas beyond a certain dosage, only fractions of super plasticizer with larger molecular weights will be preferentially adsorbed. This can lead to differences in the effectiveness of different products since they could have fractions of varying molecular weights and larger molecular fractions of varying effectiveness. Also, the response of the SPs to the variation in chemical composition of cement depends on the type of superplasticizer.
Admixture type and dosage: For all SPs, the rate of increase in the fluidity of the paste or workability of concrete decreases as the dosage increases until there is no significant increase in the fluidity (beyond a dosage often denoted as the saturation dosage). Further increase in the superplasticizer dosage could lead to segregation and set retardation rather than assist in increasing the workability of the concrete, not to mention the cost implications. It is also well known that almost all SPs increase the length of the dormant period and slow down the hydration process.
Time of addition of the SP: The amount of admixture adsorbed reduces when cement hydrates; in other words, adsorption is greater on unhydrated compounds compared to the hydrated phases. Delayed addition has shown to result in lesser participation of the polymer in the formation of the organo-mineral phase.
- Cement composition and fineness: The finer the cement, the higher the specific surface area, and consequently, the water demand for a given workability is also expected to be higher. The amount of SP required for certain workability would be higher for finer cement. The amount of SP adsorbed would also depend on the fineness, with finer cements causing more SP adsorption.
India Specific Scenario
In India, there is an increasing tendency to use synthetic gypsum instead of natural gypsum during cement production. However, the rate of solubility of synthetic gypsum and its effects on the initial adsorption and consequent fluidity are not clear. Moreover, synthetic gypsum could increase the setting time from 3 to 10 hours. Such retardation can be eliminated if the phosphogypsum is neutralized with lime, and content of water soluble phosphates and fluorine is reduced to below 0.02%. In addition, the use of cements high in alkali causes workability problems in concrete without any admixtures while, cements low in alkali result in poor rheology of the concrete in concretes using sulphonate based admixtures.
Added to the cement-superplasticizer interactions, the effect of an “inert” component on the workability could be significant (for example, when there is some interaction between a superplasticizer and fine sand particles leading to a high loss of workability during transportation), and early-age cracking increases even when the concrete is of good quality (which can occur when fillers absorb the bleed water, causing higher plastic shrinkage, or when a high paste content causes excessive thermal shrinkage). Another complication is that the type of mixing can also affect SP effectiveness.
The placing of concrete at high ambient temperatures adds a new dimension to the problem of incompatibility. Low temperature has been reported to decrease fluidity. Conversely, increase in the temperature causes higher rate of slump loss. The influence of temperature on cement-SP interaction is also closely associated with the cement composition.
In India, cement standards are not very stringent, and enable manufacturers to adjust their product in many different ways. For example, while the minimum fineness is specified for different grades of cement, there is no control on the maximum. Thus, a manufacturer could use the same composition and grind cement to different finenesses, and still have the same end product. Such a situation might lead to variations in the super plasticizer demand.
Additionally, the requirements of chemical composition of the cement are also not stringent, and large ranges are acceptable. This could result in significant variability in the cement properties, even from the same manufacturing plant. From the viewpoint of use of water reducers, there is insufficient knowledge among users regarding the limitations of different types of chemicals.
Studies on cement-water reducer interactions in India have been limited to the workability evaluation of concretes containing these chemicals, in specific regions where rapid slump loss has been observed in concreting operations. There have not been any investigations to understand the physico-chemical nature of this interaction. Thus, the results from these studies are not broad-based.
There is a distinct need for the characterization of Indian cement and admixture properties, in order to understand the nature of their interactions. Moreover, the wide range of cements used, varying transportation duration and climatic conditions necessitate more fundamental studies that could explain the mechanisms of interaction and help establish methods for identifying incompatibility in practical situations.
In addition, simple methodologies are required to be able to identify systems prone to undesirable effects due to such interactions and to further understand the fundamental nature of admixture behaviour in cement-based systems.
Prof. Ravindra Gettu
Prof. Manu Santhanam
Department of Civil Engineering, Indian Institute of Technology Madras, Chennai
Department of Civil Engineering, Amrita Vishwa Vidyapeetham, Coimbatore