🕑 Reading time: 1 minuteSuperplasticizers are used to fluidify the concrete in the field. Most of the superplasticizers helped to maintain high slump values for a time of 30 to 45 minutes. But some of the Portland cement do no behave well with the use of superplasticizers. This problem is understood only by the detailed study on the mode of action of superplasticizers.
- Cement and Superplasticizer Reactivity and Compatibility
- Rheopump to Study the Behavior of Superplasticizers and Cement
- Problems in Choosing Appropriate Superplasticizer of High-Performance Concrete
- Selection of Superplasticizer for High-Performance Concrete
Cement and Superplasticizer Reactivity and CompatibilityIt is clearly impossible to explain the minute details about the working of superplasticizers. But among many problems, the most faced issue is the slump-loss problem which is observed in the making of high-performance concrete. When the intention is to create a large slump value with a lower water/ cement ratio the mentioned problem is met. Generally, 120 to 135 L/m3 of mixing water is used in the manufacture of high-performance concrete compared to a quantity of 160 to 180L/m3 of water in ordinary concrete. This value is dependent on the entrained air content, the maximum size of the coarse aggregate, nature and other related factors. On one hand, the rheology of the high-performance concrete is mostly influenced by:
- the rate at which different Portland cement mineral phases react with the water molecules
- the rate at which molecules of superplasticizers are trapped by compounds that are formed newly by the contact of water and the Portland cement
- Whether the cement is having lower rheological activity
- Whether the superplasticizers used will not compete with the crystals of ettringite
Rheopump to Study the Behavior of Superplasticizers and CementAn empirical test was developed at the University of Sherbrook. The method is proved to be efficient and extremely reliable. A great variety of cement that varies in the content of tri calcium aluminate, tetra calcium alumino ferrite, limestone filler, di and tri calcium silicates and cement with different fineness works compatible with the test. The whole arrangement is called the rheopump. The test comprises an arrangement of small modified pump that will recirculate a cement grout having a water cement ratio of 0.35. The grout consists of a reference superplasticizer of naphthalene. The dosage is based on the type of cement that is used in the preparation of the grout. This is dosed for a time of four minutes. As shown in the figure-1, the flow time taken for 1L of the prepared grout through the Marsh Cone is measured. This marsh cone is used to check the fluidity of bentonite in the petroleum industry. Once the measurement of initial time is completed, the grout is placed in another plastic container and subjected to continuous agitation until the next measurement. The next set of measurement will be taken 40 minutes later. Hence the initial reactivity and the compatibility of the chosen cement with the specified superplasticizer is understood within an hour by employing a single bag of cement. The rheopump have never failed even used with concrete for slump loss measurement.
Fig.1: Schematic Diagram Representing RheopumpThe rheopump can also be used to find the retarder dosage whenever it is necessary to slow down the slump loss or to find the compatibility of a different variety of superplasticizer with a given type of cement.
Problems in Choosing Appropriate Superplasticizer of High-Performance ConcreteIt is very essential to carefully choose the different constituents to make a high-performance concrete. The concrete producers lack a complete knowledge of this particular area and the following problems are faced. 1. Problems with the physical Nature: Because of a limited number of bins and service to the regular customer, the facilities allow a little latitude in this area. 2. Problems due to an Economical Nature: Based on the geographic location, resources that are locally and economically available are demanded first then selecting others. 3. Problems with the Social Nature: Limit the Superplasticizer selection due to the tie up with known cement or quarry companies. The concrete producers may have a personal dealing with any of the cement companies that limit the variety of selection.
Selection of Superplasticizer for High-Performance ConcreteCertain general rules that each concrete producer must strictly follow to bring the desired result are mentioned below:
Type of SuperplasticizerBy only focusing on the solid content it is recommended to use naphthalene superplasticizers when compared with the melamine. The naphthalene superplasticizers are marketed with a solid content from 40 to 42 percent. The melamine superplasticizers are marketed for a solid content of 22 to 30 %. New melamine superplasticizers are said to have solid content of 40 percent. The experience show that the efficiency of the superplasticizers depends on the following parameters:
- Quantity of solids present
- Quality of the solids
- The length of the molecular chains
- The amount of impurities that are present
- The amount of residual sulphates present