Why should Superplasticizers be Compatible with Cement?

Superplasticizers offer us high workability for normal strength concrete and optimum workability for low water content in concrete to produce high strength concrete. In any purpose, when they are used at very low dosages, fewer problems are arrived. When high dosages of superplasticizers are required in concrete mix, the situation becomes completely different.At first we have to decide when high dosages are required. To produce a normal strength concrete with high workability, a very low dosage is considered enough; the objective is to produce flowing property to concrete or to place a thick section of concrete having reinforcement congestion.

Admixture-cement compatibility becomes important, for high dosage of superplasticizers; it is not enough to consider cement and superplasticizers separately to comply with respective standards. Large dosage of superplasticizers can be achieved by applying re-dosage; which is considered has less influence on superplasticizers-cement compatibility. But often re-dosing is not possible.

So superplasticizers-cement compatibility has to be established in these cases; a perfect match of both materials will ensure retention of expected high workability for expected long period. The duration is often (60-90) minutes, but a retention period of even more up to 120 min can be achieved.

Now we have to be established required dosage of superplasticizers, at first, before assessing compatibility. The approach followed usually is to

• Determine water reduction as percentage relative to same workability of concrete without superplasticizers or other water reducing admixtures.

• According to BS 1881 : Part 105 or ASTM C230, we can use following table method.

• Mini-slump test introduced by Kantro can be applied alternatively.

 

Marsh cone was recommended by Aïtcin et al to determined time required to flow a specified amount (measured by volume) of grout having known amount of superplasticizers and cement through an specified orifice. The time required is called Marsh Flow-time, which is usually decreased with increase in dosage of superplasticizers. But this reduction is valid up to a certain limit; dosage exceeding this limit will show little important in workability (i.e. time reduction).

Thus optimum dosage is found. Excess application of superplasticizers is not only affect economy but also it will induce segregation. It is found that the difference between workability at 5 min and 60 min is very small after mixing as found in Marsh Cone test.

Different properties of cement like fineness, type of calcium sulfate in cement as retarder, C₃A content in cement can affect performance concrete with particular superplasticizer. It is very important to verify suitability of superplasticizers rapidly but full scale test is required in determining dosage of superplasticizers in laboratory.

A finer cement within concrete mix should be associated with higher dosage of superplasticzers to achieve a particular workability. A higher C₃A content in cement will reduce effectiveness of superplasticizers. Often the manufacturers recommended dosages are found not enough for a special concrete to achieve specified objective.

In selecting a suitable combination of superplasticizer and cement, it is sometimes found easier to keep cement type fixed and varying superplasticizers, whereas, in other situations, selection of suitable cement for a particular workability may be required. Thus any random combination of both materials will not be considered satisfactory; reliable means to establish compatibility of them must be checked before application.

To make superplasticizers effective we must not allow superplasticizers to react with C₃A during mixing; when react with C₃A, it is called fixing of superplasticizers by C₃A. if they are fixed with

C₃A there sufficient supperplasticizers will not available there to achieve and maintain high workability.Now question is how to restrict fixing? If calcium sulfate exist in Portland cement is released fast enough into mix and they react with C₃A before reaching superplasticizers to that. Thus compatibility between Portland cement and superplasticizers means the combination that abstaining reaction between C₃A and superplasticizers.

In case of very low water/cement ratio, the compatibility issue is of prime concern as various ingredients of wet mix compete for water to make wet their surface and subsequent early hydration. The solubility of calcium sulfate is crucial point, as this rate will be low if there have less water to receive sulfate ions and due to high cement content (water/cement ration is low) there will be more, thus control on reaction between superplasticizers and C₃A is very difficult. Thus we cannot forecast compatibility of given combination under water/cement ratio of 0.25 from test results from identical materials using water/cement ration of 0.5.

Though comparatively less amount of C₃A exist in cement (around 10.8%), its nature and micro-structural relationship with other phases make it very important. An immediate stiffening property of cement paste is rendered by C₃A. The violent reaction of pure C₃A lead to flash set.

Flash set produce many problems, but here our interest is workability. It is prevented by mixing gypsum

(CaSO₄.2H₂O) with cement clinker.
Obviously gypsum is the source of SO₄^— ions but these
can be arrived in cement from other sources like Anhydrite (CaSO₄ )
or hemihydrate (CaSO₄.1/2H₂O).

The solubility & rate of solubility of calcium sulfate both are influenced by superplasticizers, again dosages and their types of superplasticizers influence solubility. In our present state of knowledge, conversion of those qualitative factors to predict compatibility of Portland cement and superplasticizers is not possible. Experimental prediction of rheological properties for a particular combination of them is essential.

Let’s conclude the factors that control compatibility; these are as follows:

For cement:

• Content of C₃AF and C₃A

• Reactivity of C₃A which is controlled by sulferisation rate of clinker and morphological origin of C₃A.

• Content of calcium sulfate of cement

• Ultimate form of calcium sulfate like gypsum, anhydrite or hemihydrate in grounded cement

For superplasticizers:

The considerable factors are

• Length of molecular chain

• Position of sulfonate group in molecular chain

• Type of center-ion i.e. calcium or sodium ions

• The availability of residual sulfates; it is believed to have influence on deflocculation properties of cement

Now what is ideal cement and ideal superplasticizers?

Ideal cement:

Depending on above factors, an ideal cement that is considered perfectly compatible from rheological aspect can be concluded as

• A moderately fine cement approximately up to 400 m²/kg as determined in Blaine method

• C₃A content is very low to have controlled reactivity of it by SO₄^— ions liberated from sulfate content in cement.

 

Ideal superplasticizer:

A super plasticizer can be said to be ideal which have:

Sufficient long molecular chains, sulfonate groups should take place β-position in condensate of sodium salt of naphthalene and formaldehyde sulfonates. Residual sulfates of the superplasticizers and their content depends on solubility and content of sulfates present in cement which is investigated to be used with this superplasticizers; that is the necessary factor is availability of sufficient soluble sulfates in concrete mix.

The preceding discussion can help us to exclude or select an appropriate or proper superplasticizers and cements. The subsequent step is to testing samples in laboratory. The samples are made based on trial-and-error selection of different combination of superplasticizers and cement. Tests on neat cement paste are conducted to establish a rheological best combination of them.

The performance of a particular superplasticizers in combination with a given type/brand of cement can be determined by measuring time required to flow a fixed amount of neat cement paste of those materials through Marsh flow cone (a funnel of standard size and dimension). Obviously time required to flow will be decreased with respective increase in dosage of superplasticizers; but there have a saturation point i.e. further increasing of dosage will no longer beneficial.

Loss of workability can be examined, by delaying test of cement paste, here excess time required to make this cone empty is measured. Compatible combination of superplasticizers and Portland cement will yield small loss of workability (small delay to empty) between tests conducted at (5~60) minutes. They will also show a clear saturation point including optimum dosages of superplasticizers beyond which addition of superplasticizers will not yield benefit.

Now there have many brand of cement and few commercial superplasticizers in market; such tests stated above make it easy to narrow to select few types of cement that are compatible with one or more superplasticizers. To make decision about final combination of superplasticizers and cement, tests should be conducted on trial mix of concrete (not neat cement paste) as only these tests give us true reliable data about strong gain and slump loss.