CL305 Concrete TechnologyTerm Paper I: Construction Chemicals and Admixtures Retarders

Prepared by Ronak Kamdar (13BCL033)

Nirma University Institute of Technology October 2014

Table of Content

What are Retarders? Composition of Retardation Mechanism of Retardation Interaction of Retarder with Cement Effects of Retarders on the Drying Shrinkage of Cement Paste at Various Stages of Hydration Result of Setting Time Experiments1) Commercial Retarders2) Pure Retarders

Effects of large retarder overdose Applications Usage of retarders as per IS 9103:1999 Usage of retarders as per ASTM C494 Cost of retarders References

What are Retarders?

According to IS 9103:1999,An admixture which delays the setting of cement paste, and hence of mixtures, such as mortar or concrete containing cement.

So basically retarders are chemical formulations which retard, but do not "kill" the set of the mortar at the surface of concrete. When the underlying concrete has hardened, the retarded mortar surface can be flushed off with a stream of water and/or removed by scrubbing with a stiff brush. Since these compounds do no "kill" the set, if they are left on the concrete or unintentionally splashed on other fresh concrete, they will permit the concrete to eventually attain a set and achieve full strength.

Retarders are classified as type B or type D chemical admixtures for portland cement concrete in ASTM Designation C494. Their primary function is, as the name implies, to retard the setting time of concrete. Many problems of hot weather concreting, mass concrete, multi-lift concreting, continuous placement, and others have been helped by the use of retarders.

As a secondary effect, many retarders reduce the water requirement to produce a concrete of desired workability and produce a concrete of higher strength with equal cement content. Some retarders, additionally, produce many small air bubbles, much the same as those produced by air-entraining agents in concrete mixtures. Hence, the use of retarders is also beneficial to other properties of concrete such as workability, strength, and durability.

Composition of Retardation

Retarder can be formed by organic and inorganic material. The organic material consists of unrefined Ca, Na, NH4, salts of lignosulfonic acids, hydroxycarboxylic acids, and carbohydrates. The inorganic material consists of oxides of Pb and Zn, phosphates, magnesium salts, fluorates, and borates. Commonly used retarders are lignosulfonates acids and hydroxylated carboxylic (HC) acids, which act as Type D (Water Reducing and Retarding Admixtures). The use of lignosulfonates acids and hydroxylated carboxylic acids retard the initial setting time for at least an hour and no more than three hours when used at 65 to 100oF.A study performed on the influence of air temperature over the retardation of the initial set time (measured by penetration resistance as prescribed in ASTM C 403 92) shows that decreasing effect with higher air temperature. The table below describes the effect of air temperature on retardation of setting time:Table: Air Temperature and Retardation of Initial Setting TimeAdmixture TypeDescriptionRetardation of initial setting time (h:min) at temperature of

30oC40oC50oC

DHydroxylic acid4:571:151:10

DLignin2:200:420:53

DLignosulfonates3:371:071:25

BPhosphate-based---3:202:30

Mechanism of Retardation

Many water reducers have a retarding tendency. Therefore, some of the ingredients in water reducers, such as lignosulfate acids and hydroxycarboxylic acids, are also a basis for set-retarding admixtures. Other important materials used in producing set retarders are sugars and their derivatives.Mechanisms of set retardation were studied by many researchers. Several theories have been offered to explain this mechanism. A review of these theories was presented by Young (1972). The role of retarding admixtures can be explained in a simple way: the admixtures form a film around the cement compounds (e.g., by absorption), thereby preventing or slowing the reaction with water. The thickness of this film will dictate how much the rate of hydration is retarded. After a while, this film breaks down, and normal hydration proceeds. However, in some cases when the dosage of admixtures exceeds a certain critical point, hydration of cement compounds will never proceed beyond a certain stage, and the cement paste will never set. Thus, it is important to avoid overdosing a concrete with a retarding admixture.Other factors influencing the degree of retardation include the w/c, cement content, C3A and alkali contents in cement, the type and dosage of the admixture, and the stage at which the retarder is added to the mix. The effectiveness of retarder is increased if its addition to the fresh concrete is delayed for a few minutes.

Interaction of Retarder with Cement

1. C3A, C4AF, hydrated C3S , and, possibly, hydrated C3A adsorb retarders from solution.

2. Retarders that have chemically different structures act differently in their effects on cement hydration.

3. Not only the aluminate phase but also the silicate phase in cement plays a significant role in the setting process.

4. Retarders are more effective when they are added to cement of low C3A and low alkali content.

Effects of Retarders on the Drying Shrinkage of Cement Paste at Various Stages of Hydration

Figures 1 and Figure 2 show, respectively, the shrinkage of the cement paste bars as affected by the addition of commercial retarders and selected pure chemicals when the pastes were dried in a vacuum oven at 105 to 110C for 24 hours. The shrinkage values were plotted against the degree of hydration of the samples, determined as previously explained. For clarity, data points are not shown in the figure. For citric acid, however, data points only are shown in Figure 2 because of the lack of enough points to draw a complete curve.

At the degree of hydration of about 50%, which roughly corresponds to an age of 2 days, all the samples shrank practically an equal amount, approximately 0.8%. The cement paste alone, with no admixture, always had a smaller shrinkage than the others at this point and at most later ages. At the degree of hydration of 85 to 90%, the rate of increase in shrinkage was reduced for all pastes except that with calcium chloride.

When the degree of hydration was between 2 and 4 5% the shrinkage behavior of the samples was changed significantly. All hydroxycarboxylic acids such as retarder A, glycolic acid and citric acid increased the shrinkage considerably. Sucrose had an effect about equal to that of these acids, but the carbohydrate retarder (S) , conversely, minimized the shrinkage. The cement paste with lignosulfonate retarder (L) shrank more or less in the same way as did the cement paste with no admixture, but showed less shrinkage at early ages. Addition of 3-hydroxy-2-butanone or hydroquinone , which have no acid group, but only hydroxyl1 or carbonyl groups, resulted in a smaller shrinkage of 79 cement paste at this early age. Calcium chloride increased the shrinkage at all stages of hydration and showed the largest shrinkage of all hydrations higher than 50%. When either the concentration of citric acid was increased enough to result in a relative retardation of 200% or the water-cement ratio of plain paste was changed from 0.40 to 0.43, the shrinkage of the pastes was practically unchanged.

Figure 1 Shrinkage of vaccum over-dried cement pastes at various stages of hydration (commercial retarders)

Figure 2 Shrinkage of vaccum over-dried cement pastes at various stages of hydration (pure retarders)

Result of Setting Time Experiments

Commercial Retarders

Figures 3, 4, and 5 show the results of setting time experiments for the three commercial retarders when they were used at different concentrations, expressed as weight per cent of the retarder-solids based on the cement. In these figures, values of penetration resistance are plotted against elapsed time after initial contact of the cement with mixing water (plus admixture). The general expectation that the higher the concentration of retarder, the longer the setting time, is apparent. However, it is noteworthy that the retarders A (hydroxycarboxylic acid) and S (carbohydrate) tend to accelerate the initial hydration of cement when they were added at the highest concentration whereas the retarder L (lignosulfonate) retards all stages of the hydration evenly up to 4000 psi even at higher concentration.In order to compare the effects of concentration of retarders at different penetration levels, Figures 6, 7, and 8 were made from the same results. It is observed that relative retardation of the mortars is smaller at higher penetration resistances. This is an important and necessary property for properly retarded concrete. Another interesting point is that the increases in setting time at the penetration resistance levels of 500, 1000, and 4000 psi are in close agreement and the retardation at 500 psi level appears to be a mean retardation in the resistance range of 100 to 4000 psi. Since initial setting time corresponds approximately to the last time when revibration of concrete can be made, the result would indicate that initial setting time is a practical and convenient standard point for evaluating retarders. Manufacturer's suggested dosages are also shown in each figure by a shaded range. These dosages produce increases in setting time of roughly 30 to 90% for all the retarders. The resulting range of retardation is typical of that of concrete (49) . This result implicitly affirms the adequacy of the penetration resistance test as conducted on mortar specimens in this work. In the same figures, it should be noted that the lignosulfonate retarder (L) required more than twice as much dosage as the others(A and S) did to obtain a certain amount of retardation.

Figure 3 - Penetration Resistance vs Elapsed Time for Mortar with Retarder L

Figure 4 - Penetration Resistance vs Elapsed Time for Mortar with Retarder A

Figure 5 - Penetration Resistance vs Elapsed Time for Mortar with Retarder S

Figure 6 - Effect of Concentration of Retarder L on Setting Time of Mortar

Figure 7 - Effect of Concentration of Retarder A on Setting Time of Mortar

Figure 8 - Effect of Concentration of Retarder S on Setting Time of Mortar

Pure Chemicals

Strong retarders such as tartronic acid, d-tartaric acid, mucic acid, gluconic acid, 2-ketoglutaric acid, citric acid, sucrose, and 2 ,4 ,6-trihydroxybenzoic acid have large values of T500/T50. T500 is defined as the initial setting time of each sample relative to that of a control sample, and was obtained as follows: A relationship between elapsed time and penetration resistance was drawn for each sample in the same way as was done for the commercial retarders, and an initial setting time was determined. This time was then expressed in terms of a percentage of that of an appropriate control sample without admixture. In the same way, T50, T100, and T4 0Q0, which are relative setting times at the penetration resistance of 50, 100, and 4000 psi, respectively, were determined. Again, calcium lignosulfonate is not a strong retarder. Glycolic acid, methyl glycolate and a-hydroxyacetamide are strong retarders at higher concentrations. Only l,3-dihydroxy-2-propanone showed an unusual setting when it was added at the rate of 0.5%. It accelerated early72 hydration of the cement to reach a penetration resistance of 2 00 psi in 2 hr , but extremely retarded the later hydration. This behavior was similar to that observed in the hydration of a Type I cement clinker without any gypsum addition (Figure 8).

Figures 9 and 10 show the effects of concentration of admixture on the relative initial setting time of mortar specimens. It is seen that strong retarders can delay the setting of mortars almost indefinitely even at a relatively low concentration while some other chemicals accelerate the setting when added at higher concentration.The results seem to divide themselves into two groups. In Figure 14, whatever the magnitude of the retardation, additional increments of concentration produced an increasingly larger relative retardation, i.e., the curves are concave downward. In Figure 15, on the other hand the opposite is true; the instances showed less additional effect at higher concentrations. Indeed, sometimes the effect was reversed.

Figure 9 - Penetration Resistance vs Elapsed Time for Mortar with 1, 3-Dihydroxy-2-Butanone

Figure 10 Concentration of admixture vs initial setting time of mortar (No. 1)

Figure 11 Concentration of admixture vs initial setting time of mortar (No. 2)

Effects of large retarder overdose

A large amount of retarding admixtures is available in the market. Some of their desirable effects are described by the manufacturers, but there is no information regarding some other effects such as the effect of overdosing, with no allowable over dosage limit being specified. Such overdosing can ocuur in practice due to faults in a retarder dispenser at a batching plant.

The following main conclusions can be drawn based on this investigation that used the retarder Pozzolith 300R, an admixture made to satisfy the ASTM Type B and D requirements.

1. An overdose of retarder up to 3 times the normal dosage can be easily accommodated in concrete strength gain nit being appreciably affected.2. Concrete with retarder overdoses of up to even 6 times the normal dosage will eventually reach or even exceed their corresponding 28 day strengths.3. Equivalent cube strength results from cores are progressively lower than the corresponding cube tests results as retarder dosages increase4. Reductionsin cement setting time for the specimens with a retarder are greater for the effect of a 2.5m/s wind than for a temperature increase of 220C above the ambient 31oC5. The question as to whether the interior quality of retarder overdosed concretes is better or poorer than the surface quality could not be resolved unambiguously, and requires further investigation.

Applications

Creation of exposed aggregate surfaces Precast panels Decorative sidewalks and walkways Bond improvement for water-proofing materials Slip-resistant surfaces Formulations for both horizontal and vertical To offset the accelerating effect of hot weather on the setting time of concrete Placing concrete in large piers and foundations, cementing oil wells, or pumping grout or concrete over considerable distances

Usage of retarders as per IS 9103:1999

Usage of retarders as per ASTM C494

Note:A: Water ReducingB: RetardingC: AcceleratingD: Water Reducing and RetardingE: Water Reducing and AcceleratingF: Water Reducing, High RangeG: Water Reducing, High Range and Retarding

Cost of retarders Sodium Gluconate retarder concrete admixture agentUS $700-800 /Metric Ton( FOB Price)20 Metric Tons(Min. Order)150000 Metric Ton/Metric Tons per Year(Supply Ability) Polycarboxylate retarder concrete admixture purity higher than 40%US $1000-2000 /Ton( FOB Price)10 Tons(Min. Order)100000 Ton/Tons per Month(Supply Ability) Sodium Lignosulphate retarder concrete admixture shampoo slsUS $190-290 /Metric Ton( FOB Price)1 Metric Ton(Min. Order)5000 Metric Ton/Metric Tons per Month(Supply Ability) PCE 50% concrete retarder admixtureUS $2250-2280 /Ton( FOB Price)16 Tons(Min. Order)1500 Ton/Tons per Month(Supply Ability) JK-06 SG Sodium Gluconate Set Retarding concrete admixture14 Tons(Min. Order)300,000 Ton/Tons per Year(Supply Ability)

References

http://www.alibaba.com/showroom/retarder-concrete-admixture.html http://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Admixture/AdmixturesMain.htm http://en.wikipedia.org/wiki/Retarder_(chemistry) www.euclidchemical.com/.../Concrete%20Surface%20Retarder.pdf www.euclidchemical.com/...retarders/concrete-surface-retarders/ www.basf-admixtures.com HomeProduct Search www.cement.org/cement-concrete-basics/concrete.../chemical-admixtures www.sfu.ca/~nmallika/Nuwan/.../Effect_of_large_retarder_overdose.pdf docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2188&context=jtrp http://sintef.se/upload/Hardening%20Retarders%20for%20Massive%20Concrete.pdf https://law.resource.org/pub/in/bis/S03/is.9103.1999.pdf http://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Admixture/LinkTab.htm