Set retarding admixtures are water soluble chemicals that have little or no other effect than to delay the setting of the cement. They do not plasticize significantly and have little or no effect on the water demand or other properties of the concrete. Set retarding water-reducing admixtures not only delay the setting of the cement but are also efficient in plasticizing concrete or reducing its water demand. Most commercially available retarders are of this type.
These may be also be used in conjunction with sulphonated naphthalene/melamine-formaldehyde condensates or polycarboxylates to produce retarding high range water reducing admixtures. The retarder molecule chemically adsorbs onto the cement particle in a mechanism similar to that described for water reducers. The main difference is the strength of the chemical bond that is formed. This strongly links the retarder molecule onto the cement surface, blocking and slowing down the rate of initial water penetration into the cement [Older,1992]. Retarder molecules also chelate calcium ions in solution, slowing the crystallization of portlandite. These two mechanisms slow the growth of hydration products, delaying the stiffening and setting of the cement but once initial hydration starts, the retarder molecules are swamped and normal hydration proceeds.
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As with water reducing admixtures, ultimate strength gain is increased with increasing water reduction Retardation of set allows the slower formation of a more ordered, smaller, denser cementitous matrix. This has the effect of increasing ultimate strength relative to an unretarded mix with the same water cement ratio [Dodson, 1990].
Acceleration of strength by heat produces the opposite effect, with the rapid formation of a coarse matrix. This explains why steam cured precast concrete rarely produces the same ultimate strength as concrete cured at normal temperatures and produced from the same concrete.
Retarding admixtures do not have a significant effect upon initial workability. However, they generally have a beneficial effect upon workability retention, particularly at elevated temperature [Older,1992].
Retarding water reducing admixtures, have a pronounced effect upon workability. Typically, an increase in slump of 60-100mm results from the addition of a dosage of 0.25% by weight cement. Set retarding high range water reducing/plasticizing admixtures may be used to enable workability to be increased to a greater extent, at a typical dosage level of 0.3 to 1.0% [Dodson, 1990].
2.3 Slump loss
Retarding admixtures are useful for helping to reduce slump loss, particularly at elevated temperature but it is still important to have a high initial workability.
Retarding water reducing admixtures are very effective at reducing slump loss when used to increase the initial workability of the mix, but less so when used as a water reducer. Indeed, if water reduction is taken at the expense of high initial workability, initial slump loss may be slightly faster and will slow when about half the initial slump is reached.
2.4 Setting time
The prime function of a retarder is to extend the setting (stiffening) time of concrete, usually in order to prevent the formation of cold joints between deliveries of concrete. Even if workability has fallen to almost zero slump, fresh concrete can be vibrated into, and will bond with, a preceding, older pour.
In hot weather, even a small delay in deliveries or a short breakdown of the pump can result in the first concrete pours setting before subsequent pours can be placed and vibrated to form a monolithic joint. In deep pours, if concrete placed early starts to set, the heat generated can cause faster setting of concrete above it and again lead to cold joints. In this situation, retarder dosage can be progressively reduced as the pour proceeds.
2.5 Air entrainment
Retarding admixtures do not normally entrain air, and some types, especially those based on hydroxycarboxylic acid, may actually reduce air content. This may cause these retarded mixes to feel harsher and have more tendency to bleed.
Most types of retarder can be used effectively in combination with an air entraining agent.
The total volume of bleed water arising from concrete is often related to its setting time because once setting starts, bleeding stops. Thus retarded concretes are always more prone to bleed. Any reduction in air tends to aggravate this potential problem.
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The plasticising component of a retarding water reducing admixture may help to offset this effect and some types are formulated to slightly air entrain in order to reduce bleed.
2.7 Heat of hydration
Retarding admixtures do not reduce the heat output of concrete but do serve to delay the time of peak temperature rise by exactly the same time interval by which it was retarded. In small sections this may allow slightly more heat dissipation and so peak temperature may be a little lower [Older,1992]. In thick sections there will be no reduction in peak temperature and there is evidence that the peak temperature may even be increased slightly.
2.8 Volume deformation
Creep and drying shrinkage are not significantly affected by the inclusion of retarding admixtures.
If the concrete is water reduced by the use of a retarding water reducing admixture, then drying shrinkage will be reduced.
Provided that the concrete is correctly cured, then retarded concrete should be stronger and just as durable as equivalent plain concrete. However, because of the extended plastic stage, more attention needs to be paid to protecting the concrete before it sets. Retarded water reduced concrete will have a lower water content than the equivalent plain concrete, and will be correspondingly more durable [Dodson, 1990].
MECHENISM OF RETARDING ADMIXTURES
Retarding admixture is an admixture that retards the setting of concrete. A retarding admixture causes cement set retardation by one or more of following mechanisms:
(1) Adsorption of the retarding compound on the surface of cement particles, forming a protective skin which slows down hydration;
(2) Adsorption of the retarding compound on to nuclei of calcium hydroxide, poisoning their growth, which is essential for continued hydration of cement after the end of induction period;
(3) Formation of complexes with calcium ions in solution, increasing their solubility and discouraging the formation of the nuclei of calcium hydroxide.
(4) Precipitation around cement particles of insoluble derivatives of the retarding compounds formed by reaction with the highly alkaline aqueous solution, forming a protective skin .
3.1 Detailed Explanation
According to the first mechanism, a retarding admixture is adsorbed on the surface of cement particles. This layer of retarding admixture around the cement particles acts as a diffusion barrier. Due to this diffusion barrier, it becomes difficult for the water molecules to reach the surface of the unhydrated cement grains and hence the hydration slows down, and the dormant period (period of relatively inactivity) is lengthened. Due to the slow hydration, no considerable amount of the hydration products giving rigidity to the cement paste will be formed and thus the paste remains plastic for a longer time. Later, when the admixture is removed from solution by reaction with C3A from cement or by some other way it is removed and incorporated into the hydrated material, further hydration is eliminated [Dodson, 1990]. On first contact of water with cement grains (C3S and C2S) calcium ions and hydroxyl ions are rapidly released from the surface of the cement grains. When concentration of these ions reaches a critical value (at which the solution becomes saturated), the hydration products calcium hydroxide and calcium silicate hydrate start to crystallize from the solution and then hydration proceeds rapidly.
According to the second mechanism, a retarding admixture incorporated into cement paste is adsorbed on the calcium hydroxide nuclei and prevents its growth until some level of super saturation is reached during the induction period of hydration. Thus, retarder lengthens the induction period by causing an increase in the level of calcium hydroxide super saturation before crystallization begins. This is analogous to the poisoning of crystal growth of calcium hydroxide by the retarding admixture as both calcium and hydroxyl ions are present in the solution but unable to precipitate as a result of poisoning of the calcium hydroxide nuclei.
According to the third mechanism, a retarding admixture incorporated into cement paste forms some kind of complexes with calcium ions released by the cement grains during the first few minutes. Formation of the complexes increase the solubility of cement, i.e., increased concentration of Ca2+, OH, Si, Al and Fe in the aqueous phase of the cement pastes will occur when hydrated in the presence of the retarding admixture. Thus the calcium ions and hydroxyl ions will accumulate in solution and will be unable to precipitate to form calcium hydroxide. For example, when ordinary Portland cement is hydrated in sucrose solution, lime is solubilised and a sucrose calcium complex (R - -O -Ca+ - -OH) is formed in which Ca+ - -OH group is attached to the five membered ring (R) of the sucrose molecule. Such sucrose-calcium complex will be able to become absorbed on the growing calcium hydroxide nucleus [Gahtani,1998]. The adsorption of the complex on the calcium hydroxide nucleus will inhibit its growth as the calcium and hydroxyl ions will not be able to precipitate. In this way, hydration is retarded.
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The fourth mechanism is similar to the first but here some kind of insoluble derivatives of retarder are formed by reaction with the highly alkaline solution as pH of the solution rises to over 12 within few minutes after first contact of water with cement. For example, inorganic salt admixtures (borates, phosphates, zinc and lead salts etc.) give insoluble hydroxides in alkaline solution. The cement hydration is suppressed through the precipitation of protective coatings of these insoluble derivatives around the cement grains [Gahtani, 1998].
Three different types of cements were used for setting time tests. These are denoted as type-A, type-B. Type-A and type-B cements are pozzolanic type cements, which approximately correspond to the ASTM type IP. Type-A cement is obtained by adding 6-20% calcined clay to the normal Portland cement clinker during manufacturing while in type-B cement the calcined clay ranges from 21 to 35%. Their compound composition cannot be calculated by using Bogue's or other such formula.
Mixing Water and Retarding Admixture:
Normal tap water was used as mixing water. The retarding admixture used was ASTM C 494 type D admixture. Its density was about 1.02 mg/ml and its chloride content was claimed nil. The amount of the admixture incorporated into the pastes was expressed in ml/100g of cement indicated as percentage.
Cement pastes were prepared for determination of consistency and setting times tests. The cement content and w/c ratios were kept constant for all tests for a given cement type. The amounts of cement and water used per test are shown in Table.
Table1. Amounts of Cement and Water Used per Test.
A Vicat apparatus was used for determination of both the standard consistency and setting times of pastes. The apparatus was similar to that recommended by the ASTM C 187-77 and C 191-77 except the minor difference in the needle and ring (mold) dimensions. The needle of the apparatus was 1.13mm in diameter and 46mm long. The ring had an inside diameter of 90mm at the base and 80mm at its top.
4.2 Determination of standard consistency and setting times
For standard consistency determination, the procedure of the ASTM C 187- 77 was followed and for setting time determination, the Turkish Standard 19 (TS-19) was followed. The TS-19 nearly follows the ASTM C 191-52 procedure with minor amendments as described below: The initial set is said to have taken place when the needle (1.13mm dia.) of theVicat apparatus ceases to pass 3-5 mm above the bottom of cement paste taken in the Vicat mould. Final set is said to have occurred when the needle penetrates the cement paste to a maximum depth of 1mm. In both cases, the setting time is reckoned from the moment when mixing water is added to the cement.
4.3 Curing Conditions
In order to simulate the approximate normal and adverse outdoors climatic conditions, the following three categories of curing conditions were provided to the test specimens:
(1) First curing condition (CC-I): Temperature = 220C, Relative Humidity = 55-65%
(2) Second curing condition (CC-II): Temperature = 350C, Relative Humidity = 35-45%
(3) Third curing condition (CC-I): Temperature = 500C, Relative Humidity = 25-35%
For maintaining the desired curing conditions, a temperature controllable cabinet was used. The required relative humidity at various temperatures was obtained by placing saturated salt solutions (sodium nitrate at 220C, potassium carbonate at 350C and potassium chloride at 500C).
4.4 Test Results and Discussion
Setting time tests with varying admixture contents were performed under the specified curing conditions. An average of three test readings was taken as the final reading. To compare the changes occurred in setting times by incorporation of the admixture, the setting time of cement paste with out admixture content under CC-1 was used as reference. The setting times were recorded in minutes. These results are presented in the following tables and figures.
Table 2. Setting Times of Type-A Cement Containing Varying Admixture Contents under Different Curing Conditions.
Table 3. Relative Retarding Effect of Admixture on the Setting Times of Type- A cement Under Different Curing Conditions in Comparison with the Reference Setting Times.
Fig1. Effect of the Admixture on Initial Setting Time of Type-A Cement.
Fig 2. Effect of the Admixture on Final Setting Time of the Type-A Cement.
Table 4. Setting Times Of Type-B Cement Containing Varying Admixture Contents Under Different Curing Conditions.
Table 5. Relative Retarding Effect of Admixture on the Setting Times of the Type-B cement Under Different Curing Conditions in Comparison with the Reference Setting Times.
Fig3. Effect of the Admixture on Initial Setting Time of the Type-B Cement.
Fig 4. Effect of the Admixture on Final Setting Time of the Type-B Cement
The results reveal that for each of the three types of cements, high temperatures and low relative humidity reduced both the initial and final setting times. This trend is in agreement with most of the relevant published works of other researchers. Higher curing temperatures and low relative humidity accelerate the hydration of cement; consequently the necessary amount of the hydration products giving rigidity to the cement paste is formed with in shorter period. Thus, setting times are lowered. The temperature effects on setting times in the range of 22 - 350C are greater than in the range 35 - 500C. For example, for the type-A cement paste without admixture, the initial setting time were reduced by about 40% when comparing 35 to 220 C and 21% when comparing 50 to 350C.
The addition of the retarding admixture caused marked retardation (i.e., setting times are extended) for each of the two cements under the three curing conditions. When admixture is incorporated into cement pastes, the rate of hydration slows down. Consequently, the necessary amount of the hydration products giving rigidity to the cement paste will require longer time. Thus, cement pastes having retarding admixture remains plastic for longer time.
The results also reveal that for constant admixture content, the set-retarding tendency decreased at higher temperatures and low relative humidity. In case of the type-A cement, the highest admixture content (0.375%) caused an increase of 342% in setting times under CC-I, 169% under CC-II and 44% under CC-III. with respect to the reference setting times. At elevated temperature, the reaction between C3A and gypsum is also activated resulting into a relatively large amount of ettringite 3CaO.Al2O3.3CaSO4.31H2O) during the early stage of hydration. The lower retarding tendency of the admixture at elevated temperatures is probably due to the adsorption of the admixture on the ettringite. Consequently, lower concentration of the admixture is left to retard the C3S hydration.
In case of the type-B cement, the initial setting times were shortened by adding the admixture to the pastes while the final setting times were extended. This behavior of the admixture was observed under each of the three curing conditions. The exact cause of this abnormal behavior of the retarding admixture to accelerate the initial set is not known. However, in the author's opinion this may be due to the addition of the greater percent of pozzolana to this cement. It is reported by some researchers that in addition to the reaction between lime and pozzolana, some other reaction between C3A or its hydration products and pozzolana can occur . There may be some reaction between the pozzolana and the admixture to form some compounds giving rigidity to the paste earlier than that obtained by the hydration products of the cement. The final setting is again obtained due to the hydration products of the cement as usual. Thus, care should be exercised while using retarding admixture with pozzolanic cements. Trial tests should beperformed before use to confirm the behavior of any retarding admixture with such cements.
(1) High temperature and low humidity accelerated the setting of cement pastes for all mixes with and without the retarding admixture.
(2) The retarding admixture successfully retarded cement setting under each curing condition.
(3) The retarder showed lower retarding tendency at higher temperatures and lower humidity.
(4) The loss in setting times (with respect to the reference setting times) at 35oC was recovered by adding 0.125% of the admixture to the mix while at 500C, it was recovered by adding 0.25% of the admixture.
(5) With the type-B cement, the admixture showed accelerating effects on initial set. So, caution is needed when using retarders with pozzolanic type cements.