Set retarding alloies are H2O soluble chemicals that have small or no other consequence than to detain the scene of the cement. They do non plasticise significantly and hold small or no consequence on the H2O demand or other belongingss of the concrete. Set retarding water-reducing alloies non merely detain the scene of the cement but are besides efficient in plasticising concrete or cut downing its H2O demand. Most commercially available retarders are of this type.
These may be besides be used in concurrence with sulphonated naphthalene/melamine-formaldehyde condensates or polycarboxylates to bring forth retarding high scope H2O cut downing alloies. The retarder molecule chemically adsorbs onto the cement atom in a mechanism similar to that described for H2O reducing agents. The chief difference is the strength of the chemical bond that is formed. This strongly links the retarder molecule onto the cement surface, barricading and decelerating down the rate of initial H2O incursion into the cement [ Older,1992 ] . Retarder molecules besides chelate Ca ions in solution, decelerating the crystallisation of portlandite. These two mechanisms slow the growing of hydration merchandises, detaining the stiffening and scene of the cement but one time initial hydration starts, the retarder molecules are swamped and normal hydration returns.
As with H2O cut downing alloies, ultimate strength addition is increased with increasing H2O decrease Retardation of set allows the slower formation of a more ordered, smaller, denser cementitous matrix. This has the consequence of increasing ultimate strength relation to an unretarded mix with the same H2O cement ratio [ Dodson, 1990 ] .
Acceleration of strength by heat produces the opposite consequence, with the rapid formation of a harsh matrix. This explains why steam cured precast concrete seldom produces the same ultimate strength as concrete cured at normal temperatures and produced from the same concrete.
Retarding alloies do non hold a important consequence upon initial workability. However, they by and large have a good consequence upon workability keeping, peculiarly at elevated temperature [ Older,1992 ] .
Retarding H2O cut downing alloies, have a marked consequence upon workability. Typically, an addition in slack of 60-100mm consequences from the add-on of a dose of 0.25 % by weight cement. Set retarding high scope H2O reducing/plasticizing alloies may be used to enable workability to be increased to a greater extent, at a typical dose degree of 0.3 to 1.0 % [ Dodson, 1990 ] .
2.3 Slump loss
Retarding alloies are utile for assisting to cut down slack loss, peculiarly at elevated temperature but it is still of import to hold a high initial workability.
Retarding H2O cut downing alloies are really effectual at cut downing slump loss when used to increase the initial workability of the mix, but less so when used as a H2O reducing agent. Indeed, if H2O decrease is taken at the disbursal of high initial workability, initial slack loss may be somewhat faster and will decelerate when about half the initial slack is reached.
2.4 Setting clip
The premier map of a retarder is to widen the scene ( stiffening ) clip of concrete, normally in order to forestall the formation of cold articulations between bringings of concrete. Even if workability has fallen to about zero slack, fresh concrete can be vibrated into, and will bond with, a preceding, older pour.
In hot conditions, even a little hold in bringings or a short dislocation of the pump can ensue in the first concrete pours puting before subsequent pours can be placed and vibrated to organize a massive articulation. In deep pours, if concrete placed early starts to put, the heat generated can do faster scene of concrete above it and once more lead to cold articulations. In this state of affairs, retarder dose can be increasingly reduced as the pour returns.
2.5 Air entrainment
Retarding alloies do non usually entrain air, and some types, particularly those based on hydroxycarboxylic acid, may really cut down air content. This may do these retarded mixes to experience harsher and hold more inclination to shed blood.
Most types of retarder can be used efficaciously in combination with an air entraining agent.
The entire volume of bleed H2O originating from concrete is frequently related to its scene clip because one time puting starts, shed blooding Michigans. Therefore retarded concretes are ever more prone to shed blood. Any decrease in air tends to worsen this possible job.
The plasticizing constituent of a retarding H2O cut downing alloy may assist to countervail this consequence and some types are formulated to somewhat air entrain in order to cut down bleed.
2.7 Heat of hydration
Retarding alloies do non cut down the heat end product of concrete but do function to detain the clip of peak temperature rise by precisely the same clip interval by which it was retarded. In little subdivisions this may let somewhat more heat dissipation and so peak temperature may be a small lower [ Older,1992 ] . In thick subdivisions there will be no decrease in peak temperature and there is grounds that the peak temperature may even be increased somewhat.
2.8 Volume distortion
Creep and drying shrinking are non significantly affected by the inclusion of retarding alloies.
If the concrete is H2O reduced by the usage of a retarding H2O cut downing alloy, so drying shrinking will be reduced.
Provided that the concrete is right cured, so retarded concrete should be stronger and merely every bit lasting as tantamount field concrete. However, because of the drawn-out plastic phase, more attending demands to be paid to protecting the concrete before it sets. Retarded H2O reduced concrete will hold a lower H2O content than the tantamount field concrete, and will be correspondingly more lasting [ Dodson, 1990 ] .
MECHENISM OF RETARDING ADMIXTURES
Retarding alloy is an alloy that retards the scene of concrete. A retarding alloy causes cement set deceleration by one or more of following mechanisms:
( 1 ) Adsorption of the retarding compound on the surface of cement atoms, organizing a protective tegument which slows down hydration ;
( 2 ) Adsorption of the retarding compound on to karyons of Ca hydrated oxide, poisoning their growing, which is indispensable for continued hydration of cement after the terminal of initiation period ;
( 3 ) Formation of composites with Ca ions in solution, increasing their solubility and detering the formation of the karyon of Ca hydrated oxide.
( 4 ) Precipitation around cement atoms of indissoluble derived functions of the retarding compounds formed by reaction with the extremely alkalic aqueous solution, organizing a protective tegument.
3.1 Detailed Explanation
Harmonizing to the first mechanism, a retarding alloy is adsorbed on the surface of cement atoms. This bed of retarding alloy around the cement particles acts as a diffusion barrier. Due to this diffusion barrier, it becomes hard for the H2O molecules to make the surface of the unhydrated cement grains and therefore the hydration slows down, and the hibernating period ( period of comparatively inaction ) is lengthened. Due to the slow hydration, no considerable sum of the hydration merchandises giving rigidness to the cement paste will be formed and therefore the paste remains fictile for a longer clip. Subsequently, when the alloy is removed from solution by reaction with C3A from cement or by some other manner it is removed and incorporated into the hydrated stuff, farther hydration is eliminated [ Dodson, 1990 ] . On first contact of H2O with cement grains ( C3S and C2S ) Ca ions and hydroxyl ions are quickly 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 merchandises calcium hydrated oxide and Ca silicate hydrate start to crystallise from the solution and so hydration returns quickly.
Harmonizing to the 2nd mechanism, a retarding alloy incorporated into cement paste is adsorbed on the Ca hydrated oxide karyon and prevents its growing until some degree of ace impregnation is reached during the initiation period of hydration. Therefore, retarder lengthens the initiation period by doing an addition in the degree of Ca hydrated oxide ace impregnation before crystallisation begins. This is correspondent to the toxic condition of crystal growing of Ca hydrated oxide by the retarding alloy as both Ca and hydroxyl ions are present in the solution but unable to precipitate as a consequence of toxic condition of the Ca hydrated oxide karyon.
Harmonizing to the 3rd mechanism, a retarding alloy incorporated into cement paste forms some sort of composites with Ca ions released by the cement grains during the first few proceedingss. Formation of the composites increase the solubility of cement, i.e. , increased concentration of Ca2+ , OH, Si, Al and Fe in the aqueous stage of the cement pastes will happen when hydrated in the presence of the retarding alloy. Thus the Ca ions and hydroxyl ions will roll up in solution and will be unable to precipitate to organize Ca hydrated oxide. For illustration, when ordinary Portland cement is hydrated in sucrose solution, calcium hydroxide is solubilised and a sucrose Ca composite ( 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 composite will be able to go absorbed on the turning Ca hydrated oxide nucleus [ Gahtani,1998 ] . The surface assimilation of the composite on the Ca hydrated oxide karyon will suppress its growing as the Ca and hydroxyl ions will non be able to precipitate. In this manner, hydration is retarded.
The 4th mechanism is similar to the first but here some sort of indissoluble derived functions of retarder are formed by reaction with the extremely alkalic solution as pH of the solution rises to over 12 within few proceedingss after first contact of H2O with cement. For illustration, inorganic salt alloies ( borates, phosphates, Zn and lead salts etc. ) give indissoluble hydrated oxides in alkalic solution. The cement hydration is suppressed through the precipitation of protective coatings of these indissoluble derived functions around the cement grains [ Gahtani, 1998 ] .
Three different types of cements were used for puting clip trials. These are denoted as type-A, type-B. Type-A and type-B cements are pozzolanic type cements, which about correspond to the ASTM type IP. Type-A cement is obtained by adding 6-20 % calcined clay to the normal Portland cement cinder during fabricating while in type-B cement the calcined clay ranges from 21 to 35 % . Their compound composing can non be calculated by utilizing Bogue ‘s or other such expression.
Blending Water and Retarding Admixture:
Normal tap H2O was used as mixing H2O. The retarding alloy used was ASTM C 494 type D alloy. Its denseness was about 1.02 mg/ml and its chloride content was claimed nil. The sum of the alloy incorporated into the pastes was expressed in ml/100g of cement indicated as per centum.
Cement pastes were prepared for finding of consistence and puting times trials. The cement content and w/c ratios were unbroken changeless for all trials for a given cement type. The sums of cement and H2O used per trial are shown in Table.
Table1. Sums of Cement and Water Used per Test.
A Vicat setup was used for finding of both the criterion consistence and puting times of pastes. The setup was similar to that recommended by the ASTM C 187-77 and C 191-77 except the minor difference in the needle and ring ( cast ) dimensions. The acerate leaf of the setup 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 consistence and scene times
For standard consistence finding, the process of the ASTM C 187- 77 was followed and for puting clip finding, the Turkish Standard 19 ( TS-19 ) was followed. The TS-19 about follows the ASTM C 191-52 process with minor amendments as described below: The initial set is said to hold taken topographic point when the acerate leaf ( 1.13mm Defense Intelligence Agency. ) of theVicat setup ceases to go through 3-5 millimeter above the underside of cement paste taken in the Vicat mold. Final set is said to hold occurred when the needle penetrates the cement paste to a maximal deepness of 1mm. In both instances, the scene clip is reckoned from the minute when commixture H2O is added to the cement.
4.3 Curing Conditionss
In order to imitate the approximate normal and inauspicious out-of-doorss climatic conditions, the undermentioned three classs of bring arounding conditions were provided to the trial specimens:
( 1 ) First bring arounding status ( CC-I ) : Temperature = 220C, Relative Humidity = 55-65 %
( 2 ) Second bring arounding status ( CC-II ) : Temperature = 350C, Relative Humidity = 35-45 %
( 3 ) Third bring arounding status ( CC-I ) : Temperature = 500C, Relative Humidity = 25-35 %
For keeping the coveted hardening conditions, a temperature governable cabinet was used. The needed comparative humidness at assorted temperatures was obtained by puting concentrated salt solutions ( sodium nitrate at 220C, K carbonate at 350C and potassium chloride at 500C ) .
4.4 Test Results and Discussion
Puting clip trials with changing alloy contents were performed under the specified hardening conditions. An norm of three trial readings was taken as the concluding reading. To compare the alterations occurred in puting times by incorporation of the alloy, the scene clip of cement paste with out alloy content under CC-1 was used as mention. The puting times were recorded in proceedingss. These consequences are presented in the undermentioned tabular arraies and figures.
Table 2. Puting Timess of Type-A Cement Containing Varying Admixture Contents under Different Curing Conditions.
Table 3. Relative Retarding Consequence of Admixture on the Setting Times of Type- A cement Under Different Curing Conditions in Comparison with the Reference Setting Times.
Fig1. Consequence of the Admixture on Initial Setting Time of Type-A Cement.
Fig 2. Consequence of the Admixture on Final Setting Time of the Type-A Cement.
Table 4. Puting Timess Of Type-B Cement Containing Varying Admixture Contents Under Different Curing Conditions.
Table 5. Relative Retarding Consequence of Admixture on the Setting Times of the Type-B cement Under Different Curing Conditions in Comparison with the Reference Setting Times.
Fig3. Consequence of the Admixture on Initial Setting Time of the Type-B Cement.
Fig 4. Consequence of the Admixture on Final Setting Time of the Type-B Cement
The consequences reveal that for each of the three types of cements, high temperatures and low comparative humidness reduced both the initial and concluding scene times. This tendency is in understanding with most of the relevant published plants of other research workers. Higher bring arounding temperatures and low comparative humidness accelerate the hydration of cement ; accordingly the necessary sum of the hydration merchandises giving rigidness to the cement paste is formed with in shorter period. Thus, puting times are lowered. The temperature effects on puting times in the scope of 22 – 350C are greater than in the scope 35 – 500C. For illustration, for the type-A cement paste without alloy, the initial scene clip were reduced by about 40 % when comparing 35 to 220 C and 21 % when comparing 50 to 350C.
The add-on of the retarding alloy caused pronounced deceleration ( i.e. , puting times are extended ) for each of the two cements under the three bring arounding conditions. When alloy is incorporated into cement pastes, the rate of hydration slows down. Consequently, the necessary sum of the hydration merchandises giving rigidness to the cement paste will necessitate longer clip. Therefore, cement pastes holding retarding alloy remains fictile for longer clip.
The consequences besides reveal that for changeless alloy content, the set-retarding inclination decreased at higher temperatures and low comparative humidness. In instance of the type-A cement, the highest alloy content ( 0.375 % ) caused an addition of 342 % in puting times under CC-I, 169 % under CC-II and 44 % under CC-III. with regard to the mention puting times. At elevated temperature, the reaction between C3A and gypsum is besides activated ensuing into a comparatively big sum of ettringite 3CaO.Al2O3.3CaSO4.31H2O ) during the early phase of hydration. The lower retarding inclination of the alloy at elevated temperatures is likely due to the surface assimilation of the alloy on the ettringite. Consequently, lower concentration of the alloy is left to retard the C3S hydration.
In instance of the type-B cement, the initial scene times were shortened by adding the alloy to the pastes while the concluding scene times were extended. This behaviour of the alloy was observed under each of the three bring arounding conditions. The exact cause of this unnatural behaviour of the retarding alloy to speed up the initial set is non known. However, in the writer ‘s sentiment this may be due to the add-on of the greater per centum of pozzolana to this cement. It is reported by some research workers that in add-on to the reaction between calcium hydroxide and pozzolana, some other reaction between C3A or its hydration merchandises and pozzolana can happen. There may be some reaction between the pozzolana and the alloy to organize some compounds giving rigidness to the paste earlier than that obtained by the hydration merchandises of the cement. The concluding scene is once more obtained due to the hydration merchandises of the cement as usual. Therefore, attention should be exercised while utilizing retarding alloy with pozzolanic cements. Trial trials should beperformed before usage to corroborate the behaviour of any retarding alloy with such cements.
( 1 ) High temperature and low humidness accelerated the scene of cement pastes for all mixes with and without the retarding alloy.
( 2 ) The retarding alloy successfully retarded cement puting under each bring arounding status.
( 3 ) The retarder showed lower retarding inclination at higher temperatures and lower humidness.
( 4 ) The loss in puting times ( with regard to the mention puting times ) at 35oC was recovered by adding 0.125 % of the alloy to the mix while at 500C, it was recovered by adding 0.25 % of the alloy.
( 5 ) With the type-B cement, the alloy showed accelerating effects on initial set. So, cautiousness is needed when utilizing retarders with pozzolanic type cements.