Concrete admixtures and retarders are popular materials used to enhance the mechanical properties of concrete and mortar. Nowadays, concrete retarders are highly preferred in hot weather to allow time to properly mix, transport, and place concrete wherever necessary.
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Many construction chemical manufacturers provide concrete retarders but to get the best results it is always recommended by the experts to choose the best ones. Nevertheless, in this blog, we will explore all about concrete retarders including their mechanisms, types, benefits, and more.
To give idea about the concrete retarders, it is additives that are added to cement-based mixtures like mortar or concrete to delay the setting time of the cement paste. These are commonly known as concrete retarders, which are water-soluble additives that get easily dissolved in water.
When you add the concrete retarder in the concrete mix can result in extending the setting time of the concrete by approximately up to one hour. It basically slows down the hardening in warm or hot weather to allow construction workers more time to mix, deliver and pour the concrete. In fact, concrete retarders reduce the pace of the process while conserving water.
A concrete retarder is a type of admixture that temporarily stops the hydration process upon addition to the mixing water or on application to the surface of fresh concrete, thereby allowing additional time for the concrete to rest before it begins to harden.
The mechanism involved in such retarders depends on both the type of retarder and cement involved. It is worth noting that the retarder effects are temporary or for a short period of time, and after a certain period, its mechanism fades, and the watering goes back to normal.
Below are the four key ways retarders slow down the setting process:
A retarding additive forms a protective coating on the surfaces of the cement particles. The barrier reduces access by water molecules to the unhydrated cement, hence it retards the hydration reactions. As a result, the cement paste thus remains workable for a longer time due to the insufficient hydration produced to initiate hardening. The retarder complex with the C3A of cement and comes out of solution, incorporating into the hydrating material.
When water is added to it, the surfaces of cement particles release calcium and hydroxyl ions into the mix. When their concentration increases up to a certain critical amount, C2S and CS start crystallizing. In this process, the crystal nuclei of calcium hydroxide absorb the retarding admixture that delays further crystal growth until the point where the cement becomes supersaturated.
At the start of the process, calcium ions given out by the cement grains make some complex with other ions. It is easier to dissolve cement when these complex forms. In the liquid phase of the cement paste, during hydration with a retarding admixture, the concentrations of Ca²⁺, OH⁻, Si, Al, and Fe are increased. This buildup of calcium and hydroxyl ions interferes with the association between these ions and thus slows down the hydration process.
During precipitation, the retarder interacts with the highly alkaline solution to form insoluble derivatives. The phenomenon resembles with adsorption. In a few minutes of mixing water with cement, the pH of the solution goes beyond 12. These insoluble compounds create protective coatings around the cement particles, which slows down the hydration process. The coating forms a diffusion barrier that restricts the ability of water to reach the cement particles effectively.
This is all about the mechanism of concrete retarders and now let’s check out the types of concrete retarders.
There are broadly two categories of concrete retarders which are as follows:
Now, let’s understand how retarding additives acts on the concrete’s properties.
The above-mentioned effects can be observed when the concrete retarders are added to the concrete mixture. However, the best concrete retarder doesn’t affect the concrete properties after the setting time effect. Keep reading to know the various uses of concrete retarders.
There are various applications of concrete retarders to get the desired outcome in particular situations. The following are the uses of concrete retarder:
Now that you are aware of retarder additive uses, let’s have insights into the advantages of concrete retarders.
There are several advantages of concrete retarder which are as follows:
Each material has its own set of advantages and disadvantages when applied. Let’s explore some of the limitations of using concrete retarders.
The best concrete retards are always used in many great construction projects, however, you should also choose a branded one to get the best results in the construction process. Let’s have a look at one of the top-quality dry mix raw materials that can be used with the concrete admixtures.
Sodium Gluconate which is also known as Sodium Salt of Gluconic Acid which is a dry mix raw material, can be used as a setting time retarder. Our Sodium Gluconate is formulated under strictly controlled conditions through the fermentation of glucose. To give an idea about it, it is a white crystalline powder and easily soluble in water.
Our Sodium Gluconate is non-toxic, non-flammable, and non-corrosive. In short, Sodium Gluconate is an environment-friendly and biodegradable construction chemical. When it is added to cement, it inhibits hydration and slows down the setting time of the concrete mix.
To wrap up, a concrete retarder’s role is impressive in the construction industry as it delays the hydration process. It keeps the concrete workable and easier to handle for an extended period. Concrete retarders are specifically useful in hot and warm temperatures to allow time for workers to deliver, transport, and pour the concrete. However, it is very essential to buy this construction chemical from authorized and genuine manufacturers like Sakshi Chem Sciences Pvt Ltd.
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Introduction
Many important characteristics of concrete are influenced by the ratio (by weight) of water to cementitious materials (w/cm) used in the mixture. By reducing the amount of water, the cement paste will have higher density, which results in higher paste quality. An increase in paste quality will yield higher compressive and flexural strength, lower permeability, increase resistance to weathering, improve the bond of concrete and reinforcement, reduce the volume change from drying and wetting, and reduce shrinkage cracking tendencies (PCA, ).
Reducing the water content in a concrete mixture should be done in such a way so that complete cement hydration process may take place and sufficient workability of concrete is maintained for placement and consolidation during construction. The w/cm needed for cement to complete its hydration process ranges from 0.22 to 0.25. The existence of additional water in the mixture is needed for ease of concrete placing and finishing (workability of concrete). Reducing the water content in a mixture may result in a stiffer mixture, which reduces the workability and increases potential placement problems.
Water reducers, retarders, and superplasticizers are admixtures
for concrete, which are added in order to reduce the water content
in a mixture or to slow the setting rate of the concrete while retaining
the flowing properties of a concrete mixture. Admixtures are used to modify
the properties of concrete or mortar to make them more suitable to work
by hand or for other purposes such as saving mechanical energy.
If you want to learn more, please visit our website cement retarder chemicals.
Water reducing admixtures (WRA)
The use of WRA is defined as Type A in ASTM C 494. WRA affects mainly the fresh properties of concrete by reducing the amount of water used by 5% to 12% while maintaining a certain level of consistency, measured by the slump as prescribed in ASTM C 143-90. The use of WRA may accelerate or retard the initial setting time of concrete. The WRA that retards the initial setting time more than three hours later is classified as WRA with retarding effect (Type D). Commonly used WRA is lignosulfonates and hydrocarboxylic (HC) acids. The use of HC acids as WRA requires higher water content compared to the lignosulfonates. Rapid bleeding is a problem for concrete treated with HC acids.
Increase of slump is different according to its type and dosage. Typical
dosage rate is based upon the cementitious material content (milliliters
per hundred of kilograms). The figure below illustrates the influence of
dosage of Lignosulfonates and HC acid on slump. It is shown in the figure
that HC acids give a higher slump compared to lignosulfonates with the
same dosage.
WRA has been used primarily in hot weather concrete placing, pumping,
and tremie. Careful concrete placement is required, as the initial setting
time of concrete will take place an hour earlier. It is also shown that
the use of WRA will give a higher initial concrete compressive strength
(up to 28 days) by 10% compared to the control mixture. Other benefit of
using WRA is that higher concrete density is achieved which makes the concrete
less permeable and have a higher durability.
Retarding admixtures
The use of this admixture is defined in ASTM C494. There are two kinds of retarders, defined as Type B (Retarding Admixtures) and Type D (Water Reducing and Retarding Admixtures). The main difference between these two is the water-reducing characteristic in Type D that gives higher compressive strengths by lowering w/cm ratio.
Retarding admixtures are used to slow the rate of setting of concrete. By slowing the initial setting time, the concrete mixture can stay in its fresh mix state longer before it gets to its hardened form. Use of retarders is beneficial for:
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 (Neville). The table below describes the effect of air temperature on retardation of setting time:
Table 1 Air Temperature and Retardation of Initial Setting Time Admixture Type Description Retardation of initial setting time (h:min) at temperature of 30oC 40oC 50oC D Hydroxylic acid 4:57 1:15 1:10 D Lignin 2:20 0:42 0:53 D Lignosulfonates 3:37 1:07 1:25 B Phosphate-based --- 3:20 2:30The use of retarding admixture has the main drawback of the possibility of rapid stiffening, where rapid slump loss will result in difficulty of concrete placement, consolidation, and finishing. An extended-set admixture has been developed as another retarding admixture. The advantages of this admixture compared to the conventional one is the capability to react with major cement constituents and to control hydration and setting characteristics of concrete while the conventional one will only react with C3A.
Careful usage of retarder is required to avoid excessive retardation,
rapid slump loss and excessive plastic shrinkage. Plastic shrinkage is
the change in fresh concrete volume as surface water evaporates. The amount
of water evaporation is influenced by temperature, ambient relative humidity,
and wind velocity. Proper concrete curing and adequate water supply for
surface evaporation will prevent plastic shrinkage cracking. The amount
of water needed to prevent plastic shrinkage cracking is given by the chart
below:
The extended-set admixture is widely used as a stabilizing agent for
wash water concrete and fresh concrete. Addition of extended-set admixture
enables the reuse of wash water to the next batch without affecting concrete
properties. This admixture can also be used for long haul concrete delivery
and to maintain slump. Factors affecting the use of this admixture include
the dosage rate and the ambient temperature of the concrete.
Superplasticizers (High Range Water reducer)
ASTM C494 Type F and Type G, High Range Water Reducer (HRWR) and retarding admixtures are used to reduce the amount of water by 12% to 30% while maintaining a certain level of consistency and workability (typically from 75 mm to 200 mm) and to increase workability for reduction in w/cm ratio. The use of superplasticizers may produce high strength concrete (compressive strength up to 22,000 psi). Superplasticizers can also be utilized in producing flowing concrete used in a heavy reinforced structure with inaccessible areas. Requirement for producing flowing concrete is defined in ASTM C . The effect of superplasticizers in concrete flow is illustrated in the chart below:
Figure 3 Relation between Flow Table and Water Content of Concrete with and without Plasticizers (Neville, ).Another benefit of superplasticizers is concrete early strength enhancement (50 to 75%). The initial setting time may be accelerated up to an hour earlier or retarded to be an hour later according to its chemical reaction. Retardation is sometimes associated with range of cement particle between 4 – 30 m m. The use of superplasticizers does not significantly affect surface tension of water and does not entrain a significant amount of air. The main disadvantage of superplasticizer usage is loss of workability as a result of rapid slump loss and incompatibility of cement and superplasticizers.
Superplasticizers are soluble macromolecules, which are hundreds of times larger than water molecule (Gani, ). Mechanism of the superplasticizers is known as adsorption by C3A, which breaks the agglomeration by repulsion of same charges and releases entrapped water. The adsorption mechanism of superplasticizers is partially different from the WRA. The difference relates to compatibility between Portland Cement and superplasticizers. It is necessary to ensure that the superplasticizers do not become fixed with C3A in cement particle, which will cause reduction in concrete workability.
Typical dosage of superplasticizers used for increasing the workability of concrete ranges from 1 to 3 liters per cubic meter of concrete where liquid superplasticizers contained about 40 % of active material. In reducing the water cement ratio, higher dosage is used, that is from 5 to 20 liters per cubic meter of concrete. Dosage needed for a concrete mixture is unique and determined by the Marsh Cone Test.
There are four types of superplasticizers: sulfonated melamine, sulfonated naphthalene, modified lignosulfonates and a combination of high dosages of water reducing and accelerating admixtures. Commonly used are melamine based and naphthalene based superplasticizers. The use of naphthalene based has the advantage of retardation and affecst slump retention. This is due to the modified hydration process by the sulfonates
Admixtures Dispensers
The basic function of a dispenser as defined in ACI Bulletin E4-95 is:
References:
Chemical Admixtures for Concrete, ACI Committee 212.3R-91 Report.
Chemical and Air Entraining Admixtures for Concrete, ACI Education Bulletin No. E4-95.
Dodson, Vance, Concrete Admixtures, VNR, .
Gani, M.J., Cement and Concrete, Chapman & Hall, .
Komatska, S. H. and Panarese, W. C., Design and Control of Concrete Mixtures, PCA, .
Ramachandran, V. S., Concrete Admixtures Handbook, Properties, Sciences, and Technology, 2nd edition, .
Aitcin, P., Jolicoeur, C., and MacGregor, J., Superplasticizers: How They Work and Why They Occasionally Don’t, Concrete international, May .
Information compiled by Titin Handojo.
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