1. Admixture for concrete
SET RETARDERS
Admixture for concrete
5/7/2019
İlayda Bengisu Küçük

2. Contents: 5/2/2019 Setting on concrete Mechanisms of retardation
Effects of usage of retarding admixtures upon the properties of concrete
Types of retarders
references
5/2/2019

3. Setting on concrete:
The setting of concrete means the loss of the elastic property of concrete and the state that not able to work on it. The transportation, compaction, and other processes applying to fresh concrete are made before the setting. Setting time means the duration between mixing the materials (cement and water) and the moment the cement paste (or concrete mixed with aggregates) loses its elastic property. Also, there are some definitions like the initial and final setting time.
But in some cases such as high-temperature climate, high-speed wind and/or the low moist in the air may cause some negative effects on concrete listed below:
1- Increasing the water to obtain the consistency of concrete.
2- Setting of concrete before the expected time.
3- Increase at a plastic cracking on the concrete surface.
etc. [1]

4. Setting on concrete: (cont’d)
The duration of the setting is affected by these factors:
1- Due to the hot weathers the duration of hydration gets shorter and the setting time gets shorter.
2- The cement that been keeping for a long time cause taking moisture from the environment and it retards the setting.
3- Using water more than it needs makes the setting time longer. [4]
Retarding admixtures are those compounds which are used to slow down the chemical process of hydration due to which the concrete remains plastic and workable for a longer time than normal concrete without changing its mechanical properties. It is also used to overcome the accelerating effect of hot weather on setting time of concrete we mentioned before. [2]
The setting of concrete is a transformation process that starts with a suspension of particles and ends when these particles are connected in a hardened state. The most important action during the setting of concrete is the hydration of cement. [3]

5. Mechanisms of retardation:
Cement, normally, contains 4 types of minerals: alite (C3S), belite (C2S), aluminate (C3A) and ferrite (C4AF). Clinker sulfate and gypsum is the main material of cement so they also contain them. When water is added to the cement, the chemical reactions occurs and it’s an exothermic reaction that released an amount of heat. A graph of this heat (H) in function of the time (fig. 1) gives and shows of the reaction speed. Three principal reactions can be remarkable on this graph. After adding water, the clinker sulphates and gypsum produce an alkaline and sulphate rich solution. After mixing, the C3A reacts with the water and forms an aluminate-rich gel which reacts with the sulfate-rich solution and forms ettringite (I).

6. Mechanisms of retardation: (cont’d)
C3A+CS̅ H2+10H → C4AS̅ H12
C4AS̅ H12+3CS̅ H2+26H → C6AS̅ 3H32 (Ettringite)
Also some ettringite may occur shown in reaction below.
C3A+3CS̅ H2+26H → C6AS̅ 3H32 (Ettringite) [1]
This reaction produces a lot of heat but takes only a few minutes. It’s represented by the first peak in the graph and is followed by a dormant period that takes much longer than the first period (a few hours) also a period which almost no reactions occurs (II). Up to the lowest point of the graph (first lowest), the paste is workable. The main period of hydration (III) starts after the dormant period. The alite and belite start to react and calcium silicate hydrate (CSH) and calcium hydroxide (CH) is formed. This is the moment where the concrete’s strength starts to increase and workability drops. [3]

7. Mechanisms of retardation: (cont’d)

8. Mechanisms of retardation: (cont’d)
So at the end of the reaction we have the following hydration products: calcium silicate hydrate (CSH), calcium hydroxide (CH) and two groups of minerals. These are the main informations to understand the mechanisms of retardation.
Adding a retarder, dissolved in the mixing water or sprayed on the surface of the concrete, temporarily interrupts the hydration reactions, especially at the lowest point of the graph, after reaction I, which creates a longer dormant period. There are four different mechanisms of actions between retarders and cement to interrupt those reactions. The mechanisms that appear depend on the combination of the type of retarder and the type of cement. Most retarders normally act by several actions. It’s also important to realize that the mechanisms of retardation are temporary. After a predictable period, the effects of the mechanisms disappear and the hydration continues. Below, all four mechanisms are described.

9. Mechanisms of retardation: (cont’d)
1-Adsorption: On the surface of the cement particles, a retarding admixture is adsorbed. This layer of retarding admixture creates a protective skin (diffusion barrier) around the cement particles. Due to this diffusion barrier the water molecules are hindered to reach the surface of the unhydrated cement particles and the hydration is slowed down. The result is that there is no considerable amount of hydration products to give rigidity to the cement paste so the paste remains plastic for a longer period. The retarding admixture is removed from the solution, among other by reacting with the C3A from the cement, and is include into the hydrated material.
2-Nucleation: When water is added to the cement, calcium ions and hydroxyl ions are released from the surface of the cement particles. When a critical value of the concentration of those ions is reached, the hydration products C2S and CS start to crystallize. A retarding admixture, which is incorporated into the cement, is adsorbed by the calcium hydroxide nuclei, which prevents the growth of the calcium hydroxide nuclei until some level of super saturation. So the induction period has been extended because of the increase of the level of calcium hydroxide super saturation before crystallization starts.

10. Mechanisms of retardation: (cont’d)
3-Complexation: During the first few minutes, some kind of complexes with calcium ions, released by the cement grains, are formed. The formation of those complexes causes an increased solubility of the cement. During the hydration, in the presence of a retarding admixture, an increased concentration of ca2+ , OH- , si, al and fe will occur in the aqueous phase of the cement paste. The accumulation of the calcium and hydroxyl ions in the solution prevent the precipitation of those ions to form calcium hydroxide. In that way, hydration is retarded.
4-Precipitation: Precipitation is nearly similar to adsorption but in the case of precipitation some kind of insoluble derivatives of retarder are formed by a reaction with the highly alkaline solution. Because of that, the ph of the solution rises over 12 after the first few minutes of the contact between water and cement. The precipitation of protective coatings of these insoluble derivatives around the cement particles suppresses the cement hydration. The protective coating acts as a diffusion barrier so the water molecules can’t make a good contact with the cement particles.

11. Effects of usage of retarding admixtures upon the properties of concrete:
1. It improves workability of concrete and helps in better placement and handling. 2. It delays the setting time of concrete in order to give time for it to be placed. 3. Due to the retarding action, the 1-day strength is reduced but the ultimate strength of concrete is improved. 4. The appearance of the concrete is greatly improved and finished concrete has better texturing. 5. The rates of drying shrinkage and creep increase due to the addition of retarding admixtures which can cause cracks and long-term failure. 6. The addition of retarding admixtures also causes some air to be entrained in the concrete. 7. Reduces bleeding and segregation where poor sand grading are unavoidable.

12. There are two categories of retarders and some of them shown in graph
There are two categories of retarders and some of them shown in graph. But The most commonly used retarders are calcium sulfate (gypsum), common sugar, lignosulfonic acids. [2,3]
Types of retarders:
Organic retarders
Inorganic/chemical retarders
Lignosulphonaties
Borates
Phosphonates
Sugars (saccharides)
Salts of Pb, Zn, Cu, As, Sb
Hydroxycarboxylic acids and their salts

13. Thank you

14. References:
[1]Erdoğan Turhan, beton, odtü yayıncılık, 2007, sf: [2] Retarders.pdf (International journal of scientific & engineering research, volume 5, issue 4, april- 2014) [3]file:///C:/Users/ASUS/Downloads/Concrete_setting_retarder_mechanism.p df (Pieter BALLIEU, Björn NOYELLE, Jef PAUWELS, Thesis Study from Student Master of Science in Civil Engineering, UGent, Gent, Belgium ) [4]