1. CHEMICAL ADMIXTURES FOR CONCRETE
CIVL586
CHEMICAL ADMIXTURES FOR
CONCRETE
Prof. Dr. Özgür EREN
Department of Civil Engineering
Eastern Mediterranean University-Gazimağusa-N.Cyprus
2017

2. CHAPTER 1 AN INTRODCUTION TO, AND A CLASSIFICATION OF, CEMENT ADMIXTURES

3. CHAPTER 1
Introduction
Admixtures are materials that are added to concrete to give new properties (fluid or plastic stage or set or cured condition).
They are different than additives for cement (during production) to control grinding or setting.
Example: Gypsum

4. Classification of admixtures
What are the admixtures?
What is available?
How do they work?
What are the benefits and shortcomings?

5. Classification of admixtures
There are 9 main categories and 5 sub-groups.
See Table 1.1
Regional variation can change the importance of these admixtures (cold, hot climates, etc).

6. Accelerating admixtures (category C1 admixture data sheet)
Shorten the setting times of cement and/or increase the rate of strength build-up.
Early removal of formworks.
Reduction in contractors’ time schedule.
Divided into 2:
1. chloride containing
2. chloride free

7. Calcium chloride: causes reinforcement corrosion.
It is not used in some countries for RC members.
Used especially for unreinforced concrete.
Use calcium formate for reinforced concrete.
They (acclerators) increase rate of hydration of cement.

8. Retarders (catergory C2-admixture data)
Slow down the rate of setting of cement.
Retarding/water reducing admixtures assist in hot weathering concreting.
Eliminated cold joints for large number of pours (vibration time is extended).

9. Materials comprising retarders are in the main:
Unrefined lingnosulphonates containing sugars.
Modifications and derivatives of the first group.
Hydrocarboxylic acids and their salts (sodium tartrate)
Modifications to the above class.
Carbohydrates including sugar
Heptonates which are related to the sugars and starches.

10. Properties Type 1, 2, 3, 4, 6: water reducing
Type 5: not water reducing
Retarding admixtures are adsorbent on C3A phase in cement forming a film around cement grains and preventing or reducing the reaction with water.
It is interesting that some accelerators can act as retarders depending on concentartion (low concentrtions).
See Figure 1.1 (comparison of chemicals wrt setting).
See Figure 1.2 (retention of workability using a retarder)

14. Water-reducing or plasticising admixtures (category C3-data sheet admixture chart)
Addition of a plasticiser allows:
1. Greater workability to be achieved for a given w/c ratio,
2. or, retains the workability (or consistency) whlist reducing the water content.
Denser and stronger concrete (save cement with maintaining strength)
What about durability (permeablity and porosity)?
See Figure 1.3

16. How they work?
Water reducing agents are characterised by having detergent-like properties.
These are surface active agents.
They carry an unbalanced charge of electricity and if put into water tend to migrate to the surface with the electrically-charged or active end sticking into water, whlist the tail is out in the air.
See Figure 1.4

18. If we put a surface active agent into a suspension of cement particles in water 2 things happen:
The surface-active agent’s tail is adsorbed on to the surface of the cement with the negative charge protruding into the water.
As a result the cement particles do not collect together and more surface area is available for reaction with water.
Also, water that may have been trapped inside a cement particle flog is released. (see Figure 1.5)

20. 2. Entrapped air is more readily removed since orientation of the surface-active agent prevents the air bubble from attaching to cement particles.
(see Figure 1.6).

21. Why they are used?
If concrete is being made to a given workability (slump), then the addition of water-reducing agent gives rise to the following possibilities:
Concrete having greater workability may be made without the need for more water and so strength losses are not encountered.
By maintaining workability, but at a lower water content, concrete strengths may be increased without the need for further cement addition.
Whlist maintaining the w/c ratio and workability concrete can be made to a given strength specification at a lower cement contents than would otherwise be required.
See Table 1.2 (results for concretes for each of the possibilities)

23. Most water reducing agents entrain air due to their surfactant properties.
This can be offset by including in this type admixture an air-detraining agent (tributyl-phosphate) which is reflected in the amount of air entrained and the water reduction obtained (see Table 1.3).

25. Types of materials- 1. lignosulphonate salts
Naturally occuring materials.
Obtained by pulping wood (using lime and sulphorous acid mixture).
Described as calcium lignosulphonates.
If sugars are present retardation may accur; alternatively, a refined, sugar-reduced product may be used which avoids excessive retardation.

26. Types of materials- 2. polyhydroxy compounds (carboxylic acid based)
They are similar in function to the lognosulphonates and may be sensitive to over-dosing leading to excessive retardation.

27. Types of materials- 3. Superplasticisers
They are based on synthesised condensates.
They are group of melamine-formaldehyde and naphtalene-formaldehyde.
Produces slumps in excess of 200mm with no increase in water content.
Water reduction: as high as 30% may be achieved.
These chemicals should also offset bleed and segregation in order to be totally effective.

28. Acclerating/water reducing admixtures (category C4-admixture data sheet)
Two categories available.
Those containing calcium chloride:
Those not containing calcium chloride.
mixture of calcium chloride blended with a plasticiser water-reducing agent.
Increases the rate of hydration.
Rapid increase in build-up of strengths.
Useful at low ambient temperatures.
(see Table 1.4 & 1.5)

30. RESULT: saving in time, labour and materials.
This is achieved by the calcium chloride reacting mainly with C3A/C3S phases (responsible from setting and strength development) in cement.
Use of these admixtures aids cold weather working and lessens frost damage risk.
Allows release and re-use of formwork and moulds.
Allows earlier handling of precast concrete.
Allows earlier finishing of concrete floors and screeds.
RESULT: saving in time, labour and materials.
Application: flooring and screeding (time reduces 50% between initial and final trowelling)

31. Retarding/water reducing admixtures (category C5-admixture data sheet)
Mixture of
[conventional water-reducing agents] +
[sugars (or hydrocarboxylic acids or their salts)]
Setting time & rate of strength build-up are affected.
(see Table 1.6)

33. Main use:
Reduces the rate of heat generation & reduction in thermal cracking).
Ease the making of placing of large pours.
Doubt: strength/time figures contradict the simple explanation. So, more work is needed on this topic.
Retarding admixtures: very effective in maintaining workability for long periods and help in offsetting the effect of high ambient temperatures (see Table 1.7).

35. Air-entraining admixtures (category C6-admixture data sheet)
Concrete containing small air bubbles (0.05 mm/1.25 mm) spaced at gaps less than or equal to 0.4 mm (bubble size is one thousand times that of cement paste capillary pores) evenly distributed throughout its bulk is demonstrably more durable to freeze/thaw action than normal concrete (see Figure 1.7)

37. Improves workability of concrete at plastic stage.
Freeze/thaw generates up to 8 MPa pressure.
Use of admixture gives better resistance against desructive action of de-icing salts.
Improves workability of concrete at plastic stage.
Handling and placing done with less segregation & bleeding.

38. You never get something for nothing!
Air entrainment reduces strength.
10-50% loss in compressive strength for
10-15% air entrainment by volume of mortar.
So, be carefull on mix design.

39. Materials comprising this group are:
Natural wood resins & their soaps.
Certain fats & oils (olive oil, stearic acid).
Lignosulphonates.

40. Waterproofers (category C10 & M3-admixture data sheet)
It is difficult to obtain waterproof concrete.
In practice, concrete is often porous, and absorbs and transmits moisture.
Water – repelling admixtures & permeability reducers?? They are different!

41. Repellents:
Line the pores and make them hydrophobic (increases the contact angle between water and concrete and prevents passage of water at low pressures but not the passage of vapour).
Waterproofers (permeability reducers)
Blocks the pores and prevent water flowing at moderate/medium pressure gradients.

42. Pumping aids (category C9-admixture data sheet)
These admixtures impart cohesiveness to the mix so that when a slug of concrete is subjected to a pressure gradient there is little or no separation of the liquid and solid phases. So concrete remains intact.
Admixture has a secondary effect of lubricating the walls of the lines so that slug flow is induced rather as toothpaste is extruded from a tube.