What are concrete admixtures?
In the world of concrete, some materials are used as an admixture to change concrete properties. If the user wants to improve the concrete properties as intended, he should employ concrete admixtures. Each one of the admixture materials is used to alter a specific characteristic of concrete. Therefore, to identify the types of concrete admixtures, it is better to read this article to the end.
What are concrete admixtures?
Concrete admixtures are special materials that are added during mixing to increase the properties of fresh concrete. A variety of admixtures are added to the concrete mixture to improve the behavior of concrete in different weather conditions.
By changing the properties of hardened concrete, admixtures minimize the cost of construction and enhance the concrete quality during mixing, transportation, curing, and overcoming certain emergency cases in concreting operations.
The purpose of using concrete admixtures
- To increase or decrease the setting time of the fresh concrete mixture
- To improve or increase the efficiency and fluidity of the concrete mixture
- To maximize the strength and durability of concrete
- To reduce the hydration heat
- To decrease concrete segregation and water bleeding during concreting
- To reduce the permeability of concrete
- To achieve other desirable properties
Advantages of concrete admixtures
- Admixture materials in concrete can accelerate its setting time.
- Some admixtures contain enzymes that act as an antibacterial agent.
- Admixture materials added to concrete can reduce the initial strength but will increase the final strength of concrete.
- Admixture materials help reduce the hydration heat and decrease the possibility of thermal cracking in concrete.
- Improve the concrete strength due to the melting of ice on concrete.
- Improve the workability of concrete (using, concreting, and curing).
- Admixture materials create initial resistance in concrete.
Types of concrete admixtures
Water-reducing admixtures are added to concrete mixtures, mortars, or grouts to increase workability or fluidity without increasing the water content. This workability is a major property of concrete that is modified by adding water; however, the strength and durability of concrete will be affected if more water is added than the required amount.
These materials also increase the bond between concrete and steel in addition to improving the efficiency and strength of concrete. It also prevents cracking, concrete segregation, water bleeding, and other problems.
Normal plasticizers reduce the water demand value by 10%, mid-range plasticizers by 15%, and super-plasticizers by 30%. From calcium, sodium, and ammonium lignosulfonates are usually used as water-reducing additives.
2.Set retarding admixture
The set retarding admixtures are the kinds of concrete admixtures that reduce the initial rate of cement hydration and increase the setting time of cement paste. This type of concrete admixture can be used in cases where the area is prone to temperature variations and concrete must be transported over a long distance.
Fast setting in some conditions may lead to a discontinuity in the structure; poor bonding between surfaces causes undesirable voids in the concrete. The set retarding admixtures are suitable to eliminate these types of problems.
Retarding admixtures are usually calcium or gypsum sulfates. Some other admixtures used as retarders are starch, cellulose products, regular sugar, acid salts, and other materials.
These admixtures are used to decrease the initial setting time of the fresh concrete mixture. The initial hardening process of concrete accelerates using these materials; hence those are also called accelerators. These accelerators also enhance the concrete resistance in the initial stages by increasing the hydration value.
Fast hardening of the concrete is useful in many cases such as early form removal, lower period of curing, and emergency repair works for construction in low temperature regions.
The only accelerators used in these concrete admixtures are silica fume, calcium chloride, finely divided silica gel and other materials. Calcium chloride is cost-effective and fast-setting admixtures are commonly employed.
The main function of concrete foaming admixtures is to increase concrete durability under freezing, dehydrating, and melting conditions. Adding these materials to the concrete mixture leads to create millions of air bubbles throughout the mixture and improves the concrete properties.
Foaming admixtures improve the mixture’s workability, prevent segregation and concrete bleeding, and reduce in the unit weight and tensile strength of the concrete. These admixtures improve the chemical resistance of concrete against temperature and other atmospheric effects and will decrease the cement or sand or water contents in the concrete.
The commonly used foaming admixtures include Vinsol resin, Darx, Tipol, Chicol, and other materials. These additives are actually made of natural wood resins, alkaline salts, animal and vegetable fats, oils, etc.
Pozzolans can be included in the list of concrete admixtures. These admixtures are employed to prepare dense concrete mixtures that are suitable for water-retaining structures such as dams, and reservoirs. In addition, these materials will also reduce the hydration heat and thermal shrinkage.
The best pozzolanic materials under minimum content provide the best result and prevent many hazards such as alkali reactions, leaching, and sulfate attack or reduce their influence. The naturally available pozzolanic materials include clay, shale, volcanic tuffs, and artificial available pozzolanic materials include fly ash, silica fume, blast furnace slag, rice husk ash, etc.
6.Damp proofing admixtures
Damp-proofing or water-proofing admixtures are used for creating an impermeable structure of concrete to water and prevent moisture creation on the concrete surface. These admixtures in addition to their waterproof properties, also act as accelerators in the early stages of concrete hardening.
Damp-proofing admixtures are available in the market in the forms of liquid, powder, paste, and other forms. The main ingredients of these admixtures contain aluminum sulfate, zinc sulfate, aluminum chloride, calcium chloride, etc.
7.Concrete gasifier admixtures
When concrete gasifier admixtures are used, the hydroxide resulting from cement hydration reacts and forms small bubbles of hydrogen gas in the concrete.
The range of bubble formation in concrete depends on many factors such as the admixture content, chemical composition of cement, temperature, etc. The bubbles aid the concrete to neutralize the problems of sedimentation and concrete bleeding. Concrete gasifier admixtures are also used to produce lightweight concrete.
To resist concrete bleeding, a small amount of gasifier concrete mixture, which is generally 0.5 to 2% of cement weight, is employed for the mixture. Aluminum powder, activated carbon, hydrogen peroxide, and other materials are commonly used in gas-forming chemical additives.
Air-detraining admixtures are types of concrete additives. These materials are utilized to eliminate excess air in concrete voids. Some of the mixtures used in air bubbles are tributyl phosphate, silicones, water-soluble alcohols, etc.
9.Anti-alkaline expansion admixtures
Anti-alkaline expansion admixtures occur with the reaction of alkaline cement with silica in the aggregates. This admixture creates a gel substance and causes volumetric expansion of concrete, which may lead to decomposition of alkaline materials.
Using pozzolanic additives prevents the reaction of alkaline materials and in some cases used with anti-alkaline expansion admixtures. These additives are used to minimize the risk of alkaline reactions, aluminum, and lithium powder salts.
Anti-wash admixtures in concrete are specially used for underwater concrete structures. This material also protects the concrete mixture from washing under water pressure and improves concrete cohesion. This type of concrete admixture is prepared from natural or synthetic rubber and cellulose thickeners.
According to the grout requirements, grout admixtures are added to the mixtures to enhance the grouting properties. Sometimes there is a need for fast-setting grout and sometimes a need for slow-setting grout to expand into deep cracks or fissures.
In these admixtures, accelerators such as calcium chloride, triethanolamine, and other materials are used during fast-setting grout placement. In the same way, retarders such as mucic acid and gypsum are utilized to decline the speed of grout setting.
Therefore, different admixtures are employed as grout additives based on their condition and structure. Gas producing additives such as aluminum powder are mixed with grout materials to be used in concrete members.
12.Corrosion inhibitor admixtures
The steel corrosion in reinforced concrete structures occurs generally, and when these structures are subjected to salt water, industrial fumes, chlorides and others, using the corrosion inhibitor admixtures intensifies in order to prevent or to diminish the corrosion process.
In general, corrosion inhibitors are among the types of concrete admixtures. Sodium benzoate, sodium nitrate, sodium nitrite can be mentioned as corrosion inhibitor admixtures used in reinforced concrete.
13.Concrete adhesive admixtures
Concrete adhesive admixtures are used to create a strong bond between old and new concrete surfaces. In general, there is a failure possibility for the fresh concrete surface due to weak bonds with the old surface, if the fresh concrete is poured over a hardened concrete surface.
To create a strong bond, concrete adhesive admixtures are added to the cement slurry or mortar, which will be applied before pouring the fresh concrete. This kind of admixtures is used for pavement coatings, repair works, etc. Adhesive admixtures are water emulsions and are made from natural rubber, synthetic rubber, polymers such as vinyl chloride, polyvinyl acetate and others.
14.Antifungal and bacterial admixtures
To prevent from growing bacteria, germs and fungus on hardened concrete structures, it is recommended to use admixtures with fungicidal, germicidal and insecticidal properties. These characteristics can be created by adding additives such as polyhalogenated phenols, copper compounds and dieldrin emulsions, etc.
Color admixtures are pigments that create color for the ready-mix concrete. Additives used to produce color should not affect the strength of concrete. General colored additives are added to cement, thereafter colored cement can be obtained, which can be utilized to make colored concrete.
The ultimate conclusion:
Admixtures are the materials that are added during the mixing of concrete in order to increase the properties of fresh concrete. These admixtures are other than aggregate, water and cement, and improve the concrete’s resistance against coldness, sulfates, etc. Furthermore, all kinds of concrete admixtures are effective in hardening, efficiency, resistance, and many properties of concrete.