The dynamic development of the construction industry stimulates the emergence of increasingly modern additives and admixtures for concrete on the market. They represent a wide range of products with various chemical structure and properties. These substances are added in small amounts to concrete mixtures and interact with its components, ensuring the desired effect. As a result, the modified concrete has improved features that translate into greater structural and design capabilities.
Chemistry for concrete – differences between additives and admixtures used in construction
According to the definition included in PN-EN 934-2 “Admixtures for concrete, mortar and grout. Part 2. Concrete admixtures. Definitions, requirements, conformity, marking and labelling”, an admixture for concrete is a material added during the mixing process of concrete in a quantity not more than 5 % by mass of the cement content of the concrete, to modify the properties of the mix in the fresh and / or hardened state. While a concrete additive is a substance whose content exceeds 5% of the cement mass. This applies to ingredients that are not the basic raw materials for the production of concrete, i.e. sand or aggregate.
So much to choose from – what are the types of admixtures?
The admixtures for concrete may be both organic and inorganic compounds. The ones that modify more than one feature are called multi-functional admixtures. When using more than one type of admixtures, it is important to remember about their mutual influence. Before a concrete mix is prepared, it is necessary to check the compatibility of additives and the effectiveness of their operation with cement.
Admixtures are classed according to their functions. There are several distinct classes of chemical admixtures: plasticizing, liquefying, air-entraining, viscosity-enhancing, accelerating the bonding and hardening reactions, retarding (delaying the rate of setting), and sealing
It is a group of plasticizers that, without changing the consistency of concrete, reduce the amount of water by about 2-5%. With the use of these substances, the concrete mix becomes plastic and its compressive strength increases. From a chemical point of view, these are usually phosphates or polycyclic derivatives. This type of admixtures is used, among others for ready-mixed concrete, road and floor concrete.
High-range water-reducing admixtures
This group includes superplasticizers that reduce the amount of mixing water by 12-40%. The best results can be obtained by using polymer concrete admixtures. As a result of their action, the concrete mix is liquefied, the porosity of the concrete decreases, and its durability and strength increases. Superplasticizers are used, among others, in the production of sprayed concrete, self-compacting concrete and concrete elements.
Air-entraining admixtures for concrete
Admixtures of this type enable mixing of air bubbles into the concrete mass and spreading them evenly throughout its volume. With this modification, the concrete becomes frost-resistant and less absorbent, but its compressive strength decreases. The addition of 1% air to the concrete mass means a decrease in strength by about 5Mpa. The best results can be obtained by using organic acid salts as admixtures. Air-entraining admixtures are used, among others for bridge, surface and hydrotechnical concrete.
Anti-washout admixtures for concrete
Admixtures from this group prevent loss of water from the concrete mix, in other words – improve its retention. As a result, they increase the cohesion of concrete and improve the durability of its top layer. They are particularly important in concrete constructions placed under the water surface, in the production of paving stones or concrete flooring.
Admixtures accelerating the bonding and setting of concrete
This group consists of substances that shorten the time of altering the state of the concrete mix from plastic to hardened state. For this purpose, inorganic compounds such as nitrates, chlorides and metal fluorides as well as low molecular weight polyethylene glycols are usually used. This type of admixtures is used in the production of waterproof, fast-setting concrete and sprayed concrete. These strength additives increase the hydration heat, but may worsen the workability of the concrete mix.
Retarding admixtures for concrete
These are usually metal oxides, sugars or phosphates that physically hinder or slow down the cement binding process, so that the concrete remains in fluid form for longer. The retarding admixtures are used when there is a need to combine layers of concrete together. The slower concrete setting rate caused by the additive can last up to several hours. Admixtures are used for concreting at high temperatures, in the production of architectural concrete and mixtures transported over long distances.
Damp-proofing admixtures for concrete
These are additives used for watertight concrete, and their task is to reduce the capillary absorption of the concrete mix by blocking the capillary pores. Waterproofing admixtures make it possible to obtain concrete surfaces that are resistant to water and to aggressive chemical agents. Some of them also work well as anti-frost admixtures for concrete. They reduce the moisture absorption of concrete and increase its durability. For this reason, they are used for the production of concrete exposed to unfavourable conditions.
What factors to consider when choosing chemistry for concrete?
The main factors determining the choice of chemistry for concrete are the economic aspect, i.e. the cost of modifications with the use of an admixture, and technological aspect, i.e. the evaluation of the ease of use and the additive effect on the process conditions. A multitude of factors affecting the effectiveness of admixtures causes a lot of confusion. It is therefore important to focus on the most important factors, such as:
- properties and chemical composition of admixture,
- presence, type and properties of other admixtures or additives for concrete,
- the size of the application and the moment of addition of the admixture in relation to the moment of mixing the cement with water,
- mixing time and temperature of concrete mass,
- compatibility of cement with admixture.
Admixtures and additives for concrete are currently the fastest growing product group in construction chemicals. Thanks to the research and development of these substances, concrete is becoming a modern multifunctional material. Its modified properties enable the implementation of many interesting projects of building constructions, which until now remained only in the imagination of architects.