Concrete has some definitive core ingredients like cement, water, and aggregates. Apart from these traditional components, the users add some additional ingredients that serve dome specific purposes. These add-ons, called admixtures, are usually incorporated into the concrete while mixing. The project managers decide the additives based on the type of project and terrain and inform the companies in advance to obtain accurately synthesized ready mix concrete. Properties like the heat of hydration, acceleration or retard setting time, water reduction, workability, dispersion, and several others can be modified using admixtures.

Difference between admixtures and additives

The major differentiating criteria between the two is that admixtures do not exceed five percent of cement content by mass while mixing and additives can exceed this percentage in the ready-mix concrete. Admixtures incudes of mineral as well as chemical elements and materials like sand and aggregates are considered to be additives. Some factors like compatibility of the cement with the admixture, the mixing time, and the temperature of concrete mass have to be thought out beforehand. Let us discuss different types of chemical admixtures and their uses.

Air-entraining admixtures

These are the king of admixtures that ensures that the air present in the concrete mixture is stabilized and the water and air are infolded and mechanically enveloped during mixing. The air bubbles are mixed thoroughly to enable frost resistance and make it less absorbent. This addition also decreases the strength by approximately 5 Mpa due to the incorporation of less that one percent of air. These are mostly used for bridge, surface, and hydrotechnical concrete.

Water reducer admixtures

These are added before the pouring of the mixtures on-site. The main use of such admixtures is to decrease the porosity that is advantageous during the freeze-thaw cycle as well as makes the final product extra durable. These are mostly used in cases of concreting at a location with hot weather. The water-reducing salts act as surfactants and are able to reduce the surface tension of the liquid. It enhances the workability of the concrete by preventing the flocculation of cement particles. The most recent generation of water reducing materials includes carbon copolymers that have a significantly high capacity due to the inherent compacting property.

Accelerating admixtures

Accelerators are responsible for the quick curing of concrete. They also aid in the development of the strength of the concrete. These admixtures are suitable for locations with cold weather conditions. They function effectively by decreasing the rate of hydration and speeding up and shortening the duration of time required for the concrete to set. Calcium chloride, the initial accelerator has been replaced with calcium nitrite which also acts as a corrosion inhibitor. This provides more potential for the ready-mix concrete to strengthen. The conversion of the state of concrete mix from plastic to a stiff and hardened state has to be completely appropriated, which is assisted heavily by accelerating admixtures.

Retarding admixtures

Retarding  admixtures consists of a group of metal oxides, sugars, and phosphates that slow the process of concrete curing. The work is antagonistic to the accelerating admixtures by shortening the time taken for the transition from the plastic to the hardened state of the concrete. They do so by slowing the cement binding and delaying the chemical reactions that occur when concrete starts setting, which in turn enables the concrete to remain fluid for long periods of time. They are useful while transporting ready-mix concrete over long distances and also for concreting at high temperatures.

Plasticizers

These are materials that are extremely useful in high-strength mixes since they tend to increase the workability of the concrete. Plasticizers also increase the slump pumpabilty without the addition of water. Sometimes also known as superplasticizers, they have the capacity to convert the stiff concrete mix into one that can flow and mould easily. Like plasticizers, there are several other admixtures that can modify the properties of concrete. Ready-mix concrete can be incorporated with minerals like fly ash, silica fumes, pozzolans, other bonding admixtures, and corrosion inhibitors to enhance the performance in multiple ways. New types of admixtures are being experimented extensively to achieve environmental benefits as well as reduce pollution. Some are being tested for providing self-healing properties to the concrete and absorb pollution.