What is concrete and what additives does it contain?

Concrete can be made from different types of cement and pozzolans, furnace slag, different additives, polymers, fibers, etc. In general, concrete is a product that is obtained from the mixture of water with cement and various aggregates and the reaction of water with cement, and it has special characteristics.

Setting and hydration of cement and concrete production

In the process of concrete production, when cement is placed in the vicinity of water, the constituent phases of cement start to hydrate and produce hydrated calcium silicate, which leads to hardening and hardening of concrete.

In the figure below, you can see the hydration reaction of calcium silicates.

In the picture above, H is the symbol of water molecule.

The first reaction is related to the hydration of tricalcium silicate and the second reaction is related to the hydration of dicalcium silicate.

As you can see, due to the hydration of calcium silicates, calcium hydroxide is produced. Compared to dicalcium silicate (assuming the same initial weight), tricalcium silicate produces more than twice as much calcium hydroxide with almost the same amount of water.

A few important points:

1- The difference between the components of cement before setting and after setting

Before setting, cement has a very small amount of free lime (a small amount of lime remains unreacted during the clinkering stage), but after setting, hydrated free lime becomes one of the main components.

2- Penetrating silicate hardeners are supposed to react with this free lime produced as a result of the setting process and convert them into hydrated calcium silicate to improve and increase the properties of concrete surfaces. Therefore, for this reason, it is said that it is necessary to allow the concrete setting period (28 days) to be completed and then apply these primers on the concrete surface.

3- Dandruff that appears on fresh concrete surfaces after a few weeks is mainly related to these reactions and the migration of calcium hydroxide to the surface.

Effects of produced free calcium hydroxide on concrete properties

Hydrated free lime is not harmful to concrete, but it causes at least the following two factors.

1- Creation of carbonated scale

To learn more about the mechanism of carbonated dandruff production and ways to recognize and eliminate them, refer to the article on recognizing different types of dandruff on this website.

2- washing out of the concrete and increasing the pores of the concrete

If the concrete is exposed to continuous washing or continuous rain. Lime in concrete is dissolved by water and removed from the surface, which makes the surface more permeable.

Quality of water used in concrete production

The quality of water in concrete is important because the impurities in it may have an adverse effect on the setting of cement as well as the strength of concrete and cause stains on the surface of the concrete and even rusting of the reinforcement.

In most mixes, water suitable for concrete is water that is suitable for drinking. As a general rule, any water whose pH is between 6 and 8 and does not taste salty can be used for concrete.

The amount of water used in concrete

The amount of water used in concrete is very important. In order to complete the process of cement reaction with water (hydration), a certain amount of water is needed.

If this amount is less than that, part of the cement does not receive enough water for reaction and remains unreacted.

If more than the required amount of water is added to the concrete mixture after the completion of the reaction, some water will remain freely inside the concrete, which will cause its porosity after the concrete hardens and, as a result, reduce its strength.

The amount of water required to complete the reaction is defined as the water-cement ratio parameter. This ratio for ordinary Portland cement is about 25%. With this amount of water, the concrete will lack the necessary efficiency and fluidity, and usually the ratio of water to cement used in construction workshops is more than this amount.

The relative humidity must be at least 80% for the hydration process to proceed or continue. If the relative humidity is less than 80%, the treatment is stopped, and if the relative humidity returns to above 80%, the hydration process will start again.

After concreting, attention should be paid immediately to the curing process.

Curing is keeping the concrete moist until the reaction between cement and water is complete. This action can be done by temporary insulation, water spraying or steam production.

From a practical point of view, it is recommended to keep concrete moisture for 7 days. If this is not possible, the minimum concrete curing time should not be less than 2 days.

Pozzolan and microsilica gel

Pozzolans (volcanic ash) are siliceous, or siliceous-aluminum materials that alone have no adhesion value or have low adhesion value, but in a very fine form in the vicinity of moisture during a chemical reaction with calcium hydroxide released as a result of hydration in Normal temperature produces compounds with cement properties.

Micro silica is actually a super pozzolan and can play an effective role in the quality of concrete.

Aggregates

Aggregates in concrete make up almost three quarters of its volume, so their quality is of particular importance. In fact, their physical, thermal and sometimes chemical properties affect the performance of concrete.

Size and size of aggregates

Concrete is generally made of aggregates of different sizes, the maximum diameter of which is between 10 mm and 50 mm. Particle size distribution is commonly known as aggregate gradation.

In general, grains with a diameter greater than four or five millimeters are called sand and smaller ones are called sand. foot limit