Concrete can be prepared from different types of cement as well as pozolans , kiln slag, additives , polymers, fibers, etc. In general, concrete is a product that is obtained from a mixture of water with cement and various aggregates and the reaction of water with cement and has special properties.
When cement is exposed to water (concrete production), the phases of the cement begin to hydrate and produce hydrated calcium silicate, which hardens the concrete.
The figure below shows the hydration reaction of calcium silicates.
In the picture above, H is the symbol of the water molecule.
In the picture above, 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, calcium hydroxide is produced by hydrating calcium silicates. Tricalcium silicate produces more than twice as much calcium hydroxide than dicalcium silicate (assuming the same initial weight) with almost equal water consumption.
1- Difference between cement components before hydration and after hydration
Cement has a very small amount of free lime before hydration (a small amount of lime remains unreacted in the clinker stage) but after hydration, hydrated free lime becomes one of the main components.
2- Penetrating lithium silicate hardeners react with the same free lime produced by the hydration process and while converting them to hydrated calcium silicate, they improve and increase the properties of concrete surfaces. Therefore, it is said that the hydration period of concrete (14 days) should be allowed to be completed and then these liquid concrete hardeners should be applied on concrete surfaces.
3- Calcium carbonate deposition, which occurs on fresh concrete surfaces after a few weeks, is mainly related to the same reactions and the migration of calcium hydroxide to the surface.
Hydrated free lime is not harmful to concrete but causes at least two of the following factors.
1- Creating carbonate sediment
For more information on the mechanism of production of carbonate efflorescence and ways to identify and remove them, refer to the post on recognizing different types of efflorescence on this website.
2- Washing and getting out of the concrete and increasing the number of concrete pores
If the concrete is exposed to constant washing or continuous rain. Lime in concrete is dissolved by water and removed from the surface, which makes the surface more permeable.
The water quality of concrete is important because the impurities in it may adversely affect the setting of cement as well as the strength of concrete and cause stains on the concrete surface and even rusting of the reinforcement.
In most mixtures, water is suitable for concrete, which is suitable for drinking. As a general rule, any water with a pH between 6 and 8 that does not taste salty can be used for concrete.
The amount of water consumed inside the concrete is very important. In order to complete the process of cement reaction with water (hydration), a certain amount of water is required.
If this amount is less than that, part of the cement will not receive enough water to react and will remain unreacted.
If more than the required amount of water is added to the concrete mix, after completing the reaction, some water remains freely inside the concrete, which after hardening of the concrete will cause it to become hollow and consequently reduce the strength.
The amount of water required to complete the reaction is defined as a parameter of the ratio of water to cement. This ratio is about 25% for ordinary Portland cement. With this amount of water, concrete will not have the necessary efficiency and flow, and usually the ratio of water to cement used in construction workshops is more than this amount.
Relative humidity must be at least 80% to cure or continue the hydration process. If the humidity is less than 80%, the processing is stopped and if the relative humidity returns above 80%, the hydration process will start again.
Immediately after concreting, the necessary attention should be paid to the curing process.
Curing is the retention of moisture in the concrete until the reaction between cement and water is complete. This can be done by temporarily insulating, spraying water or producing steam.
From a practical point of view, it is recommended to keep the concrete moist for 7 days. In cases where this is not possible, the minimum concrete curing time should not be less than 2 days.
Pozzolans (volcanic ash) are silica, or silica-alumina materials that alone have little or no adhesion value, but are very finely exposed to moisture during a chemical reaction with calcium hydroxide released by hydration. At normal temperatures, compounds with cementitious properties are formed.
Micro silica is actually a super pozzolan and can play an effective role in the quality of concrete.
Aggregates in concrete make up about three-quarters of its volume, so their quality is of particular importance. In fact, their physical, thermal and chemical properties affect the performance of concrete.
Concrete is generally made of aggregates of different sizes with a maximum diameter of 10 mm and 50 mm. Particle size distribution called aggregate granulation is common.
Grains larger than four or five millimeters in diameter are generally called sand and smaller grains are called sand. The lower limit of sand is usually 0.1 mm or slightly less.
Additives are divided into two groups: chemical additives and mineral additives.
The common types of concrete admixtures are as follows.
Concrete hydration accelerator (quick arresters)
Retarding admixtures used to Reduces setting speed of concrete
Foamed concrete admixtures
Concrete plasticizers used to reduce the amount of concrete water.
Pigments that can be used to change the color of concrete and beauty.
Concrete adhesive
Concrete hardener
There are additives that accelerate the hardening of concrete and increase the initial strength of concrete. Some examples of accelerators include sodium carbonate, potassium carbonate, sodium fluoride, sodium aluminate, iron salts, and calcium nitrite.
There are additives that delay the setting time of concrete. These materials are very useful in very hot weather, which shortens the normal setting time of concrete, and also to prevent the formation of cracks due to setting in successive concreting.
Examples of reducing agents include sugar, hydrocarbon derivatives, and soluble zinc salts.
For example, if we add 0.05 weight of sugar cement to the concrete with a precise control, its hydration will be delayed for about four hours. Consumption of 0.2 to 1 percent by weight of cement prevents the cement from setting.
As mentioned above, if water is used only in the amount required to hydrate the cement components, the concrete will not have any flow and will not be usable. On the other hand, adding too much water also causes pores in the concrete. For this reason, lubricants are used to reduce water consumption.
plasticizers is a substance that either reduces the amount of water in the concrete mix without changing the flow, or increases the flow of concrete water without changing the flow rate, or causes both a decrease in water and an increase in flow rate.
Water-reducing additives contain various compounds of organic matter, the most common of which are lingosulfonates or naphthalene formaldehyde sulfonates and polycarboxylates.
Basically, the use of sulfonates and carboxylates accelerates the dispersion process. Because cement is adsorbed on the surface of the particles and they are negatively charged, which causes the particles to repel each other. This process increases the efficiency in a certain ratio of water to cement.
Fiber concrete is a type of concrete that is made of fibers and mixes cement, water, aggregates and additives with the fibers. The fibers increase the cohesion, tensile strength, reduce concrete cracks and increase the softness of the concrete.
The type of fibers and their size depends on the type of concrete used and the tensile strength considered. The fibers can be glass fibers or metal fibers or polymer fibers and their size is usually 3 to 20 mm.
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