It generally takes about 24 to 48 for newly poured concrete to dry or harden enough to be able to walk on the surface. Cement is the binding ingredient in concrete.
Composition of Concrete
Concrete is a versatile construction material composed of several ingredients that work together to create a strong and durable product. The primary components of concrete are cement, aggregates, water, and often additional admixtures or additives. Here is a breakdown of each component:
Cement: Cement acts as the binder in concrete. The most commonly used type of cement is Portland cement, which is made by heating limestone, clay, and other materials to high temperatures and then grinding them into a fine powder. When mixed with water, cement undergoes a chemical reaction known as hydration, which forms a paste that hardens and binds the aggregates together.
Aggregates: Aggregates are the inert granular materials that make up the bulk of concrete. They provide stability, strength, and volume to the mixture. The two main types of aggregates used in concrete are fine aggregates and coarse aggregates.
Fine aggregates: These are small particles with sizes typically less than 4.75 mm (0.1875 in). Examples include sand, crushed stone dust, and natural or manufactured fine aggregates.
Coarse aggregates: These are larger particles with sizes ranging from 4.75 mm (0.1875 in) to several inches. Common coarse aggregates include gravel, crushed stone, and recycled concrete.
Water: Water is an essential component in the concrete mixture. It is required for the hydration of cement and allows the mixture to flow and be workable. The amount of water used in concrete should be carefully controlled to achieve the desired consistency and strength.
Admixtures/Additives: Admixtures are additional materials added to the concrete mixture to modify its properties or enhance its performance. They can be classified into several types:
Plasticizers or water reducers: These admixtures reduce the water content needed for workability without sacrificing the strength of the concrete.
Superplasticizers: These are highly effective water reducers that can significantly increase the workability of concrete without increasing the water content.
Accelerators: These additives speed up the setting and hardening of concrete, reducing the curing time.
Retarders: Retarders, on the other hand, slow down the setting time of concrete, allowing more time for placement and finishing.
Air-entraining agents: These additives create microscopic air bubbles in the concrete, which improves its resistance to freeze-thaw cycles.
Pigments: Pigments can be added to produce colored concrete for aesthetic purposes.
Other admixtures: There are various other admixtures available that can impart specific properties to concrete, such as corrosion inhibitors, waterproofing agents, and shrinkage reducers.
Hydration Process
The hydration process is a chemical reaction that occurs when water is added to cement, resulting in the hardening and setting of concrete. It is a complex series of reactions that transform the cement into a solid and durable material. Here is an overview of the hydration process:
Dissolution: When water is added to cement, it starts to penetrate the dry particles and react with the compounds present in the cement, primarily calcium silicates. The water molecules break down the compounds and dissolve them, forming ions in the solution.
Nucleation: As the dissolved ions move within the water, they begin to form new compounds through chemical reactions. Small crystals, known as nuclei, start to form and grow within the mixture.
Hydration of cement compounds: The primary compounds responsible for the hydration process are tricalcium silicate (C3S) and dicalcium silicate (C2S), which are the main constituents of Portland cement.
C3S hydration: C3S reacts rapidly with water, releasing calcium ions (Ca2+) and hydroxide ions (OH-) into the solution. The calcium ions combine with the hydroxide ions to form calcium hydroxide (CH), also known as lime. Additionally, the C3S compounds react with the water molecules, forming calcium silicate hydrate (C-S-H) gel, which is the main binder responsible for the strength and durability of concrete.
C2S hydration: C2S reacts more slowly with water compared to C3S. It undergoes a similar process of releasing calcium and hydroxide ions, which then react to form additional calcium hydroxide and contribute to the formation of the C-S-H gel.
Formation of calcium hydroxide: The hydration process also produces a significant amount of calcium hydroxide (lime), which does not contribute much to the strength of concrete but fills the voids between the solid particles. Over time, some of the calcium hydroxide can react with carbon dioxide in the air, forming calcium carbonate (a process known as carbonation).
Setting and hardening: As the hydration reactions progress, the C-S-H gel and calcium hydroxide continue to form and fill the spaces between the cement particles, resulting in a solid matrix. This process leads to the initial setting of concrete, where it transitions from a plastic state to a stiff, solid state. With time, the matrix becomes denser and gains strength through further hydration and the formation of additional C-S-H gel.