The reaction of a compound/mixture with water is known as hydration. When sand, gravel and water is mixed with portland cement, concrete is formed. This is a form of synthetic rock.
The key is to understand what happens when it is mixed with water. Apart from hydration, there are a number of reactions that occur at the same time. As the process of reactions evolves, the combined process of the hydration process bind the individual particles of the mixture to make “concrete” and forming a solid mass.
Speaking of phases, there are aluminate, belite,alite and the ferrite phase. Along with them negligible amount of clinker Sulphate and gypsum are present. When all these are added to the clinker, a grey powder of sorts is produced. Exothermic reactions (that generate heat) produce water in most of the cases and an idea of how the minerals are reacting can be gauged by the heat generated. This process is known as conduction calometry.
The Hydration of Products:
The products which arise from the reactions of water and cement are known as hydration products. Four main types of product emerge out of this:
- Calcium Hydroxide – The chemical formula is referred to as CA (OH) 2 and it is formed from the process of alide hydration.
- Hydro garnet – Normally found in autoclaved cement products, they are a part of various combinations. They evolve out of the ferrite or C3A hydration.
- Calcium silicate hydrate- This is the main component that comes out of the hydration process and it happens to be the pillar of the concrete strength.
- Afm and Aft phases- These two minerals are vital components of cement. Monosulfate and ettringite are the two phases of Afm and Aft repectively. The former tends to occur in the later process of the reaction and evolves a day or two after mixing while the latter is present in the earlier stages of the reaction and evolves as rod like crystals. The common feature in both these phases is that they contain a lot of water.
Models of the C-S-H:
In general there are three accepted models of the C-S-H structure which are depicted below
- Munich – It is a three dimensional model on the basis of colloidal particles. In fact the chemical nature of the model tends to be secondary and water is attracted to the solid particles reducing the pressure on the particle interaction.
- Feleda and Sereda- This is a model based on the array of single layers which forms irregular interlayer space. Water can move in and out of the layers even after drying.
- Powers Braunauer- This is a model which is based on clay-like configuration of patterns. Water is absorbed on the surfaces and the sheets can be arranged randomly. With the help of strong dying, the interlayer water can be removed.
The major fact which emerges from all these models is that water has a profound effect on it. The moment water is removed, a comprehensive sort of stress is created. As soon as the interlaying water is taken, the particle as well as the sheet bonding is affected.
Process of Hydration:
After having a look at the products along with the models, one needs to understand the process of hydration of portland cement. The processes of reaction of two calcium silicates which make up a substantial portion of the cement are similar. With regards to this aspect, calcium silicon hydrate along with calcium hydroxide is used. Both are crystalline particles but the former tends to occupy less space. An insight into the process is as follows:
Stage 1: A heat generation of rapid order takes place for close to 15 mins. The calcium and hydrogen ions are released from the surface and when certain levels of critical concentrations are reached, the evolution of calcium hydroxide and calcium silicate hydroxide begins. The initial reactions are dependent on the temperature.
Stage 2: This stage tends to be the dormant period and the cement is forced to become plastic for a period of 2 to 4 hours. The process of reaction tends to slow down.
Stage 3: This is the acceleration period as the silicate hydrates rapidly along with critical concentration of ions. The entire hardening takes place and the final set is released and the time period is generally for 4 to 8 hours.
Stage 4: This is known as the deceleration stage .The overall rate of reaction tends to slow down resulting in an independent diffusion reaction.
Stage 5: This stage is referred to as the steady stage and the temperature has less effect on the hydration stage. The reaction process is constant and is for a period of 12 to 24 hours.
The individual reaction of minerals tend to be less effective than the combined reaction of the hydration of cement. The hydration of cement can be split into several small components. It is observed that the aluminate and the ferrite stages tend to react first and then the reactions tend to carry over to the silicate phase.