The presence of water in cement will result in the hydration process (Exothermic Reaction - Heat Evolution). This reaction will result in hydration products of silicates and aluminates that will bring a hard mass over a period of time. This hard mass can be called as Hydrated Cement Paste (HCP).
Phases of Concrete
Concrete is considered to have two-phase materials. They are the:
- Paste Phase (Cement Phase)
- Solid Phase (Aggregate Phase)
The properties and quality of the paste phase influence largely on the overall properties of the concrete when compared with the aggregate phase. The strength, permeability, elastic properties, durability and volume change of the concrete are highly influenced by the paste structure.
Interfacial Transition Zone (ITZ)
As mentioned above, even if the cement phase influences the majority of the properties of concrete, the presence of aggregate will affect the cement phase in certain means. The cement particles in water cannot arrange sufficiently as in the case of aggregate particles. They lose the ability to be packed closely. This will create a phenomenon called the "wall effect", where the aggregate particle will exert some sort of shear stress on the cement paste during the mixing procedure. This process will result in the separation of water from the cement particles.
The above process will result in the formation of a thin layer between the cement and the aggregate. This layer will have fewer amount of cement particles but have higher water content. This separate zone is called as Interfacial Transition Zone (ITZ).
Features of Interfacial Transition Zone (ITZ)
The ITZ is a plane of weakness and has a greater influence on the mechanical properties and behavior of concrete. The zone is composed of the same bulk cement paste. The quality of the cement paste is very poor in the transition zone.
The drying shrinkage reasons or temperature variation will result in the vaporization of the water in the zone. This will create voids in the plane. Even before the application of external loads, these voids will break to form small micro cracks. These smaller cracks will propagate with further load application to form large cracks. These voids in the plane hence will affect the overall durability of the concrete. Final bond failure will result as the cracks go on propagating. Hence the quality of concrete will depend on the quality of ITZ.
The ITZ is:
- Weakest Link and the critical one in the Concrete Mixture
- Higher Water Cement Ratio
- Higher Porosity
- Not Uniform in Nature- depends on the aggregate phase
- Overall Thickness 20-40 £gm
- Larger the aggregate particles larger is the ITZ
The general properties of concrete like:
- Concrete is Weak in Tension and Good in Compression
- Concrete is Brittle in Nature
are all due to the properties of ITZ.
HCP structure in Microscopic Level
At the microscopic level, the Hydrated Cement Paste Structure Consists of the following elements.
Solids in HCP
The various solids that are present in HCP are:
- Calcium Silicate Hydrate ( C-S-H): The volume of C-S-H in HCP is 50% to 60%. This imparts Vander walls force, which will influence the strength of HCP. This hydration product has a higher surface area. This is not a well-defined compound.
- Calcium Hydroxide ( Ca(OH)2): This is also called as Portlandite. This constitutes 20 to 25%volume of HCP. This has a lower surface area. This is not a desirable product from hydration. This product is soluble in water and gets leached out making the concrete porous. This is alkaline in nature and maintains the pH value of concrete at around 13.
- Calcium Sulfoaluminate Hydrate: This product has a minor role in the HCP characteristics. The presence of this product will result in making the concrete vulnerable to sulfate attack. These are needle-shaped plate products called ettringite.
Voids in HCP
The voids present in HCP are:
- Interlayer Space in HCP
- Capillary Voids
- Air Voids
Water in HCP
The water present in HCP are:
- Capillary Water
- Adsorbed Water
- Interlayer Water
- Chemically Combined Water