JNTU B.Tech III CONCRETE TECHNOLOGY Previous Question Paper (March-2021 - R18/R16)

The structure of hydrated cement, or cement paste, is a complex, porous solid formed by the chemical reaction between cement and water. It consists of four primary components: calcium silicate hydrate (C-S-H) gel, calcium hydroxide (CH), unhydrated cement grains, and capillary pores.

The C-S-H gel is the most important constituent, forming a dense, poorly crystalline, fibrous mass that provides the primary strength and binding properties to the paste. Calcium hydroxide forms as hexagonal plate-like crystals, which are relatively soluble and contribute little to strength but help maintain the alkaline environment necessary for steel reinforcement protection. Unhydrated cement grains act as inert fillers within the matrix, while the capillary pore system consists of voids originally occupied by excess mixing water. As hydration progresses, the C-S-H gel fills these spaces, reducing porosity and increasing the density and strength of the hardened cement paste.

The water/cement (w/c) ratio is the most critical factor governing the microstructure and performance of concrete. A lower w/c ratio results in a denser, less porous cement paste matrix because there is less excess water to leave behind capillary voids upon evaporation or consumption during hydration.

In the cement paste matrix, a high w/c ratio increases the total volume of capillary pores, which significantly reduces compressive strength and increases permeability, making the concrete more susceptible to chemical attack and environmental degradation. Conversely, a low w/c ratio promotes a more continuous and interconnected C-S-H structure, leading to higher strength and durability.

The interfacial transition zone (ITZ), the region between the cement paste and aggregate particles, is particularly sensitive to the w/c ratio. Because of the 'wall effect' where aggregate particles hinder the packing of cement grains, the ITZ naturally has a higher local w/c ratio than the bulk paste. This results in a more porous, weaker zone characterized by larger, oriented calcium hydroxide crystals and ettringite. A lower overall w/c ratio helps mitigate this effect by reducing the bleeding of water toward the aggregate surfaces, thereby densifying the ITZ and improving the bond strength between the paste and the aggregate.