w/c ratio, porosity and Powers' concept

Effect of w/c ratio on porosity

Porosity of the paste is the volume of pores (gel pores plus capillary pores). Mixing water in excess of what hydration needs stays in the paste as capillary pores (empty or water-filled). About 0.23 of the cement mass is chemically combined and about 0.15 is held in gel pores, so full hydration needs $w/c \approx 0.38$. For higher $w/c$, the extra water makes capillary pores. With higher $w/c$:

• Capillary porosity is higher and pores are larger and interconnected.
• Strength falls and permeability rises sharply; a pore volume of about 25% and above makes capillaries continuous.
• Durability is therefore worse (carbonation, chloride and sulphate ingress, freeze-thaw).

Powers' concept

Powers and Brownyard treated the paste volume as cement + water, and found that hydrated cement (gel including gel pores) occupies about 2.06 times the volume of the cement. For 1 g cement, specific volume 0.319 cm$^3$:

$$\begin{aligned} \text{Gel volume} &= 0.657\alpha, \qquad \text{Total paste} = 0.319 + w/c \\ \text{Capillary porosity } P_c &= \frac{w/c - 0.36\alpha}{w/c + 0.32} \end{aligned}$$

Here $\alpha$ is degree of hydration. Capillary porosity is the space left in the paste after subtracting unhydrated cement and gel. Strength follows the gel/space ratio, $f_c = 240x^3$.

Proper w/c ratio for durability (IS 456, Table 5)

Thus a lower w/c (0.40 to 0.45) is required in marine, cold or aggressive environments, and 0.50 to 0.55 is acceptable for mild weather.