Water requirement per 50 kg bag of cement: approximately how many litres of water are needed for complete hydration and to fill gel pores (not counting extra water for workability)? (Select the nearest standard value used in mix design concepts.)

Introduction / Context:
In concrete technology, understanding the theoretical water demand of cement helps distinguish between water needed for chemical hydration and that added for workability. A classic figure is the water-to-cement ratio required to hydrate cement fully and fill gel pores without leaving capillary water.

Given Data / Assumptions:

• Water required for complete hydration is about 0.25 by mass of cement.
• Water to fill gel pores adds roughly 0.15 by mass of cement.
• Total theoretical water ≈ 0.25 + 0.15 = 0.40 (often rounded to ≈0.38 to 0.40 depending on reference).
• One bag of cement = 50 kg.

Concept / Approach:
Compute the theoretical litres needed using w/c ≈ 0.37–0.40 (commonly taken as ≈0.38). Multiply by bag mass to obtain litres of water (density ≈ 1 kg/L). This yields a value close to 19 L. Among the options, 18.5 L is the nearest standard selection used in many exam problems.

Step-by-Step Solution:

Verification / Alternative check:
Concrete technology texts repeatedly cite ≈0.38 as the theoretical w/c for hydration plus gel pore water; practical mixes use higher water or plasticizers to achieve workability while maintaining strength targets.

Why Other Options Are Wrong:

• 16.5 L: too low for hydration + gel pores.
• 20.5–25 L: exceed theoretical needs; represent workability additions rather than stoichiometric requirement.

Common Pitfalls:
Confusing theoretical water with total mix water; assuming more water always improves strength (it reduces it by increasing capillary pores); ignoring admixture use to reduce water demand.

Final Answer:
18.5 litres