Oven-drying temperature for water content when gypsum may be present When determining natural water content of a soil suspected to contain gypsum, the sample should be dried for a longer period at a temperature not exceeding:

Introduction / Context:
Gypsum-bearing soils contain chemically bound (crystal) water. Standard oven drying at 105–110°C can drive off part of this water of crystallization, causing an overestimation of natural moisture content. A reduced drying temperature avoids this error.

Given Data / Assumptions:

• Soil may contain gypsum (calcium sulfate dihydrate).
• Goal is to measure only free (pore) water, not water of crystallization.
• Extended drying time is acceptable to achieve equilibrium at lower temperature.

Concept / Approach:

Use a lower-temperature oven (about 60°C) for gypsum-bearing soils so that crystal water remains intact while pore water evaporates, yielding an accurate natural water content.

Step-by-Step Solution:

Verification / Alternative check:

IS/ASTM notes recommend reduced temperatures for soils with gypsum or organic matter to prevent chemical alteration during drying.

Why Other Options Are Wrong:

80–110°C risks releasing structural water; 95–110°C are typical for non-gypsiferous soils only.

Common Pitfalls:

Using 110°C by habit; not checking mineralogy before testing; insufficient drying time at 60°C.

Final Answer:

60°C