## **Core Concept**
The strength of gypsum products, commonly used in dentistry for making dental casts, dies, and other applications, is a critical property that determines their durability and suitability for various clinical and laboratory procedures. Gypsum products are primarily composed of calcium sulfate, and their strength can vary based on their type and the conditions under which they set. The strength is often evaluated to ensure that the product can withstand the forces it will encounter during use.

## **Why the Correct Answer is Right**
The strength of gypsum products is generally expressed in terms of **compressive strength**. Compressive strength is a measure of the maximum stress a material can withstand while being compressed before failing. For gypsum products, compressive strength is a critical parameter because it directly relates to their ability to resist fracture and deformation under load. This property is particularly important in dental applications, where gypsum products are used to create models of teeth and surrounding tissues. The compressive strength is usually measured in units of pressure, such as megapascals (MPa) or pounds per square inch (psi).

## **Why Each Wrong Option is Incorrect**
- **Option A:** Tensile strength refers to the maximum stress a material can withstand while being stretched or pulled before failing. While important for some materials, tensile strength is not the primary measure used for gypsum products.
- **Option B:** Flexural strength, or bending strength, measures the material's ability to resist deformation under load when it is supported at two points and loaded at the center. Although relevant for some applications, it is not the standard measure for gypsum products.
- **Option C:** Shear strength refers to the material's ability to resist shear stress, which causes a material to deform by sliding along a surface parallel to the direction of the stress. Shear strength is important in certain contexts but not the primary measure for gypsum products.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the compressive strength of gypsum products increases with the density of the set material. Therefore, controlling the water-to-powder ratio during mixing is crucial to achieve the desired strength. Improper mixing can lead to porosity and reduced strength, affecting the accuracy and durability of dental restorations.

## **Correct Answer:** D. Compressive strength.