In soundness testing, which material is commonly used to determine the loss of quality?

In soundness testing, sodium sulfate is commonly utilized because it is effective in simulating the potential expansion of concrete materials when subjected to moisture and temperature fluctuations. The primary goal of soundness testing is to assess the ability of aggregate particles to maintain their integrity under these conditions.

Sodium sulfate specifically reacts with the components in the concrete or aggregate, leading to expansion that can cause cracking or other forms of deterioration. This reaction helps to quantify the material's resistance to such degrading factors, which is crucial for ensuring long-term durability in concrete structures.

The other materials listed do not serve the same purpose in the context of soundness testing. Cement paste may provide insights into the properties of the binder but does not specifically relate to the evaluation of aggregate soundness. Gypsum is often used in conjunction with cement but is not the standard for testing soundness. Sand, though a significant component of concrete, is not used in its pure form for assessing soundness; instead, it's typically part of a larger aggregate mix. Thus, sodium sulfate stands out as the appropriate choice for soundness testing.