Question

1. what is specific surface area according to gb 175 - 1999? 2) how to measure the setting time of cement. 3) what are reasons for poor dimensional stability of cement.

Explanation:

1. According to GB 175 - 1999, the specific surface area of cement is the total surface area per unit mass of cement, usually measured in square centimeters per gram ($cm^{2}/g$) or square meters per kilogram ($m^{2}/kg$). It is an important property related to cement reactivity.
2. The setting - time of cement can be measured using a Vicat apparatus. The initial setting time is the time elapsed from the addition of water to the cement paste until the Vicat needle penetrates to a certain depth, and the final setting time is when the needle no longer penetrates the paste significantly.
3. Reasons for poor dimensional stability of cement can include excessive free lime or magnesia content in the cement clinker, improper manufacturing process, or inappropriate storage conditions that can cause volume changes during hydration.

Answer:

1. The specific surface area of cement according to GB 175 - 1999 is the total surface area per unit mass, measured in $cm^{2}/g$ or $m^{2}/kg$.
2. Use a Vicat apparatus to measure the setting - time of cement.
3. Excessive free lime or magnesia content, improper manufacturing process, and inappropriate storage conditions can cause poor dimensional stability of cement.