Question

期中测验（水泥）

1. what raw materials are used to produce portland cement?
2. could you please briefly describe the process of producing portland cement? the main process of producing silicate cement is as follows:
3. please write down the formulas for the hydration reactions of portland cement (4 formulas) and provide explanations.
4. what is mechanism of modifying setting time by gypsum?
5. what are the main factors influencing the setting and hardening of portland cement? water cement ratio, blend of gypsum, temperature and humidity, curing age.
6. what does the stability of cement refer to? it refers to the process of reducing and averaging volume of the cement in the process of cement hardening. poor dimensional stability refers to uneven volume change happened in the process of cement hardening, such as cracking, bending.
7. what are reasons for poor dimensional stability? three reasons are too much disassociated f - cao, too much disassociated f - mgo and too much gypsum.
8. what methods are there to evaluate the dimensional stability of cement?
9. what do the strength grades 42.5 and 42.5r indicate?
10. what types of corrosion does silicate cement suffer from?
11. what are the benefits of adding mineral mixtures, such as fly ash or silica fume, to cement?
12. what are the characteristics of high - alumina cement?

Explanation:

1. Raw materials for Portland - cement include limestone, clay, and iron - bearing materials. Limestone provides calcium, clay supplies silica, alumina, and iron - bearing materials add the necessary iron content.
2. The production process involves crushing and grinding raw materials, blending them in proper proportions, heating in a kiln to form clinker, and then grinding the clinker with gypsum.
3. Hydration reactions of Portland cement involve reactions of calcium silicates, calcium aluminates, etc. with water. For example, tricalcium silicate ($C_3S$) reacts with water to form calcium - silicate - hydrate (C - S - H) gel and calcium hydroxide.
4. Gypsum retards the setting time of cement by reacting with tricalcium aluminate ($C_3A$) to form ettringite, which coats the $C_3A$ particles and slows down its reaction with water.
5. Main factors influencing setting and hardening are water - cement ratio (higher ratio may slow hardening), gypsum blend (controls setting), temperature (higher temperature can accelerate reactions), humidity (needed for hydration), and curing age (strength develops over time).
6. Cement stability refers to the volume change during hardening. Poor stability causes cracking due to uneven volume change.
7. Reasons for poor dimensional stability are free $CaO$, free $MgO$, and excessive gypsum. Free $CaO$ and $MgO$ react slowly with water after hardening, causing expansion.
8. Methods to evaluate dimensional stability include the Le - Chatelier method and autoclave method.
9. Strength grades 42.5 and 42.5R: 42.5 indicates the specified compressive strength of the cement after 28 days. The 'R' in 42.5R means rapid - hardening, which reaches a certain strength faster than the non - 'R' type.
10. Silicate cement can suffer from chemical corrosion such as sulfate attack, acid attack, and alkali - aggregate reaction.
11. Benefits of adding mineral mixtures like fly ash or silica fume include improving workability, reducing heat of hydration, enhancing durability by reducing permeability and resisting chemical attacks.
12. High - alumina cement has high early strength development, good resistance to sulfate attack, and high heat resistance, but it has poor long - term strength stability under certain conditions.

Answer:

1. Limestone, clay, iron - bearing materials.
2. Crushing and grinding raw materials, blending, heating in a kiln to form clinker, grinding clinker with gypsum.
3. Reactions of calcium silicates, aluminates with water. For example, $C_3S + H_2O

ightarrow C - S - H+Ca(OH)_2$.

1. Gypsum reacts with $C_3A$ to form ettringite, retarding setting.
2. Water - cement ratio, gypsum blend, temperature, humidity, curing age.
3. Volume change during hardening; uneven volume change causes cracking.
4. Free $CaO$, free $MgO$, excessive gypsum.
5. Le - Chatelier method, autoclave method.
6. 42.5: 28 - day compressive strength; 42.5R: rapid - hardening type.
7. Sulfate attack, acid attack, alkali - aggregate reaction.
8. Improve workability, reduce heat of hydration, enhance durability.
9. High early strength, good sulfate resistance, high heat resistance, poor long - term strength stability under some conditions.