Lec 5

Question 1

Setting and hardening of concrete are the result of
….. and ….. processes taking place between ……..

Answer 1

Chemical, physical

Cement and water

Question 2

Hydration of cement compound

Answer 2

They are independently of the others, but still can be interactions between hydrating compounds

Question 3

Why dry cement does not have cementitious properties

Answer 3

Because cement is hydration material

Question 4

The chemical reaction between cement and water is called

Answer 4

Hydration

Question 5

Hydration stages

Answer 5

1. Setting: initial setting (mixing to plastic state for 1 hour) final setting (lose plasticity for 10 hours)
2. Hardening: concrete strength gradually increases to form solid state

Question 6

Steps to become hydration products

Answer 6

Component elements (O2,Si,Ca,Al,Fe)
Component oxides (CaO, SiOw, AI2O3,Fe2O3J
Cement compounds (C3S,C2S, C3A, C4AF)
Portland cements
Hydration products (gel, Ca(OH)2)

Question 7

https://ibb.co/hdF0Mvv
https://ibb.co/phQ8f4b

Which one is the better reaction?
Why the C-S-H is various?

Answer 7

The C2S reaction, because they have less CH

The variety of C-S-H is still a mystery for civil engineer, we can use both.

Question 8

What is the principal hydration products?

Answer 8

Calcium silicates hydrate

Question 9

The Calcium silicates (C-S-H and CH) reactions are exothermic. What does it mean?

Answer 9

The heat is released during the reaction progress. The heat will raise the temperature of concrete

Question 10

https://ibb.co/ZhyGHbD

Answer 10

C3S calorimetric curve of heat evolution

Question 11

What is the common feature pr CRMs

Answer 11

They exhibit pozzolanic behavior to affect the extent

Question 12

Pge 14

Answer 12

Active silica (SiO2) not cementitious, chemically react with calcium hydroxide (CH) at ordinary temperatures to form cementitious compounds (C-S-H)

Question 13

What is the reaction of Hydration of C3A

Answer 13

C3A + 3CSH2 + 26H -> C6AS3H32

It involves reaction with sulfate ions that are supplied by the dissolution of gypsum

C6AS3H32 is called calcium sulfoaluminate hydrate (ettringite)

Question 14

Ettringite is …… hydration product only while there is as ample supply of …..

Answer 14

Stable
Sulfate available

Question 15

If all sulfate is consumed before C3A has completely hydrated, then ettringite transforms to another…….., containing …..:

Answer 15

Calcium sulfoaluminate hydrate
Less sulfate

2C3A + C6AS3H32 + 4H -> 3C4ASH12

Question 16

https://ibb.co/pnGYzsk

Answer 16

Tricalcium aluminate C3A + gypsum

Hydration exotherm

Question 17

Describes the C4AF hydration reaction

Answer 17

C4AF forms similar hydration products to C3A. The reactions are slower and involve less heat.
Changes in the composition of the ferrite phase affect only the rate of hydration.
F can subs for A in the hydration products

C4AF + 3CSH2 + 21H -> C6(A,F)S3H32 + (F,A)H3
C4AF + C6(A,F)S3H32 + 7H -> 3C4(A,F)SH12

Question 18

The reaction between C3A, gypsum and water produces calcium sulfoaluminate hydrate, also known as

Answer 18

Ettringite

Question 19

If there is no supply of sulfate (no added gypsum) to the initial period of adding C3HA to water, what will happen?

Answer 19

hydration products will be formed quickly, and a flash set will occur, accompanied by a large amount of heat of hydration and immediate condensation.

Question 20

What is the use of gypsum in hydration reaction?

Answer 20

controls/delays the generation of hydration heat.

Question 21

Describes C3A

Answer 21

has a fast hydration rate, generates a lot of hydration heat, and is not good for concrete quality.

Question 22

What is produced for second reaction of C3A and C4AF if no enough gypsum?

Answer 22

Monosulfoaluminate

Question 23

What are flash set and false net

Answer 23

FLASH SET: If the proper amount or form of calcium sulfate is not available to control the calcium aluminate hydration, rapid stiffening takes place. Flash set cannot be dispelled, nor can the plasticity be regained by further mixing without the addition of water.

FALSE SET: Temperature > 100 C. In such conditions, gypsum dehydrates, loses water and goes into the gypsum hemihydrate. This leads to stiffening due to the rapid reformation of secondary gypsum. Additional mixing without added water breaks up these crystals to restore workability. Ettringite precipitation can also contribute to false set.

Question 24

https://ibb.co/mtMJK2J

Answer 24

Crystalize to increase solid volume

Forms ettringite with sulfate and water and crystallize again

Continuous hydration with C3A and dissociation again

Particle of first part

Needle tube (ettringite)

Hexagon flakes (monosulfoaluminate)

Question 25

https://ibb.co/VjwDWPr
Explain the informations given by the graph

Answer 25

1. The hydration heat peak of C3A delays with addition of gypsum
2. Cements with same fineness:
   
   • Too little gypsum will shift the exothermic peak forward;
   • Too much gypsum will shift the exothermic peak back.
3. Generally control the C3A exothermic peak behind the C3S hydration exothermic peak to maintain the high workability of the concrete.

Question 26

https://ibb.co/XtXJM8W
https://ibb.co/YXYbPyj

Answer 26

1. Ettringite as first formed in Portland cement paste
2. CH
   3.Monosulfoaluminate (from C3A)
3. Ettringite needles
4. CH
5. Unhydrated cores
6. C-S-H
7. CH pores.
8. Capillary

Question 27

https://ibb.co/RvB9tJR

Answer 27

a. Initial
b. 7 days
c. 28 days
d. 90 days
1. Unhydrated material
2. Water-filled capillary pores
3. C-S-H
4. Calcium hydroxide