Lec 5 Flashcards
Setting and hardening of concrete are the result of
….. and ….. processes taking place between ……..
Chemical, physical
Cement and water
Hydration of cement compound
They are independently of the others, but still can be interactions between hydrating compounds
Why dry cement does not have cementitious properties
Because cement is hydration material
The chemical reaction between cement and water is called
Hydration
Hydration stages
- Setting: initial setting (mixing to plastic state for 1 hour) final setting (lose plasticity for 10 hours)
- Hardening: concrete strength gradually increases to form solid state
Steps to become hydration products
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)
https://ibb.co/hdF0Mvv
https://ibb.co/phQ8f4b
Which one is the better reaction?
Why the C-S-H is various?
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.
What is the principal hydration products?
Calcium silicates hydrate
The Calcium silicates (C-S-H and CH) reactions are exothermic. What does it mean?
The heat is released during the reaction progress. The heat will raise the temperature of concrete
https://ibb.co/ZhyGHbD
C3S calorimetric curve of heat evolution
What is the common feature pr CRMs
They exhibit pozzolanic behavior to affect the extent
Pge 14
Active silica (SiO2) not cementitious, chemically react with calcium hydroxide (CH) at ordinary temperatures to form cementitious compounds (C-S-H)
What is the reaction of Hydration of C3A
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)
Ettringite is …… hydration product only while there is as ample supply of …..
Stable
Sulfate available
If all sulfate is consumed before C3A has completely hydrated, then ettringite transforms to another…….., containing …..:
Calcium sulfoaluminate hydrate
Less sulfate
2C3A + C6AS3H32 + 4H -> 3C4ASH12
https://ibb.co/pnGYzsk
Tricalcium aluminate C3A + gypsum
Hydration exotherm
Describes the C4AF hydration reaction
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
The reaction between C3A, gypsum and water produces calcium sulfoaluminate hydrate, also known as
Ettringite
If there is no supply of sulfate (no added gypsum) to the initial period of adding C3HA to water, what will happen?
hydration products will be formed quickly, and a flash set will occur, accompanied by a large amount of heat of hydration and immediate condensation.
What is the use of gypsum in hydration reaction?
controls/delays the generation of hydration heat.
Describes C3A
has a fast hydration rate, generates a lot of hydration heat, and is not good for concrete quality.
What is produced for second reaction of C3A and C4AF if no enough gypsum?
Monosulfoaluminate
What are flash set and false net
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.
https://ibb.co/mtMJK2J
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)
https://ibb.co/VjwDWPr
Explain the informations given by the graph
- The hydration heat peak of C3A delays with addition of gypsum
- Cements with same fineness:
- Too little gypsum will shift the exothermic peak forward;
- Too much gypsum will shift the exothermic peak back.
- Generally control the C3A exothermic peak behind the C3S hydration exothermic peak to maintain the high workability of the concrete.
https://ibb.co/XtXJM8W
https://ibb.co/YXYbPyj
- Ettringite as first formed in Portland cement paste
- CH
3.Monosulfoaluminate (from C3A) - Ettringite needles
- CH
- Unhydrated cores
- C-S-H
- CH pores.
- Capillary
https://ibb.co/RvB9tJR
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
