Final Prep Flashcards
Steps for the Production of Portland Cement
- Stone crushed to 125mm and again to 20mm size
- Raw materials are either ground to powder and blended, or ground, mixed with water to form slurry, and then blended
- Raw mix is kiln burned to partial fusion
- Adding Gypsum to clinker
Calcium Oxide
CaO
-> C
Silicon Dioxide (Silica)
SiO2
-> S
Aluminum Trioxide (Alumina)
Al2O3
-> A
Water
H2O
-> H
Sulphur Trioxide
SO3 _
-> S
Iron Oxide
FeO Fe2O3
->F
Calcium Hydroxide
Ca(OH)2
-> CH
Tricalcium Silicate
Alite
C3S
50 wt%
Dicalcium Silicate
Belite
C2S
25 wt%
Tricalcium Aluminate
Aluminate
C3A
12 wt%
Tetracalcium Aluminoferrite
Ferrite
C4AF
8 wt%
Gypsum
Gypsum
CS_H2
4-6 wt%
Type GU
General Use
Suitable for all uses where the special properties of other types are not used
Composition of GU cement
Tricalcium Silicate (C3S) 50 wt%
Dicalcium Silicate (C2S) 25 wt%
Tricalcium Aluminate (C3A) 12 wt%
Tetracalcium Aluminoferrite (C4AF) 8 wt%
Gypsum (CS_H2) 4-6 wt%
Type HE
High Early - provides strength at an early period, usually a week or less
Used when forms need to be removed as soon as possible or when the structure must be put into service quickly
In cold weather, it permits a reduction in the length of curing period
Sulphate Attack
Sulphates in moist soil or water may enter the concrete and react with the hydrated C3A, resulting in expansion, scaling and cracking of concrete
Why does sulphate attack lead to cracking?
Formation of the expansive product ettringite
Type MS
Used where precaution against modern sulfate attack is important
Contains no more than 8% tricalcium aluminate (C3A)
Type HS
Uses in concrete exposed to severe sulfate action - principally where soils or groundwater have a high sulfate content
Gains strength more slowly than Type GU
High sulfate resistance - low tricalcium aluminate (C3A), no more than 5%
What is crucial to the performance of any concrete exposed to sulfates?
Low water to cementitious materials ratio
Low permeability
Type LH
Used where the rate and amount of heat generated from hydration must be minimized
Develops strength at a slower rate than other cement types
Intended for use in massive concrete structures, where the temperature rise resulted from heat generated during hardening must be minimized
Heat of hydration
The heat energy released during the chemical reactions between water and the components of Portland Cement, specifically the hydration of cement particles
-> Highest for C3S and C3A
How are low heat cements produced?
Increasing the amount of C2S and lowering the amount of C3S
-> C3S constitutes the majority of the phases in clinker
Canadian Types of low heat producing Portland Cements
MH
LH
Compare the appearance of alite and belite
Alite: light, angular crystals
Belite: dark, rounded crystals
Hydration Reaction for C3S
2C3S + 6H = C3S2H3 + 3CH
ΔH = -500 J/g
Hydration Reaction for C2S
2C2S + 4H = C3S2H3 + CH
ΔH = -250 J/g
Hydration of the silicates
calcium silicate + water = calcium silicate hydrate + lime
Negative enthalpy = exothermic
Calcium Silicate Hydrate
CSH
No specific chemical composition because it is neither completely crystalline nor entirely amorphous
-> Fibrous Nature
Hydration of the Aluminates
C3A + 6H = C3AH6
Aluminate + Water = Tricalcium Aluminate Hydrate
C3A Compound in the absence of Gypsum
Will react vigorously with water to produce a large amount of hydration product in a very short time. This causes the setting of the paste within a few minutes and thus impairs the fluidity of the mixture
Flash Set
The almost immediate stiffening caused by the hydration of C3A
-> robs the paste of it fluidity, also not desirable from the strength standpoint
Gypsum (Role, explanation, time of adding)
To prevent flash setting
In the presence of gypsum, the C3A reacts to form ettringite, which builds up as a layer around the C3A particle and prevents further hydration temporarily
Added to the cement clinker at the time of grinding
Hydration of Aluminates in the presence of Gypsum
C3A + CS_H2 + 26H = 3C6AS_3H32
C3AF + 2CH + 14H = C4AFH13 + AFH3
Monosulphatealuminate
In the event of less than adequate gypsum, the result of ettringite reacting further with C3A to form a compound containing lower amounts of sulphate
Basis for Sulphate Attack
C4AS_H12 + 2CS_H2 + 16H = C6AS_3H32
Monosulphate+gypsum+water=ettringite
Notes on CH
- Negative effect on strength
- Positive effect on the long-term response of reinforced concrete
- Highly alkaline -> highly basic nature of pore fluids, forming a passive layer around steel preventing corrosion
- Easily Soluble, increased porosity, bad for durability
Main Difference Between Mineral and Chemical Admixtures
Chemical admixtures are water soluble, mineral admixtures are mostly inert when in water
Mineral Admixtures
Replace cement to achieve better strength and durability
Chemical Admixtures
Required to achieve ease of placement and better durability
Water Reducing Admixture Categories
- Salts and derivatives of Lignosuphonates
- Salts and derivatives of Hydro-Carboxylic Acids
- Polymeric Materials
WRA Categories
Conventional -> salts and derivatives of lignosuphonates or hydro-carboxylic acid
Superplasticizers (High range WRA) -> salts and derivatives of lignosuphonates and polymeric materials
Flocculation
Small particles (like cement grains) stick together or “clump” together in a loose, flock-like structure
-> caused by opposing charges on adjacent particles, causing attraction forces
Purpose of WRA
Molecules of WRA and HRWRA interact to neutralize surface charges
-> improves workability and reduces water demand
Air Entraining Admixture
Hydrophilic (water loving) molecules and Hydrophobic (water fearing) molecules align themselves favourable in the mix such that a stable air bubble is formed
Pozzolanic Reaction
Used to convert CH to CSH to reduce ecological footprint and improve concrete strength and durability
CH + S + H = CSH
Non-Hydraulic Cement
A binder based on calcium oxides alone
- Calcium oxides react with water directly, and become fluid once dissolved, allowing for placement and being shaped into forms, solidifying when dry to become load-bearing
Fly Ash
Any fine particulate precipitated from the stack gases of industrial furnaces burning solid fuel
Fly Ash Class F
C = < 5%
S = > 50%
A = 20-30%
Fly Ash Class C
C = 20-30%
S = > 30%
A = 15-25%
Slag
A hydraulic cement consisting essentially of silicates and aluminosilicates of calcium developed in a molten condition simultaneously with iron in a blast furnace
C = 35-45%
S = 32-38%
A = 8-16%
Silica Fume
A by-Product of the smelting process in production of silicon metal or ferrosilicon alloys
S > 90%
Due to fineness and high amorphous silica content, highly reactive pozzolan
Increases water demand
Metakaolin
A by product of the glass and porcelain manufacturing industries
S = 30-35%
A = 55-65%
Pozzolans Effect
A densification of the paste leading to lower permeability and higher strength
Long Term Requirements
Hardened concrete
strength
durability
volume stability
Short Term Requirements
Plastic State (fresh concrete)
Transport ability
handling
placing
finishing
Important Fresh Concrete Properties (5)
Workability
Consolidation
Air Content
Segregation
Bleeding
Workability
The property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished
Consolidation
The agitation of concrete to remove all entrapped air and eliminate unwanted voids
Air Content
Future Resistance to freeze-thaw
Segregation
The tendency for sand-cement mortar to separate from the coarse aggregate or the cement mortar to separate from fine aggregates
Bleeding
The tendency for water to rise to the surface, causing weakness or dustiness of the surface of the concrete
Rheology
The science that deals with the deformation and flow of materials under stress
Thixotropic
A time-dependent fluid
Slump Test
A measure of consistency of concrete
Slump Factors ( 4+ 2- )
Increase Slump:
- Higher w/c ratio
- Round Aggregates
- Fly Ash
- WRA, AEA
Decrease Slump:
- Higher aggregate/cement ratio
- Silica Flume
Setting
The onset rigidity of fresh concrete
Hardening
The change in measurable strength
Initial Set
When the paste begins to stiffen considerably
- Vicat needle time of penetration 25 mm into the paste
Final Set
When the paste has become rigid to the point that it can sustain some load
- Vicat needle time of penetration 0 mm into the paste
False Set
Rapid stiffening of the concrete shortly before mixing without evolution of much heat
- Occurs due to the crystallization of gypsum
Flash Set
Rapid Development of rigidity with evolution of considerable heat
- Occurs due to the rapid hydration of C3A
