The document discusses the chemical compounds found in Portland cement and how they react during hydration, including tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. It examines how the compounds influence properties like heat evolution, setting time, strength development and durability when reacted with water. The inclusion of supplementary cementitious materials like fly ash, slag, and silica fume can improve sulfate resistance, reduce permeability, and limit alkali-silica reaction.
2. Chemical Compounds of Portland
Cement
Polished thin-section
examination of Portland
clinker shows alite (C3S)
as light, angular crystals
3. Chemical Compounds of Portland
Cement
The darker, rounded
crystals are belite
(C2S)Scanning electron
microscope (SEM)
micrograph of alite
(C3S) crystals in
Portland clinker.
5. Portland Cements
Influence of Compounds
C3S Hydration:
Heat of Evolution
Setting characteristics
Hardening properties
Early Strength Development
C2S Hydration:
Heat of Evolution
Setting characteristics
Hardening properties
Later Strength Development
6. Portland Cements
Compound Influence
Fineness
Finer the cement, the faster the hydration,
higher initial strength
3 and 7 day strengths are increased by
fineness
Increased water demand of high fineness
cement may decrease strength
7. Heat Evolution
Heat evolution as a function of time for cement paste.
Stage 1 is heat of wetting or initial hydrolysis (C3A and C3S
hydration).
Stage 2 is a dormant period related to initial set.
Stage 3 is an accelerated reaction of the hydration products
that determines rate of hardening and final set.
Stage 4 decelerates formation of hydration products and
determines the rate of early strength gain.
Stage 5 is a slow, steady formation of hydration products
establishing the rate of later strength gain.
9. Heat of Hydration
Heat of hydration
Compound Name Bogue Compound
Cal/g
Tricalcium Silicate C3S 120
Dicalcium Silicate C2S 62
Tricalcium
C3A 207
Aluminate
Tetracalcium
C4AF 100
aluminoferrite
10. Relative Volumes
Relative volumes of the
major compounds in the
microstructure of
hydrating Portland
cement pastes (left) as a
function of time and as a
function of the degree of
hydration.
11. Relative Volumes
Values are given for an
average Type I cement
composition:
C3S=55%, C2S=18%, C3A=10
% and C4AF=8%. “AFt and
AFm” includes ettringite
(AFt) and calcium
monosulfoaluminate (AFm)
and other hydrated calcium
aluminate compounds.
13. Reactivity of Cement Compounds
Relative reactivity of
cement compounds.
The curve labeled
“overall” has a
composition of 55%
C3S, 18% C2S, 10%
C3A, and 8% C4AF, an
average Type I cement
composition.
14. Compound Influence on Setting Time:
Time of Final Set
C3S is major influence in final set time
Fineness of cement
Temperature of mixture
(Nearly all exothermic reactions are accelerated by heat)
Admixtures
Mass of Cementitious Material
Type of Cementitious Materials
Fly ash and slag are slower in winter
Large mass of fly ash may be slower
15. Setting Time of Cement
Coagulation-Crystalline layer of ettringite is thixotrophic network of high surface area off of
which C-S-H can grow.
Coagulation-Crystalline
C-S-H
network of ettringite
H2O
time
•C3A
•C3S •C3A
•C2S •Gypsum
•C4AF •H2O
C3S+H2O
•Gypsum
Thixotrophic
Hardening
Mixture
Mixture
17. Portland Cements
Alkalies (K2O & Na2O) - usually expressed as Na2Oeq
or total alkalies
Present in all raw materials
More soluble alkalies -
Shorter set times
Higher early strength
Lower 28 day strength
Lower AEA dose
Higher WR dose
Alkali - reactive aggregates (opal, chert)
“Low alkali cement” contains less that 0.60% total
alkalies
18
18. Portland Cements
Alkalies (K2O & Na2O) - usually expressed as Na2Oeq or total
alkalies
Present in all raw materials
More soluble alkalies -
Shorter set times
Higher early strength
Lower 28 day strength
Lower AEA dose
Higher WR dose
Alkali - reactive aggregates (opal, chert)
“Low alkali cement” contains less that 0.60% total alkalies
20. Sulfate Attack
Sulfate attack is often the
most severe at the location
of the most wetting and
drying, which is usually near
the soil line.
Here concrete posts have
been attacked by sulfates
near the soil line. The
concrete is in better
condition deep within the
soil where it is moist.
22. Alkali-Silica Reactivity
ASR Expansion vs. Total Equivalent Alkali
ASR 12 Week Percent Expansion
0.5 Y = -3.2E-02 + 7.88E-02X + 0.417862X**2
R-Sq = 84.1 %
0.4
0.3
0.2
0.1
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Total Equivalent Alkali Content (%)
Alkali Silica correlation at 14 weeks is not as great as at greater ages.
23. Fly Ash and Natural Pozzolans
Reduced Permeability & Diffusivity
Resistance to ASR
Consumption of Ca(OH)2
Reduction in water mobility
Resistance to Sulfate (Low CaO pozzolans)
Dilution of C3A
Consumption of Ca(OH)2
Iron stabilizing effect
24. Blast Furnace Slag Impact
Reduced Permeability & Diffusivity
Resistance to ASR (>35%)
Reduction in water mobility
Resistance to Sulfate
Dilution of C3A
Iron stabilizing effect
25. Silica Fume Impact
Higher Strength
Reduced Permeability & Diffusivity
Resistance to ASR
Consumption of Ca(OH)2
Reduction in water mobility
Resistance to Sulfate (Low CaO pozzolans)
Consumption of Ca(OH)2
Sulfate attack is often the most severe at the location of the most wetting and drying, which is usually near the soil line. Here concrete posts have been attacked by sulfates near the soil line. The concrete is in better condition deep within the soil where it is moist.