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.

1. CON 123
Cementitious Materials
Session 5
Chemical Properties

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.

4. Chemical Compound Reactions
Chemical
Name Formula Hydrate
Reaction
Tricalcium Silicate
Tricalcium Silicate C3S C-S-H
Hydrate
Dicalcium Silicate
Dicalcium Silicate C2S C-S-H
Hydrate
Tricalcium
C3A Ettringite C6AS3H32
Aluminate
Tricalcium Calcium Mono-
C3A C4ASH12
Aluminate sulfoaluminate
Calcium Sulfate
Gypsum CaSO4:2H2O CHS2
Dihydrate
Calcium
Calcium oxide CaO CH
Hydroxide
Magnesium Magnesium
MgO MH
oxide Hydroxide

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.

8. Heat Evolution

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.

12. Portland Cements
Chemical Compound Contribution to Strength

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

16. Time of Set for Portland Cements

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

19. Portland Cements

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.

21. Effect of C3A on Sulfate Resistance
Sulfate Attack 4 Month % Expansion
Y = 0.116075 - 3.71E-02X + 5.80E-03X**2
R-Sq = 0.320
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
4 5 6 7 8 9 10 11 12
C3A

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

26. Chemical Properties
Please return to Blackboard and watch the
following video:
 Video 1: Cement Hydration