T2-5: Sulfate Attack Flashcards
(17 cards)
What source contains high levels of sulfate?
Many natural and polluted ground waters
How can sulfate solutions attack Portland cement-based concrete?
Sulfate solutions can attack Portland cement-based concrete:
- leads to formation of AFt (ettringite) and sometimes gypsum
- the expansion and microcracking associated with sulfate attack is often attributed to AFt formation
What is the chemical composition of Ettringite?
NB. shorthand = Ettringite
Why is ettringite often considered an undesirable product?
Its formation is associated with expansion, and thus with structural failure
Despite a volume shrinkage, volume of solids more than doubles
[NAQ] volume relationships between C3A and gypsum
[NAQ] volume relationships for AFm to AFt
[NAQ] volume relationships for Type K cement
[NAQ] volume relationships for mine cement
Why is there expansion in some cases and not others (x2)?
What affects crystal morphology?
- Consistency of material when ettringite is formed; ettringite will not occur if paste is sufficiently fluid
-
Crystal morphology (shape and size); the time that they form and location of crystals in microstructure are of primary importance
- presence of Ca(OH)2 [calcium hydroxide] affects the morphology of the ettringite and where in the microstructure it is deposited
Where does expansion of cements occur?
When does the formation of ettringite produce:
a) high stresses
b) more evenly distributed
Expansion occurs at the surface of the anhydrous phase, from the orientated growth of ettringite crystals
a) Produces high stresses, especially when its formation is concentrated in relatively few places
b) Stresses are more evenly distributed if precipitation occurs more evenly throughout material
Where is the zone of ettringite formation?
In sulfate attack:
a) what extra cations are needed for the conversion of AFm to AFt
b) which phase do they initally come from
a) Ca2+ [calcium ions]
b) Ca(OH)2 [calcium hydroxide]
What ions are removed from the C-S-H during sulfate attack?
What is this called and what effect does it have
Ca2+ (calcium ions)
Decalcification; weakens or destroys the integrity of the paste
Summarise the process of attack by sodium sulfate (x4)
Summary: AFt (ettringite) formation, CSH decalcification
- AFm replaced by AFt; Ca(OH)₂ decreases; no C-S-H decalcification
- Ca(OH)₂ disappears; C-S-H partially decalcifies; gypsum forms in veins
- Cracking appears parallel to surface; gypsum linked to cracks
- Further decalcification; material loss from cracking or dissolution (more extensive loss at corroded edge)
Is external sulfate attack manifested by:
a) expansion or cracking, or
b) loss of cohesion and strength
Why?
Loss of cohesion of strength
- These properties are primarily due to the C-S-H
- Expansion is due to the enhanced swelling of the (decalcified) C-S-H
What phase can the C-S-H be described as?
What does the decalcification of the C-S-H do?
The binding phase
Weakens/destroys the integrity of the paste
What does C-S-H stand for
Calcium silicate hydrate
