42907 Chap.8 Sulfate attack, DEF and aggressive soils
Sulfate attack, DEF and aggressive soils
Week 6 covers the one deterioration process in the design project that is not modelled from first principles. Sulfate converts the monosulfoaluminate left by hydration into ettringite, which grows as needles at pressures the lecture puts above 150 MPa. The same crystal forming early is harmless; forming late in rigid concrete it cracks and spalls.
The chapter reads the aggressive soil tables, including the permeable against low permeability soil columns that are routinely taken the wrong way round, and sets out the prevention list, the low aluminate cement and the curing temperature ceiling.
What this chapter covers
- 01
Monosulfoaluminate plus sulfate plus water giving ettringite, and the expansion that follows
- 02
Early ettringite formation as benign, delayed ettringite formation as damaging
- 03
Four severity drivers: concentration, cation type, pH and the microstructure of the paste
- 04
Magnesium sulfate attacking the calcium silicate hydrate rather than the aluminates
- 05
External sources: high sulfate soils and groundwaters, pollution, sea water, and acid sulfate soils
- 06
Biogenic sulfuric acid from anaerobic bacteria in sewers and treatment plants
- 07
The three preconditions: permeability, water and a sulfate rich environment
- 08
Internal attack from over sulfated cement or gypsum contaminated aggregate, in a sulfate free environment
- 09
The positive feedback: attack raises permeability, which admits more of everything
- 10
The aggressive soil table, its four chemistry bands and its two soil condition columns
- 11
Why the permeable soil in groundwater is always the harsher column
- 12
Notes that change the answer: chlorides suppressing expansion, sulfate resisting cement, protective coatings, pH sampling
- 13
The prevention list, and low permeability as the best protection
- 14
Delayed ettringite formation from steam curing, and the temperature ceilings that control it
Classifying the footing of a bridge in a mangrove
- +1Check the magnesium gate first. Below one gram per litre, so the sulfate soil table applies. At or above that figure the building code would have refused to classify the surface and sent it to specific assessment.
- +1Pick the soil condition. A silty clay is a low permeability soil, so it is condition B rather than A, one step milder than a sand in the same water would have been.
- +1Enter the band. Over 10,000 parts per million in the groundwater with a pH below 4 is the bottom row of the table.
- +1Read the class from the right code. The building code gives B2 for that row and condition; the bridge code, written for a hundred years, gives C1 for the same ground. The design life decides which table you are in.
- +1Follow the consequences. A severe class in acid sulfate ground attracts a recommendation for sulfate resisting cement and for a protective coating, and the footing is cast against the ground, so the cover read from the class carries the cast against ground increase on top.
Key terms
- Ettringite
- An expansive calcium sulfoaluminate hydrate formed when sulfate reaches the aluminate hydration products. Forming in fresh paste it regulates setting harmlessly; forming in rigid hardened concrete it cracks and spalls it.
- Delayed ettringite formation
- Ettringite forming at late ages in concrete that is already rigid, producing expansion that the material cannot accommodate. High curing temperature and continued moisture are its principal triggers.
- External sulfate attack
- Attack in which the sulfate arrives from outside, through the porosity, from soil, groundwater, sea water or industrial effluent. Because the sulfate has to diffuse in, porosity is the control variable.
- Internal sulfate attack
- Attack in which the sulfate was already inside the concrete, from over sulfated cement or from gypsum contaminated aggregate. It can occur in a completely sulfate free environment, so only the constituents can prevent it.
- Acid sulfate soil
- Naturally occurring sediment containing iron sulfides, found in mangroves, salt marsh, tidal areas and low lying coastal floodplains. It is exactly the ground bridge foundations tend to be built in.
- Soil conditions A
- High permeability soils such as sands and gravels that sit in the groundwater. For identical chemistry this column always gives the more severe class, because a permeable soil keeps delivering fresh sulfate.
- Soil conditions B
- Low permeability soils such as silts and clays, or any soil above the groundwater. It is the milder of the two columns because delivery of sulfate is limited by diffusion rather than by flow.
- Brucite
- Magnesium hydroxide, which together with magnesium silicate hydrate is the evidence of magnesium sulfate attack. Its presence indicates that the calcium silicate hydrate itself, rather than the aluminates, is being destroyed.
Sulfate attack, DEF and aggressive soils FAQ
Why can cover not protect against sulfate attack the way it protects against corrosion?
Because nothing is happening to the steel. The paste itself is being destroyed, throughout the thickness the sulfate has reached, so adding cover simply adds more material to be attacked. That is why the whole prevention list is aimed at permeability and mix composition rather than at geometry, and why the design project asks you to design the footings for aggressive ground by the Standard rather than by modelling a front.
Does sulfate resisting cement solve every sulfate problem?
No. It works by carrying less tricalcium aluminate, which is the phase that forms the monosulfoaluminate the attack converts. Magnesium sulfate attacks the calcium silicate hydrate instead, replacing the calcium in it and destroying the binding properties directly, so a low aluminate cement does not address it.
The building code says the same thing in its own note: sulfate resisting concrete is adequate for sodium sulfate conditions, and magnesium sulfate conditions need specific consideration.
Why is the same crystal harmless once and destructive later?
Because of when it forms rather than what it is. Early ettringite formation happens within hours in a mix that is still plastic, so the volume change is accommodated and the reaction simply regulates the setting time. Delayed ettringite formation happens at late ages in concrete that is already rigid, where the same volume change has nowhere to go and produces cracking and spalling. The distinction is mechanical, not chemical.
How can steam curing cause an attack years later?
By building the conditions for internal sulfate attack into the element at the casting yard. High temperature during curing raises the risk of delayed ettringite formation, with the danger zone around 85 degrees, and the damage appears not long after the heat cycle ends and worsens in humid storage.
Australian road authorities cap curing temperature in their specifications around 70 to 75 degrees, and one of them caps the product of duration and temperature as well.
Why do chlorides appear in a note about sulfate attack?
Because they change the outcome. The code note records that in the presence of chloride ions, sulfate attack generally shows little disruptive expansion, except under wetting and extreme drying where crystallisation causes surface fretting. A marine footing therefore has an unusual combination: the chloride that threatens the steel partly protects the paste.
It is worth a sentence in a report, because it is counter intuitive and it is in the Standard.
Assessment move
Learn this chapter as two tables and one distinction. The two tables are the aggressive soil classification, with its four chemistry bands and two soil condition columns, and the prevention list. The distinction is early against delayed ettringite formation, which is the answer to why the same reaction appears in every concrete and only sometimes destroys it.
Practise the magnesium gate as the first move of any soil classification, since it can stop the process before the chemistry is even read. Be very careful with the two soil columns: write down whether the soil is permeable and whether it is in the groundwater before you look at the table at all.
And remember that this is the process the design project handles by the Standard alone, so the examinable skill is reading the table, not running a model.
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