42907 Chap.7 Cements and supplementary cementitious materials
Cements and supplementary cementitious materials
Week 5 judges three binders against six durability mechanisms, and the examinable part is that two of the six run the other way. Fly ash, slag and silica fume all reduce chloride ingress, sulfate expansion, alkali silica expansion and the heat of hydration; they all increase carbonation depth and, except for silica fume, lower early age strength.
The chapter also sets out the four clinker phases, why a sulfate resisting cement is simply a low aluminate cement, and the compliant binder proportions the bridge code requires once the exposure class becomes severe.
What this chapter covers
- 01
The four clinker phases, their mass ranges, their reaction rates and their heats of hydration
- 02
Tricalcium aluminate: the smallest phase, the most heat, and poor resistance to sulfate attack
- 03
Gypsum controlling the hydration of that phase, and the 3.5 per cent cement sulfate cap
- 04
The seven Australian cement types, and sulfate resisting cement as a low aluminate cement
- 05
One tonne of carbon dioxide per tonne of cement, and about 8 million tonnes a year from Australian manufacture
- 06
The pozzolanic reaction: amorphous silica plus portlandite gives secondary calcium silicate hydrate
- 07
Why pozzolanic reaction is slow, and the dilution effect on early strength
- 08
Why slag is not a pozzolan, and needs activation by the alkalis, lime and heat of the cement
- 09
Fly ash, slag and silica fume compared on origin, particle size, typical replacement and availability
- 10
Class F against Class C fly ash, and which one eastern Australia produces
- 11
Silica fume working chemically and by packing, and why it raises early strength where the others lower it
- 12
The six mechanisms, four favourable and two unfavourable, and the curing that softens both unfavourable ones
- 13
Compliant binder proportions by exposure class, and why a blend becomes mandatory at the severe classes
Writing the binder line for a tidal member
- +1Classify, then look up the compliant range. A tidal surface is the most severe marine class, and for that class the code range for slag is 50 to 70 per cent of the binder by mass. A plain cement mix is not compliant at all.
- +1Apply the project cap. The instruction is to use the maximum the code allows, but supply limits slag to 50 per cent, which is inside the code range. The specification is 50 per cent slag with 50 per cent general purpose cement, fixed by the cap rather than by the code.
- +1Check the mechanism this choice hurts. Fifty per cent is above the 35 per cent threshold at which slag begins to worsen carbonation. A tidal surface is wet almost all the time, so the carbonation front there is slow regardless and chloride is unambiguously the governing load. The penalty is real but it is not on the governing mechanism.
- +1Pay for it in curing. The penalty is softened by good curing, and marine concrete already attracts an extended curing requirement, so specify the longer period and say that it is doing double duty.
Key terms
- Clinker phase
- One of the four compounds that make up Portland cement clinker, each with its own reaction rate, heat output and contribution to strength. Their proportions define the cement type.
- Tricalcium aluminate
- The clinker phase that reacts fastest and evolves the most heat per gram, at 6 to 12 per cent of the clinker. It has poor resistance to sulfate attack, which is the reason sulfate resisting cement exists.
- Sulfate resisting cement
- A cement defined by a low tricalcium aluminate content, around 4 per cent against about 12 in a general purpose cement. It is adequate for sodium sulfate conditions but not for magnesium sulfate ones.
- Pozzolanic reaction
- Amorphous silica reacting with the portlandite left by cement hydration to form additional calcium silicate hydrate. It densifies the pore network but consumes the alkalinity reserve that resists carbonation.
- Dilution effect
- The loss of early age strength that follows from replacing cement, which reacts quickly, with a material that reacts slowly. It is why blended concretes need longer before formwork is stripped or loads applied.
- Supplementary cementitious material
- An industrial by product used as part of the total binder content, chiefly fly ash, ground granulated blast furnace slag or silica fume. Each recovers a waste stream and cuts the cement, and therefore the carbon, in the mix.
- Class F fly ash
- Fly ash from black coal, carrying less than 10 per cent calcium oxide and behaving as a pure pozzolan. It is what New South Wales, Queensland and Victoria produce, since the coal burnt there is black.
- Packing effect
- The physical densification produced by silica fume particles about one hundredth the size of cement grains, which fit into the spaces between them. It reduces capillary porosity immediately rather than waiting for a reaction.
Cements and supplementary cementitious materials FAQ
Do supplementary binders always improve durability?
No, and treating them as uniformly good is a shipped defect waiting to happen. Across the six mechanisms this chapter tracks, four improve and two get worse: carbonation depth increases and early age strength falls. The four that improve are chloride ingress, sulfate expansion, alkali silica expansion and the heat of hydration.
A binder choice that is right for a submerged member can therefore be the wrong choice for a sheltered one on the same structure.
Why is slag described as not being a pozzolan?
Because it is semi hydraulic rather than pozzolanic. Its hydration products are a calcium silicate hydrate broadly similar to the one cement makes, but with a lower calcium to silicon ratio, and because of that low ratio it needs activating: the alkalis, the lime and the heat of the Portland cement beside it start the reaction. Batching slag and water alone does not produce a binder.
One practical consequence is that slag substitutes for cement one for one, with no change to the sand, the aggregates or the admixtures.
Why does silica fume behave differently from the other two on early strength?
Because part of what it does is geometric rather than chemical. Particles about one hundredth the size of cement grains pack into the spaces between them and cut the capillary porosity as soon as the concrete is placed, without waiting for any reaction. It is also an extremely reactive pozzolan, being over 92 per cent amorphous silica.
The combination raises early strength and shortens setting time, where fly ash and slag lower the first and lengthen the second.
When does the code stop allowing a plain cement mix?
At the severe exposure classes. The bridge code tabulates compliant binder proportions as a range from a minimum to a maximum for each class. In the mild classes the range still begins at 100 per cent Portland cement with no replacement at all; from the second severe class upward the minimum itself carries a replacement, so a plain mix is non compliant.
For the two most severe classes the course states the clean form as 25 to 40 per cent fly ash or 50 to 70 per cent slag.
Assessment move
Build the six row direction table from memory and then check it, because reproducing it is the single most useful thing this chapter gives you and reversing one row is a marked error.
Attach a mechanism to each direction rather than memorising the sign: the pozzolanic reaction densifies the pore network, which helps everything that has to travel through it, and consumes the portlandite, which hurts the one process that has to react through it. Learn the four clinker phases by behaviour rather than by name, since the extraction of that table is easy to misread.
For the specification itself, practise writing the binder line with both the code range and the project cap in it, and say which one bound the answer.
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