Silica Fume (Microsilica) — Pozzolanic Performance Addition

Primary uses

• Pozzolanic reaction
• Permeability reduction
• Cohesion and rheology modification

Applications

• High-performance and architectural precast mixes
• GFRC matrices
• Repair mortars requiring low permeability
• Shotcrete and infrastructure concrete

Advantages

• Reduces permeability substantially, which is central to durability in de-icing salt and marine exposure
• Contributes to high strength and a dense matrix in demanding precast and GFRC work
• Widely accepted in Canadian durability-led specifications for infrastructure work

Limitations to consider

• Very high fineness increases water demand, so it is almost always paired with a high-range water reducer
• Dispersion is difficult; poor mixing leaves agglomerates that do nothing for durability
• Reactivity and contribution to the mix are established by trial batches and testing, not by catalogue values
• Dosage, addition rate, water content and binder proportions are decided by your own mix design and trial batches, not by a catalogue figure. Supplier literature ranges are a starting point for that development work and are not reproduced here as a specification.
• Where the finished element goes into a building, the performance that matters is the performance of your element under the applicable code and project specification, not the property of a single ingredient. Testing and conformity evidence for the finished product are the responsibility of the producer and are established by project-specific testing.
• Mix ratio, pot life, working time, demould time, full-cure schedule, hardness, tear strength, elongation, shrinkage, viscosity and temperature resistance are properties of one specific supplier system. They are never quoted here from memory or by analogy with a similar product; they are read from the supplier data sheet for the exact material offered and confirmed at quotation.

How to choose the right grade

• Start from the element being produced and the specification it has to satisfy, because binder, aggregate and admixture choices are decided together rather than one at a time
• State the placing method — poured, vibrated, sprayed or pressed — since rheology requirements follow from it
• Describe the curing regime and the plant conditions, including whether production continues through a Canadian winter
• State the durability requirement driving its use, and let the mix designer set the replacement level
• Confirm the superplasticiser it will be run with, since the two are effectively one decision
• Mould-making and casting behaviour depends on the pattern, the shop temperature and humidity, the mixing method and the operator. A production-representative trial on your own pattern and in your own shop conditions is the only honest way to qualify a material before a run is committed.
• Exact specification is confirmed against supplier documentation at quotation.

Typical grades

• Densified and undensified products as the supplier classifies them
• Slurry products where the plant is equipped for liquid dosing
• Buyers search this category by grade name, and those names describe what the workshop needs rather than what is asserted here. Which grade a specific offer corresponds to, and what the supplier declares for it, is read from the technical documentation issued for the exact material quoted.
• Materials in this category interact: rubbers, releases, sealers, resins, pigments and fillers can inhibit cure, stain a surface or shorten mould life when they are combined outside the combinations the supplier has tested. State every material already in the process at enquiry so compatibility is checked against supplier documentation rather than discovered on the shop floor.
• Reactive materials age in the container, and moisture-sensitive systems age faster once opened. Order against real consumption rather than against a discount, and confirm the shelf life the supplier states for the exact product and pack.