High Performance Concrete Flashcards
What is the typical compressive strength of concrete?
20-30MPa
What’s the definition of a high performance concrete?
Compressive strengths ~120 MPa
What are pozzolanas?
> silica-bearing fine minerals
volcanic ash, diatomaceous earth, burnt clay, zeolites
fly ash (amorphous silica byproduct from coal production), blast furnace slag
What are the qualities of pozzolanas?
> fine particle size = open porosity
How is CSH formation during the hydration of cement compare to CSH formation using Pozzolanas?
> the formation of CSH fibrils with pozzolanas is MORE GRADUAL than via hydration of cement.
substitution of pozzolana and lime for cement can LOWER COST, but it can also REDUCE HEAT EVOLUTION, which can be an issue for massive constructions
What do pozzolanas do?
> these particles react with Ca(OH)2 hexagonal platelets to form more CSH fibrils
What is a highly effective, common pozzolana? What is it made of?
> microsilica
microsilica is a byproduct of the production of silicon, it is a condensed mist of SiO2
What does microsilica do in cement?
> it results in a denser packing of particles to fill in the voids between cement particles
gel formation on alite particles is INHIBITED
Si4+ and Ca2+ RAPIDLY NUCLEATE and GROW on microsilica particles
this attenuates the available Ca2+, which can nucleate Ca(OH)2. The microsilica further removed already formed Ca(OH)2 to form more CSH fibrils
net result of more CSH fibrils forming mechanical bonds with the aggregate
resulting concrete has increased abrasion resistance because AGGREGATES are HARDER TO PULL OUT-UTS is raised
What are the relative sizes of cement particles and microsilica particles?
cement: 1-100um
microsilica: 100nm
Where is the zone of weakness in concrete?
> there is a porous zone adjacent to aggregate particles with a HIGH CONCENTRATION OF CSH FIBRILS on aggregate surfaces, forcing a LOW LOCAL concentration of CSH fibrils
this is the zone of weakness
