TUT 04 : SCMs and admixtures Flashcards
name four common supplementary cementing materials and describe where they come from
- blast funace slag - waste from smelting iron
- fly ash - residue from burning coal
- silica fume - residue from silicon production
- calcined clay - modified natural mineral
name all four types of chemical admixtures and explain why one would use them
- air entraining. - to add entrained air to resist freeze-thaw damage
- water reducing - to reduce w/c and or increase workability
- set accelerating - to increase early strength developpement
- set retarding - to increase workability time
explain how air entrainment helps protect against but does not prevent freeze-thaw damage
- impermeable entrained air bubbles are formed in cement paste
- water fills permeable entrapped air bubbles and freezes and begins to expand
- cracks form to find a place for water t expand but when there is no space to accommodate the expansion, the cracks are large
- cracks follow path of least resistance and reach entrained air voids (crack easily), cracks enter entrained air voids, provide space for water to expand in
- damage is not prevented becasue cracks still occur but mititgates gamage by reducing severity of cracking
what does SCM mean
supplementary cementitious materials
- also know as admixtures
what is SCM’s (use)
finely grained particles added to portland cement concrete and mortar mixes to obtain specific engineering properties
what is a chemical admixture
adding small amounts of materials (dissolved in mixing water) to concrete
- changes physical properties of fresh and hard concrete
what are the types of SCM’s
- cementitious and pozzolanic
what are the advantages of scm’s
- economic
- sustainable
- increases durability
- alternative to cement and other materials
example of cementitious scm and what is needed for it to react
- blast furnace slag - comes from smelting of iron
- reacts with water alone
example of pozzolanic scm and what is needed for it to react
- fly ash - residue from coal production
- silica fume - residue from silicon production
- calcified clay - from modified clay
needed to react: calcium hydroxide and water
what are the microscopic properties of CH
- thin, hexagonal plates
- higher surface to volume ratio
- highly reactive
- leaches during efflorescence
what are the microscopic properties of CSH
- dense, amorphous, crystalline structure
- interstitial pores are too dense for water flow
- responsible for impermeability
what are the 4 common types of chemical admixtures
- water reducing
- air entraining
- set retarding
- set acceleration
how does air entraining work?
- surfactants act at air-water interface
- create stable microscopic bubbles of foam from mixing
- have one hydrophobic end and one hydropholic end
- results in stable bubble that persits through hydration by volume of concrete
whats the difference between entrained air and entrapped air
- entrained air:
intentional (added by using admixtures), consistent in size, uniformely dispersed, waterproof even after concrerte hardens - entrapped air:
accidental added by mixing, placing and consilidation mistakes - reduced by proper compaction
