SCMs, Admixtures & Alternative Cements

Air Entraining Agent - how does it work

• Disperse trapped air evenly

• organic molecules - negative hydrophobic heads.

• when form voids, heads stick out repelling other bubbles → even dispersion of bubbles/

Air Entraining Agents - pros (3), cons(1)

• freeze thaw resistance → the equal voids allow for water to expand upon freezing

• ↑ workability & cohesion

• ↓ strength bc of voids

superplasticisers - how do they work & therefore the main pro

• organic molecules absorbed into surface of cement grain

• each have a slightly neg charge → cement grain neg charge

• each grain repels each other → prevents formation of ‘flocks’

• Flocks = grains surrounding water molecule

• Therefore, allows water to escape → water surrounds surface of each grain

• IMPRVES WORKABILITY bc can now flow! - w/out changing water content

Accelerators (mechanism & 3 uses)

• ↑ rate of hardening → by ↑ hydration to CSH

• chloride based ones banned bc steel reinf

• cold weather, repairs, precast

Retarders (mechanism & 4 uses)

• delays setting of concrete so can still flow

• coats surface of cement grains → prevent access of water so delays hydration

• pumping, transportation, hot weather, large pours

What is a Type 1 addition? (and why is it added)

Limestone

Hydration rate ↑ bc limestone not inert

↑ effective w/c early on

reacts w C3A

fine so fills voids → ↑ density

Type 2 additions

Pozzolans (aka SCMs)

• dont have cementitious properties but react w lime & water → ↑CSH gel

OPC vs cements w type 2 (3)

• slower reaction

• ↑CSH gel

• CSH has ↓ C/S ratio

GGBS properties

Slow hydration and heat evolution → ↑ strength and durability

chloride resistance

Pozzolanic Reaction

Silica + Portlandite (CH) → CSH

PFA (2 pros, 1 con maybe)

• spherical glassy particles - dont need to grind & ↑ workability

• ↓ porosity

• less effect on strength & permeability than GGBS

Silica Fume (2 pros)

• added to high strength concrete

• Fills capillary porosity

• ↑ Workability

Metakaolin (2 pros, 2 cons)

• good chloride resistance → bc ↓ permeabilty

• v reactive

• Expensive

• Not a waste product

RHPC - Rapid Hardening

• Fast heat evolution

• limited to small sections

LHPC - Low Heat

• good for large pours

• hardening rate determined by fineness

• Addition of GGBS will ↓ temp and ↑ permeability

Modified PC ( Sulphate Resisting)

• Low C3A content

• reduces expansion of concrete

Required properties of alternative cements (5)

1) cheap

2) Lower CO2

3) Raw material availability

4) durable

5) pH sufficient to passivate steel