Materials- Blended Cements and Steel Reinforcing Flashcards
What does SCM stand for in blended cements?
Supplementary cementitious materials
Effects of using SCMs
Most react more slowly than Portland cement which can reduce early strength but can improve final properties such as strength and durability. Also leads to reduction in CO2 emissions in manufacture of concrete.
What are the 3 main types of SCMs?
Pozzolans, ground granulated blast furnace slag, limestone
What are SCMs defined as?
A powder which itself is not cement (i.e doesn’t generate useful strength when mixed with water), but which reacts and contributes to material performance when blended with Portland cement and water.
What is involved in the pozzolanic reaction?
The pozzolans are sources of SiO2 and/or Al2O3 (often waste materials which are cheap and don’t contribute much CO2 release). They react with CH (portlandite) to form more C-S-H or C-A-S-H. Reaction generally slower than cement hydration.
How does pozzolanic reaction improve strength and durability?
Don’t need much portlandite for its protective properties so there is often a surplus. Usually reduces early strength. Then fills in pore spaces at longer age, refines pores. Reduces permeability. Prevents damaging agents entering concrete.
Examples of pozzolanic additives
Fly ash (by-product from coal-fired power generation). Natural pozzolans (volcanic earth). Heat-treated (calcined) clays. Silica fume (by-product of semiconductor Si manufacture). Rice husk ash (by-product from burning agricultural waste). Other waste silicate glass materials.
Describe fly ash as a pozzolan
Medium sized spherical particles (many are hollow). Reacts fairly slowly. Can use up to about 55% in Portland cement.
Describe calcined clay used as a pozzolan
Heat treatment of clays at 600-800°C gives disordered, highly reactive phase. Kaolinite clay goes to metakaolin. Halloysite clay goes to metahalloysite. Forms pozzalinic and additional AFm phases. However, takes up a lot of water so maximum blending ratio with cement is 35%.
Describe silica fume as a pozzolan
Reaction CH+xS->C-S-H
Very small SiO2 particles (0.2μm diameter). High water demand so can’t use greater than 10%. Unusually, provides high early strength development. Densifies pore structure to reduce permeability and improve concrete durability.
Rice hull (husk) ash as pozzolan
Remnant ash is mainly silica in small particles. Fairly pure source of silica. Much cheaper than silica fume in most parts of world. Need to control carbon content carefully. Acts same as other silica sources.
Examples of other potentially pozzolanic wastes
Various metallurgical slags. Bottle, window glass. Solid waste incineration ash. Plasma vitrification residue. Brick and ceramic wastes. Many more.
How does slag work as a SCM?
Reacts by process similar to pozzolanic reaction to form C-S-H (and C-A-S-H). Separately classified because it is much more reactive (therefore better) than normal pozzolans. Is a by-product of iron manufacturing. Has composition similar to clinker but lower Ca content. Can use up to 95% replacements for Portland cement.
Limestone as a SCM
Has CaCO3. Used in this way unburned. Usually very finely ground for reactivity. Cheapest of all admixtures. Small amount (<5%) can give improved properties but more doesn’t react and reduces strength. Combination with pozzolans can give slight increase in strength by forming extra AFm phases.
Categories of Portland cement
CEM 1 Portland cement (>95% PC)
CEM 2 Portland-composite cement (65-95% PC and 1 pozz/LS)
CEM 3 Blastfurnace cement (5-65% PC and slag)
CEM 4 Pozzolanic cement (45-89% PC and 1 pozz)
CEM 5 Composite cement (20-65% PC cement and slag and 1 other SCM)
