SCMs and Admixtures Flashcards
What are the two types of SCMs?
Supplementary cementitious materials. Pozzolanic and cementitious
What are admixtures and how do they work?
Finely divided materials (powders) which are added to Portland cement concrete and mortar mixes to get specific properties. They are usually dissolved in the mixing water.
What are the advantages of scms?
They reduce cost because they reduce the amount of expensive cement needed. They enhance durability. Are sustainable and environmentally friendly because they are usually byproducts (waste materials)
What is the full name of slag and what is it?
Blast furnace slag. Waste from steel blast furnaces. When combined with only water it results in CSH.
What are the three pozzolans and what do they need in order to react w water?
Fly ash (from burning coal), silica fume (dust from silicon manufacture), calcined clay (from modified clay). They need CH to react with water and result in CSH. They are very small and round in shape. Silica is smallest.
What are the 4 types of chemical admixtures?
Air entraining, set retarding, set accelerating and water reducing.
What is the mechanism of air entraining admixtures?
Surfactants act at the air-water interface and create stable bubbles during mixing. Polar hydrophobic and hydrophilic ends. Protect against freeze thaw damage.
What is the mechanism of water reducing admixtures?
Decrease in w/c ratio for same workability. Increase in strength and less air. Acts as dispersants: decrease in inter particle resistance which prevents flocculation and changes the surface properties of the cement grains (ionic repulsion) which frees the water.
Increases strength ultimate strength and decreases air content
What is the difference between water reducing and plasticizer?
Water reducing: decrease in wc ratio for same workability. Plasticizer: increase in workability for same wc ratio.
What is the set accelerating mechanism?
Sets faster BUT increases drying shrinkage AND increases corrosion potential. Provides additional calcium species into early hydration reactions which increases rate of early strength development. Good for cold weather
What is the set retarding mechanism?
Slows down early hydration reactions and decreases rate of early strength development. Creates a layer around the cementitious particles which slows down hydration reactions. Increases workability time without affecting ultimate strength. Good for hot weather
