Cement Replacement Materials

• Cement replacement materials or mineral admixtures are materials used to contribute to the properties of hardened concrete through hydraulic or possolanic activity or both.

• Types:1. Fly ash( pulverized-fuel ash:pfa)2. Ground Granulated Blast Furnace Slag(GGBFS)3. Silicon fume.4. Natural pozzolans.

• Fly ash ( pulverized-fuel ash: pfa): It is the ash precipitated electrostatically or mechanically from exhaust gases of coal-fired power stations. It has spherical particles and high fineness which give it advantages from water requirements.

• Ground Granulated blast-furnace slag(ggbs): Waste product in production of pig iron. It is a mixture of lime, silica and alumina. It generally reduces water demand and improves workability. Blending of PC and ggbs produces portland blast furnace cement.

• Silica fume: Also known as microsilica or condensed silica fume. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy. It is highly reactive, the smallness of the particles speed up the reaction with calcium hydroxide. Very small particles of silica fume can enter the space between the particles of cement and thus improves packing.

- silica fume is usually incorporated in the mix at the batcher.

- blended cement containing silica fume( 6 ½ to 8% by mass)

• Natural Pozzolans: These are materials with presence of moisture, chemically reacts with the calcium hydroxide released by the hydration of Portland cement to form calcium silica hydrate.

• How to use these materials? These materials are used as:

1. Supplementary cementing materials added to concrete as part of the total cementing system. Or

2. Used as a partial replacement of Portland cement or blended cement.

• Why use these materials?These materials are used to improve a particular concrete

property.• Optimum amount: to be established by testing / trial mix.• Why testing:1. To determine whether the material is indeed improving

the property of concrete.2. To determine the correct dosage rate. The correct

dosage rate is important as an over dose can be harmful while underdose may not achieve the desired effect.

• These materials react differently with different cement.

• Two or more of these materials can be combined to optimize concrete properties. Mixtures using three cementing materials called ternary mixtures.

• High performance concrete: It is concrete having a high durability, low permeability as well as high strength. The strength sometimes reaches in excess of 80 Mpa( 12000 psi)

• What makes the concrete a high performance? - very low w/c ratio (as low as 0.2) - high dosage of superplasticizer. - High quality aggregate. - mixtures of fly ash or GGBF slag. - Add 5-15% of silica fume by mass of cementitious

materials.

• One advantage of high strength concrete is that concrete columns sections can be reduced in size or less amount of steel.