UNIT- 1 Cement, Aggregates and Admixtures Portland Cement Concrete is made by portland cement, water and aggregates. Portland cement is a hydraulic cement that hardens in water to form a water-resistant compound. The hydration products act as binder to hold the aggregates together to form concrete. The name portland cement comes from the fact that the colour and quality of the resulting concrete are similar to Portland stone, a kind of limestone found in England. 1.1.1 Manufacture of Portland cement Portland cement is made by blending the appropriate mixture of limestone and clay or shale together and by heating them at 1450oC in a rotary kiln. The sequence of operations is shown in following figure. The preliminary steps are a variety of blending and crushing operations. The raw feed must have a uniform composition and be a size fine enough so that reactions among the components can complete in the kiln. Subsequently, the burned clinker is ground with gypsum to form the familiar grey powder known as Portland cement.
The raw materials used for manufacturing Portland cement are limestone, clay and Iron ore. a) Limestone (CaCO3) is mainly providing calcium in the form of calcium oxide (CaO) CaCO3 (1000oC) → CaO + CO2 b) Clay is mainly providing silicates (SiO2) together with small amounts of Al2O3 + Fe2O3 Clay (1450oC) → SiO2 + Al2O3 + Fe2O3 + H2O c) Iron ore and Bauxite are providing additional aluminium and iron oxide (Fe2O3) which help the formation of calcium silicates at low temperature. They are incorporated into the raw mix. Limestone High temperature 2 3 CaOoSiO2 CaOoSiO2 Clay (1,450 oC) Iron Ore, Bauxite 3 CaOoAl2O3 4 CaOoAl2O3oFe2O3 d) The clinker is pulverized to small sizes (< 75 μm). 3-5% of gypsum (calcium sulphate) is added to control setting and hardening. The majority particle size of cement is from 2 to 50 μm. A plot of typical particle size distribution is given below. (Note: “Blaine” refers to a test to measure particle size in terms of surface area/mass)
Chemical composition a) Abbreviation: CaO = C, SiO2 = S, H2O = H, SO3 = S (sulphur trioxide) Al2O3 = A; Fe2O3 =F, Ca(OH)2 = CH, Thus we can write 3 CaO = C3 and 2 CaOoSiO2 = C2S. b) Major compounds: Compound Oxide composition colour Common name Weight percentage C3 S white Alite 50% C2 S white Belite 25% C3 A white/grey --- 12% black Ferrite 8% Tricalcium Silicate Dicalcium Silicate Tricalcium Aluminate Tetracalcium Aluminoferrite C4AF Since the primary constituents of Portland cement are calcium silicate, we can define Portland cement as a material which combine CaO SiO2 in such a proportion that the resulting calcium silicate will react with water at room temperature and under normal pressure. c) Minor components of Portland cement The most important minor components are gypsum, MgO, and alkali sulphates. Gypsum (2CaSO4o2H2O) is an important component added to avoid flash set. Alkalies (MgO, Na2O, K2O) can increase pH value up to 13.5 which is good for reinforcing steel protection. However, for some aggregates, such a high alkaline environment can cause alkali aggregate reaction problem.
Hydration The setting and hardening of concrete are the result of chemical and physical processes that take place between Portland cement and water, i.e. hydration. To understand the properties and behaviour of cement and concrete some knowledge of the chemistry of hydration is necessary. A) Hydration reactions of pure cement compounds The chemical reactions describing the hydration of the cement are complex. One approach is to study the hydration of the individual compounds separately. This assumes that the hydration of each compound takes place independently of the others. I. Calcium silicates Hydration of the two calcium silicates gives similar chemical products, differing only in the amount of calcium hydroxide formed, the heat released, and reaction rate. 2 C3S + 7 H → C3S2H4 + 3 CH 2 C2S + 5 H → C3S2H4 + CH The principal hydration product is C3S2H4, calcium silicate hydrate, or C-S-H (non-stoichiometric). This product is not a well-defined compound. The formula C3S2H4 is only an approximate description. It has amorphous structure making up of poorly organized layers and is called glue gel binder. C-S-H is believed to be the material governing concrete strength. Another product is CH - Ca(OH)2, calcium hydroxide. This product is a hexagonal crystal often forming stacks of plates. CH can bring the pH value to over 12 and it is good for corrosion protection of steel. II. Tricalcium aluminate Without gypsum, C3A reacts very rapidly with water: C3A + 6 H → C3AH6 The reaction is so fast that it results in flash set, which is the immediate stiffening after mixing, making proper placing, compacting and finishing impossible. With gypsum, the primary initial reaction of C3A with water is : C3A + 3 (C S H2) + 26 H → C6A S 3H32 The 6-calcium aluminate trisulfate-32-hydrate is usually called ettringite. The formation of ettringite slows down the hydration of C3A by creating a diffusion barrier around C3A. Flash set is thus avoided. Even with gypsum, the formation of ettringite occurs faster than the hydration of the calcium silicates. It therefore contributes to the initial stiffening, setting and early strength development. In normal cement mixes, the ettringite is not stable and will further react to form monosulphate (C4A S H18).