LECTURE 2 CEMENT

Cement Manufacturing: - Raw materials: Limestone, chalk, silica, alumina & iron oxide→ found in nature as clay or shale. - These are grinded into very fine powder and burned in large rotary kilns at 1400⁰ C. The material fuses into clinker( 3-25 mm in diameter) → it is cooled and ground to a fine powder then gypsum added → the result is portland cement.

Cement The cool clinker(very hard) is mixed with gypsum to prevent flash setting of the cement.

Cement Chemistry of Cement Main compound in portland cement: - Tricalcium silicate C₃S - Dicalcium silicate C₂S - Tricalcium aluminate C₃A - Tetracalcium aluminoferrite C₄AF

Cement C₃S & C₂S responsible for the strength of hydrated cement paste. C₃A generate the maximum heat and C₂S generate the minimum heat. Due to this C₃A is responsible for most of the undesirable properties of concrete. However C₃A is beneficial in the manufacturing of cement because it facilitates the combination of lime and silica.

Cement C₃A contribute little to the strength of cement paste at early ages i.e. responsible for initial setting of cement. C₄AF is present in small quantities and do not affect the behavior of cement paste significantly. Excess gypsum causes expansion & disruption of the set cement.

Cement

Cement

Cement Minor compound of cement ( refer to quantities and not effect) MgO, K₂O, Na₂O → amount only to few % of the mass of cement. - K₂O & Na₂O react with water and release the alkalis sodium hydroxide and potasium hydroxide in the cement paste. These react with some aggregates and produce alkali aggregate reaction causing disintegration of concrete and also affect the rate of gain strength in concrete.

Cement Hydration of cement - Cement paste is the product of reaction of cement and water. Silicates and aluminates react with water and produce a hard mass. There are some impurities exist in calcium silicates: impure C₃S(alite) and impure C₂S(belite). These affect the hyderated silicates.

Cement - The product of hydration of C₃S & C₂S is C-S-H gel(60%) + Ca(OH)₂(30%) -Calcium hydroxide is produced by treating lime with water: CaO + H2O → Ca(OH)2 The calcium silicate hydrates occupy the largest proportion of the volume and govern the mechanical properties. The spaces between the particles( known as gel pores or capillary pores) occupy about 30% of the C-S-H volume.

Cement Reaction of C₃A with water is very rapid and lead to a flash set. This is prevented by addition of gypsum to the cement clinker. Understand Fig. 2.1 in the book

How can the heat of hydration of cement be reduced? The hydration of cement compounds is exothermic. The temperature at which hydration occurs affects the rate of heat development. Therefore, heat of hydration of cement can be reduced by: cooling the ingredients of the mix 2) cooling the surface of the concrete 3) insulate the entire surface of the concrete.

Types of Cement Ordinary Portland Cement(Type 1): It is the most common cement used in general concrete construction when there is no exposure to sulfates in the soil or in ground water. Sulfate-resisting cement(type 5): It is generally used for underground concrete structures(substructure) or where concrete touching the ground. It has a low C₃A content so as to avoid sulfate attack from outside the concrete.

Rapid –hardening Portland cement(type 3): The strength of this cement develops rapidly because It has high C₃S content and high fineness. It is used when formwork is to be removed early for re-use or where sufficient strength for further construction is required quickly. Low-heat Portland cement(type 4): It used in dams and large bulk of concrete. It has low heat of hydration and low content of C₃S & C₃A.

Portland blast –furnace cement(type 1S): It used in mass concrete because of a lower heat of hydration and in sea-water construction because of a better sulfate resistance( due to lower C₃A content). It is also used to reduce alkali-aggregate reaction.