(i) Decay of concrete and its prevention.

(ii) Setting and hardening of cement.

FE > Semester 1 > Applied Chemistry 1

Marks : 03

Years : DEC 2015

1. Decay of concrete and its prevention

Various types of aggregate undergo chemical reactions in concrete, leading to damaging expansive phenomena. The most common are those containing reactive silica, that can react with the alkalis in concrete (K2O and Na2O, coming principally from cement).

Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, calcium leaching, physical damage and chemical damage (from carbonation, chlorides, sulphates and distilled water). This process adversely affects concrete exposed to these damaging stimuli.

Advantages

• Techniques for arresting and preventing further degradation include: Cathodic protection; Chloride extraction; Re-alkalisation; repair of cracks and surface coatings.

• One novel proposal for the repair of cracks is to use bacteria. BacillaFilla is a genetically engineered bacteria designed to repair damaged concrete, filling in the cracks, and making them whole again.

• Techniques for filling holes left by the removal of spalled or damaged concrete include: mortar repairs; flowing concrete repairs and sprayed concrete repairs.

• Various techniques are available for strengthening concrete structures, to increase the load-carrying capacity or else to improve the in-service performance. These include increasing the concrete cross-section, adding material such as steel plate or fibre composites to enhance the tensile capacity and providing additional pre-stressing.

1. Setting and hardening of cement

Setting and hardening is the most important property of cement and set of addition of water to cement. After mixing with cement with water. It formsa plastic mass called cement paste and hydration reaction begins resulting in the formation of gel and crystalline products. The interlocking of crystals binds the inherit particle of aggregate into rock like material.

Setting is defined as stiffening of original plastic mask due to initial gel formation. Hardening is defined as development of strength due to crystallisation. Initial setting of cement paste occurs due to hydration of tri calcium aluminate and gel formation of tetra calcium alumina ferrite.

Hydration of dicalcium silicate and tri calcium silicate results in formation of gel, which constitutes 50% of hydrated cement and contributes to initial setting and strength of cement.Final setting and hardening of cement paste is due to formation of gel and crystallisation of calcium hydroxide and hydrated tri calcium aluminate