Defect of Lime

Defect of Lime

1- Magnesium carbonates: Lime stones contain magnesium carbonate in varying proportions. Presence of this constituent allows the lime to slake and set slowly, but imparts high strength. Further, the production of heat and expansion are low. The magnesium lime stones are hard, heavy and compact in texture. In burning limestone, the magnesium carbonate is converted to magnesium oxide at a much lower temperature whereas calcium carbonate is oxidised at a little higher temperature. By the time calcium carbonate is oxidised most of the magnesium oxide formed is over burnt. Magnesium lime stones display irregular properties of calcinations, slaking and hardening. Up to 5 per cent of magnesium oxide imparts excellent hydraulic properties to the lime.
2- Clay: It is mainly responsible for the hydraulic properties of lime. It also makes lime insoluble in water. The percentage of clay to produce hydraulicity in lime stones usually varies from 10 to 30. If, it is in excess, it arrests slaking whereas, if in small quantities the slaking is retried. Thus, limes containing 3-5 per cent of clay do not display any hydraulic property and do not set and harden under water. Whereas, when clay is present as 20-30 per cent of lime, the latter exhibits excellent hydraulic properties and is most suitable for aqueous foundations.
3- Silica: In its free form (sand) has a detrimental effect on the properties of lime. Limes containing high percentage of free silica exhibit poor cementing and hydraulic properties. Limes containing 15-20 per cent of free silica are known as poor limes.