Impurities in Limestone

Following are some of the impurities in limestone

• Magnesium Carbonate. Limestones 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 limestones 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 limestones display irregular properties of calcination, slaking and hardening. Up to 5 per cent of magnesium oxide imparts excellent hydraulic properties to the lime.
• 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 retarted. 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.

• 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.
• Iron Compounds Iron occurs in small proportions as oxides, carbonates and sulphides. They are converted into Fe2O3 at lower temperatures of calcination. At higher temperatures iron combines with lime and silicates and forms complex silicate compounds. Pyrite or iron sulphide is regarded to be highly undesirable. For hydraulic limes 2-5 per cent of iron oxide is necessary.
• Carbonaceous matters Carbonaceous matters in lime are seldom present. Its presence is an indication of the poor quality of lime.
• Sulphates Sulphates, if present, slow down the slaking action and increase the setting rate of limes.
• Alkalis When pure lime is required the alkalis are undesirable. However, up to 5 per cent of alkalis in hydraulic limes do not have any ill effect but improve hydraulicity.

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