Tests for Strength of Lime Mortars
Two bricks are joined flat in a cross fashion one over the other with 12 mm mortar joint. Bricks are thoroughly soaked in water before joining and cured for 7 days after they are jointed. Load required to separate them at joint gives the adhesive strength of mortar which should not be less than 1.4 kg/sq. cm for I lime to 2 sand mortar. For testing tensile strength, briquettes are made (as for cement test), and cured for 24 days by immersion in water. A good hydraulic lime should have an ultimate tensile strength of at least 7 kg/sq. cm and a fat lime 2.8 kg/sq. cm, with 1 : 3 lime : sand mortar. The compressive strength should be 4 to 5 times its tensile strength. Mortar consisting of 1 part good quality kankar lime and 1 part sand should develop a compressive strength of over 50 kg/sq. cm after 3 months and twice that after two years. A pure surkhi mortar gives a breaking strength of about 5.6 to 6.3 kg/sq. cm if left in dry air, and 21 to 25 kg/sq. cm if left immersed under water.
Compressive strength of 1:4 cement-sand mortar after 3 months is over 210 kg/sq. cm. Simple method of testing the strength and suitability of a particular limestone or kankar is to burn the limestone, produce lime, mix it with the required proportion of sand and test the mortar as described above.