This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
(a) It is unquestionably more economical; but if the percentage added (or that which replaces the cement) is more than about 5 per cent, the strength of the mortar is sacrificed. The percentage of loss of strength depends oh the richness of the mortar.
Fig. 7. Bottomless Box for Measuring Sand.
(c) It always makes the mortar work more easily and smoothly. In fact, a rich cement mortar is very brash; it will not stick to the bricks or stones when striking a joint. It actually increases the output of the masons to use a mortar which is rendered smoother by the addition of lime.
The substitution of more than 20 per cent of lime decreases the strength faster than the decrease in cost, and therefore should not be permitted unless strength is a secondary consideration and the combination is considered more as an addition of cement to a lime mortar in order to render it hydraulic.