Why use mass ratio for masonry mortar and volume ratio for plastering mortar?

From the traditional experience, the sand particle size, actual strength and moisture content in cement mortar mixture ratio are always closely related to the construction level. Generally speaking, the cement mortar plastering for building exterior walls is generally prepared according to the mass ratio of 1: 2.5- 1: 3. To put it simply, the cement mortar with the mass ratio of 1:3 is to use one part of cement and three parts of sand, and the proportion of component water should be controlled at about 0.6 as far as possible. To sum up, the mass ratio of cement mortar for plastered external wall is equal to 1: 3: 0.6. The density of cement mortar with this mass ratio can generally reach 2000Kg/m3. 1 and cement mortar 1: 3 are configured according to the mass ratio of cement and sand 1: 3, and the content of cement mortar per m3 is: cement weight: sand weight = 0.404t:1.508t; 2. Mixed mortar 1: 1: 6 is prepared according to the mass ratio of cement, lime paste and sand 1: 1: 6, that is, the content of mixed mortar per m3 is: cement weight: lime paste weight: sand weight = 0.202t: 0.2/kloc-.