In article ,
The Natural Philosopher writes:
I used a mix in my wall varying from about 1:1:5 cement/lime/sand to
1:1:8 when I got a bit low on sand and cement to 3:1:15 when I nearly
ran out of cement.

The lime:cement ratio is important. At most ratios, they
interfere with each other and result in much reduced strength.
However, the 1:1 ratio happens to be good.
There's a BRE report on this.

--
Andrew Gabriel