Getting the ratios of the materials correct consistently is the secret to good cement render or bricklayers mortar.
We all know that the lime mortar joint will erode over time. To slow down that process you create a shell of lime on the surface of the mortar joint. This done by smoothing the joint with a trowel while it contains moisture on the face.
The 2 main materials used in cement that comply to AU standards are cement and lime. The 3rd material is sand which will vary and can have an adverse effect on the final mortar mix.