mortarwalls
We have a number of "slabs" that are just wider than a brick, which are set on top of walls in the garden. A number of them have come lose and I need to remove the cement mix that used to hold them down and set them again.
What mix of cement and sand should I use, and are there other things I should mix in to ensure the mortar has a good "grip" on the stone that sits on top of the wall?
It is not so much about mortar as it is about the working process. You should clean out the old crumbling mortar to make place for the new one. Remove the old mortar so that all parts are firm and nothing is crumbling - you can even scratch out a bit of the old mortar. The very important part is to wash the wall properly with water so that all parts hold firm and there is no dust - otherwise the new mortar would just fall out. Than press the new mortar in the joints and fill them, make them smooth on surface.
As for the mortar the lime content should not be important for the lifetime, but you can add a bit for better workability. Generally there should be no or very little lime in a stone wall. The new mortar should consist of 1 part of portland cement and 2-3 parts of sand.
If you do it properly (well prepared surface, good mortar), the new mortar should not crumble or dust for tens of years.
A personal experience: it is a very time consuming monotonous work, but the result can be very good if done with care.
It's all about particle size, which is classified using the Wentworth scale or The Krumbein phi (φ) scale.
Sharp Sand, also known as Concrete Sand is a coarse sand with larger particles. This type of sand is typically used in concrete.
φ scale - 1 to 0
Size range - 1/2 to 1 mm (0.020–0.039 in)
Builder's Sand, also known as Plasterer's, Mason, or Bricklayer's Sand is a finer grade sand with smaller particles. It's often mixed with water and cement to make mortar for laying bricks/blocks.
φ scale - 3 to 2
Size range - 125–250 µm (0.0049–0.010 in)
Jointing Sand, also known as Beach Sand is very fine sand with very small particles. This type of sand is used for grouting joints in paving or patio slabs.
φ scale - 4 to 3
Size range - 62.5–125 µm (0.0025–0.0049 in)
Best Answer
you could use a Construction adhesive like Liquid Nails or Beats the Nail. This works well for cap stones, since they are usually not a structural part of the wall.