The grout should be suitable for joints up to 15mm in width and designed for external or internal conditions as your case may be. Generally grouts are water mix but where some flexibility may be needed then a latex additive or similar should be used in place of the water.

Experience indicates coarser grout works well in relatively large joints and is easier to use. In wet areas, however, finer, less porous grout should be used.

It is important to wait only the interval recommended by the adhesive and grout manufacturer before grouting to minimise dirt ingress between the pebbles and avoid the opportunity for other trades to dislodge pebbles.