Most of my line is laid on medium density cement blocks with cosmetic ballast using aquarium gravel held in place with diluted PVA. I took this pic this morning of a boxcar I'm going to weather but it shows the ballast fairly well. This has been in place for 7 years now and although there has been some erosion at the edges it has held up well. I used Wickes builders adhesive mixed about 2:1. I found that if I went for a thicker 50/50 mix it tended to leave lumps of glue that pooled on the surface and spoiled the effect. For some reason the areas at the rear of the layout wore away more quickly and after about 4 to 5 years I had to re-ballast these areas (I probably started to over dilute the adhesive by that stage). When re-ballasting I tried several methods including mixing grit with cement powder and Rowlands mix but they did not work well - didn't even last a year.
If you like moss, you will notice that it grows quite happily in the grit/PVA mix
If you like moss, you will notice that it grows quite happily in the grit/PVA mix