you can substitute marble dust for sand as per sand cement render / screed mix. Just joined this site. am also doing my own house and want a polished concrete / marble finish on my external and bathroom renders. Was told to look at systems but finishes are crap with K-rend / Weber / etc so thinking of mixing my own s/c on traditional battens / eml with marble dust for sand (50 / 50) in float coat which you can then wax / lacquer etc if you want it really shiny. Read a scientific article about it. Need to use Portland or type s cement. good news is that the marble improves the cured strength of the mix, makes it less water permeable and also more moisture retentive during cure so less need to keep wet. Bad news (for me) is that it's a heavier (denser) mix as the dust fills all the gaps around larger sand particles. Probably have to watch suction (if doing walls) and maybe add retarder to begin with until you see how it behaves. Micro-cement is really just smaller particles in mix and bigger prices on the tin. If you know what you're doing (which I don't, here - see name), you always adjust mixes for ambient conditions/ backgrounds etc or seek to control those conditions. For contractors etc, this is most economically done with prefab backgrounds (boards / screeds) and pre-mixed coatings. It's cheaper and better make your own if time isn't a factor but end result is. Where did you find your marble dust? Have looked and 25kg bags are 15 quid which seems a bit much for waste material. Also, any advice on my external rendering? It's the weight...Is there possibility of using Parex basecoat on board and then making float coat mix. Adhesion? do you need to polymerise the mix to bond with the base? Thanks
