I need white concrete for a countertop sample, but the only white concrete-like product is white mortar. If I just add an equal sized bag of gravel to the mortar, will it be close enough to use as concrete? If not, what else do I need to add, and at what proportions?
Mortar is similar to concrete in that they both contain Portland cement and sand. The 2 different ingredients are that mortar has hydrated lime and concrete has larger aggregates (stones). Using mortar in place of concrete isn't a good idea as the two have dissimilar compressive strengths. If you are only able to use the white mortar you may be able to make it concrete-like by adding a shovel full of Portland cement and than mix (1) shovel of gravel with (2) shovels of plaster sand and (1) shovel of mortar. The added Portland cement and sand may produce a strong enough mix, but the mortars' hydrated lime is the weak link. As been commented on, it may be easier to seek-out a pre-mixed white cement if possible and use dye for a brighter color. Look for concrete dye at your local building supply store. They should have several colors available including white. A pint is the typical liquid amount. Depending on the shade of white you desire (brilliant down to greyish white) you may need several pints. You will have to experiment with proportions to get the right shade of white, but as was pointed out in comments you may be using more dye than is cost effective if you want a "white" white.