I think you could make it look real nice if you use white Portland Cement for the most contrast or common Portland for a little less contrast. Drive ten or twelve penny nails part way into the logs on both top and bottom about every eight inches to give the mortar something to hang on. You might also paint the area to be covered with mortar with a bonding agent like WeldCrete by Larsen's Products. After a while you will develop the feel for the timing and when it has partially set brush it with a damp soft brush to raise some sand. This will leave a nice even sand finish.