I've demolished the old brick fireplace,and I'm left with what you can see in the picture. I am planning on using a surround made from reclaimed timber. I would like to tile the area between the surround and the opening. Should I use about fifteen tons of sand/cement mix to bring it flush with the fireplace?. Should I use lime in the mix?. The fire is to be used in the winter,using either logs or coal. Thanks.