Calculating cement and sand quantity.

phoenix76x

Hi all, I have tried on line and come to a stand still. I am concreting a new hearth in my fireplace with a 4:1:1 mix (sand, lime and cement). This is a mix I found online which is recommended for high heat areas. Please correct me if this is wrong.  I have calculated the area to be 0.07m3 but I am stuck with how much to buy. I was thinking 100kg sand 25kg lime and 25kg cement??  I would much rather have a little over than not enough!!

FreeBear

http://www.bricksmith.co.uk/mortar-calculator.cfm

Nearest volume in the drop-down is 0.1m³... Go with 0.7m³ and divide the results by 10.

I get 14Kg cement, 7Kg lime, and 104Kg of sand for a 1:1:6 mix. But you probably want some aggregate in the mix, so look for a concrete calculator instead... https://www.diydata.com/general_building/concrete_calculators/concrete_calculator_2_m.php

A general purpose mix - 25Kg cement, 75Kg of sand, and 100Kg of 20mm gravel (rounded to the nearest 25Kg). No real need for lime as a hearth is not going to get that hot - You'll probably have some tiles or slab of stone on top...

Tip - Don't try to get the concrete slab perfectly smooth & level. Near enough is just fine. Finish it off with a layer of self leveling compound.

phoenix76x

Thanks for the reply, the plan was to not use any aggregate and add a cement dye and use as is with no additional tiles. This will be formed with a timber frame(to be removed when set) and raised from floor level by approx 50mm. Does this change anything or would u still recommend 1:1:6? 

FreeBear

At 50mm thick, you will need aggregate mixed in. A slab of pure mortar will end up cracking, especially if you are going over a suspended wooden floor.

A suggestion - Put down a slab of concrete, and then mix some dye in with a bucket of self leveling compound. Pour on to the concrete slab, and when set, remove the timber frame. You may well find the edges full of air pockets and the odd void or two. Either trowel some coloured SLC around the edge or use a painted strip of wood to finish it off.

A strip of wood trim will be better in the long term as you will find the edges & corners of the slab prone to chipping. So some form of trim will help to protect it.