cast hearth problems (powdery)

[sonomacast]

I cast my hearth in segments deciding that the bottom will be the top (A perfect flat finish (like casting for polished concrete) However yesterday I struck the forms and flipped the slabs to find the (now top) concrete is very powdery (apparently I used too much release agent which stopped the curing reaction) I guess I cant seal it as food is going to be cooked on it, Any ideas of how I can sort this mess. Polish perhaps? Thank you

[david s]

Re: cast hearth problems (powdery)

If you used too much release agent you will have to remove this first. If you used oil then use a degreaser and wash it off with water. I make my floors in the same way and cast them upside-down against polycarbonate, which gives a very smooth surface. Sometimes if vibration has been insufficient there are some tiny holes in which case I use a brew of equal parts kaolin (white clay), lime, white cement and calcined alumina. I mix this into a paint like consistency and paint it on working the mixture into any of the tiny holes. It sets extremely hard and fires on to create a permanent coating. You could substitute talc or whiting for the calcined alumina, it is just a fine filler. Don't apply the mixture if the casting is too damp or it won't stick and harden properly. Likewise don't apply it if the casting is too dry. About two days out of the mould works well for me. It is very similar to the home brew mortar, but more cement rich and all the ingredients are white, which gives you a nice white surface.

[sonomacast]

Re: cast hearth problems (powdery)

I have just brushed and vacumed all the loose cement, it now looks like it was cast onto retarder (it wasn't it looks like/peble dash).
If I give it a coat of slurry and polish it afterwards how will it cope with the heat and general abuse it will suffer? any ideas. thank you

[david s]

Re: cast hearth problems (powdery)

You could try sieving out the courser aggregate in a sample of castable and paint that over a test area, not sure how it will work.

[Neil2]

Re: cast hearth problems (powdery)

Additional layers may just spall off. If you have an angle grinder, grinding and polishing may be the best way to go.

[sonomacast]

Re: cast hearth problems (powdery)

I sieved out all the aggregate and mixed the cement with water to make a slurry which I bagged into the troublesome area (after a thorough brushing and vacuum. After a few days I polished it with Diamond pads working up to a 3000 grit pad but cutting through the origional aggregate. Looks good and feels tough,