Marble is a natural stone used for many decorative applications, including fireplace surrounds. Natural marble is more delicate and porous than other types of stone or masonry material, which raises the risk of damage. Impact as well as general wear and tear can cause the marble surround to crack or chip in places. Repairing these cracks or chips can be done with a little time and patience, without the need to replace the entire surround.
Cover your eyes with a good pair of goggles, to keep pieces of marble or hardened cement from flying into your eyes.
Remove any loose marble pieces or grout within the joints of the surround with a dampened cotton swab or shop vacuum.
Dampen a soft cloth with acetone. Rub the dampened cloth over the entire surface of the marble fireplace surround and allow it to dry.
Use a hammer and a metal chisel to remove the marble pieces around the cracked or chipped area. Use the cloth that you dampened with acetone to clean the edges of the marble pieces.
Use the chisel and hammer to chip away the mortar that was behind the marble pieces. Chip away enough mortar so the repaired marble will fit flush beside the remaining marble on the fireplace surround.
Sand the edges of the marble pieces with fine-grit sandpaper. This abrasion will give the edges a slight etching for the epoxy to adhere better.
Lay the marble pieces that you chiselled out on a flat surface, and rearrange them so they are in the shape of the chiselled-out hole and the marble pattern looks natural.
Squeeze a tiny bead of epoxy glue onto the edges of the marble pieces, and wait 30 seconds for the epoxy to turn tacky. Fit the broken marble pieces together, and hold for five minutes. Use a soft cloth to wipe away any epoxy glue that squeezed through the cracks.
Allow the epoxy glue to set for 24 hours before putting the mended marble back on the fireplace surround. Sand down any glue that may have squeezed up and hardened.
Mix thinset mortar in a large container according to the package directions. Use a pointed trowel to apply a thin layer of mortar on the area you chipped away earlier. Scrape the point of the trowel over the thinset mortar layer to etch it so the marble will have a strong bond.
Push the repaired marble piece into the thinset mortar and wiggle it into place. Allow the mortar to dry for up to 12 hours.
Place pre-mixed grout into a grout bag, and squeeze it into the marble joints. Use your finger to smooth the joints to match the depth of the surrounding marble joints. Wipe any excess grout that gets on the marble surface with a dampened cloth. Allow the grout to dry for 12 to 24 hours.