You will first want to inspect the entire area and identify any cracks that might be in the concrete. Once you have found them, repair them using a mixture of water and thinset mortar. Make sure the mixture is smooth. Use the trowel and cover any visible cracks with the mortar. Try to smooth out the mortar so that it is level with the surrounding area.
If you do not want to use the mortar to fill up the various cracks on the fireplace crown then you can always use some caulking. Remove any debris from the cracks and fill them up with caulking. You will have to use the brush to get a nice even coat of caulking in the cracks. Once you are done allow it to dry.
Using the hammer and chisel you can gently remove the crown for repair. Make sure that you collect all of the broken pieces of brick and concrete while removing it.
Now you will want to prepare the Mason’s Cement by adding gravel. The ideal mix is to use one part gravel for every two parts of cement. Make sure that the mixture of cement has a decent consistency. Do not forget to slowly add water to the mixture until you feel that it is ready.
You will want to add the cement mixture to where the crown was placed on the flue of the chimney. It is important to note that you will need to adjust for the length of the flue while applying the cement. A good rule of thumb is to go two inches below or above the six inch mark of the flue. You will need to use the trowel to apply the cement to the crown. After you have completed it, it is a good idea to let it dry for around 24 hours. Once it is all dry you can easily apply some water sealant paint to the entire crown to protect it in the future.