Before you throw down big bucks for us to ship a bag of Portland Cement to you, check with your local Masonry &/or Landscape Supply as this is a commonly stocked item.  Portland Cement is also an everyday item at Big Box stores like Home Depot, Lowes, Rona and Menards.  Depending on your location, prices can range from $5 - $10 per 60lb bag of Portland Cement.   100lb bags are a few dollars more (but really heavy).

High-Temperature Mortar Mix Recipe:

3 Parts Silica Sand
1 Part Hydrated Lime
1 Part Fire Clay
1 Part Portland Cement

How to make High-Temperature Mortar