How to install vent cap over cement pipe?
That flue pipe looks like "Transite" pipe which was used on some gas only appliances before about 1960. When I got into the heating business in the late 1960's nobody was still using it. There were better alternatives. If it is transite, it probably contains asbestos. I don't know of any caps available for use on this product. You could take a standard 6" cap and slit the bottom connection in about 6 sections and bend the slit pieces inward and use a large hose clamp to hold the cap to the old pipe. You could "GOOGLE" transite to learn about this pipe. Hope this helps.