Installing masonry under weather protection and/or a covered mixing station

I have not been able to find any references to any chemical reaction to fresh or curing mortar being discolored by or reacting to diesel fumes. I am not considering heavy black exhaust smoke from trucks or buses. What I am concerned about is the life safety issue of the effect of the exhaust on the workers. When fossil fuel burns it creates carbon monoxide (CO). It must be well-ventilated, and I don't see any ventilation equipment in your photo. The greatest impact that the heated work area will cause is the reduction in humidity. This will impact the setting time of the mortar, the need for retempering the mortar, and the tooling time of the mortar. Each one of those can cause variations in the color of the joints. If you use steam heat to offset the dryness you could cause ice to form on or about the enclosure. I believe a review of cold weather construction and CFR 1910 and 1926 requirements for handling exhaust and enclosures would be a minimum requirement to provide good life safety and quality masonry work.  I hope we've been able to help. Call me if you see the need.

Be safe.

Jerry Painter, FASTM