To join new concrete to old concrete, will it work to use a paste of pure Portland as a join?

Concrete what? Slab? Wall? Sidewalk? How thick?

Sidewalk can use an expansion joist.

On slab can be pinned together with rebar.

Chuck

No. Portland cement paste cannot be used to bond old concrete to new concrete. There are bonding agents available for that purpose, under the right conditions. Cement paste has one basic purpose where it excels, and that is it's capacity to bond the individual components of a concrete mix together.