A bonding agent is pretty much what I’ve just explained … it’s a substance or additive that helps make your ‘tufa (or concrete) recipe stick or adhere to an existing layer.
Commercially made bonding agents are located in the cement products isle at stores like Home Depot or Lowe’s.
A DIYer’s “Homemade” Bonding Agent
Since commercially made bonding agents can get a bit pricey, many frugal do-it-yourselfers have discovered that Elmer’s brand White Glue or Wood Glue can be substituted with great success (and again, you’ll save a little money, too). The basic chemistry of the Elmer’s brands is almost identical to the commercial bonding products.