Picture the mix of concrete accented with natural stone. Design applications mixing these two elements introduce a layered, textural dimension to a hard-scape theme. Transitioning from concept to application means pulling together the elements that make this work, which requires finding the right kind of adhesive to bind the natural stone to the existing concrete.
The Mechanical Bond
Porous natural stones form the best mechanical bond with adhesives. Adhesive spread over the surface of the stone readily enters the pores -- or small holes -- through the stone's surface, forming a continuous bond with the adhesive outside of the pores. The process is called wetting, where the adhesive forces the air out of the pores and fills them up. Instead of a smooth, flat layer between the stones and the concrete, the adhesive forms "roots" in the pores of both the stones and concrete, creating a very strong bond with the natural stone once it dries and cures. Of the three major types of natural stones, limestone and marble are the porous ones. Granite -- the third type of natural stone -- rates as a non-porous stone, as do some types of marble.
The Chemical Bond
Concrete's porous nature makes it a durable substrate -- or binding surface -- for an adhesive that will bind natural stone. A chemical reaction occurs between the concrete and the adhesive, and then the adhesive and the natural stone. On the molecular level, combinations of hydrogen, metallic, covalent and ionic bonds form that link the composition of the concrete and the stone to the adhesive. This type of bond is the strongest since it chemically reconfigures the surfaces of the concrete and the natural stone to interact and bind with the adhesive.
Acrylic and Polyester Adhesives
Acrylic and polyester adhesives can bind concrete and natural stone, but each exhibit characteristics that do not make them the top choice for this application. Acrylic adhesives bind better to natural stone than polyesters because of their ester groups, and are suitable for the non-porous granite and marble applications. Although acrylic adhesives are weather resistant and their versatility makes them suitable for thick and thin adhesive applications, acrylic adhesives shrink when cured.
Polyester adhesives set relatively fast but do not contain enough binding factors, such as ester groups, that form a strong chemical bond. Polyester also shrinks and is subject to forming weak bonds once the adhesion receives stress. Polyester adhesives work for binding porous natural stone to the porous concrete -- a bond that fosters more of a mechanical bond than a chemical bond.
Epoxy is the Best
The bond between natural stone and an epoxy adhesive is the strongest and best bond. Complementing the mechanical bonding of the natural stone, the epoxy adhesive and concrete, the epoxy introduces a very strong chemical reaction between its resin and the hydroxyl groups in its amine hardener. Classified as a two-component adhesive, the epoxy parts are kept separate until ready to use. After mixing its parts together -- on the spot and at a precise ratio -- the epoxy adhesive has a short window of time where it remains tacky to form its bond to the natural stone. Once set and cured over a few hours, the bond's strength, lack of shrinkage and weather resistance makes the epoxy durable enough to maintain the bond with the weight of the natural stone.
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