There are many great chemical admixtures to improve the durability of concrete exposed to a wide range of conditions. Most important will be superplasticizers which will allow concrete to be mixed and placed at low w/c (
As mentioned by Ara Jeknavorian already, there are many chemical admixtures (especially superplaticizers < reduction of W/C ratio> and special organic polymers), which are improving the resistance of concrete. The resistance of concrete against ????????? has to be specified! The major parameter is the reduction in permeability of concretes to stop transportation/exchange processes. Very effective admixtures are neutral water-soluble/translucent Silica Sol solutions (sometimes called "Nanosilica" < particle size smaller than 7 nm and at SiO2 concentrations > 30%> with Tyndall effect). Those products are commercially available.