A lower ratio leads to higher strength and durability, but may make the mix more difficult to work with and form.
Cementitious materials include cement and supplementary cementitious materials such as ground granulated blast-furnace slag (GGBFS), fly ash (FA), silica fume (SF), rice husk ash (RHA), metakaolin (MK), and natural pozzolans.
Most of supplementary cementitious materials (SCM) are byproducts of other industries presenting interesting hydraulic binding properties.
After reaction with alkalis (GGBFS activation) and portlandite (Ca(OH)2), they also form calcium silicate hydrates (C-S-H), the "gluing phase" present in the hardened cement paste.
SCMs also help reducing the clinker content in concrete and therefore saving energy and minimizing costs, while recycling industrial wastes otherwise aimed to landfill.
[1] However, a fresh concrete with a w/c ratio of 0.35 may not mix thoroughly, and may not flow well enough to be correctly placed and to fill all the voids in the forms, especially in the case of a dense steel reinforcement.
Such diluted concrete immediately loses any official certification and the responsibility of the contractor accepting such a deleterious practice is also engaged.