Clause 7.2.2 (Revised): All concrete used in toboggan construction must be comprised of at least 30% cementing materials by mass. Cementing materials are any materials having cementing properties or contributing to the formation of silicate compounds. Examples include portland cement, hydraulic cement, fly ash, slag, silica fume, and metakaolin.
1) Does 30% by mass refer to the mass of dry materials?
2) Does this mean that the use of portland cement, or any type of cement is not required for the mix as long as we use other cementing materials?
1) The mass requirement refers to the mass of the concrete, not the mass of the dry materials. As such, you need to include the weight of all materials used in the mix, including water and admixtures.
2) Correct. You may elect to not use Portland Cement in your mix design if you so choose, so long as your mix design adheres to Clause 7.2.2.