A multi-function composition for incorporation into settable composite materials is provided. The composition is formulated as an additive to modify the density of the composite material and increase the rate of hardening or strength development of the material. The composition of the additive generally includes an alkaline activation compound such as sodium silicate and a modified low density siliceous material having at least one region morphologically altered by a chemical, such as a partially digested region. The additive can be in slurry form, in powder form, or in an agglomerated particle form. The additive can be produced using a two-stage process in which a siliceous material is reduced in particle size, combined with an alkali compound in a solution and then digested in an atmospheric or pressurized vessel. In some implementations, the solution can be spray dried to form agglomerated particles containing the alkaline activation compound and the low density siliceous particle having one or more partially digested regions.