Effects of an eco-silica source based activator on functional alkali activated lightweight composites

In this paper, alkali activated slag-fly ash lightweight composites with moderate densities between around 1200 and 1500 kg/m3 are prepared and characterized. An eco-olivine nano-silica is applied to prepare sustainable silicate based activators to replace commercial sodium silicates. Na2O contents of 2.0, 3.5 and 5.0 wt% are investigated in order to reach a suitable balance between performance, costs and application. The results show the positive effect of density and Na2O content on strength, while strength increment between Na2O dosage of 3.5% and 5.0% is limited. The reduction of Na2O content shows a dramatic delay of the reaction process up to around 3 d, but shows negligible effect on the typical Si-O bonds. An increased Na2O content benefits the formation of reaction products, including the contents of hydrotalcite and carbonates. Besides, the thermal conductivity and acoustical absorption properties of the lightweight products are characterized; the phase transition zones between lightweight aggregate and binder matrix are evaluated by SEM. The calculation on the carbon footprint shows an evident advantage of using alkali activated materials to replace Portland cement, also the utilization of olivine nano-silica further reduces the carbon emission of the activator by around 25%.