Abstract 
The current thesis assesses the effectiveness of different active materials predominantly made up of SiO2 for their use as a supplementary cementicious material (SCM) in traditional Portland cement systems or as an additional silica source in non-conventional systems of alkali activation. The main aim was to obtain materials with an improved mechanical performance, higher durability and a lower environmental impact.
Silica fume is the mineral admixture most commonly used in the manufacture of high performance concrete. However, its commercialization in a densified state leads to a decrease in its reactivity degree as a consequence of the agglomeration and low degree of micro-particle dispersion during the concrete mixing. The application of a high frequency ultrasonic treatment improves the dispersion of the silica fume particles, leading to an increased compressive strength and pore refinement in mortars. This is attributed to the improvement of pozzolanic reactivity through the SiO2 micro-particles dispersion in order to promote higher portlandite consumption and the formation of C-S-H gel with higher degree of cross-linkage and longer chains. The results suggest that the use of sonicated silica fume as a SCM in traditional Portland systems can reduce the quantity of mineral admixture required to reach a specific performance with the additional advantage of obtaining a highly densified structure with lower permeability and improved mechanical properties. Likewise, the silica fume activation through sonication treatment leads to obtaining a high volume of particles with sizes lower than 1 ?m and whose effectiveness as pozzolanic materials is quite high compared to other nano or micromaterials made up of SiO2. The present thesis shows silica fume along with other materials made up of SiO2, such as rice husk ash, microsilica, colloidal dispersion of silicic acid (SiO2 nano-particles) can successfully be used as an additional silica source in alkali activation systems. These silica sources along with hydroxide solution can be used as an alternative alkali activator, which can be used as a substitute to the commercial silicate solutions. The manufacture of commercial silicate solutions has a high environmental impact due to high energy consumption and green house gas emission during production. Therefore, the production of alkali activator based on an alternative silica source, which can be wastes, industrial sub-products or other type of materials are an attractive alternative for the development of construction materials that fulfill a major sustainability criteria. 
This thesis assesses the effect of alternative alkali activators based on an alternative silica source modified chemically with a hydroxide solution SiO2/MOH; M=K+ or Na+) and its performance in alkali-activated fly ash binders. Binders based on commercial silicate exhibit a higher reactivity degree associated with an increase of  geopolymer gel. However, the binder produced with alternative activators presented a lower water demand and a reduced permeability, regardless of the type of alkaline cation used. The Na-geopolymers had a higher strength and lower permeability compared to K-geopolymers. Zeolitic products were identified as a crystalline reaction product. Wide ranges of zeolitic structures were identified when commercial silicate was used. These results suggest that there exists a variation of Si availability in the system as well as alkalinity degree, depending on which alkali activator is used. The results also show the high potential for the use of alternative alkali activators in geopolymer technology.