## 1 Introduction
An alternative is to replace fly-ashes with microsilica (silica fume), another artificial product obtained by the reduction process of quartzite during the production of alloys like iron-silicon or metallic silicon. Microsilica is constituted of granules smaller than fly-ashes. Their origin is from oxidation, hardening and condensation of silica fume at 2000 \({}^{\circ}\)C, following a process similar to the natural pozzolan [30]. Microsilica is utilized in many countries to make concretes with high durability [31]. They are composed of spherical granules with a diameter of 0.1 \(\upmu\)m; this size allows the granules to fill all of the empty spaces of concrete. Microsilica is mainly comprised of dioxide of silicon; dioxide activates the transformation of hydrates of calcium (dangerous for concrete durability) in an insoluble, stable and effective product to fill all the pores. The inclusion of silica fume at high replacement levels significantly increases the autogenous shrinkage of concrete due to the refinement of pore size distribution that leads to a further increase in capillary stress and more contraction of the cement paste [32]. In fact, microsilica fume addition produces a positive influence on the dynamic and static mechanical properties of concrete, especially on the resonant frequencies, the dynamic and static moduli of elasticity, damping ratio and strength [33; 34]. In particular, the results obtained by [PERSON] et al. [28; 29] evidence that microsilica additions or replacements reduce both the dynamic modulus of elasticity and damping ratio of concrete. In other studies [35; 36; 37], some fibers (like carbon or steel fibers) were added to silica fume concretes, obtaining an improvement in the tensile and compressive strengths of concrete. [PERSON] and [PERSON] [38], and [PERSON] et al. [39] determined the influence of silica fume over a wide range of water-binder ratios and cement replacement percentages. In particular, the incorporation of blast-furnace slag into silica fume concrete reduces the water demand and this combination offers increased resistance to the alkali-silica reaction expansion and chloride ingress than the use of one of these materials alone [40]. [PERSON] et al. [41] and [PERSON] et al. [42] showed how the increase of fly-ashes replacement in concrete clearly reduces the chloride penetration (CP) and the steel corrosion in concrete. The last characteristic is also explained because these fine by-products make the microstructure of concrete denser, reducing its diffusivity [43; 44]. In the present paper, the good results of experimental tests carried out on concretes in which sand is totally replaced by blast-furnace slag and microsilica, or by blast-furnace slag and fly-ashes are shown and discussed. Mechanical characterization tests and water absorption tests are performed. Blast furnace slag was integrated in the finest fraction by means of appropriate percentages of microsilica or fly-ashes. The tests, although preliminary by nature, confirm the possibility to obtain concretes with satisfactory mechanical properties and lower water absorption values using either fly-ashes or microsilica. Therefore, the results and comparisons reported in this paper are important bases towards the aim of carrying out wider research in this field.