CEE Oct-Dec 2012

Key difference between Microsilica and Aluminosilica - · Microsilica Aluminosilica 1. By-product of ferrosilicium and silicium production, 1. not specifically produced for concrete Manufactured product. It is only produced for use as concrete additive 2. Quantities are depending on the metal industry and 2. the economic development Quantities are not depending on other industries and are unlimited, therefore reliable availability 3. Quality of the product has a higher deviation 3. because it is only a by-product High quality standards for end product because every step in production is controlled Alummosillcates Comparison of Strength Development between Microsilica and Aluminosilicates During cement hydration there is surplus of calcium hydroxide. The added condensed silica fume's Si0 2 reacts with surplus of calcium hydroxide, which are greater amounts of calcium silicate hydrate, which are denser and stronger than calcium hydroxide. The pozzolanic reaction and the filler-effect lead to a compaction of the cement paste and the conversion of CH crystals into CSH gel leads to homogenous paste. The phenomenon of dense packing in the interface zone of aggregates also contributes to increase the strength of the concrete on account of aggregates fully contributing their strength of concrete with silica fume is greater than those of the matrix, indicating the contribution of the aggregate of microsilica (50 : 50 with water) have all the benefits in transportation, dispensing methods, mixing times and dispersions to get the desired effect in durable concrete for tunnelling segments. New generation aluminosilicates: New generation aluminosilicates based on special nano-crystalizers have been developed. These new materials improve the properties that are crucial for the durability of high performance concrete. In addition to reducing chloride migration , an exceptional chemical and resistance to aggressive media of the concrete can be achieved with aluminosilicates. The concrete structure is simultaneously reinforced right down to nanoscale, density is improved and compressive and flexure strength as well as abrasion resistance of the high-performance concrete is increased. There is also a significant reduction of micro-crack formation , which makes it particularly suitable for the production of tunnelling concrete. Aluminosilicate reduce the proportion of portlandite by way of a pozzolanic reaction that changes it into the aluminosilicate crystals into calcium silicate hydrate. In addition to the unique resistance against acids a crystalline micro-reinforcement within the concrete structure is achieved. This reduces the risk of micro-crack formation, rendering concrete impermeable. Due to high homogeneity and reduced tackiness compared with microsilica based concrete, workability is improved significantly. In many instances this enables the production of high-performance concrete that can be pumped. In addition, a distinct improvement of the building structure's aesthetics is gained due to the fair appearance of the concrete surface. Aluminosilicates perform over some of the disadvantages of microsilica : Graded for dispersion in concrete • Graded particle size Optimizes mixing time within concrete Good dispersion reduces unreacted material in the mix and increases passivation by C-S-H gel on aggregate surface Material if agglomerated improve strength of the mix Reduces risk of alkali silica reaction by agglomeration of aluminosilicate particles. All in all the use of PCE admixtures and microsilica or aluminosilicate slurries in addition to the standard ingredients in concrete, plus excellent mix-design practices can facilitate the production of high performance concretes resistant to aggressive media, suitable for use in tunnelling applications. Courtesy: Indian Cement Review, Sep. 2012, Pp32-33. 57