Drying Shrinkage and Durability Studies on Alkali Activated Slag Concrete Using Different Activators

What is it about?

For every ton of cement clinker produced, an approximately equal amount of carbon - dioxide is released into the atmosphere (Davidovits, 1991). The world cement industry contributes some 7% to the t otal man-made CO2 emission (Malhotra, 1999). This leads to the search for more environmentally viable alternatives to Portland cement. One of these alternative materials is alkali-activated slag (AAS), in which ground granulated blast furnace slag (GGBS) is usednot as a partial replacement for cement but as a sole binder itself in the production of concrete. This will produce an environmentally friendly concrete. The use of slag cement has advantages due to its excellent cementitious properties over ordinary Portland cement (OPC), but the disadvantage of the low early strength has limited its use. Various studies had investigated ways to enhance the reactivity of the slag. One of the economic ways of activation is alkali activation. Slag has latent hydraulic properties. If GGBS is placed in water alone, it dissolves to a small extent but a protective film deficient in Ca2+ is quickly formed, which inhibits further reaction. The reaction continues if the PH is kept sufficiently high. The pore solution of a Portland cement, which is essentially one of alkali hydroxides, is a suitable medium. The supply of K+ and Na+ ions is limited, but these ions are only partially taken up by the hydration products, and the presence of calcium hydroxide ensures that the supply of OH ions is maintained. The slag can be similarly activated by OH ions supplied in other ways such as addition of sodium hydroxide or silicate (Taylor, 1997).

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Why is it important?

The production of OPC is responsible for about 7% of the world’s CO2 emissions, lead to the search for more environmentally viable alternative to cement. One of those alternative material is alkali-activated slag (AAS) where ground granulated blast furnace slag is used not as a partial replacement to cement but as a sole binder in the production of concrete. The durability performance of alkali-activated slag concrete with sodium silicate, with and without addition of hydrated lime, sodium hydroxide with lime and sodium carbonate as activators are studied. All the activators are used at 8% Na2O (by weight of slag) and 4% of hydrated lime by total weight of solid binder content if used as a retarder. Drying shrinkage was measured up to 40 days. The d urability potential of AAS concrete was investigated by testing for oxygen permeability and porosity. Among different AAS concrete Na 2C03 activated concrete (SC8) showed less shrinkage, which is little bit higher than OPC concrete with same w/c (CM2) but lower than OPC/Slag blended mix (SLG60) where as water-glass activated slag showed highest drying shrinkage. Among AAS concrete, L.S.S activated slag concrete without addition of lime (SS8) showed lower porosity, which was even little bit lower than the OPC concrete having same w/c ratio (CM2) at the age of 28days. NaOH activated slag concrete (SS8L4) showed high porosity than other AAS concrete in both water and dry curing conditions, whereas Na 2CO3 activated slag concrete showed comparable porosity with OPC concrete (CM2). Therefore, the use of SC as an activator for slag concrete is recommended, since it results in adequate strength, comparable or lower shrinkage to control mixes and almost equal or comparable porosity.