Comportamento do cimento supersulfatado ao ataque por sulfatos

Abstract

With the issuance of approximately 150 million tons of carbon dioxide from the Portland cement production the search for alternative binders has gained importance. The supersulphated cement (SSC) presents itself as an excellent alternative to Portland cement, mostly because of the non presence of clinker in its composition, mainly responsible for greenhouse gas emissions, and the use of other manufacturing byproducts materials. Generally, this kind of special cement consisting of 80-85% of blast furnace slag with 10-15% calcium sulfate and even small percentages of alkali activator. The aluminum ions, calcium and silica present in the slag reacts with the calcium sulfate in an environment with a high pH elevated by the alkaline activator to form mainly ettringite and hydrated calcium silicate, conferring resistance to the mixture. In addition to this, alternative sources of calcium sulphate, such as phosphogypsum may be used. A major advantage displayed by the SSC is its resistance to aggressive environments such as those with presence of sulfates, due to non presence of calcium hydroxide in the hydration products. Attack by sulfates are considered the most extreme situation imposed on cementitious materials, that because the sulfate ion (SO4-2) reacted with the components of the cement resulting in the formation of compounds that expand its structure and decrease its resistance. However, sulphates attack detailed mechanisms are not fully understood. The aim of this study was to analyze the performance of different traces of supersulphated cemente in exposure to harsh environments, as well as its comparison with Portland cement. SSC samples were prepared with 10% and 20% of gypsum and 10% of phosphogypsum as the sources of calcium sulfate. Portland cement resistant to sulphates with high early strength was also used in order to expose them to solution of calcium sulphate and verify the dimensional change of the samples, as proposed by NBR 13583: 2014. The test, with 84 days of duration, showed a considerable expansion for Portland cement while the SSC samples with phosphogypsum showed low expansion and samples with gypsum almost non expansion. The investigation of the formed compounds showed the presence of calcium hydroxide which explains the greater expansion of Portland cement samples, and a significant amount of ettringite in samples with phosphogypsum which match the expansion by this trait.