O uso do metacaulim como fonte de alumina em cimentos supersulfatados

Abstract

Metakaolin is a material derived from the calcination of kaolinite clays (kaolin) whose predominant composition is hydrated aluminum silicate (Al2Si2O5(OH)4). While kaolin is widely employed by the ceramics industry, the use of metakaolin has been growing in the cement industry as pozzolanic material in Portland cements. Considerable attention hasbeen given to special cements, which can reduce CO2 emissions and, consequently, reduce the impacts caused by the cement industry, one of which is supersulfated cement (SSC). This is composed of blast furnace slag (80-85%), calcium sulphate (10-15%) and an alkaline activator, which may be Portland cement or hydroxide. SSCs have their performance influenced by the percentage of alumina present in blast furnace slag. Slags with higher alumina content (greater than 13%) tend to form more hydrated product and less pores, resulting in higher compressive strengths. In Brazil, the slag has a medium content of alumina (7 to 11%). The goal of this study was to investigate the metakaolin as source of alumina in supersulfated cements to achieve greater compressive strength. For this, reference cement pastes were formulated in the initial proportion of 85% blast furnace slag, 15% calcium sulphate and 5% of Portland cement (as activator). The metakaolin was used as a replacement of slag in 5, 10, 15 and 20%. The compressive strength and the formation of the hydrated compounds were investigated at 7 and 28 days. Although the proportions with 15 and 20% have higher alumina contents, these pastes had the lowest compressive strengths and the fastest hydration reactions. This fact may be related to the high fineness of the metakaolin, which had low workability in the mixture. The pulps with proportions of 5 and 10% showed satisfactory results for compressive strength, higher to reference, andmore formation of ettringite.