ORIGINAL ARTICLE Phase transformations and mechanical strength of OPC/ Slag pastes submitted to high temperatures Alessandra Mendes Æ Jay Sanjayan Æ Frank Collins Received: 12 February 2007 / Accepted: 10 April 2007 / Published online: 25 May 2007 Ó RILEM 2007 Abstract Ground granulated blast furnace slag (GGBFS or ‘‘slag’’) is a by product of the steel industry and is often used in combination with ordinary Portland cement (OPC) as a binder in concrete. When concrete is exposed to high temper- atures, physical and chemical transformations lead to significant loss of mechanical strength. Past studies have reported changes in concrete where OPC is 100% of the binder, but there is a lack of published data on slag blended cements. This work provides better understanding of how slag blended cement pastes behave when exposed to high temperatures, when the critical transformations occur, and what the consequences in the structure of these pastes are. Thermogravimetric analysis made it possible to identify when the transformations occurred and the changes in mechanical strength in the cement paste. A unique outcome of this work is the lower damage presented by slag blended cements after exposure to high temperatures Keywords Cement Á Slag Á Compressive strength Á High temperatures Á Thermogravimetric analysis 1 Introduction The use of slag as a partial substitute for Portland cement (OPC) in blended cements and in ready mixed concrete not only lessens the amount of unused waste that is produced by the steel industry, but most importantly reduces the carbon dioxide emissions arising from cement making. A key property of concrete is the durability when exposed to high temperatures (e.g. to provide resis- tance when exposed to accidental fire). Past studies have reported the concrete behavior when exposed to high temperatures where OPC is 100% of the binder, but there is a lack of published data on slag blended cements. Two effects of high temperature on exposed mortars were reported [1]: (i) a differential movement between sand particles and cement paste due to different coefficients of thermal expansion; (ii) breakdown of cement paste, regardless of whether the sand is present or not. This investigation focuses on OPC and slag blended cement pastes and therefore examines Item (ii). Lea and Stradling [1] reported the relation between the coefficient of expansion of OPC paste and the dissociation of water from the hydrated paste. Their work recognized that OPC paste expands at temperatures up to 938C then continually contracts up to 491 8 C. Calcium hydroxide, (Ca(OH) 2 ), one of the products of the hydration of OPC, decomposes at approximately 4008C into calcium oxide (CaO) and water. A. Mendes (&) Á J. Sanjayan Á F. Collins Department of Civil Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia e-mail: Alessandra.mendes@eng.monash.edu.au Materials and Structures (2008) 41:345–350 DOI 10.1617/s11527-007-9247-8