Comparative performance of alkali activated slag/metakaolin cement pastes exposed to high temperatures

This paper presents a study on the effect of temperature exposure of binders of blast furnace slag (BFS) and metakaolin (MK) in BFS-MK weight ratios of 100-0, 50-50, and 0-100 activated with sodium silicate of modulus Ms = SiO2/Na2O = 1 and 5, 10 and 15% Na2O. A blended ordinary CPC-30R Portland cement reference was used. Pastes were subjected to exposure up to 1200 degrees C and the performance was evaluated in terms of compressive strength, residual strength, volumetric shrinkage, physical appearance and microstructural changes at different temperatures. All the binders retained more than 30 MPa after exposure to 800 degrees C for 4 h; specimens of MK and CPC-30R experienced the highest strength losses of 42 and 56% respectively, while those of 100-0 and 50-50 showed minor losses of similar to 20%. After heating at 1200 degrees C the samples showed microstructural damage and more than 65% of strength losses. XRD indicated that the 100-0 and 50/50 binders are prone to form crystalline phases as akermanite, nepheline and nosean at temperatures greater than 1000 degrees C, while 0-100 geopolymeric binders preserved mostly an amorphous structure even at 1200 degrees C with some traces of mullite. The dehydration of C-A-S-H and N-A-S-H altogether with the crystallization of the binder gel induced the formation of highly porous microstructures. (C) 2017 Elsevier Ltd. All rights reserved.