ORIGINAL ARTICLE Decalcification of alkali-activated slag pastes. Effect of the chemical composition of the slag C. Varga • M. M. Alonso • R. Mejı ´a de Gutierrez • J. Mejı ´a • F. Puertas Received: 7 April 2014 / Accepted: 11 September 2014 / Published online: 3 October 2014 Ó RILEM 2014 Abstract Portland cement decalcification and its effects on paste microstructure and mechanical strength have been widely studied. Decalcification in alkali activated slag (AAS) pastes is still not fully understood, however. The present study therefore explored the process in AAS cement pastes, acceler- ated by submerging specimens in concentrated ammo- nium nitrate solutions (NH 4 NO 3 ) for 3–21 days to induce leaching. Two AAS pastes were prepared with slag of different origins (Spanish and Colombian) and chemical compositions. OPC pastes were used as a reference. The findings showed that decalcification has a more adverse impact on OPC than AAS pastes strength. BSEM/EDX and 29 Si MAS NMR data nonetheless confirmed that Ca leaches out of C–A–S–H gels (formed in AAS pastes) to an extent that depends on the nature of the prime material. OPC pastes were shown to generate more silica gel with a very low Ca content (Q 3 and Q 4 units). Moreover, the higher the percentage of such units, the lower was mechanical strength. Decalcification in slag with lower MgO and higher Al 2 O 3 contents leads to the formation of smaller amounts of silica gel. The resulting gel was more compact and stable due to more intense chain cross-linking a possible tri-dimen- sional structure. Keywords Decalcification Á Leaching Á Alkali- activated slag (AAS) cements Á Mechanical strength Á C–A–S–H gel 1 Introduction Portland cement decalcification has been studied and described by a number of authors [16, 17, 31]. The hydrated compounds in OPC pastes (portlandite (Ca(OH) 2 ), C–S–H gel and ettringite) are dissolved or decompose in acid media, leaching (primarily Ca 2? and OH - ) ions into the aqueous phase and often from there to the outer medium. This interaction between cement hydrated phases and the acid medium induces the formation of soluble calcium salts and aluminium hydroxides from dissolved portlandite and decom- posed ettringite, respectively, and calcium silicate hydrates with low Ca/Si ratios if the phase attacked is C–S–H gel. These processes are associated with increases in porosity and declines in strength. Alkali-activated materials (AAMs) are known to be much more stable in acid media than Portland cement systems [5, 27, 28, 35]. The higher performance of alkali-activated slag (AAS) in this respect is due to the C. Varga Á M. M. Alonso Á F. Puertas (&) Eduardo Torroja Institute for Construction Science (IETcc-CSIC), Madrid, Spain e-mail: puertasf@ietcc.csic.es R. Mejı ´a de Gutierrez Á J. Mejı ´a School of Materials Engineering, Composite Materials Group (CENM), Universidad del Valle, Santiago de Cali, Colombia Materials and Structures (2015) 48:541–555 DOI 10.1617/s11527-014-0422-4