ORIGINAL ARTICLE Slag‐fly ash and slag‐metakaolin binders: Part II—Properties of precursors and NMR study of poorly ordered phases Kaushik Sankar 1 | Andre Sutrisno 2 | Waltraud M. Kriven 1 1 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 2 NMR/EPR Laboratory, School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois Correspondence Waltraud M. Kriven, Department of Materials Science and Engineering, University of Illinois at Urbana- Champaign, Urbana, IL. Email: kriven@illinois.edu Funding information US Army Corps of Engineers through the Construction Engineering Research laboratory (CERL), Champaign though Dr. Ghassan Al-Chaar Abstract Sodium silicate‐activated slag‐fly ash binders (SFB) and slag‐metakaolin binders (SMKB) are room‐temperature hardening binders that have excellent mechanical properties and a significantly lower carbon footprint than ordinary Portland cement (OPC). The aim of this study was to use nuclear magnetic resonance (NMR) spectroscopy to study the nanostructure of poorly ordered phases in SFB by varying slag/fly ash ratio, curing time, and curing temperature. Fly ash was completely substituted with metakaolin and the effect of this substitution on the poorly ordered phases was studied. It was observed that the proportion of geopolymer was generally higher in SMKB when compared to SFB. Although C– N–A–S–H and geopolymer coexisted in SFB and SMKB, C–N–A–S–H was the major product phase formed. The mean chain length (MCL) and the structure of C–N–A–S–H gel were estimated as a function of time, temperature, and slag/fly ash ratio. The MCL was found to have a negative correlation with slag/fly ash ratio and Ca/(Si+Al) ratio, but positive correlation with curing temperature. The average Si/Al atom ratios for geopolymers were also estimated. Lastly, the increased proportion of five‐coordinated aluminum (Al(V)) in metakaolin resulted in the decreased unreacted metakaolin in the hardened binder but did not increase the geopolymer content. 1 | INTRODUCTION Concrete is the most widely used construction material in the world. It generally contains ordinary Portland cement (OPC), water, fine aggregate, and coarse aggregate. How- ever, the problem in using OPC is that its manufacturing process releases significant amounts of CO 2 into the atmo- sphere. Estimates by Gartner 1 and Damtoft et al 2 indicate that the manufacture of 1 ton of OPC releases around 0.8‐ 0.9 tons of CO 2 into the atmosphere. Hence, there is a need for greener alternative binders that could compete with OPC in terms of cost, strength, setting time and dura- bility. Slag‐fly ash binders (SFB) and slag‐metakaolin binders (SMKB) are viable alternative binders to OPC. The precursors for SFB are inexpensive, industrial byproducts, namely, ground granulated blast‐furnace slag and class F fly ash. If slag and fly ash are sourced properly, the binders can lead to significant reductions in carbon emissions. 3,4 Additionally, the binders have excellent compressive and flexural strengths, as well as reasonable set times. 5,6 Previ- ous literature reports many SFB mixture formulations. 6–12 SMKB can be processed by mixing an alkali silicate solu- tion with amorphous blast furnace slag and metakaolin. SFB and SMKB are alternative binders that have a high scope for large‐scale commercialization and widespread use. However, the structure and evolution of poorly ordered phases in these blended binders must be understood. Previ- ous research proved that geopolymers [Q 4 (mAl) units] and calcium silicate hydrate type gels (calcium silicate hydrate Received: 20 August 2018 | Revised: 6 November 2018 | Accepted: 12 November 2018 DOI: 10.1111/jace.16224 J Am Ceram Soc. 2018;1–24. wileyonlinelibrary.com/journal/jace © 2018 The American Ceramic Society | 1