The effect of d-gluconic acid as a retarder of ground granulated blast-furnace slag pastes

• 1 wt.% d-gluconic acid retards the GGBFS suspension for at least 28 days.
• The retarder extends the dormant period of the hydration process.
• The retarder prevents not only the formation of ettringite but also that of strength-giving phases.
• No transformation of sulfite to sulfate is indicator for a retarded GGBFS suspension.

Ground granulated blast-furnace slag (GGBFS) is a latent hydraulic binder, which has also the ability to hydrate autonomously after contact with water. The rate of hydration strongly depends on the composition of the slag and the pH-value of the mixing water or activator. The retarded GGBFS suspension and an alkali activator can be used in a special two-component binder system for variable applications. d-gluconic acid (C6H12O7) was found to be a suitable retarder for GGBFS, which retarded the hydration at least 28 days. The retarder extends the dormant period of the hydration process and prevents mainly the formation of strength-giving phases. The interaction of GGBFS and retarder was investigated by heat flow calorimetry, measurement of non-purgeable organic carbon (NPOC), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetry (TG) and scanning electron microscopy (SEM). The basis for this study was a constant binder content, various concentrations of the retarder and different durations for the retarding effect. The concentrations were 0.05, 0.25 and 1.00% by weight of the mixing water. The samples were investigated between 0 and 28 days. At the end, one mixture was optimized for a shelf time of at least 28 days without separation.

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