Effect of sulfate on C-S-H at early age

Elise M.J Bérodier,Arnaud C.A Muller,Karen L Scrivener

doi:10.1016/j.cemconres.2020.106248

Elise M.J Bérodier, Arnaud C.A Muller + Show 1 more

Open Access

https://doi.org/10.1016/j.cemconres.2020.106248

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Abstract

The effect of increasing sulfate contents on the hydration of white cement was studied with a multi-technique approach. Quantitative X-Ray Diffraction (XRD) revealed that adding more gypsum in the cement resulted in more ettringite formation. Combining isothermal calorimetry, advanced in-situ 1H nuclear magnetic resonance (NMR) and early scanning electron microscopy (SEM) observations, it was possible to correlate the depletion of the gypsum with a change in C-S-H morphology, from needle-like to agglomerated C-S-H, together with a change in C-S-H gel pore content. Nevertheless, the content of C-S-H interlayer water remained similar and no change in C-S-H pore sizes were observed when increasing the sulfate content. The data allowed to calculate the volume composition of hydrated cement pastes at 3 days of hydration. The more gypsum was added to the cement, the lower was the bulk C-S-H volume due to lower gel pore content and the higher was the content of capillary porosity.

Highlights

Gypsum is a critical component of Portland cements
This allowed for the first time correlating the reaction kinetics measured by calorimetry with the phase assemblage, composition and morphology of hydrates measured by X-Ray Diffraction (XRD), nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM)
One of the main findings of this study is that there was a clear reduction of calcium silicate hydrates (C-S-H) gel water by the addition of extra calcium sulfate

Summary

Introduction

Gypsum is a critical component of Portland cements. About 4–5% of gypsum is usually added to the clinker during grinding. The first function of the gypsum is to control the reaction of the C3A phase and to avoid flash set. Too much gypsum has been seen to cause expansion due to ettringite formation. In the cement and concrete industry, the optimum gypsum is typically chosen empirically by checking both setting time and strength development. Isothermal calorimetry can alternatively be used to adjust the gypsum content from the intensity and position of the aluminate peak (ideally located between 10 and 24 h of hydration). Despite the primary function of calcium sulfate as a set regulator, its impact on the phase assemblage, on the C-S-H at early age (< 2 days), and on the strength development is more complicated to understand and less documented

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