Formation of intermediate phases during
hydration of C₃S

1 Hydration heat development of a slowly reacting C₃S (w/s=0.50, 20 °C)

2 Formation of a continuous envelope of reaction products on the surface of the C₃S particles in the early phase of hydration [12]

3 Formation of isolated clusters of the intermediate phase (here designated as C-S-H) on the surface of the C₃S particles immediately after contact with water [11]

4 Results of ²⁹Si MAS NMR spectroscopy [13]: Top – single pulse spectrum of the non-hydrated nano C₃S (v_r=6 kHz, 60 s pulse repeat time, 1344 pulses); Center – single pulse spectrum of the nano C₃S after a 5 minute hydration period (v_r=6 kHz, 60 s pulse repeat time, 1360 pulses); Bottom – ²⁹Si{¹H} CP spectrum of the nano C₃S after a 5 minute hydration period (v_r=3 kHz, 10 s pulse repeat time, CP contact time=1 ms, 2304 pulses)

5 XPS analysis results of the two references (individual data points and data fit of the Si2p binding energies)

6 XPS analysis of a C₃S sample after 30 min. hydration (individual data points, data fit and contribution of the references to the overall peak)

7 Schematic representation of various geometric situations for evaluation of the XPS results. A: uncovered C₃S surface before contact with water, B-F: different degrees of covering and layer thicknesses that all would produce an XPS result of 44 % C₃S to 56 % intermediate phase

8 Electron micrograph of the C₃S sample after 30 minutes hydration. Some very small C₃S particles (diameter smaller than 25 nm) and a few hydrate phases are discernible. The actual C₃S surface is relatively smooth and no clusters as assumed in Figs. 7B-7E are discernible (resolution of micrograph: 2 nm)

9 Change in the Gibbs free energy during the hydration of C₃S in a single step (left) or in two steps (right). For this calculation, thermophysical data [17, 18] and the following reaction equation were used: C₃S + 3.917 H₂O ➞ 1.7Ca(OH)₂-SiO₂-0.917H₂O + 1.3 Ca(OH)₂

C₃S-Partikel in an early stage of the reaction. On the surface, etch pits (diameter about 20 nm) and first C-S-H phases are visible

The described model concepts can be employed as the basis for the development of binding agents possessing a significantly lower energy content and thus generating lower specific carbon dioxide emissions.

1 Introduction

The setting of cement is an important technological process. Nowadays, more than 2 billion t of cement are manufactured and processed all around the world. In addition to the technological parameters of the produced cements, such as water demand, setting times and compressive strength development, the industry places very great importance on the environmental impact of the cement manufacturing process. The main focus is placed on carbon dioxide emissions and the consumption of thermal and electrical energy. It can be assumed that cement production capacities will continue to...

TEXT Dr.-Ing. habil. Frank Bellmann¹, Dipl.-Ing. Thomas Sowoidnich¹, Prof. Dr. habil. Denis Damidot², Prof. Dr.-Ing. Horst-Michael Ludwig¹ ¹Bauhaus-Universität Weimar, Weimar/Germany
²Ecole des mines de Douai, Douai cedex/France