Influence of calcium sulfate and calcium hydroxideon the hydration of calcium sulfoaluminate clinker

1 Influence of gypsum addition on hydration heat flow of CSA pastes

2 Influence of anhydrite addition on hydration heat flow of CSA pastes

3 Influence of combined portlandite and gypsum addition on hydration heat flow of CSA pastes

4 Influence of combined portlandite and anhydrite addition on hydration heat flow of CSA pastes

$5 X-ray diffraction analysis of CSA clinker after various hydration times$

5 X-ray diffraction analysis of CSA clinker after various hydration times

Thermogravimetrische Analyse des CSA-Klinkers zu verschiedenen Hydratationszeiten

$7 X-ray diffraction analysis of CSA clinker blended with gypsum (1:0.88 molar ratio) after various hydration times$

7 X-ray diffraction analysis of CSA clinker blended with gypsum (1:0.88 molar ratio) after various hydration times

8 Thermogravimetric analysis of CSA clinker blended with gypsum (1:0.88 molar ratio) after various hydration times

$9 X-ray diffraction analysis of CSA clinker blended with portlandite (1:0.88 molar ratio) after various hydration times$

9 X-ray diffraction analysis of CSA clinker blended with portlandite (1:0.88 molar ratio) after various hydration times

10 Thermogravimetric analysis of CSA clinker blended with portlandite (1:0.88 molar ratio) after various hydration times

11 Development of ettringite content of (a) CSA/gypsum and (b) CSA/anhydrite mixtures, estimated from TGA analyses

12 SEM images of CSA-based mortars, water/binder = 0.70, sample age 24 hours, (a) pure CSA clinker, (b) CSA/gypsum 1:0.88, (c) CSA/anhydrite 1:0.88, (d) CSA/portlandite 1:0.88; C = unhydrated clinker, E = ettringite, M = monosulfate, M* = C₄AH_x, G = gel-like areas, Q = quartz aggregate

13 Influence of gypsum and portlandite on the chemical composition (EDX spot analyses) of the hydrate phases in CSA based mortars, sample age 24 hours

SEM images of CSA-based mortars; CSA/anhydrite 1:0.88
(C = unhydrated clinker, E = ettringite, G = gel-like areas)

Summary: Calcium sulfoaluminate cements represent a low CO₂-alternative to ordinary Portland cements. In order to better understand the hydration mechanisms of this type of binder, a commercially available calcium sulfoaluminate clinker was investigated that has been blended with different amounts of gypsum, anhydrite and portlandite. It was found that the hydration of the main hydraulic phase ye’elimite (C₄A₃S−) can be accelerated and controlled by the addition of calcium sulfate of sufficient reactivity. In the presence of gypsum or anhydrite, ettringite and Al(OH)₃ form as hydration products. When the calcium sulfate is consumed, monosulfate forms instead of ettringite. The addition of calcium hydroxide leads to very rapid initial hydration reactions forming calcium aluminum hydrate C₄AH_x, which causes a very rapid setting of the binder. Compared to Portland cements, mortars based on calcium sulfoaluminate cements are able to reach higher compressive strengths up to 100 MPa after 28 days.

1 Introduction

The production of Portland cement clinker counts for about 5 % of the total man-made CO₂-emissions. A promising low-CO₂ alternative is the production of alternative clinkers based on calcium sulfoaluminate (C₄A₃S−) [1]. They can be made from calcium sulfate, limestone and bauxite at a temperature of about 1250 °C [2]. Compared to alite (1.80 g CO₂ per ml of the cementing phase), calcium sulfoaluminate releases only 0.56 g CO₂ per ml cementing phase [1]. In addition, this type of clinker can be ground easier compared to ordinary Portland cement. Depending on the raw meal...