Phase Stability of Alkali-Activated Fly Ash-Slag-Silica Fume Cements: An XRD Analysis

Author(s): Z. Zhang and H. Wang
Paper category: Proceedings
Book title: 3rd International RILEM Conference on Microstructure Related Durability of Cementitious Composites
Editor(s): Changwen Miao, Wei Sun, Jiaping Liu, Huisu Chen, Guang Ye and Klaas van Breugel
Print-ISBN: 978-2-35158-188-9
e-ISBN: 978-2-35158-189-6
Publisher: RILEM Publications SARL
Publication year: 2016
Pages: 365-373
Total Pages : 9
Language : English

Abstract: Alkali-activated cement (geopolymer) based on fly ash has been regarded as a greener alternative to Portland cement. With the encouraging commercialisation environment and the removal of the standardisation and regulatory barriers, alkali-activated concretes are gradually adopted by industries, with several noticeable application projects constructed recently.
However, a question still remains in both academics and industries: are alkali-activated concretes durable? This study aims to investigate the microstructural evolution of fly ashbased alkali-activated cements with addition of 0-40% slag and 0-15% silica fume, which are the most commonly used blends for the current fly ash-based alkali-activated cement manufacturing. The X-ray diffractometric (XRD) analysis revealed that the amorphous phases in sodium silicate activated fly ash-based cement could transform into crystalline analcimelike zeolite over time (or by accelerated hydrothermal ageing scheme). Associating with the phase transformation could be changes of pore sizes and porosity, which lead to strength loss; however, as the remnant raw materials can slowly hydrate in this process and the formed gel may compensate the increased pore volume, thus the strength loss may be not evident. By incorporating slag, the stability of amorphous phases was significantly increased. The role of alkalis on the phase stability was further examined by a comparative study of samples with and without leaching treatment. The results showed that the free alkalis in hardened materials could still activate or assist the hydration of residual raw materials after two years of ageing at ambient conditions. The calcium containing phases were much more stable than those sodium aluminosilicate hydrates under the accelerated hydrothermal ageing conditions. From the long term of view and for the purpose of engineering applications of this new type of materials, this study suggests that the incorporation of slag and silica fume in fly ash-based alkaliactivated cements can increase their phase stability, which might have advantages on their durability.

Online publication : 2016
Publication type : full_text
Public price (Euros) : 0.00