Microstructural Evolution of Calcium Aluminate Cements Hydration with Silica Fume and Fly Ash Additions by Scanning Electron Microscopy, and Mid and Near‐Infrared Spectroscopy

Abstract

Calcium aluminate cement (CAC) is less commonly used as ordinary Portland cements (OPCs) for structural concrete, is relatively expensive, but may have certain advantages when used for solidification of wastes; it introduces rapid strength gain and has a higher resistance to chemical attack than OPC. However, the most widely identified degradation process suffered by CAC is the so‐called conversion of hexagonal calcium aluminate hydrate to a cubic form. Mixes of CAC with silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with binders of CAC–SF and CAC–FA, and their evolution over time. Mid infrared and near infrared spectroscopy have been used to assess the microstructure of these formulations. Microstructural characterization was completed by backscattering electron microscopy observation and microanalysis.

Studies on the influence of spent FCC catalyst on hydration of calcium aluminate cements at ambient temperature

“…There are opinions that silica reacts in the hydrating CAC to produce various crystalline hydrates with changing proportions of Ca, Al and Si, such as: C 3 AS 3-x H 2x , (0 \ x \ 3), and also different zeolite-type phases [4,6,13,14].…”

“…The strätlingite is a stable hydrate in the normal temperature and it improves strength properties after long time [1,5]. Ground granulated blast furnace slag [3,5], fly ash [4,6,7], silica fume [4,6,8], microsilica [13] or natural pozzolans [9] introduced into CAC paste can react to produce this stable phase, although the amount of C 2 ASH 8 in such complex system depends on the ability of the mineral admixtures to release silica. Chemical activity of silica can be increased in the presence of proper ions which act as a catalyst activating the surface of these extra grains.…”

“…Silica component of the mineral addition introduced into hydrating CAC system gives possibility of alternative reaction way for the metastable hydrates by formation of new aluminate hydrate-containing silica-certain C-A-S-H phases-instead of conversion into the stable cubic hydrogarnet. Strätlingite, also called gehlenite hydrate-C 2 ASH 8 is most often proposed to be formed [2][3][4][5][6][7][8][9][10][11][12][13]. The strätlingite is a stable hydrate in the normal temperature and it improves strength properties after long time [1,5].…”

“…However, the stable phases are C 3 AH 6 (hydrogarnet) and AH 3 (gibbsite), and other phases inevitably convert to these with time and/or at higher temperature as well as in favourable humidity. As a result, porosity of such cement material and its permeability increase with loss of strength of hardened concrete [1][2][3][4][5][6].…”

“…They especially offered to use appropriate additions introduced into the cement system to avoid, or at least reduce, the hydrate conversion and its negative effect in CAC composites [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”