To reduce CO2 emissions the cement industry has for decades diluted the Portland cement fraction of concrete binders with Supplementary Cementitious Materials (SCM). Defining the reactivity of these aluminosilicate-based materials and their interaction with clinker phases has however challenged the research community and limited their replacement levels. In the present study, to quantify the reactivity of SCMs during early hydration, aluminosilicate glasses with compositions representative of blast furnace slag and fly ash systems were synthesised and exposed to different activator solutions in a continuously stirred closed system reactor for a period up to 3 h. Solution compositions were measured from the very first minutes of dissolution with complementary solid surface analysis. The experimental conditions were designed to avoid precipitation, however an initial Ca maximum in the first 30 min of exposure to the activating solution was a common feature in most dissolution profiles with a subsequent rapid decline, most likely attributable to Ca-reincorporation on the reacting surface. Surface specific analysis confirmed Ca enrichment on the surface and also detected Al enrichment, suggesting the formation of a Ca-modified aluminosilicate phase by a dissolution reprecipitation mechanism on the surface of the glass. Differing chemistries are thought to be responsible for the Ca and Al re-integration on the reacting surface depending on the pH of the solution; near-neutral conditions favour Ca-readsorption and surface condensation reactions, whereas alkaline solutions favour Ca-reintegration via covalently bound phases. The Ca concentration in solution was also observed to control glass alteration. Decreased dissolution rates were in fact observed as Ca concentrations in solution were increased supporting the formation of a C-A-S-H phase on the surface, the formation of which was instead suppressed when a Ca chelating chelating agent (EDTA) was added to the solution resulting in increased glass dissolution. Experiments using in situ AFM and LAOICPOMS are also reported and the significance of the findings to the early hydration reactions of a blended cement system is discussed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:675576 |
| Date | January 2015 |
| Creators | Newlands, Katrina |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=227976 |
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