Phase relations of the high Na-low Ca part of CaO-B2O3-Na2O-H2O system, relevant for the chemistry of boron in hydrating cement, were explored on a fine grid basis at 25°C and 55°C. The structure of several observed phases were determined and their diffraction patterns indexed. The experimental phase equilibria of calcium-borate hydrates and calcium-aluminate-borate hydrates were also assessed by calculation with good agreement. Solid solution formation between ettringite (AFt) or monosulphate (AFm) and their boron analgoues, which are candiate boron hosts in cement, was demonstrated by chemical analysis, FTIR and XRD. The size of the unit cells was found to be related to the series of substitutions in the structure. FTIR showed that boron is in [B3O3(OH)5]2- units in B-AFm. In a cement environment, B-AFt or mixed borate/sulphate AFt are stable and have very low solubility, AFm phases are metastable. Incorporation of simulated boron-containing waste water into cementitious matrtix (OPC, ground OPC clinker and BFS/OPC 9:1 blend) were investigated. Boron was used as boric acid, dissolved in the mixing water (4-12%) or sorbed in ion exchange resin (equivalent to 8% boric acid). The strong retardation effect of borates on cement hydration was confirmed. Instant precipitation of calcium-borate hydrates on the cement grains, inhibiting further access to water, is assumed to be responsible for this. Addition of NaOH at a necessarily high Na/B ratio to the mixing water keeps boron initially in solution and results in proper set. Matrix performance was investigated by calorimetry, pore fluid expression and leach testing. The resulting products were characterised by XRD and SEM. Confinement of boron in the major component of hydrated cement, in C-S-H gel, proved to be effective, although it could not be attributed to the formation of any crystalline boron compound. The use of blended cements (e.g BFS/OPC 9:1) instead of pure OPC is recommended due to its relatively low heat evolution, low permeability and low leachability.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:592297 |
| Date | January 1993 |
| Creators | Csetenyi, Laszlo Jozsef |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU554416 |
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