The necessity to reduce CO2 emissions has increased the driving force for the cement research community to develop alternatives to traditional Portland cement (PC). One of the alternative cements being investigated is calcium sulfoaluminate cement (CS ̅A), first developed commercially in China in the 1970's. CS ̅A cements are produced by clinkering together limestone, bauxite, clay and calcium sulfate to produce mainly ye'elimite (C4A3S ̅) and belite (C2S). Due to the presence of ye'elimite, SO3 content is typically between 6 – 10 wt%. The abundance of elemental sulfur arising from the desulfurisation of oil and gas and its use to produce CS ̅A clinkers were the starting points of the Green Concrete Project (GCP), which aimed to develop a novel approach to produce CS ̅A cement via the combustion of elemental sulfur, recycling this by-product and reducing the dependence on hydrocarbon fuels. Integrating the current knowledge on the production of CS ̅A, a new generation of CS ̅A belite calcium sulfoaluminate (BCS ̅A) and belite-ye'elimite-ternesite (BYT) clinkers incorporating the combustion of elemental sulfur has been successfully developed and produced in a pilot plant production process. A high-temperature thermodynamic model was developed to predict stable assemblages and to complement and validate experimental results. Within the frame of the GCP, the title thesis focused on collecting conditional thermodynamic data for ye'elimite and ternesite (enthalpy of formation) that were determined experimentally using isothermal conduction calorimetry. The enthalpies of formation of ye'elimite and ternesite at 25 °C were determined to be -8523 kJ/mol and -5993 kJ/mol, respectively. The thesis was also focus on characterising two clinkers of interest (designated BCS ̅A and BYT) obtained from a pilot plant trial (with the novelty of sulfur combustion). The obtained clinkers, BCS ̅A and BYT, have a distinctive mineralogy where alpha prime belite (α΄-C2S) and ternesite (C5S2S ̅) are present, respectively. Both phases are candidates to replace a significant amount of ye'elimite and therefore reduce the need of expensive bauxite to establish a more sustainable cement. The alpha prime polymorph of belite proved to be more reactive than the beta. However, actions had to be taken to control the fast setting of this BCS ̅A cement. To provide a solution to the fast setting problem a variety of retarders were investigated: 0.5 wt% citric acid with an additional 5 wt% gypsum showed the best results. A fundamental solution was also Abstract 5 investigated in order to avoid the use of any retarders. The results showed that the clinkering temperature should be around 1300 °C. The hydration of ternesite in the BYT cement was found to be moderate. Therefore, a thorough investigation was conducted to understand and find ways to enhance its reactivity. It was found that the addition/presence of 0.4 wt% MgO, 0.2 wt% K2O and 0.1wt% Na2O in the raw meal required for the synthesis of ternesite, can increase its reactivity. Furthermore, it was found that particle size distribution above 600 m2/kg can also accelerate its reactivity. Single-phase chemically activated ternesite hydrated rapidly at 25 °C, achieving compressive strengths of ≈ 30 and ≈ 65 MPa at 28 and 90 days respectively, with C-S-H and gypsum as hydration products.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:731618 |
| Date | January 2017 |
| Creators | Skalamprinos, Solon |
| 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=234036 |
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