• Pol Segura, Isabel

Abstract

Around 4 billion tonnes of Portland cement are manufactured worldwide every year. For every tonne of Portland cement produced, around 0.6 tonnes of CO₂ are released to the atmosphere. This leads to an annual contribution of approximately 8% of the global CO₂ anthropogenic emissions. Most of the carbon emissions from cement production are inherent to the chemistry of cement and the high temperatures required for its processing (up to 1450 ℃). This means that by changing the main raw material with a non-carbonate based precursor, the CO₂ emissions could be significantly reduced.<br/><br/>One of the promising non-carbonate based materials are alkali-activated materials (AAMs), which include geopolymers. It has been demonstrated that geopolymers can potentially have similar mechanical properties to Portland cement and outperforming them with respect to acid and fire resistance. On top of that, several investigations indicate that geopolymers can reduce the CO₂ emissions by more than 50%, when compared to Portland cement. Nevertheless, geopolymers also have limitations that constrain their further widespread use. This includes high prices of some raw materials, handling of very alkaline components in the production process and uncertainties on which mix designs lead to the best products.<br/><br/>The main goals of the project were to study the two main geopolymer production methods (two-part and one-part), focusing on the use of calcined clay as the main aluminosilicate precursor, and to develop an optimal and efficient geopolymer cement plant.<br/><br/>The first three chapters are considered introductory chapters, were the objectives, the background and testing methods for the rest of the studies are explained. Chapter 2 specifically provides the framework of this study, including a literature summary on geopolymer synthesis and a review on the main relevant technologies that are already available in the construction industry and applicable to geopolymer cement and concrete production. Moreover, it includes an environmental and economic assessment, which estimated that geopolymer manufacturing could reduce by more than 68% the CO₂emissions associated with Portland cement’s production and that the geopolymer price level largely depends on the aluminosilicate and alkali activators sources used.<br/><br/>Chapter 4 consists in an experimental study conducted to assess the influence of the alkali reagent concentration and the water content on fresh and hardened properties of two-part metakaolin geopolymers. The test results showed that the increase of water (water-to-solids &gt; 0.52) and the increase of NaOH (Na₂O/Al₂O₃&gt;1) had the largest impact, showing a detrimental effect on both fresh and hardened properties. Moreover, the best results were obtained when using a SiO₂/Al₂O₃ molar ratio of 3.14, a Na2O/Al₂O₃ molar ratio of 0.97, and a water-to-solids ratio of 0.51, which led to a compressive strength of 44.1 MPa after 28 days on 40 mm cube specimens.<br/><br/>Chapter 5 compares the one-part and two-part mixing methods with two aluminosilicate precursors, metakaolin and ground granulated blast-furnace slag (GGBFS), using identical mix<br/>designs with both preparation methods. The results revealed that using one-part mix delays the setting time, increases the heat of reaction, decreases the shrinkage and reaches between 80 to 85% of the compressive strength of the two-part mix. Moreover, it was observed that the extent of reaction in two-part alkali-activated mixes was higher than for one-part.<br/><br/>Chapter 6 characterizes and evaluates kaolinite, montmorillonite, bentonite, halloysite, and illite clay as potential precursors in two-part geopolymers. The main advantage of using clays as precursors is that they are widely abundant and constitute 2/3’s of the Earth’s crust. However, up to date, calcined high-grade kaolin seems to be the most suitable for geopolymerization due to its composition (58 wt% SiO2, 38 wt% Al₂O₃). From this study, it has been observed that calcining a commercial kaolinite and halloysite at 750 ℃ for half an hour led to compressive strengths above 15 MPa, and thus they could become potential precursors for two-part geopolymers in low-strength applications.<br/><br/>Chapter 7 studies the substitution of commercial sodium silicate by silica-rich wastes. This investigation is mainly motivated due to the high-energy consumption, and large CO₂ emissions associated to the production of sodium silicate, as well as, representing 50 to 80% of the total cost of geopolymers. The alternative silica-rich wastes that have been studied in this project are rice husk ash, wheat straw ash, glass waste and silica fume. The results indicated that silica fume and waste glass have a great potential as substitutes of sodium silica...

Topics

• impedance spectroscopy
• Carbon
• glass
• glass
• strength
• Sodium
• cement