Early hydration of Portland Cement Compounds Synthesis and Hydration of Alite and Calcium Aluminate Bachelor Thesis Högskoleingenjörsprogrammet Kemiteknik MATTHIAS MOLÈN Institutionen för Kemi- och Bioteknik CHALMERS TEKNISKA HÖGSKOLA Göteborg, Sverige 2014 Abstract Cement is an adhesive substance primarily used as the bonding agent in concrete. Concrete is one of the most commonly used building materials and is formed when water, cement and an aggregate (usually sand or gravel) is mixed. The most used cement is called Portland cement. It is hydraulic cement which means it hardens even if it is under water. Portland cement consists mainly of five components which are gypsum and the four clinker phases: alite, belite, aluminate and ferrite. In this work alite and aluminate were synthesized and their hydration reaction studied by using infrared spectroscopy and isothermal calorimetry. The main hydration reaction of alite occurs during the first 24 hours and peaks after 10-15 hours. The hydration reaction of aluminate is very fast and occurs immediately. The addition of gypsum leads to a slower reaction rate. With 10 wt% gypsum added the reaction occurred after 2-3 hours and with 20 wt% gypsum it occurred after 10 hours. Contents Abstract ................................................................................................................................................... 2 1 Introduction .......................................................................................................................................... 4 2 Aim........................................................................................................................................................ 4 3 Theory ................................................................................................................................................... 5 3.1 Notations ....................................................................................................................................... 5 3.2 Portland Cement ........................................................................................................................... 5 3.3 Alite ............................................................................................................................................... 6 3.4 Belite .............................................................................................................................................. 7 3.5 Aluminate ...................................................................................................................................... 7 3.6 Ferrite ............................................................................................................................................ 7 3.7 Reaction Products of Alite and Belite ............................................................................................ 7 3.8 Reaction Products of Aluminate and Ferrite ................................................................................. 7 3.8.1 AFm Phases ............................................................................................................................. 7 3.8.2 AFt Phases .............................................................................................................................. 7 3.9 Infrared Spectroscopy ................................................................................................................... 8 3.10 Isothermal Calorimetry ............................................................................................................... 8 3.11 X-Ray Diffraction .......................................................................................................................... 8 4 Experimental ........................................................................................................................................ 9 4.1 Synthesis ........................................................................................................................................ 9 4.1.1 Aluminate ............................................................................................................................... 9 4.1.2 Alite ........................................................................................................................................ 9 4.2 Hydration of the Synthesized Phases .......................................................................................... 10 4.2.1 Calorimetry ........................................................................................................................... 10 4.2.2 IR ........................................................................................................................................... 10 5 Results and Discussion........................................................................................................................ 11 5.1 Synthesis ...................................................................................................................................... 11 5.2 Hydration of the Synthesized Phases .......................................................................................... 13 5.2.1 Calorimetry ........................................................................................................................... 13 5.2.2 Infrared Spectroscopy .......................................................................................................... 15 6 Conclusion .......................................................................................................................................... 20 7 Acknowledgements ............................................................................................................................ 21 8 References .......................................................................................................................................... 22 1 Introduction Cement is an adhesive substance primarily used as the bonding agent in concrete. Concrete is one of the most commonly used building materials and is formed when water, cement and an aggregate (usually sand or gravel) is mixed. The most used cement is called Portland cement. It is a hydraulic cement which means it hardens even if it is under water. Portland cement got its name in 1824 when Joseph Aspdin took patent for it. 2013 was the cement production in the world about 4 billion tonnes, where China accounted for 2.3 billion tonnes [1]. The environmental impact from the cement production is big. To produce one tonne of cement is about 4-5 GJ required and the CO2 emission from the process is approximately 0.7-0-8 tonnes. Portland cement contains of five main components: alite, belite, aluminate and ferrite, which are the clinker phases, and gypsum. Alite, aluminate and ferrite are the most important components during the early phases of the cement hydration and belite is more important for the final strength in the cement. The gypsum is added to the cement to delay the hydration reaction of aluminate and ferrite and to make it less exothermic. 2 Aim Cement have been used for a long time and is today one of the most building materials, still there are much to learn about the chemistry behind it. This is because of the complexity of the cement-water system which makes it very hard to study and understand. The aim in this work is to synthesize the triclinic form of alite and the cubic form of aluminate and study their hydration reaction using infrared spectroscopy and isothermal calorimetry. 3 Theory 3.1 Notations Abbreviations used in this work: Alite (Ca3SiO5) = C3S Belite (Ca2SiO4) = C2S Calcium Aluminate/Aluminate (Ca3Al2O6) = C3A Ferrite Ca4Al2Fe2O10 = C4AF C-S-H = Calcium Silicate Hydrate w/s = water-to-solid ratio 3.2 Portland Cement Portland cement contains five main components which are alite, belite, aluminate, ferrite and gypsum. To produce Portland cement limestone and clay/shale are crushed and grinded and then mixed together. Sometimes are Fe2O3 (iron oxide) and SiO2 (quartz) also added. The material is then heated in a kiln (figure 1) to about 1450ᵒC where it forms clinkers [2]. Figure 1 - Rotating cement kiln [2] The material is subjected to an increasing temperature in the kiln and following reactions take place: In temperatures below 1000ᵒC calcite and clay are decomposed. In temperatures between 1000-1300ᵒC is 2CaO·SiO2 formed from the decomposed calcite and clay and SiO2. In temperatures between 1300-1450ᵒC are clinkers formed. After the kiln, gypsum is added and it is grinded to finished cement. An overview of the whole process is shown in figure 2. Figure 2 - Cement manufacturing process [2] 3.3 Alite Portland cement contains between 50-70% C3S (Ca3SiO5), which makes it the most common component in ordinary Portland cement. The hydration reaction of C3S lasts several months but most of it reacts within the first 24 h, with a peak around 10-15 h [3]. In figure 3 is the hydration reaction of C3S shown. Its hydration rate compared to belite is very high, therefore it plays a more important role in the early stages of cement hardening. The high reaction rate depends on lower thermodynamic stability and holes in the molecule structure which makes it easier for the water to react [4]. Figure 3 - Heat flow in the hydration reaction of alite during the first 30 h [5] 3.4 Belite Portland cement contains about 15-30% C2S (Ca2SiO4). C2S occurs most often in 4 different polymorphic states: α (a hexagonal phase),