Review of the South African cement and fly ash status quo and hydration chemistry of hybrid fly ash cement

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Chemical characterisation techniques

ray fluorescence (XRF)

Wilhelm Conrad Roentgen discovered X-rays in 1895, and the first industrial X-ray spectrometer was available about 60 years later. An unprecedented series of discoveries, all within a couple of decades after that of Roentgen, paved the way towards commercialized analysis using X-rays. Current X-ray spectrometers are compact and capable of analysing dozens of elements in minutes, although analysis of eight elements (Si, Al, Fe, Ca, Mg, S, Na, K) is normally sufficient for Portland cement characterisation. The materials routinely analysed at a cement plant are the raw materials used to produce cement clinker, corrective materials, alternative cementitious materials, clinkers and cements. An accurate chemical analysis of materials used in cement manufacturing is required in order to design a proper cement raw mix, determine target clinker composition and produce cement meeting desired specifications (PCA, 2004). The chemical composition of the raw materials used in this study was determined by X-ray fluorescence (XRF) fused bead analysis (PANalytical Axios). The glass beads were prepared by mixing 1 g of the sample with 5 g of fluxing agent (XRF Analytical, 100% Li2B4O7) and fusing the mixture at 1000 °C. The fine powdered samples required no additional milling prior to the analysis. The loss-on-ignition (LOI) was determined by roasting the sample at 1050 °C for 1 hour until a constant weight was achieved.

ray powder diffraction (XRD)

X-ray powder diffraction (XRD) analysis is one of the most prominent analytical techniques used for the characterisation of crystalline, fine-grained materials, such as cements. The power of XRD is in the rapid and, if carried out appropriately, reliable delivery of quantitative mineralogical data compared to traditional quantitative phase analysis methods such as Bogue calculations and optical microscopy (Snellings, 2016; Snellings et al., 2014). In cements the technique is mostly used for qualitative, i.e. phase identification, and quantitative phase analysis, but other potential uses may include the determination of polymorphic modification and state of crystallinity (Snellings, 2016; Taylor, 1997). In this study, XRD was especially valuable in identifying and trending the production and consumption of secondary hydration products like portlandite and ettringite. Unfortunately, the type of gel products expected to form during hydration are amorphous to X-rays and need to be characterised by alternative analytical techniques such as Fourier-transform infrared spectroscopy (FTIR). X-ray powder diffraction (XRD) measurements were carried out using a PANalytical X’Pert Pro powder Diffractometer an X’Celerator detector and variable divergence- and fixed receiving slits, with Fe filtered Co-Kα radiation (λ=1.789Å). The phases were identified using X’Pert Highscore plus software. The relative phase amounts (weight %) were estimated using the Rietveld method (Autoquan Program). Twenty percent silicon (Aldrich, 99% pure) was also added to each sample for the determination of amorphous content. The samples were then micronized in a McCrone micronizing mill, and prepared for XRD analysis using a back loading preparation method. XRD analysis of the fly ash specimens cured in calcium hydroxide did not include the addition of silicon, since only the crystalline phases were identified for the purpose of the study.

Particle size distribution (PSD)

Almost all powders exhibit a variety of particles distributed over a range of sizes, and the width, shape and position of this size distribution will influence many aspects of their processing. Knowledge of particle size and the distribution of sizes are therefore essential for tight process control, maintenance for quality and minimisation of costs (Mingard et al., 2009). The particle size distribution (PSD) of the raw cementitious materials were obtained by laser diffraction using a Malvern Mastersizer 2000 fitted with a Scirocco 2000 sample handling unit. Scattered light data was recorded for 25 seconds. A refractive index of 1.68 and absorption of 1 was chosen. Size data collection was performed within the recommended 10-20% obscuration range.

Field emission scanning electron microscopy (FESEM)

More often than not, microscopy data is used for the physical study of morphologies of solid products, either as a principal analytical technique or as a supporting characterisation technique. The morphology of the solid samples was studied using a Zeiss Ultra SS (Germany) field emission scanning electron microscope (FESEM), operated at an acceleration voltage of 1 kV under high-vacuum conditions. Specimens were dried and then procured by dipping carbon stubs into the powders. Excess powder was removed by gentle blowing with compressed nitrogen. The samples were sputter-coated with carbon (Emitech K550X Ashford, England) and placed in the microscope for examination.

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Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) and the derivative curve of thermogravimetric analysis (DTG) are commonly employed in material science to determine thermal characteristics of materials, rate of degradation, absorbed moisture content and kinetics of a reaction based on weight changes (Dilnesa). TGA and DTG curves of cement paste can be divided into three major parts (Dilnesa): • < 300 °C : removal of water from hydrated products which are likely to include most cement phases e.g. C-S-H. Several other minor dehydration steps attributed to removal of pore water, interlayer water and adsorbed water are also likely to take place. • 400 °C – 500 °C : removal of water via dehydroxylation of mostly portlandite • 600 °C : removal of carbon dioxide via decarbonation of mostly calcite The following reaction equations explain how the above information is applied to identify the presence of certain hydration products in cementitious materials from analysing their TGA and DTG curves (Lothenbach et al., 2016). The initial loss of water (< 100 °C) from a cement sample is associated with the loss of surface moisture if the sample was not properly dried. Between 100 and 200 °C, numerous dehydration reactions of phases present in ordinary Portland cement take place. The most prominent dehydration, dehydroxylation temperature regions for cement and hydrated cement, summarised in order of ascending temperature, are presented in Table 3.1. Table 3.1. The most prominent dehydration, dehydroxylation and decarbonation temperature regions for cement and hydrated cement. Cement phase Chemical formula Temperature range (°C).

Table of contents :

Chapter 1 – Introduction, objectives and scope
1. Introduction
2. Background
3. Aims and objectives of the study
4. Scope and limitations of the study
5. Methodology
6. Layout of the thesis
Chapter 2 – Review of the South African cement and fly ash status quo and hydration chemistry of hybrid fly ash cement
1. Introduction
2. Fly ash
  1. Production and characteristics of South African fly ash
  2. Fly ash classification according to EN 450 / SANS
  3. Global perspective of fly ash production and utilization
  4. South African perspective on fly ash production and utilization
3. Fly ash as a component in the production of blended cement
  1. The restrictions of fly ash-containing cements in the cement market (locally and globally) according to EN 197-1:2011 Edition
  2. A short review on the hydration chemistry of ordinary Portland cement
  3. A short review on the hydration chemistry of typical fly-ash (pozzolana) containing cement
4. Hybrid cement
  1. What is hybrid cement?
  2. Activation and production of high fly ash-containing hybrid cements
  3. Hydration chemistry of high fly ash-containing hybrid cements
5. Conclusion
Chapter 3 – Experimental program
1. Introduction
2. Chemical characterisation techniques
  1. X-ray fluorescence (XRF)
  2. X-ray powder diffraction (XRD)
  3. Particle size distribution (PSD)
  4. Field emission scanning electron microscopy (FESEM)
  5. Thermogravimetric analysis (TGA)
  6. Fourier transform infrared spectroscopy (FTIR)
3. Characteristics of the fly ash surface reactivity exposed to a calcium hydroxide environment
4. Sulfate optimisation of a hybrid cement produced from unclassified fly ash (UFA) and cement (MC)
  1. Introduction
  2. Setting time
  3. Early age strength development
5. Hybrid fly ash cement paste
  1. Characterisation techniques
  2. Setting time
  3. Heat of hydration
  4. Expansion (soundness)
6. Hybrid fly ash mortar tests
7. Hybrid fly ash concrete testing
  1. Mix composition
  2. Workability (Slump retention)
  3. Strength behaviour
8. Overview of experimental program
Chapter 4 – Characterisation of raw cementitious materials
1. Introduction
2. X-ray fluorescence (XRF)
3. X-ray powder diffraction (XRD)
4. Particle size distribution (PSD)
5. Field emission scanning electron microscopy (FESEM)
6. Thermogravimetric analysis (TGA)
7. Fourier transform infrared spectroscopy (FTIR)
8. Conclusion
Chapter 5 – Investigation into the effect of chemical and mechanical activation
1. Introduction
2. Characteristics of the fly ash surface reactivity exposed to a calcium hydroxide environment
3. Sulfate optimisation study of a hybrid cement produced from unclassified fly ash
(UFA) and cement
1. Setting time
2. Strength behaviour
Conclusion

Chapter 6 – Characterisation of hydrating hybrid fly ash cement paste
1. Introduction
2. Setting time
3. Heat of hydration
4. Chemical characterisation
  1. X-ray powder diffraction (XRD) analysis
  2. Thermogravimetric analysis (TGA)
  3. Fourier transform infrared spectroscopy (FTIR)
5. The effect of chemical and mechanical activation on the pozzolanic reactivity of fly ash in a high fly ash hybrid cement
6. The effect of chemical and mechanical activation on stable ettringite formation in a high fly ash hybrid cement
7. Expansion (soundness)
8. Conclusion
Chapter 7 – Mortar test results and discussion of hybrid fly ash cement
1. Introduction
2. Strength of mortars
3. Conclusion
Chapter 8 – Concrete test results and discussion of hybrid fly ash cement
1. Introduction
2. Slump retention of concrete
3. Strength of concrete
4. Conclusion
Chapter 9 – Conclusions and recommendations for future work
1. Introduction
2. Conclusions
3. Recommendations for future work
References