Hydration stoppage

Table 1.2 Advantages and disadvantages of solvent exchange and direct drying techniques for hydration stoppage ...

Mixing, casting and curing of pastes and mortars Hydration stoppage... 

This chapter, therefore, describes and compares practical approaches in sample preparation, data collection and data analysis used for quantification and characterisation of (hydrated) cements by XRD. Specific attention is paid to the effects of hydration stoppage and practical strategies towards data collection are described. The aim of the chapter is not only to present best practise but also to inform about compromises inherent in the adopted approaches and to draw attention to potential pitfalls. As an aid to data analysis, a database is given that provides crystal structures and powder diffraction identification files of common cement anhydrous and hydrate phases. Finally, the application of XRD to the study of cements is illustrated by two typical case studies of QPA on (1) an anhydrous portland cement and (2) a hydrated portland cement. 

Sample pretreatment Grinding, hydration stoppage (cf. Chapter 1)... 

D-drying (hydration stoppage) Also called dry ice drying-, removes water from cement pastes by applying a vacuum over dry ice. At regular conditions this process is very slow and cannot be used for early age samples. 

Hydration stoppage Process of stopping the hydration reaction by removal of water. Typical methods are solvent exchange, freeze-drying, vacuumdrying, D-drying and oven drying. 

Oven drying (hydration stoppage) Removal of water from hydrated samples by circulating a warm, dry air flow through the oven. Oven temperatures are between 60 and 105°C. Drying is considered complete when the sample reaches constant mass. 

Solvent exchange (hydration stoppage) Removal of water from hydrated cement samples by exchange of water by an organic solvent. Upon immersion, the solvent replaces the (free) water in the sample. Upon complete exchange, the solvent is removed by evaporation. Isopropanol is most frequently used as solvent. Solvent exchange is usually preferred for microstructural studies. 

Vacuum drying (hydration stoppage) Removal of water by evaporation in a vacuum chamber (pressure <0.1 Pa) and low water vapour pressure. Vacuum drying until constant weight does not preserve the paste microstructure and collapses the crystal structures of ettringite and AFm phases by removal of bound water. 

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