hcp - Structure of the Hydrated Cement Paste Dr Kimberly...

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1 Dr. Kimberly Kurtis School of Civil Engineering Georgia Institute of Technology Atlanta, Georgia Structure of the Hydrated Cement Paste Structure of the Hydrated Cement Paste What do we mean by “structure”? Type, amount, size, shape, and distribution of phases present macrostructure – can be seen unaided (200 µ m or larger) microstructure – must been observed with the aid of a microscope
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2 Structure of Concrete Macroscopically, concrete may be considered to be composed of 2 phases – coarse aggregate and mortar (paste + fine aggregate) or aggregate and paste. • heterogeneous distribution At the micro scale, we see that these 2 phases are not homogenous themselves! Aggregate • 60-75% of the solid volume of most concretes • The aggregate is principally responsible for the unit weight, elastic modulus, and dimensional stability of the concrete because these properties depend on the physical characteristics (strength, and bulk density) of the aggregate. • In addition, porosity, shape and texture of the aggregate are important for workability, durability, and strength. • The chemical and mineralogical composition of the aggregate is usually less important, with the exception of some deleterious and some advantageous reactions. • Aggregate phase is generally stronger, than the other 2 phases, with some exceptions.
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3 Hydrated Cement Paste Solids •C-S-H •CH •Ettringite •Monosulfate hydrate •Residual unhydrated cement Voids Entrapped air (>1mm) Entrained air (75-500um) Capillary pores (macro meso) Interlayer space (micropores) Water Capillary water Adsorbed water Interlayer water Chemically combined water Important 3rd Phase in Concrete! In addition to the coarse aggregate, fine aggregate and paste (together the “mortar fraction”), an important 3 rd phase generally exists – the transition zone (TZ) or interfacial transition zone (ITZ) the interfacial region between the coarse aggregate and the hcp 10-50 um thick “the weakest link”
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4 Structure of Concrete • Each of the phases may be heterogeneous in its composition (both solids and voids) • Relative proportions and characteristics of the phases vary with mixture composition, time, environment, etc. All of these factors make predictions of concrete behavior more challenging than predictions for other materials. Microstructure Solids •C-S-H •CH •Ettringite •Monosulfate hydrate •Residual unhydrated cement Voids Entrapped air (>1mm) Entrained air (75-500um Capillary pores (macro meso) Interlayer space (micropores) Water Capillary water Adsorbed water Interlayer water Chemically combined water
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