2 The Structure of Concrete DEFINITIONS SIGNIFICANCE COMPLEXITIES STRUCTURE OF THE AGGREGATE PHASE STRUCTURE OF HYDRATED CEMENT PASTE TRANSITION ZONE IN CONCRETE Siddharth shankar

DEFINITIONS The type, amount, size, shape, and distribution of phases present in a solid constitute its structure. The gross elements of the structure of a material can readily be seen, whereas the finer elements are usually resolved with the help of a microscope. The term macrostructure is generally used for the gross structure, visible to the human eye. The limit of resolution of the unaided human eye is approximately one-fifth of a millimiter (200 μm). The term microstructure is used for the microscopically magnified portion of a macrostructure.

Modern electron microscopes The magnification capability of modern electron microscopes is of the order of 105 times; thus the application of transmission and scanning electron microscopy techniques has made it possible to resolve the structure of materials to a fraction of a micrometer.

SIGNIFICANCE Progress in the field of materials has resulted primarily from recognition of the principle that the properties of a material originate from its internal structure. The properties can be modified by making suitable changes in the structure of a material.

COMPLEXITIES From examination of a cross section of concrete, the two phases that can easily be distinguished are aggregate particles of varying size and shape and the binding medium, composed of an incoherent mass of the hydrated cement paste (henceforth abbreviated hcp).

Macroscopic level At the macroscopic level, therefore, concrete may be considered to be a two-phase material, consisting of aggregate particles dispersed in a matrix of the cement paste.

Microscopic level At the microscopic level, the complexities of the concrete structure begin to show up. It becomes obvious that the two phases of the structure are neither homogeneously distributed with respect to each other, nor are they themselves homogeneous. For instance, in some areas the hcp mass appears to be as dense as the aggregate while in others it is highly porous.

(a) 結構之示意圖 (b)各組成所佔有體積之比例關係

混凝土之微結構 9

Three-phase theory Three phases: Aggregate Hardened cement paste (hcp) Transition (interface) zone􀂇

THREE PHASES OF CONCRETE

STRUCTURE OF THE AGGREGATE PHASE

AGGREGATE PHASE The aggregate phase is predominantly responsible for the unit weight, elastic modulus, and dimensional stability of concrete. These properties of concrete depend to a large extent on the bulk density and strength of the aggregate, which, in turn, are determined by the physical rather than chemical characteristics of the aggregate structure. In other words, the chemical or mineralogical composition of the solid phases in aggregate is usually less important than the physical characteristics such as the volume, size, and distribution of pores.

Aggregate particles Natural gravel has a rounded shape and a smooth surface texture. Crushed rocks have a rough texture; depending on the rock type and the choice of crushing equipment, the crushed aggregate may contain a considerable proportion of fault or elongated particles, which adversely affect many properties of concrete. Lightweight aggregate particles from pumice, which is highly cellular, are also angular and have a rough texture, but those from expanded clay or shale are generally rounded and smooth.

STRUCTURE OF HYDRATED CEMENT PASTE Anhydrous portland cement is a gray powder that consists of angular particles typically in the size range 1 to 50 μm. It is produced by pulverizing a clinker with a small amount of calcium sulfate, the clinker being a heterogeneous mixture of several minerals produced by high temperature reactions between calcium oxide and silica, alumina, and iron oxide.

STRUCTURE OF HYDRATED CEMENT PASTE The chemical composition of the principal clinker minerals coresponds approximately to C3S, C2S, C3A, and C4AF; in ordinary portland cement their respective amounts usually range between 45 and 60, 15 and 30, 6 and 12, and 6 and 8 percent.

Setting – Solidification of the plastic cement paste Hydration process Setting – Solidification of the plastic cement paste Initial set – beginning of solidification – Paste become unworkable – loss in consistency - not < 45 min. Final set – Time taken to solidify completely – Not > 375min. Hardening – Strength gain with time – after final set

Crystal formation of Cement

Calcium silicate hydrate The calcium silicate hydrate phase, abbreviated C-S-H, makes up 50 to 60 percent of the volume of solids in a completely hydrated portland cement paste and is, therefore, the most important in determining the properties of the paste. The fact that the term C-S-H is hyphenated signifies that C-S-H is not a well-defined compound; the C/S ratio varies between 1.5 to 2.0 and the structural water content varies even more.

Calcium silicate hydrate The morphology of C-S-H also varies from poorly crystalline fibers to reticular network. Due to their colloidal dimensions and a tendency to cluster, C-S-H crystals could only be resolved with the advent of electron microscopy. Although the exact structure of C-S-H is not known, several models have been proposed to explain the properties of the materials. According to the Powers-Brunauer model, the material has a layer structure with a very high surface area.

Calcium hydroxide Calcium hydroxide crystals (also called portlandite) constitute 20 to 25 percent of the volume of solids in the hydrated paste. In contrast to the C-S-H, the calcium hydroxide is a compound with a definite stoichiometry化學計量學, Ca(OH)2. It tends to form large crystals with a distinctive hexagonal-prism morphology. The morphology usually varies from nondescript to stacks of large plates, and is affected by the available space, temperature of hydration, and impurities present in the system.

Calcium hydroxide Compared with C-S-H, the strength-contributing potential of calcium hydroxide due to van der Waals forces is limited as a result of a considerably lower surface area. Also, the presence of a considerable amount of calcium hydroxide in hydrated portland cement has an adverse effect on chemical durability to acidic solutions because of the higher solubility of calcium hydroxide than C-S-H.

Mono-Sulfoaluminate … Ettringite

Ettringite

Calcium sulfoaluminates Calcium sulfoaluminate compounds occupy 15 to 20 percent of the solids volume in the hydrated paste and therefore play only a minor role in the structure-property relationships. It has already been stated that during the early stages of hydration the sulfate/alumina ionic ratio of the solution phase generally favors the formation of trisulfate hydrate, C6AS3H32, also called ettringite, which forms needle-shaped prismatic crystals.

Reaction rate: C3A > C3S > C4AF >C2S Chemical reactions occur at different rates.

C3S, C2S contribute mostly to strength which is in the CSH.

Heat of hydration (Cal/g) Compound 3 days 90 days 13 years C3S 58 104 122 C2S 12 42 59 C3A 212 311 324 C4AF 69 98 102

Three-phase theory -Transition zone

Three-phase theory -Transition zone A thin shell layer (10-50 μm thick) around large aggregate. Formation: Water films around large aggregate during mixing. Characteristic: Larger CH crystals; more porous framework; relatively weak.􀂇 􀂇

Transition Zone Miexhta and Monteiro: Concrete

Transition zone -Influence on concrete properties Fraction of transition zone in size is much smaller than other two phases, its influence on concrete properties is far greater. It lower the strength It increase the permeability It prompt non-linear behavior It favorites crack formation

Microstructural improvement Use of silica fume reduce the porosity of the ITZ geometrical effect (no space) reduces the amount of CH due to pozzolanic reaction