2. Phases in clinker and cement

Alite (tricalcium silicate, Ca₃SiO₅) forms the bulk of a clinker (40-70%, by mass) with crystal sizes up to about 150 µm. Alite generally displays a hexagonal crystal habit in cross section, with forms ranging from euhedral (clearly-defined crystal faces), to subhedral (poorly-defined crystal faces), to anhedral (no crystal faces). Minor oxide contents may range from about 3% to 4% by mass. Alite reacts rapidly with water and is responsible for much of the early-age (28 d) strength development [5, 9] (Table 1).

Belite (dicalcium silicate, Ca₂SiO₄) forms 15-45% of a clinker, and displays a rounded habit with crystal sizes ranging from 5 to 40 µm. Normally the β polymorph is found in clinkers, although smaller amounts of, α, α'_H, and α'₁; polymorphs may occur with minor oxides comprising up to 5%. Light microscopy of lamellar structures on etched specimens and X-ray powder diffraction data are useful for distinguishing the belite forms. Belite is less reactive than alite but does contribute to later-age strengths (>28 d). Upon hydration, both alite and belite form a poorly-crystallized calcium silicate hydrate (C-S-H) and well-crystallized calcium hydroxide.

Tricalcium aluminate, Ca₃Al₂O₆, comprises 1-15% of a clinker occurring as small 1-60 µm crystals exhibiting irregular to lath-like habit, filling the area between the ferrite crystals [6]. Tricalcium aluminate may occur as cubic or orthorhombic forms, and may contain up to 7% minor oxides. It is highly reactive with water.

Ferrite (tetracalcium aluminoferrite, Ca₂(Al,Fe)O₅), comprises between 0% and 18% of a clinker with crystal habits as dendritic, prismatic, and massive. Minor oxide content within ferrites may be up to 10% and ferrites exhibit variable reactivity with water. The ferrite and aluminate phases are sometimes referred to as the interstitial or matrix phases as they occur between, and appear to bind the silicate crystals.

Phases in lesser quantities, but still influential to performance, include periclase (MgO) and free lime (CaO). Periclase may exhibit a dendritic or equant crystal habit both within and between the other clinker constituents, ranging in size up to 30 µm. Free lime may occur as isolated rounded crystals or in masses with variable crystal size.

Alkali sulfates and calcium sulfates may also occur in clinker and are of interest since they have been found to affect hydration rates and strength development [10, 11]. Increased alkali levels in clinker are considered potentially deleterious if alkali-susceptible aggregates are used in the concrete.

Miller and Tang [12] found arcanite (K₂SO₄), aphthitalite ((Na,K)₂SO₄), calcium langbeinite (K₂SO₄ · 2CaSO₄), and soluble anhydrite (CaSO₄) in a sampling of North American clinkers, although the anhydrite was thought to be a byproduct of the laboratory extraction process. Taylor [5] lists additional alkali sulfate and sulfate phases such as thenardite (Na₂SO₄) in high Na/K clinkers and possibly anhydrite (CaS0₄) in some high-SO3 clinkers. These phases form late in the clinkering process and generally are found along crystal perimeters within the voids [5]. Cements have one or more forms of calcium sulfate added upon grinding to control setting and, depending on its classification, may also contain calcite, slag, fly ash, or silica fume.

While it is important to know the bulk phase composition of a clinker or cement, the surface area of each phase will also affect cement performance, especially at early ages. The surface area of a constituent is influenced by the clinker texture and grinding characteristics, which may be studied by microscopy. A wide range of textures and compositions are possible in cement clinker, as illustrated by the three NIST Standard Reference Clinkers (Figs. 2, 3, 4). These clinkers were selected as representative of a wide range of North American clinker production with respect to phase abundance, crystal size and distribution. The images were obtained by backscattered electron imaging in a scanning electron microscope.

Clinker 2686 (Fig. 2) is intermediate in crystal size and exhibits more heterogeneous phase distribution than the other clinkers. Alite appears as subhedral to anhedral crystals approximately 25 µm in size. Belite occurs in large clusters as rounded crystals about 15 µm in diameter, which often exhibits the internal lamellar structure. Ferrite occurs as medium- to fine-grained lath-like crystals. Aluminate is fine-grained and found between the ferrite crystals. Pericl3;se occurs as equant crystals up to 15 µm in both the matrix and within some alite crystals.

Fig. 2. NIST SRM 2686, SEM backscattered electron image.

Clinker 2687 (Fig. 3) exhibits fine crystal size and relatively homogeneous phase distribution, with the exception of belite and free lime, which occur as clusters within the microstructure. Alite occurs as anhedral grains approximately 12 µm comprising the bulk of the clinker. The matrix contains subhedral aluminate and fine-grained ferrite finely inter-grown and difficult to distinguish. Some of the pores contain alkali sulfates.

Fig. 3. NIST SRM 2687, SEM backscattered electron image.

Clinker 2688 (Fig. 4) is the most coarsely crystalline of the clinkers, exhibiting the most homogeneous distribution of the constituent phases. Subhedral to anhedral alite crystals up to 110 µm form the bulk of the clinker. Be1ite occurs as rounded, evenly distributed crystals of about 35 µm diameter. Ferrite occurs with both lath-like and dendritic habit while aluminate crystals may be found between ferrite crystals.

Fig. 4. NIST SRM 2688, SEM backscattered electron image.

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