Microstructure of the Hydrated Cement Paste

1. Composition of Portland Cement

Anhydrous State:

Portland cement exists as a gray powder composed of angular particles ranging from 1 to 50 μm in size. It is manufactured by grinding a clinker (a heterogeneous mixture of high-temperature reaction products of calcium oxide with silica, alumina, and iron oxide) with 2–5% calcium sulfate (gypsum).

Primary Clinker Compounds (approximated by oxide notation):

● C?S (Tricalcium silicate): 45–60%

● C?S (Dicalcium silicate): 15–30%

● C?A (Tricalcium aluminate): 6–12%

● C?AF (Tetracalcium aluminoferrite): 6–8%

2. Hydration Process and Microstructural Development

When Portland cement is mixed with water:

Initial Dissolution (Minutes):

● Calcium sulfate and clinker compounds dissolve, rapidly saturating the aqueous phase with ions (Ca2?, SO?2?, Al3?, OH?).

● Ettringite Formation: Needle-shaped calcium trisulfoaluminate hydrate (C?A??H??) crystals precipitate first.

Early Hydration (Hours):

● Calcium Hydroxide (CH): Large prismatic crystals form.

● Calcium Silicate Hydrates (C-S-H): Fibrous, nanocrystalline phases nucleate, filling spaces previously occupied by water and dissolving cement particles.

Intermediate to Late Hydration (Days):

● Ettringite Stability: Depending on the alumina-to-sulfate ratio, ettringite may decompose into monosulfoaluminate hydrate (C?A?H??) with hexagonal-plate morphology.

● Hexagonal-Plate Phases : Observed in undersulfated or high-C?A cements, including calcium aluminate hydrates.

3. Microstructural Features and Heterogeneity

Phase Distribution:

The microstructure of well-hydrated Portland cement paste comprises non-uniformly distributed phases with variable sizes and morphologies. Key characteristics include:

● Solid Phases: Ettringite, monosulfoaluminate, C-S-H, CH, and residual clinker grains.

● Pore Structure: Capillary pores and gel pores, influenced by initial water-cement ratio and hydration kinetics.

Mechanical Implications:

Microstructural inhomogeneity (e.g., localized weak zones, pore clustering) governs mechanical properties such as strength and durability, as these depend on extreme microstructural features rather than average conditions.

4. Rheological Influences on Microstructure

Fresh cement paste behavior critically impacts hardened microstructure:

Particle Flocculation:

Anhydrous cement particles tend to aggregate into flocs, entrapping excess water and creating localized variations in water-cement ratio (w/c).

Consequences of Flocculation (vs. Well-Dispersed Systems):

Pore Geometry: Larger, irregular capillary pores in flocculated systems.

Hydration Products: Coarser C-S-H and CH crystals due to uneven ion distribution.

Conclusion

The dynamic interplay between chemical reactions (governed by cement composition) and physical processes (e.g., flocculation, pore formation) dictates the heterogeneous microstructure of hydrated cement paste. This complexity underscores the limitations of theoretical models in predicting concrete behavior and highlights the necessity of empirical characterization methods.