Solids in the Cement Hydration Products

1. Calcium Silicate Hydrate (C-S-H)

Composition and Significance:

The calcium silicate hydrate phase, abbreviated as C-S-H, constitutes 50–60% of the solid volume in fully hydrated Portland cement paste, making it the dominant phase governing the paste’s mechanical and durability properties. The hyphenated notation "C-S-H" reflects its non-stoichiometric, amorphous nature:

● Variable Composition: The calcium-to-silica (C/S) ratio ranges between 1.5 and 2.0, while structural water content exhibits even greater variability.

● Morphological Diversity: C-S-H manifests as poorly crystalline fibers or reticulated networks, with colloidal-scale dimensions necessitating electron microscopy for resolution. Historically termed C-S-H gel due to its gel-like appearance, its exact atomic structure remains unresolved.

Structural Models:

● Powers-Brunauer Model: Proposes a layered structure with an ultra-high surface area (100–700 m2/g), where strength arises from van der Waals forces. Gel pores (solid-to-solid distances) are estimated at ~18 ?.

● Feldman-Sereda Model: Depicts C-S-H as irregular, kinked layers forming interlayer spaces (5–25 ?) of diverse shapes and sizes.

● Historical Context: Early hypotheses likened C-S-H to the natural mineral tobermorite, though this analogy is now considered oversimplified.

2. Calcium Hydroxide (Portlandite)

Characteristics and Role:

● Volume Fraction: Represents 20–25% of the solid volume in hydrated paste.

● Crystalline Structure: A stoichiometrically defined compound (Ca(OH)?) forming large hexagonal-prismatic crystals. Morphology varies from nondescript plates to stacked arrays, influenced by hydration temperature, spatial constraints, and impurities.

● Mechanical Contribution: Limited strength potential compared to C-S-H due to its low surface area and brittle crystalline nature.

3. Calcium Sulfoaluminate Hydrates

Phase Evolution and Impact:

● Volume Fraction: Occupies 15–20% of the solid volume, playing a secondary role in microstructure-property relationships.

Hydration Sequence:

● Early Stage: High sulfate/alumina ratios favor ettringite (C?A??H??), characterized by needle-shaped prismatic crystals.

● Later Stage: In ordinary Portland cement, ettringite transforms into monosulfoaluminate (C?A?H??), forming hexagonal-plate crystals.

● Durability Concern: Monosulfoaluminate’s presence increases susceptibility to sulfate attack in concrete.

● Crystal Chemistry: Both phases incorporate trace iron substitutions for aluminum in their structures.

4. Unhydrated Clinker Grains

Residual Particles and Hydration Dynamics:

● Persistence in Microstructure: Unhydrated clinker grains (1–50 μm) may remain even in mature pastes, depending on initial cement particle size distribution and hydration degree.

● Hydration Mechanism: Smaller particles dissolve first, while larger grains gradually shrink. Hydration products nucleate near particle surfaces, forming dense coatings.

● Late-Stage Morphology: In advanced hydration, limited space leads to in situ formation of dense hydration products mimicking the original clinker particle shape.

Key Structural and Mechanical Implications

● C-S-H Dominance: As the primary binding phase, C-S-H’s colloidal structure and high surface area underpin cement paste’s cohesive strength and durability.

● Heterogeneity-Driven Behavior: Microstructural extremes (e.g., large calcium hydroxide crystals, residual clinker, or localized ettringite) disproportionately influence mechanical properties over average conditions.

● Durability Challenges: Phase transformations (e.g., ettringite → monosulfoaluminate) and residual clinker grains contribute to long-term degradation mechanisms.