Hydration, Microstructure and Structure–Property Relationships

Introduction and Context

Concrete performance is governed not only by its nominal mix proportions but also by the internal structure that develops as cement hydrates. Strength, stiffness, permeability, shrinkage, creep, durability and fire resistance are emergent properties of a heterogeneous material containing hydrated cement paste, aggregates, pores, cracks and interfacial transition zones (ITZs). Advanced concrete technology therefore requires a microstructural interpretation of macroscopic behaviour.

Hydration is a sequence of dissolution, ion transport, nucleation and growth reactions. These reactions transform an initially fluid suspension of cement particles and water into a rigid, multiphase solid. The resulting microstructure is neither chemically uniform nor static. It continues to evolve through ongoing hydration, drying, carbonation, thermal exposure, chemical attack and mechanical loading.

A central principle is that structure and properties are linked through multiple scales. At the nanoscale, calcium–silicate–hydrate (C–S–H) gel develops binding and stiffness. At the microscale, capillary pores, unhydrated cement and the ITZ control transport and crack initiation. At the mesoscale, aggregate distribution and cracking influence structural response. Consequently, no single parameter explains concrete performance; water-to-binder ratio, degree of hydration, pore connectivity, curing, aggregate characteristics and damage history must be considered together.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the principal reactions and heat evolution during Portland cement hydration.

2. Identify major hydration products and relate them to concrete properties.

3. Distinguish gel pores, capillary pores, entrained air voids and cracks.

4. Explain the origin and significance of the interfacial transition zone.

5. Relate pore structure and transport pathways to permeability and durability.

6. Apply microstructural models to interpret strength, stiffness and durability.

7. Evaluate curing, supplementary cementitious materials and admixtures as microstructure-control strategies.

8. Critically assess methods used to characterise concrete microstructure.

1. Cement Hydration: Reactions and Stages

Portland cement is principally composed of alite (C3S), belite (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF), together with gypsum and minor constituents. Cement notation is abbreviated: C represents CaO, S represents SiO2, A represents Al2O3, F represents Fe2O3 and H represents H2O.

When water is added, cement particles begin to dissolve. Calcium, hydroxyl, silicate, aluminate and sulfate ions enter solution. Hydration products precipitate when the solution becomes supersaturated. The reactions are not simple balanced single-step reactions because the products vary in composition and depend on temperature, water availability, sulfate concentration and supplementary materials. Simplified representations are:

2C3S + 6H → C3S2H3 + 3CH

2C2S + 4H → C3S2H3 + CH

Here C3S2H3 denotes C–S–H approximately, and CH denotes calcium hydroxide, also called portlandite. These equations are schematic rather than exact structural formulas because C–S–H has variable calcium-to-silicon ratio, water content and degree of polymerisation.

C3S hydrates relatively rapidly and contributes strongly to early strength and heat generation. C2S reacts more slowly and contributes substantially to later-age strength while producing less calcium hydroxide per unit of silicate hydrated. C3A reacts rapidly with water; gypsum moderates this reaction and promotes formation of ettringite. In sulfate-deficient or altered conditions, ettringite may subsequently transform partly to monosulfate or other AFm phases. C4AF hydrates more slowly and forms iron-substituted aluminate hydrates.

1.1 Heat evolution and hydration periods

The rate of heat release commonly shows five stages:

• Initial reaction: rapid wetting, dissolution and early precipitation.
• Dormant or induction period: relatively low heat evolution, allowing placement and compaction.
• Acceleration period: rapid C–S–H formation and setting, with increasing stiffness.
• Deceleration period: declining reaction rate as products hinder access to unhydrated particles.
• Long-term period: slow hydration controlled by diffusion, internal moisture and available reactive phases.

The heat flow curve is useful for comparing binders and selecting temperature-control measures, but it is not a complete measure of hydration degree. Two systems can release similar heat while developing different pore structures and products.

1.2 Degree of hydration

The degree of hydration, alpha, is the fraction of cement that has reacted. It may be estimated from chemically bound water, non-evaporable water, heat evolution, quantitative microscopy or thermogravimetric analysis. A simplified heat-based estimate is:

alpha = Q(t) / Q(infinity)

where Q(t) is cumulative heat released by age t and Q(infinity) is the heat corresponding to complete hydration under the chosen reference conditions. The equation assumes that heat contribution is proportional to reacted cement and that the reference heat is known. This assumption becomes uncertain for blended cements, variable reaction products and changing temperature conditions.

Degree of hydration increases with time, adequate curing temperature and water availability. It decreases when the system self-desiccates, dries prematurely or contains slowly reacting particles. Hydration may continue at very low rates for years if moisture and unhydrated phases remain available.

2. Hydration Products and Their Functions

2.1 Calcium–silicate–hydrate

C–S–H is the principal binding phase in ordinary Portland cement concrete. It is poorly crystalline or nanocrystalline and consists of calcium–silicate layers with interlayer water and calcium ions. Its composition is variable rather than stoichiometrically fixed. C–S–H provides most of the cohesion, much of the strength and a major proportion of the solid volume of hydrated paste.

C–S–H exists across several length scales. Inner-product C–S–H forms within the boundaries of original cement grains, while outer-product C–S–H forms in the surrounding space. The distinction is descriptive rather than a separate chemical compound. C–S–H gel contains very small pores, but its packing, connectivity and water content strongly affect creep, shrinkage and stiffness.

Supplementary cementitious materials such as fly ash, ground granulated blast-furnace slag and silica fume can produce additional C–S–H or calcium–aluminosilicate–hydrate (C–A–S–H). The pozzolanic reaction consumes calcium hydroxide:

SiO2 + CH + H → additional C–S–H

This expression is simplified and does not imply a single product composition. The reaction generally refines capillary pores and may improve later-age durability, although early strength can be reduced if reaction is slow or curing is inadequate.

2.2 Calcium hydroxide

Portlandite forms as a crystalline product of silicate hydration. It contributes less to binding than C–S–H and is relatively soluble. Its high alkalinity helps maintain steel passivation, but it can react with carbon dioxide and acids. Pozzolanic materials consume part of the portlandite and often reduce the size and continuity of portlandite crystals, especially near aggregate surfaces.

2.3 Aluminate and sulfate-bearing phases

Ettringite is commonly formed during early hydration when aluminate phases react in the presence of sulfate. It consists of calcium aluminate sulfate hydrate with substantial structural water. Monosulfate and related AFm phases may form as sulfate becomes depleted. Under particular exposure conditions, delayed ettringite formation can cause expansion, but this depends on temperature history, moisture and chemical environment; ettringite itself is not automatically destructive.

3. Pore Structure and Moisture States

Concrete contains pores over a wide range of sizes. A practical classification is:

• Gel pores: very small spaces within or between C–S–H domains, commonly below several nanometres.
• Capillary pores: remnants of the original water-filled space not occupied by hydration products, generally ranging from nanometres to micrometres.
• Entrained air voids: intentionally introduced, relatively spherical voids that protect against freezing and thawing.
• Entrapped air voids: larger irregular voids caused by inadequate compaction.
• Cracks: discontinuities generated by shrinkage, thermal effects, loading, chemical reactions or poor construction.

The total pore volume is important, but pore size distribution, connectivity, tortuosity and saturation are often more decisive. A large population of isolated pores may have little influence on permeability, whereas a smaller connected network can provide an efficient transport pathway.

Water in pores may be chemically bound, physically adsorbed, capillary-held or freely mobile. Relative humidity affects the balance. Drying causes menisci to form in pores and generates capillary tension, contributing to drying shrinkage. Self-desiccation can occur in low water-to-binder ratio systems when hydration consumes internal water faster than it can be supplied.

3.1 Influence of water-to-binder ratio

The water-to-binder ratio (w/b) controls the initial volume of water-filled space relative to solid binder. At a given degree of hydration, a lower w/b generally produces lower capillary porosity and a more discontinuous pore network. However, very low w/b mixtures may hydrate incompletely because internal water becomes unavailable, and they may exhibit autogenous shrinkage.

For a cement paste, the initial capillary space is approximately related to the water content per unit binder volume. A conceptual, not universal, relation is:

P_c approximately proportional to (w/b) minus the volume occupied by hydration products.

This statement is preferable to treating a single linear formula as generally valid, because chemical shrinkage, self-desiccation, admixture effects and degree of hydration alter the actual pore volume.

4. Interfacial Transition Zones

The ITZ is the region adjacent to aggregate particles where paste microstructure differs from bulk paste. In conventional concrete it is often more porous, richer in calcium hydroxide and characterized by larger crystalline products and weaker mechanical continuity. Its formation is associated with the wall effect: cement particles cannot pack as efficiently near a relatively large aggregate surface. During mixing, bleeding may also concentrate water near aggregate particles.

A conventional ITZ may be approximately tens of micrometres thick, although its apparent thickness depends on the measurement technique, aggregate type, paste composition and definition adopted. It should not be treated as a sharply bounded layer with uniform properties. Instead, it is a gradient in composition, porosity, orientation and mechanical response.

The ITZ affects:

• Crack initiation, because it is often weaker than bulk paste.
• Permeability, because connected pores may form around aggregate particles.
• Elastic modulus, through load transfer between paste and aggregate.
• Shrinkage restraint and microcracking.
• Chemical penetration, especially when ITZ networks connect with cracks.

The ITZ can be improved by reducing bleeding, using suitable particle packing, lowering w/b, incorporating silica fume or other reactive fine materials, and ensuring adequate consolidation. Rough, porous or absorptive aggregates may improve mechanical interlock but also alter local water distribution. In high-performance concrete, the contrast between bulk paste and ITZ may be reduced, although aggregate–paste compatibility remains important.

5. Transport Pathways and Durability

Deterioration agents enter concrete through pores and cracks. Important transport mechanisms include permeation under pressure, capillary absorption, diffusion driven by concentration gradients and migration under electrical potential. These mechanisms may operate simultaneously.

For one-dimensional steady diffusion, Fick’s first law is:

J = -D dc/dx

where J is mass flux, D is the effective diffusion coefficient, c is concentration and x is distance. The equation assumes steady state, a homogeneous effective medium and diffusion-dominated transport. Concrete rarely satisfies these conditions perfectly; therefore, measured diffusion coefficients are effective parameters, not fundamental constants.

For time-dependent diffusion with constant D, Fick’s second law is:

partial c/partial t = D partial squared c/partial x squared

This idealisation assumes one-dimensional transport, constant diffusivity, no chemical binding and a stable boundary condition. Chloride binding, saturation changes, aging and cracking require more advanced models.

Permeability depends strongly on connected capillary porosity, pore constrictions and pressure gradient. Sorptivity describes the rate of capillary uptake during early absorption and is influenced by surface condition and saturation. Chloride ingress depends on diffusion, convection, binding and exposure history. Carbon dioxide transport is affected by humidity, carbonation reactions and the reduction of alkalinity. Oxygen and moisture transport are particularly important for reinforcement corrosion.

A useful conceptual hierarchy is:

microstructural refinement → reduced connectivity → lower transport rate → delayed deterioration.

This is not universally monotonic. For example, a dense concrete that develops autogenous cracking may transport fluids faster than an apparently more porous but uncracked material.

6. Structure–Property Relationships

6.1 Strength

Compressive strength is influenced by the strength of hydrated paste, aggregate strength, ITZ quality, porosity and defects. Lower connected capillary porosity generally increases strength because it reduces stress concentration and the volume of weak regions. However, strength is not determined by porosity alone. Pore shape, crack population, hydration product morphology, aggregate restraint and testing conditions are also important.

The classical water–cement ratio concept remains useful for comparable materials and curing conditions, but it should not be interpreted as a universal law. In blended systems, maturity, reaction kinetics and pore refinement may produce different strengths at the same nominal w/b.

Tensile and flexural strength are especially sensitive to cracks and ITZ defects. Concrete fails through stable or unstable crack growth from distributed microcracks, often involving aggregate–paste boundaries. Fibre reinforcement changes this mechanism by bridging cracks and transferring tensile stress after matrix cracking.

6.2 Stiffness and deformation

The elastic modulus of concrete depends substantially on aggregate modulus and volume fraction, as well as paste stiffness and ITZ quality. A dense paste does not guarantee a high concrete modulus if the aggregate is soft. Shrinkage arises from chemical shrinkage, autogenous shrinkage, drying shrinkage and thermal contraction. These mechanisms interact with restraint to produce cracking.

Creep is related to time-dependent deformation of C–S–H and movement of adsorbed water. Microstructural models commonly distinguish a recoverable viscoelastic component from irreversible viscous or microcracking components. Higher sustained stress, higher humidity sensitivity, smaller member size and younger loading age generally increase creep, but prediction remains uncertain for blended binders and high-performance concretes.

6.3 Thermal behaviour

Hydration heat can generate temperature gradients in thick members. If internal temperature rise and subsequent cooling are restrained, thermal tensile stresses may exceed early-age tensile capacity. Low-heat binders, replacement materials, staged construction, insulation control and thermal monitoring are microstructure-informed strategies for reducing risk.

7. Models for Interpreting Concrete Microstructure

7.1 Powers–Brownyard concept

The Powers approach relates hydration degree, water content and volumes of gel, capillary space and unhydrated cement. It provides a useful framework for explaining why hydration reduces capillary porosity and why w/c ratio affects strength. Its limitations include assumptions concerning uniformity, idealised hydration products and applicability to modern blended binders.

7.2 Feldman–Sereda model

This model describes C–S–H as a layered structure in which water is held in interlayer, adsorbed and capillary states. It helps interpret moisture movement, shrinkage and creep, particularly the role of surface forces and structural rearrangement.

7.3 Jennings colloidal models

Colloidal descriptions treat C–S–H as assemblies of nanoscale particles or globules with different packing states. They explain how packing density and interparticle interactions influence stiffness, porosity and drying response. These models are powerful but should be used as conceptual representations rather than complete predictive descriptions of all cement systems.

7.4 Multiscale and computational models

Modern research combines nanoscale characterisation, pore-network models, micromechanics, finite-element analysis and reactive transport simulation. Homogenisation methods estimate effective properties from constituent phases; discrete models represent cracking and particle interactions; and digital-image methods reconstruct three-dimensional pore structures. These tools require careful calibration, resolution assessment and uncertainty analysis. A visually detailed model is not necessarily a reliable predictive model.

8. Microstructural Characterisation

Scanning electron microscopy with energy-dispersive spectroscopy provides morphology and approximate elemental composition. Backscattered electron imaging can distinguish unhydrated cement, hydration products and pores after suitable preparation. X-ray diffraction identifies crystalline phases but is less sensitive to poorly crystalline C–S–H. Thermogravimetric analysis estimates mass losses associated with dehydration, dehydroxylation and decarbonation. Mercury intrusion porosimetry estimates an intrusion-based pore-size distribution but is affected by ink-bottle pores, sample preparation and high-pressure damage. Nitrogen adsorption is useful for smaller pores, while micro-computed tomography reveals larger three-dimensional voids and cracks.

No technique measures the entire microstructure without bias. Drying may alter pores; impregnation may obscure features; polishing may smear soft phases; radiation may damage samples; and statistical representativeness depends on the volume examined. Reliable research therefore combines methods, documents preparation conditions and reports uncertainty.

9. Controlling Microstructure in Practice

Effective microstructure control begins with a performance-based specification rather than an isolated strength target. Important practices include:

• Select w/b according to required strength, permeability and exposure.
• Use well-graded aggregates and optimise particle packing.
• Choose supplementary cementitious materials according to reaction rate, curing and exposure.
• Use chemical admixtures to reduce water demand without impairing stability.
• Provide curing that maintains moisture and suitable temperature.
• Prevent plastic settlement, bleeding and inadequate consolidation.
• Control thermal gradients in mass concrete.
• Inspect for early-age cracking and repair defects before exposure becomes severe.
• Verify performance using permeability, sorptivity, electrical resistivity or diffusion-related tests where appropriate.

Curing is especially important in blended and low-permeability systems. A mixture may have excellent ultimate potential but poor early-age surface quality if evaporation exceeds the rate of water supply. Surface zones are often the controlling region for durability.

10. Illustrative Case: Cracking and Chloride Ingress in a Marine Parking Structure

Background

A reinforced-concrete parking structure located near a marine environment was designed with a low w/b concrete containing slag. The specified compressive strength was achieved at 56 days, but cracking was observed in exposed deck slabs within the first year. Chloride-related corrosion appeared earlier than predicted in several edge zones.

Problem

The design evaluation focused on 56-day strength and nominal cover. It did not adequately consider curing duration, early-age shrinkage, crack connectivity or the effect of construction joints. The slab surface was exposed to wind and low humidity, while wet curing was discontinued after a short period.

Analysis

Petrographic examination identified drying and autogenous microcracks intersecting the ITZ. Surface resistivity varied substantially, indicating non-uniform moisture and pore structure. Chloride profiles were inconsistent with a simple homogeneous diffusion model: cracked zones showed rapid penetration, while intact areas remained relatively resistant. The slag binder refined the uncracked paste, but inadequate curing reduced near-surface hydration. Cracks created preferential transport pathways that dominated the benefit of matrix refinement.

Approach and solution

The remedial programme included crack mapping, sealing of suitable cracks, local removal and replacement of severely damaged concrete, improved joint detailing and a revised curing procedure. For subsequent construction, curing was extended, evaporation was controlled, and temperature and humidity were monitored. Acceptance criteria were expanded to include surface resistivity and absorption testing, interpreted alongside strength.

Outcome

Follow-up inspections showed reduced moisture ingress and more uniform surface performance. The case demonstrates that a refined pore structure cannot compensate for connected cracks and a poorly cured cover zone. Durability design must therefore combine material selection, construction quality, detailing and exposure-specific verification.

Learning points

1. Strength alone is an incomplete durability indicator.

2. Cracks can dominate transport even when bulk paste is dense.

3. Blended binders require curing appropriate to their reaction kinetics.

4. Field quality assurance should measure properties related to the deterioration mechanism.

11. Challenges, Limitations and Contemporary Developments

Concrete microstructure is chemically and geometrically heterogeneous, and measurements are scale-dependent. Laboratory curing may not represent field conditions. Accelerated tests can change the controlling mechanism. Apparent diffusion coefficients vary with saturation, age and binding. Models may obscure uncertainty if input parameters are treated as exact.

Contemporary developments include low-carbon binders, calcined clay systems, limestone–calcined clay cements, alkali-activated materials, carbonated curing and engineered particles. These systems produce hydration or reaction products different from conventional Portland cement and require revised characterisation methods. Machine learning is increasingly used to relate mixture composition, imaging data and performance. Its responsible use requires representative datasets, physically meaningful variables, explainability and independent validation; prediction without mechanistic verification can conceal extrapolation errors.

Sustainability decisions should consider more than cement replacement percentage. Lower clinker content may reduce embodied carbon, but transport, curing energy, service life, repair frequency and local material availability also matter. Ethical professional practice requires transparent reporting of uncertainty and avoidance of durability claims unsupported by exposure-specific evidence.

Chapter Summary

Hydration converts cement and water into a heterogeneous solid whose performance depends on composition, morphology, pore structure and defects. C–S–H is the principal binding phase, while portlandite and aluminate-bearing phases influence alkalinity, sulfate response and chemical stability. Degree of hydration and water-to-binder ratio affect capillary porosity, but connectivity, tortuosity, saturation and cracking are equally important.

The ITZ is a graded region commonly associated with higher porosity and crack susceptibility. It can control transport and mechanical damage, particularly when connected to surface cracks. Diffusion and absorption equations provide useful idealised descriptions, but real concrete requires allowance for binding, aging, variable saturation and cracking. Microstructural models, from Powers-type volume concepts to nanoscale colloidal and multiscale computational models, support interpretation but have defined limitations.

Durable concrete is produced through integrated control of materials, particle packing, water demand, curing, temperature, consolidation, detailing and verification. Modern blended binders and digital characterisation offer major opportunities, but reliable engineering depends on combining measurements, mechanisms and uncertainty-aware judgement.

Key Terms

Hydration: Chemical and physical reaction of cement constituents with water.

Degree of hydration: Fraction of cement that has reacted.

C–S–H: Calcium–silicate–hydrate, the principal binding product of Portland cement hydration.

Portlandite: Crystalline calcium hydroxide formed during silicate hydration.

Ettringite: Early calcium aluminate sulfate hydrate formed in the presence of sulfate.

AFm phase: Family of calcium aluminate hydrate phases containing comparatively less sulfate or carbonate.

Capillary pore: Pore remaining from original mixing water space and not filled by hydration products.

Gel pore: Very small pore associated with the internal structure of C–S–H.

Entrained air: Deliberately introduced, finely distributed air voids.

Interfacial transition zone: Microstructurally distinct region near aggregate surfaces.

Chemical shrinkage: Reduction in absolute volume accompanying hydration reactions.

Autogenous shrinkage: Internal deformation caused mainly by self-desiccation in sealed, hydrating concrete.

Sorptivity: Tendency and rate of fluid uptake by capillary absorption.

Permeability: Ease with which a fluid passes through connected pores under pressure.

Diffusivity: Effective parameter describing concentration-gradient-driven transport.

Tortuosity: Increased path length and complexity experienced by a transporting species.

Pozzolanic reaction: Reaction of a siliceous or aluminous material with calcium hydroxide and water.

Reactive transport: Coupled movement and chemical reaction of species in porous media.

Homogenisation: Method of estimating effective material properties from heterogeneous constituents.

Self-desiccation: Internal drying caused by hydration consuming available pore water.

Review Questions

Short Answer Questions

1. What are the principal stages shown on a cement hydration heat-flow curve?

2. Why is C–S–H considered the principal binding phase in Portland cement concrete?

3. Distinguish capillary pores from gel pores.

4. What is the wall effect in the interfacial transition zone?

5. Why can cracks dominate chloride transport through an otherwise dense concrete?

Long Answer Questions

1. Explain the hydration reactions of C3S, C2S and C3A and discuss their contributions to strength and heat evolution.

2. Describe the pore structure of hydrated cement paste and analyse the influence of water-to-binder ratio and curing.

3. Explain the formation, characteristics and engineering significance of the interfacial transition zone.

4. Compare the Powers–Brownyard, Feldman–Sereda and colloidal models of cement microstructure.

5. Discuss how microstructure governs strength, stiffness, shrinkage, creep and durability.

Analytical / Application Questions

1. A low-water binder concrete develops high early strength but later exhibits autogenous cracking. Analyse the microstructural causes and propose material and curing interventions.

2. Two concretes have similar total porosity, but one has much lower chloride ingress. Explain how pore connectivity, tortuosity, saturation and cracking could produce this result.

3. Design a field quality-assurance programme for a marine concrete structure that verifies both matrix refinement and crack control. Justify the tests and their limitations.

Multiple Choice Questions with Answers

Question 1

Which hydration product contributes most directly to the primary binding strength of Portland cement paste?

A. Calcium–silicate–hydrate

B. Calcium hydroxide

C. Gypsum

D. Free crystalline silica

Correct Answer: A. Calcium–silicate–hydrate

Question 2

What generally happens to connected capillary porosity as hydration proceeds under adequate curing?

A. It generally decreases

B. It always increases

C. It remains exactly constant

D. It becomes entirely occupied by air

Correct Answer: A. It generally decreases

Question 3

The interfacial transition zone is commonly associated with which characteristic in conventional concrete?

A. Higher local porosity than bulk paste

B. Complete absence of calcium hydroxide

C. Perfectly uniform packing of cement particles

D. No influence on cracking

Correct Answer: A. Higher local porosity than bulk paste

Question 4

In Fick’s first law, the negative sign indicates that diffusion occurs:

A. From low concentration toward high concentration

B. From high concentration toward low concentration

C. Only through saturated pores

D. Only under an applied electrical field

Correct Answer: B. From high concentration toward low concentration

Question 5

Which material commonly consumes calcium hydroxide through a pozzolanic reaction?

A. Silica fume

B. Coarse aggregate

C. Entrained air

D. Mixing water alone

Correct Answer: A. Silica fume

Question 6

Which pore type is intentionally introduced to improve resistance to freezing and thawing?

A. Entrapped air voids

B. Entrained air voids

C. Gel pores

D. Chemical shrinkage pores

Correct Answer: B. Entrained air voids

Question 7

Why may a concrete with refined bulk paste still show rapid chloride ingress?

A. Cracks may form connected preferential pathways

B. C–S–H always dissolves in chloride solutions

C. Lower porosity necessarily increases diffusion

D. Aggregate always transports chlorides faster than cracks

Correct Answer: A. Cracks may form connected preferential pathways

Question 8

Which technique is particularly useful for identifying crystalline hydration phases?

A. X-ray diffraction

B. Slump testing

C. Rebound hammer testing alone

D. Water absorption by paper towel

Correct Answer: A. X-ray diffraction

Question 9

Self-desiccation is most closely associated with:

A. Internal drying caused by hydration consumption of pore water

B. External flooding of concrete

C. Formation of entrained air bubbles

D. Complete prevention of autogenous shrinkage

Correct Answer: A. Internal drying caused by hydration consumption of pore water

Question 10

What is a major limitation of applying an ideal constant-diffusivity model to chloride ingress in real concrete?

A. It ignores effects such as binding, aging and cracking

B. It cannot represent concentration gradients

C. It applies only to metals

D. It predicts no transport under any condition

Correct Answer: A. It ignores effects such as binding, aging and cracking