definition - w/c, w/swater to cement / solid ratio by mass - pastecement-water mix allowing setting...

Definition

- w/c, w/s water to cement / solid ratio by mass

- paste cement-water mix allowing setting and hardening to occur w/c: 0.3-0.6

- setting stiffening without significant increase in strength

- hardening development of strength

- curing storage under conditions allowing hydration: under water forthe first 24h, than under 100% humidity or air

Hydration of cement

Institut de Minéralogie et PétrographieUniversité de Fribourg

Technische MineralogieETHZ IMP 2008

Heat evolution during Portland cement hydration

I

II

III

time (hours)

heat

evolu

tion r

ate

W/k

g

Induction (dormant) period

0 10 20 30

Hydration of cement


preinduction period

acceleration period


Hydration reaction of the silicates

All clinker minerals undergo hydration reactions:

AliteC3S + (y+z)H2O = CxS Hy + zCH x+z = 3 x,y,z are not integers

CxS Hy : poorly crystallized phase, structurally similar to tobermorite C/S ratio of C-S-H 1.7 - 1.8, max range 1.2 - 2.1CH: portlanditeΔH=-500J/g

Belite

Reaction stoichiometry similar to alite, but the hydration is much slower and produces less portlandite than the hydraton of alite C/S ratio of the C-S-H phase vary between 1.6 and 2.0. ΔH=-250J/g

Hydration of cement



Hydration reaction of the aluminates

Aluminate react rapidly to aluminate hydrates

2C3A + 21H = C4AH13 + C2AH8 above 30°C both hydrate convert to a hydrogarnet C3AH6

In the presence of sulfate the reaction product is ettringite

2C3A + 3CSH2 + 6H = C3A• 3CSH32

when a surplus in C3A is present (almost always the case) ettringite reacts to monosulfate:

C3A• 3CSH32 + 2C3A + 4H = 3C3A• CSH12

Hydration of cement



Hydrated aluminate and ferrite phases are classified according to their stoichometry into AFm and AFt phases:

C3(A,F)CXn yH = AFm for Al2O3 and,or Fe2O3 plus mono CXn

C3(A,F)(CXn)3 yH = AFt for Al2O3 and,or Fe2O3 plus tri CXn

C4AH13 = C3(A)CH2 11H AFm phase C3A• 3CSH32= C3A• 3CS•32H32= C3(A)(CS)3 •32H AFt phase

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AFm and AFt phases


Temporal evolution of the hydration of clinker phases (Copeland + Kantro, 1964)

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Temporal evolution of the reactants


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Temporal evolution of the products

Temporal evolution of the hydration hydration products (Kurtis, )


Heat evolution and hydration reactions I

Hydration of cement


C3A hydrationFormation of ettringite

Ettringite coating retards further aluminate hydration

Ettringite to monosulfate transformation and further aluminate hydration

Relationship between reactions and heat evolution


• Stage 1: Initial hydrolysis characterized by dissolution of ions– Coatings form on cement particles that slow dissolution

• Stage 2: Dormant period characterized by continued dissolution of ions with nucleation control– Determines initial set

• Stage 3: Acceleration is characterized by the accelerated formation of hydration products. CH forms in solution and C-S-H forms around calcium silicate particles. – Determines final set and rate of initial hardening.– Ettringite converts to monosulfate when sulfates in solution are

used up (peak is slightly after C3S peak)• Stage 4: Deceleration is characterized by continued formation of

hydration products with diffusion control – Determines rate of early strength gain

• Stage 5: Steady state is characterized by the slow formation of hydration products – Determines rate of later strength gain

Stages of Portland cement hydration

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Heat evolution as function of grain size

Heat evolution rateduring hydration of alite as function of grain sizee.g. specific surface area.

Hydration of cement



Heat evolution during the hydration of an aluminate paste as functionof gypsum concentration. In the presence of sulfate, the aluminates transform to ettringite, which coat the grains and block further hydration.

Heat evolution during the hydration of aluminates

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Composition of the solution

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Temporal evolution of the Ca2+, SiOx, pH and C/S of products


Theories for the dormant period

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Alite hydration

Alite

The initial C-S-H pro-duct form a surface layer, which slows down the transport of water to the reactant and thus the hydration rate. The acceleration is due to a breaking of the layer due to morphological changes in the C-S-H layer

Supersaturation of the liquid in Ca(OH)2 due to the poisoning of the portlandite nuclei by silica ions slows down the dissolution of alite. With time the silica concentration decreases due to formation of first CSH and the calciumhydrox-ide becomes high enough to overcome the poisoning = end of the dormant period.

Ca2+

Ca2+

Ca2+

Ca2+Ca2+

Ca2+

poisoned port-landite nuclei The formation of the

first C-S-H I product is slowed down by the su-persaturation in Ca(OH)2. Nucleation of a second C-S-H II phase becomes possible after thermo-dynamic barriers of nucleation are overcome.

C-S-H

C-S-H I

C-S-H II


SEM micrographs of fractured C3S pastes (w/c = 0.4) in pure water at (A) 7 days, (B) 13 days, (C) 1 month of hydration

Hydration products II

Hydration of cement



Hydration products III

Early Porous C-S-H gel (Eternite shingle, 2100x)Late dense C-S-H gel (Eternite shingle, 800x)

Hydration of cement



Hydration products IV

Lathshaped AFm (sulfatefree) crystals (3900x) Ettringite (sulfatefree) crystals (1500x)

Hydrogarnets (1500x)

All images are from fiber concretesamples.

Hydration of cement



Polymerisation degree of silicate anions during the hydration cement

Structure of C-S-H gel I

Hydration of cement



Structure of 1.4 nm tobermorite, a sheet like silicate composed ofoctahedral layers and silicate chains. The silica tetrahedra can be replaced by hydroxil ions. If part the bridging tetrahedra (B) are replaced only paired groups remain explaining the dimer signal in NMR studies.

Structure of C-S-H gel II

Hydration of cement



Structure of C-S-H gel III

Hydration of cement


x

c

Structural waterAdsorbed waterCapillary poreC-S-H layerC-S-H particleC-S-H gel models


• Concrete strength, durability, and volume stability is greatly influenced by voids in the hydrated cement paste

• Two types of voids are formed in hydrated cement paste– interlayer hydration space (gel pores)– capillary voids

• Concrete also commonly contains entrained air and entrapped air• Interlayer Hydration Space

– Space between layers in C-S-H with thickness between 0.5 and 2.5 nm

– Can contribute 28% of paste porosity– Little impact on strength, permeability, or shrinkage

• Capillary Voids– Depend on initial separation of cement particles, which is

controlled by the ratio of water to cement (w/c) – On the order of 10 to 50 nm, although larger for higher w/c – Larger voids effect strength and permeability, whereas smaller

voids impact shrinkage

Hydration of cement


Voids in Hydrated Cement


w/c is 0.5 for (a)

is 1.0 for (b)

Hydration of cement


Volume relationships


Hydration mechanisms in Portland cement I

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Hydration mechanisms in Portland cement II


Hydration of cement


Hydration products I

X-ray microscopy images of C3S in a solution saturated with portlandite and gypsum after 14 min (left) and 31 min (right). The fibrous phase is C-S-H that forms at the surface (scale bar 1micron).


definition - w/c, w/swater to cement / solid ratio by mass - pastecement-water mix allowing setting...

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