- paste cement-water mix allowing setting and hardening to occur w/c: setting stiffening without significant increase in strength

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1 Definition - w/c, w/s water to cement / solid ratio by mass - paste cement-water mix allowing setting and hardening to occur w/c: setting stiffening without significant increase in strength - hardening development of strength - curing storage under conditions allowing hydration: under water for the first 24h, than under 100% humidity or air

2 Heat evolution during Portland cement hydration heat evolution rate W/kg preinduction period I II acceleration period III Induction (dormant) period time (hours)

3 Hydration reaction of the silicates All clinker minerals undergo hydration reactions: Alite C 3 S + (y+z)h 2 O = C x S H y + zch x+z = 3 x,y,z are not integers C x S H y : poorly crystallized phase, structurally similar to tobermorite C/S ratio of C-S-H , max range CH: portlandite ΔH=-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

4 Hydration reaction of the aluminates Aluminate react rapidly to aluminate hydrates 2C 3 A + 21H = C 4 AH 13 + C 2 AH 8 above 30 C both hydrate convert to a hydrogarnet C 3 AH 6 In the presence of sulfate the reaction product is ettringite 2C 3 A + 3CSH 2 + 6H = C 3 A 3CSH 32 when a surplus in C 3 A is present (almost always the case) ettringite reacts to monosulfate: C 3 A 3CSH C 3 A + 4H = 3C 3 A CSH 12

5 AFm and AFt phases Hydrated aluminate and ferrite phases are classified according to their stoichometry into AFm and AFt phases: C 3 (A,F)CX n yh = AFm for Al 2 O 3 and,or Fe 2 O 3 plus mono CX n C 3 (A,F)(CX n ) 3 yh = AFt for Al 2 O 3 and,or Fe 2 O 3 plus tri CX n C 4 AH 13 = C 3 (A)CH 2 11H AFm phase C 3 A 3CSH 32 = C 3 A 3CS 32H 32 = C 3 (A)(CS) 3 32H AFt phase

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

7 Temporal evolution of the products Temporal evolution of the hydration hydration products (Kurtis, )

8 Heat evolution and hydration reactions I C 3 A hydration! Formation of ettringite! Ettringite to monosulfate transformation and further aluminate hydration! Ettringite coating retards further aluminate hydration! Relationship between reactions and heat evolution

9 Stages of Portland cement hydration 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 C 3 S 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

10 Heat evolution as function of grain size Heat evolution rate during hydration of alite as function of grain size e.g. specific surface area.

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

12 Composition of the solution Temporal evolution of the Ca 2+, SiO x, ph and C/S of products

13 Theories for the dormant period Alite hydration Alite C-S-H The initial C-S-H product 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 Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ Ca 2+ poisoned portlandite nuclei 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. C-S-H II C-S-H I The formation of the first C-S-H I product is slowed down by the supersaturation in Ca(OH) 2. Nucleation of a second C- S-H II phase becomes possible after thermodynamic barriers of nucleation are overcome.

14 Hydration products I SEM micrographs of fractured C 3 S pastes (w/c = 0.4) in pure water at (A) 7 days, (B) 13 days, (C) 1 month of hydration

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

16 Hydration products III Lathshaped AFm (sulfatefree) crystals (3900x) Ettringite (sulfatefree) crystals (1500x) All images are from fiber concrete samples. Hydrogarnets (1500x)

17 Structure of C-S-H gel I Polymerisation degree of silicate anions during the hydration cement

18 Structure of C-S-H gel II Structure of 1.4 nm tobermorite, a sheet like silicate composed of octahedral 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.

19 Structure of C-S-H gel III C-S-H gel models x! c! Structural water Adsorbed water Capillary pore C-S-H layer C-S-H particle

20 Voids in Hydrated Cement 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

21 Volume relationships w/c is 0.5 for (a) α is 1.0 for (b)