- 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) 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!h=-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.!H=-250J/g

3 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 (AFt phase): 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(afm phase): C 3 A 3CSH C 3 A + 4H = 3C 3 A CSH 12 Temporal evolution of the products Temporal evolution of the hydration hydration products (Kurtis,1995 )

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

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

6 Theories for the dormant period Alite hydration Ca 2+ Ca 2+ Alite! C-S-H Ca 2+ Ca 2+ Ca 2+ Ca 2+ poisoned portlandite nuclei The initial C-S-H products form a surface layer, which slows down the transport of water to alite (or belite) and thus the hydration rate. The acceleration is due to morphological changes in the C-S-H layer, which make it more porous 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 and the calciumhydroxide is high enough to overcome the poisoning = end of the dormant period. C-S-H X-ray microscopy images of C 3 S 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).

7 C-S-H III Early Porous C-S-H gel (Eternite shingle, 2100x) Late dense C-S-H gel (Eternite shingle, 800x) AFm and AFt phases Lathshaped AFm (sulfatefree) crystals (3900x) Ettringite (sulfatefree) crystals (1500x) All images are from fiber concrete samples. Hydrogarnets (1500x)

8 Polymerisation of silicate anions in hydrated cement Polymerisation degree of silicate anions during the hydration cement Tobermorite, a proxy for the CSH-structure 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.

9 Structure of C-S-H gel C-S-H particles seen in HRTEM. The particles show lattice fringes il.e they are fully crystalline and contain no water interlayers. C-S-H structure derived from Small Angle Neutron and X-ray Scattering =Diffraction (SANS/SAXS). The density of the calcium silicate sheets is higher than for tobermorite 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

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