Hydration of cement Institut de Minéralogie et Pétrographie
advertisement
Hydration of cement 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 for the first 24h, than under 100% humidity or air Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Heat evolution during Portland cement hydration preinduction period heat evolution rate W/kg I II acceleration period III Induction (dormant) period 0 10 20 Institut de Minéralogie et Pétrographie Université de Fribourg 30 time (hours) Technische Mineralogie ETHZ IMP 2008 Hydration of cement Hydration reaction of the silicates All clinker minerals undergo hydration reactions: Alite C3S + (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 - 2.1 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 Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 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 Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement AFm and AFt phases 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 Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Temporal evolution of the reactants Temporal evolution of the hydration of clinker phases (Copeland + Kantro, 1964) Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Temporal evolution of the products Temporal evolution of the hydration hydration products (Kurtis, ) Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Heat evolution and hydration reactions I Ettringite to monosulfate transformation and further aluminate hydration C3A hydration Formation of ettringite Ettringite coating retards further aluminate hydration Relationship between reactions and heat evolution Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement 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 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 Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Heat evolution as function of grain size Heat evolution rate during hydration of alite as function of grain size e.g. specific surface area. Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement 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. Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Composition of the solution Temporal evolution of the Ca2+, SiOx, pH and C/S of products Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Theories for the dormant period Alite hydration C-S-H Ca2+ Ca2+ Ca2+ C-S-H I Ca2+ Alite poisoned portlandite nuclei Ca2+ 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 C-S-H II Ca2+ 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. Institut de Minéralogie et Pétrographie Université de Fribourg The formation of the first C-S-H I product is slowed down by the supersaturation in Ca(OH)2. Nucleation of a second CS-H II phase becomes possible after thermodynamic barriers of nucleation are overcome. Technische Mineralogie ETHZ IMP 2008 Hydration of cement Hydration products 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 Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Hydration products III Early Porous C-S-H gel (Eternite shingle, 2100x) Institut de Minéralogie et Pétrographie Université de Fribourg Late dense C-S-H gel (Eternite shingle, 800x) Technische Mineralogie ETHZ IMP 2008 Hydration of cement Hydration products IV Lathshaped AFm (sulfatefree) crystals (3900x) Ettringite (sulfatefree) crystals (1500x) All images are from fiber concrete samples. Hydrogarnets (1500x) Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Structure of C-S-H gel I Polymerisation degree of silicate anions during the hydration cement Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement 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. Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Structure of C-S-H gel III x c C-S-H gel models Institut de Minéralogie et Pétrographie Université de Fribourg Structural water Adsorbed water Capillary pore C-S-H layer C-S-H particle Technische Mineralogie ETHZ IMP 2008 Hydration of cement 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 Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Volume relationships w/c is 0.5 for (a) a is 1.0 for (b) Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Hydration mechanisms in Portland cement I Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 Hydration of cement Hydration mechanisms in Portland cement II Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008 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). Institut de Minéralogie et Pétrographie Université de Fribourg Technische Mineralogie ETHZ IMP 2008
