Prof. Grobéty B., Technical Mineralogy, University of Fribourg

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1 Prof. Grobéty B.,, University of Fribourg Introduction Cementitous materials Definition: Material, which binds together with solid bodies (aggregates) by hardening from a plastic state. Examples: organic polymers inorganic cements Inorganic cements - mixed with water plastic state - hydration of the components development of rigidity (setting) - steady increase of strength (hardening) - Examples: Portland cement, gypsum plasters, phosphate cements - when hardening occurs also under water: hydraulic cement - Example: Portland cement 1

2 Introduction Cement: definitions Portland cement: Hydraulic cementitous material based on clinker, a material composed of calcium silicates and aluminates, and a small amount of added gypsum/anhydrite. The clinker is made by burning mixtures of limestone and argilaceous rocks (slates). Mortar: Mixture of Portland cement, fine sand and water (used f.ex. for the construction of brick walls) Neat paste: Mixture of Portland cement and water alone (used for filling cracks and sealing small spaces) Concrete: Mixture of Portland cement, coarse and fine aggregates (rock pebbles, sand), water and chemical additives. The mechanical strength can be reinforced by the insertion of steel bars. Introduction Cement: chemical notations Chemical notation C = CaO S = SiO 2 A = Al 2 O 3 F = Fe 2 O 3 M = MgO K = K 2 O N = Na 2 O S = SO 3 T = TiO 2 P = P 2 O 5 H = H 2 O C = CO 2 LOI = loss of ignition ( H 2 O+CO 2 ) C-S-H = poorly crystallized calcium silicate hydrates HCP = hydrated cement paste PFA = pulverized fuel ash PC = Portland cement OPC = Ordinary Portland cement 2

3 Introduction Portland Cement I Chemical composition The composition of Portland Cements and puzzolanic additives cover a certain range. Introduction Portland cement II Main mineralogical components Name + Chem. Comp Approx. % in OPC Properties Belite C 2 S 20 Slow strength gain, responsible for long term strength Alite C 3 S 55 Rapid strength gain, responsible for early strength gain Aluminate C 3 A 12 Quick setting (contr. by gypsum), liable to sulfate attack Ferrite C 4 AF 8 Little contribution to setting or strength, responsible for gray color of OPC 3

4 Introduction Portland Cement III Main production steps ( Quarrying chalk in northern Jutland (Aalborg Cement) Introduction Portland Cement IV Main production steps (cont.) Chalk slurry tank (Aalborg cement) 4

5 Introduction Portland Cement V Main production steps (cont.) Preheater, rotary kilns and storage silos Introduction Portland Cement VI Main production steps (cont.) Cement silo Shipping by ship 5

6 Introduction World cement productions (minerals.usgs.gov/minerals/pubs/commodity/cement World cement production 2000 (thousand of tons): United States (includes Puerto Rico) 92,300 Brazil 41,500 China 576,000 China 576,000 China produces one third of Egypt 23,000 the world cement output! France 24,000 Germany 38,099 India 95,000 Indonesia 27,000 Italy 36,000 Japan 77,500 Korea, Republic of 50,000 Mexico 30,000 Russia 30,000 Spain 30,000 Taiwan 19,000 Thailand 38,000 Turkey 33,000 Other countries (rounded) 450,000 World total (rounded) 1,700,000 World total (rounded) 1,700,000 Introduction Swiss cement industry ( Cement plants in Switzerland cement plant klinker mills Total production 1987: t 1989: t 2000: t 6

7 Raw materials Raw materials Main raw materials Calcareous lime stones: Shales: - calcite-rich - low in dolomite - clay rich, usually dominated by illite, smectite and kaolinite. Ideal bulk composition ranges: 55-60wt% SiO 2, 15-25wt% Al 2 O 3, 5-10wt% Fe 2 O 3 Corrective constituents Sand, flyash: - adjust SiO 2 -content in quartz-poor shales Ironores, bauxite: - adjust Fe resp. Al content Additional reactive constituents, which have to be considered, may be introduced through impurities in the fuel. Up of 30% of ash is produced by the firing of brown coal. Raw materials Composition of ordinary Portland cements Major components SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O SO 3 LOI (H 2 O+CO 2 ) Minor components and traces (deleterious) few %: MgO, SrO 2 few tenth of a %: P 2 O 5, CaF 2, alkalis traces: heavy metals The composition of different cements, their minimum mechanical properties and their application is regulated by Norm SIA Norm /002 ( which corresponds to the European Norm ENV 197 ( 7

8 Klinker production Klinker phases I 1. Alite Ca 3 SiO 5 = C 3 S orthosilicate SiO 4 Ca 0.71nm O R- C 3 S projected along the c-axis Polymorphic transformations: 620 C 920 C 980 C 990 C 1060 C 1070 C T1 T2 T3 M1 M2 M3 R T: triclinic M: monoclinic R: rhombohedral Max. concentration of impurities: 1.0 wt% Al 2 O 3, 1.2% Fe 2 O 3, 1.5 % MgO impurities stabilize the M1 and or M3 in klinkers, rarely T2 is found Klinker production Klinker phases II 2. Belite Ca 2 SiO 4 = C 2 S orthosilicate α - C 2 S proj. down c-axis 0.55nm Polymorphic transformations: <500 C 630 C 1160 C 1425 O1(γ) M1(β) M2(α L ) O2(α H ) H1(α) O: orthorhombic M: monoclinic H: hexagonal Max. concentration of impurities: wt% Al 2 O 3 + Fe 2 O 3 impurities stabilize the β-phase 8

9 Klinker production Klinker phases III 3. Aluminates and ferrites Ca 3 Al 2 O 6 = C 3 A (cubic) impurities: up to 4wt% NaO up to 16% Fe 2 O 3 + SiO 2 imputirities stabilize an orthorhombic polymorph Ca 2 Al x Fe 1-x O 10 = C 4 AF x clinker : around 1.0 impurities: up to 10 wt% MgO +TiO 2 + SiO 2 Klinker production Klinker phases IV Polymorphs and composition of phases present in clinker C 3 S C 2 S early crystallized small crystals rich in substitutes: M3 late crystallized large crystals: M2 (single twins), rarely T1 (polysynthetic twins) 3-4% of substituting elements, mainly Mg, Al and Fe usually only in the M1(β) polymorph with parallel twin lamellae M2(α L ) has typical crossed twin lamellae. The transformation M2(β) M(γ) sho<uld be avoided, because the accompanying drastic volume increase leads to excessive dusting. 4-6% of substituing elements, mainly Al and Fe C 3 A polymorphs is coupled with substitution. Clinker aluminate phases are cubic (fine grained) or orthorhombic (lath shapedand twinned) 13% to 20% of substituting elements: Mg, Al, Fe, Si C 3 AF Main exchange vector Fe -2 SiMg 9

Klinker production Klinker phases V Etched

polymorphs Alite crystals with both single

complex twin lamellae (M2(αL ) polymorph)

paralllel twin lamellae crack formation along

10 Klinker production Klinker phases V Etched microstructures of the different klinker polymorphs Alite crystals with both single and polysynthetic twins Belite crystals with complex twin lamellae (M2(αL ) polymorph) Belite crystals with Belite crystals with paralllel twin lamellae crack formation along (M(β) polymorph) lamellae boundaries (M(β) (M(β) transf.) Klinker production Rotary kiln Without preheater/precalciner the kiln aspect ratio is about 30 10

11 Klinker production Klinker reactions below 1300 C Temp. range products Drying 100 C free water evaporates C release of adsorbed and crystal water Decomposition of calcite (calcining): C free lime (CaO) Decomposition of phyllosilicates: C dehydroxilated, amorphous material Formation of first clinker phases: > 800 C belite, aluminate (different phases), ferrite Formation of first melt phases: > 1000 C Klinker production Decomposition of carbonate phases I Decomposition reaction: CaCO 3 = CaO + CO 2 Equilibrium constant 1.0 K = [ CaO] [ CO P(CO 2] 2 ) = p [ CaCO 3 ] CO Rate of decarbonation is influenced by: material temperature (=> K) - gas temperature (heat transfer) - external partial pressure of CO 2 - size and purity of the calcite particles C T( C) Calcite decomposition temperature As function of CO 2 partial pressure 11

12 Klinker production Decomposition of carbonate phases II Reaction mecanism: formation of a lime layer around calcite reaction progress a Possible rate determining steps 1. heat and mass transport (CO 2 ) through the product layer 2. reaction at the calcite surface a ( 1! a) 1 3 = kt Activation energy: 196kJ/mol (Khraisha et al, 1992) reaction controlled? t Klinker production Belite formation 1. Formation of belite through solid state reaction 2. Transformation of the belite shells to belite crystal clusters lime quartz amorphous material belite 12

13 Klinker production Appearance of first melts 1. Alkali and sulfate melts 2. C-S-A melts: lowest eutecticum 1170 Klinker production Phase diagram P: typical bulk composition of Portland cement klinkers First melt appearance: 1455 C 13

14 Klinker production Klinker reactions between 1300 C and 1450 C 1. Melting reactions - Melting of ferrite and aluminate phases - Melting of part of the early formed belite 2. Formation of new phases Reaction of melt, free lime, unreacted silica and remaining belite to alite 3. Polymorphic transformation of belite 4. Recrystallization of alite and belite 5. Nodulization (clinkering) Klinker production Amount and composition melts II 35 Liquid phase (wt%) At 1450 C and above the liquid content depends on the silica modulus SM 14

Recrystallized and new formed alite replaces lime crystals lime belite amorphous material alite Klinker production Microtextures

15 Klinker production Formation and recrystallization of alite 1. Formation of melt around lime crystals 2. Crystallization of alite walls at the contacts between belite cluster and lime 3. Recrystallized and new formed alite replaces lime crystals lime belite amorphous material alite Klinker production Microtextures I (all pictures FL Smidth review 25) 0.1mm Belite clusters replacing previous quartz grains. 0.05mm Alite wall separating CaO and a belite cluster belite alite melt phase (aluminates,ferrites) lime 15

The raw mix contained few quartz grains and a well controlled carbonate grain size. Klinker production Klinker reactions during cooling 1.

16 Klinker production Microtextures II alite 0.3mm 0.2mm lime belite pores Alite crystallizing at the expense of lime and belite Well crystallized, homogeneous clinker. The raw mix contained few quartz grains and a well controlled carbonate grain size. Klinker production Klinker reactions during cooling 1. Crystallization of the restitic melt. Products: aluminates (C 3 A) and ferrites (C 4 AF) 2. Polymorphic transformations of alite and belite If cooling is too slow 3. Backreaction of alite to belite + lime 4. Recrystallization aluminates and ferrites 16

Klinker production Microtextures III belite rims 0.

02mm Backreaction of alite rims to belite plus lime

Etched thin section showing the transformation twins

17 Klinker production Microtextures III belite rims 0.04mm 0.02mm Backreaction of alite rims to belite plus lime in a belite poor clinker (fast cooling). Etched thin section showing the transformation twins in belite. Klinker production Microtextures IV 0.05mm 0.05mm Slowly cooled clinker with corroded alite phase and recrystallized belite grains. Fast cooled clinker with euhedral alite and rounded belite crystals. 17

18 Klinker production Energy balance in clinker production Temp range C wet 100 C ca. 450 C C ca. 900 C ca. 900 C C C ca C C C C Process Heating of the material Evaporation of free H 2 O Removal of H 2 O from clay heating of the material Dissociation of calcite Crystallisation of dehydrated clay Heating of the decarbonated material Heat of formation of clinker minerals Melting of liquid phases Cooling of clinker Cooling of CO 2 Cooling of H 2 O Total Heat exchange kj/kg clinker 710 (1800) Klinker production Energy costs of cement production Dry process cement plant 5000t/day Fuel Electricity Cost($/day) Process kcal/kg cement kwh/ton cement Quarry Crushers Prehomoginizing and transport Raw mill Raw meal silo Kiln feeder Kiln and cooler Coal mill Cement mill Packing plant Other total

19 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: 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 Hydration of cement Heat evolution during Portland cement hydration heat evolution rate W/kg preinduction period I II acceleration period III Induction (dormant) period time (hours) 19

20 Hydration of cement 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 Hydration of cement 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 20

21 Hydration of cement 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 Hydration of cement Temporal evolution of the reactants Temporal evolution of the hydration of clinker phases (Copeland + Kantro, 1964) 21

22 Hydration of cement Temporal evolution of the products Temporal evolution of the hydration hydration products (Kurtis, ) Hydration of cement 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 22

23 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 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 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. Technische Mineralogie ETHZ IMP

24 Hydration of cement Composition of the solution Temporal evolution of the Ca 2+, SiO x, ph and C/S of products Hydration of cement 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. 24

$Hydration of cement Hydration products II SEM micrographs of fractured C3S pastes (w/c = 0.$

25 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). 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 25

26 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) All images are from fiber concrete samples. Hydrogarnets (1500x) 26

27 Hydration of cement Structure of C-S-H gel I Polymerisation degree of silicate anions during the hydration cement 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. 27

28 Hydration of cement 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 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 28

29 Hydration of cement Volume relationships w/c is 0.5 for (a) α is 1.0 for (b) 29

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

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 -