Presentation on theme: "PC Cement Hydration PCC consists of binder and aggregates. Aggregates are typically used in two factions: fines and coarse. The binder phase normally."— Presentation transcript:
1 PC Cement HydrationPCC consists of binder and aggregates. Aggregates are typically used in two factions: fines and coarse.The binder phase normally controls the strength of the concrete. Aggregates normally serve as the filler but help to reduce shrinkage (at early ages) and creep (at later ages). Aggregates also influence the modulus of elasticity.Discussion of cementitious phase will focus on mechanical, chemical, and physical properties. Concrete phase will focus on early age characteristics, chemical admixtures, and hardened concrete properties.
2 Introduction Portland Cement Concrete Continous binder phase: the cementitious matrixBinder effect on PCC behaviorAffects permeabilityAffects strengthDispersed particulate phase: the aggregatesCoarse: #4 to 1½”Fine: #100 to #4Aggregates have a major effect on PCC behaviorServe as a fillerIncrease concrete modulus of elasticity
3 } Cementitious Phase Portland Cement Water Admixtures Liquid Mineral Workability&Strength
4 Raw materials:limestone: CaCO3Quartz: SiO2Clays: Al2O3 and Fe2O3
5 Cement Manufacture Quarrying – Raw materials Crushing Grinding Mixing Calcinated (1100C)Burned (1450C)Clinker is produced (10 mm size)Inter-ground with 5% gypsum (1-100 m)- most reactive ( <50 m)
6 Hydration process Setting and hardening processes - chemical reaction between water and cement: called hydration- forms hydration products1. Mixture is fluid as long as grains are separated.2. Hydration products grow into pore space.3. Contact between grains reduces fluidity and constitutes initial setting.4. Continued hydration results in greater strength but depends on proximity of grains. This constitutes final setting.5. Hardening stage: significant strength and continuation of drying shrinkage.
7 Hydration process Setting – Solidification of the plastic cement paste Initial set – beginning of solidification – Paste become unworkable – loss in consistency - not < 45 min.Final set – Time taken to solidify completely – Not > 375min.Hardening – Strength gain with time – after final set
8 Hydration - Exothermic Reaction 2C3S + 11H C3S2H8 + 3CH H = -500 J/g2C2S + 9H C3S2H8 + CH H = -250 J/gCalcium silicates (C3S or C2S) + waterCalcium silicates hydrate (C-S-H) + calcium hydroxideAmount of CH depends on proportion of C3S and C2SCSH - amorphous in nature, is an inexact composition,and is extremely fine (Colloidal).
10 Ferrite Phase: C4AF Forms same reaction as C3A but to a lesser degree Uses small amount of gypsumC4AF + 2CH + 14H C4(A,F)H13 + (A,F)H3Ferrite + Calcium Hydroxide + Water Tetracalcium Hydrate + Ferric Aluminum Hydroxide(product #5) (product #6)like monosulfoaluminate amorphous
11 Hydration of Portland cement Sequence of overlapping chemical reactionsHydration reactions of individual clinker mineral proceed simultaneously at differing rates and influence each otherA complex dissolution and precipitation processLeading to continuous cement paste stiffening and hardening
12 Reaction rate: C3A > C3S > C4AF > C2S Chemical reactions occur at different rates.
13 Hydration of Portland cement ReactivityCrystal size – Heating rate, burning temp.Crystal defects vs. impuritiespolymorphic form – rate of coolingFinenesse.g. C3S and C2S with impurities hydrate faster than their pure forms
14 C3S, C2S contribute mostly to strength which is in the CSH.
15 Heat of hydration (Cal/g) Compound3 days90 days13 yearsC3S58104122C2S124259C3A212311324C4AF6998102
16 Stage 1: Short; heat release due to wetting; initial dissolution Stage 2: Induction; concrete is workableStage 3: Hydration of C3S; initial and final settingStage 4: CH is depleted - C3A hydrates; CH content is adjusted so C3A does not occur in Stage 2. (flash setting).Stage 5: Slow reaction - diffusion based; pore fillingHeat differential through massive placements lead to thermal stresses and cracking.
17 Schematic view of reaction products and microstructure during hydration.
18 SEM of 7-day pasteNote the cement grain, CSH, ettringite, and monosulfoaluminate
19 SEM of portland cement paste at 28 day age A - Unhydrated cement - surrounded by dense CSHM - Monosulfoaluminatelight gray - CHDark gray - CSHBlack - porosity (not all space is filled) cement grainCHCSH
20 Bonding:35% was hydrogen bonding between sheets (weak)6% was covalent bonds (strong) Si - O - SiAs water is removed, the C-S sheets collapse and the water content, density, and surface area changeBased on saturated conditions.
21 Model of CSH CSH is extremely finely divided Exact form is unknown Amorphous and colloidal (surface properties dominate). No regular atomic arrangementActs as a space filler
23 CSH structure function of: Temperature at placement (greater covalent bonds; lower surface area)w/c (less space available)CH structureWell crystallized; large, isolated cystalsEncompass unhydrated cement grainSoluble in water (causes efflorescence)Maintains high pHFills space and reduces porosityMonosulfoaluminateReacts with sulfate ionsC4AH12 + 2CH + 2(aq) C6A3H32Causes deleterious expansion
30 Blended Cements 20 to 70% of total binding material Total = Cement & supplementary cementitious materialMost mineral admixtures are industrial by productsUse is economical, ecological, or technical in natureFly ash: coal fired power plantsBlast furnace slag: steel productionlower heat, improved durabilityFine pore structure andlower permeability with same w/cImprove workability
31 Pozzolans 2S + 3CH + 7H C3S2H8 First used by Romans CSH is of lower CaO contentLow heat and slow strength gainSimilar to increase in C2SReactivity based on surface area (silica fume)Some contain alumina (can present durability problems)Crystalline compounds (quartz); acts to dilutentsUnburned carbon may affect air entrainmentCan have a wide range of composition and reactivity
32 Blast Furnace Slag Rapidly cooled slags - to prevent crystallization (CSA)glass + H C3(SiA)2H8 (self - reacting)Forms alumin substituted CSHPresence of CH accelerated reactionMixed with cement
34 Capillary pores Gel pores SEM of 7-day paste Note the cement grain, CSH, ettringite, and monosulfoaluminate
35 Pore size distribution Volume of hydrated cement: 0.68c (includes the gel pores)Volume of gel pores: 0.18cVolume of unhydrated cement: 0.32(1 - )cVolume of capillary pores: [w/c ]cVolume of capillary pores + gel pores: [w/c ]c% Capillary Pore Volume: w/c /(w/c )2.5 nm
36 w/c controls the porosity of hardened cement paste.
37 High quality control consists of low w/c and permeability. A low w/c and adequate curing are key elements in concrete technology. A particularly low w/c can be achieved in mixes with HRWR’s and very fine pozzolans such as silica fume. These mixtures achieve compressive strength levels in the range of 50 to 130 MPa.
38 High strength and low permeability concrete Low W/C ratioProper mixture proportioningUse of superplasticizersUse of pozzolansHigh degree of hydrationGood curing