Structure of the Hydrated Cement Paste

Transcription

1 Structure of the Hydrated Cement Paste Dr. Kimberly Kurtis School of Civil Engineering Georgia Institute of Technology Atlanta, Georgia Structure of the Hydrated Cement Paste What do we mean by structure? Type, amount, size, shape, and distribution of phases present macrostructure can be seen unaided (200 µm or larger) microstructure must been observed with the aid of a microscope 1

2 Structure of Concrete Macroscopically, concrete may be considered to be composed of 2 phases coarse aggregate and mortar (paste + fine aggregate) or aggregate and paste. heterogeneous distribution At the microscale, we see that these 2 phases are not homogenous themselves! Aggregate 60-75% of the solid volume of most concretes The aggregate is principally responsible for the unit weight, elastic modulus, and dimensional stability of the concrete because these properties depend on the physical characteristics (strength, and bulk density) of the aggregate. In addition, porosity, shape and texture of the aggregate are important for workability, durability, and strength. The chemical and mineralogical composition of the aggregate is usually less important, with the exception of some deleterious and some advantageous reactions. Aggregate phase is generally stronger, than the other 2 phases, with some exceptions. 2

3 Hydrated Cement Paste Solids C-S-H CH Ettringite Monosulfate hydrate Residual unhydrated cement Water Capillary water Adsorbed water Interlayer water Chemically combined water Voids Entrapped air (>1mm) Entrained air (75-500um) Capillary pores (macro meso) Interlayer space (micropores) Important 3rd Phase in Concrete! In addition to the coarse aggregate, fine aggregate and paste (together the mortar fraction ), an important 3 rd phase generally exists the transition zone (TZ) or interfacial transition zone (ITZ) the interfacial region between the coarse aggregate and the hcp um thick the weakest link 3

4 Structure of Concrete Each of the phases may be heterogeneous in its composition (both solids and voids) Relative proportions and characteristics of the phases vary with mixture composition, time, environment, etc. All of these factors make predictions of concrete behavior more challenging than predictions for other materials. Solids C-S-H CH Ettringite Monosulfate hydrate Residual unhydrated cement Microstructure Water Capillary water Adsorbed water Interlayer water Chemically combined water Voids Entrapped air (>1mm) Entrained air (75-500um Capillary pores (macro meso) Interlayer space (micropores) 4

5 Cement Hydration Reactions 2C 3 S + 11H C 3 S 2 H 8 + 3CH 2C 2 S + 9H C 3 S 2 H 8 + CH C 3 A + 26H + 3CSH 2 C 6 AS 3 H 32 2C 3 A + 4H + C 6 AS 3 H 32 3C 4 ASH 12 3C 3 A + 12H + CH C 4 AH 13 C 4 AF + 10H + 2CH C 6 AFH 12 Pozzolanic Reaction Reaction of silica in pozzolan with calcium hydroxide: Calcium hydroxide Silica in pozzolan Water Hydration Calcium-silicate hydrate xch + ys + zh C x S y H X+Z Additional cementitious C-S-H In alumino-siliceous pozzolans (e.g. fly ash, slag and metakaolin) the alumina also participates in reactions with calcium hydroxide producing various calcium-aluminate hydrates (C-A-H) and calciumalumino-silicate hydrates (C-A-S-H). 5

6 Solids: Review 6

7 Microstructure In solids, microstructural inhomogeneities can lead to serious effects on strength and other related mechanical properties because these properties are controlled by the microstructural extremes, not by the average microstructure. Thus, the presence of voids, cracks, and other defects play an important role in determining the performance of the composite material. Why do these defects exist in concrete? Why do these defects exist in concrete? Some voids result from the intrinsic nature of the cement hydration process Other voids are introduced intentionally or unintentionally during mixing and/or placing Microcracks and cracks can develop due to mismatch between the components (i.e., different CTE, E) Microcracks and cracks can develop due to loading and environment 7

8 Microstructure Solids C-S-H CH Ettringite Monosulfate hydrate Residual unhydrated cement Voids Entrapped air (>1mm) Entrained air (75-500um Capillary pores (macro meso) Interlayer space (micropores) Water Capillary water Adsorbed water Interlayer water Chemically combined water Microstructure 8

9 Development of Microstructure Anhydrous cement Water Development of Microstructure C-S-H Ettringite CH 9

10 Development of Microstructure C-S-H Ettringite CH Development of Microstructure C-S-H Ettringite CH 10

11 Development of Microstructure C-S-H Ettringite CH Development of Microstructure C-S-H CH Monosulfate 11

12 Amount Minutes Hours Days Porosity Amount CH Ettringite Minutes Hours Days 12

13 Porosity Amount CH Ettringite C-S-H Minutes Hours Days Amount Porosity C-S-H Ettringite CH C-(A,F)-H Minutes Hours Days 13

14 C-S-H Amount CH C-(A,F)-H Monosulfate Porosity Ettringite Minutes Hours Days Relative Volume (%) Degree of Hydration Capillary porosity C-S-H Calcium hydroxide AFt/AFm calcium sulfate C 4 AF C3A C 2 S C 3 S 14

15 Hydration Products Relative Volume (%) Water-filled porosity CSH 2 C 4 AF C 3 A C 2 S C 3 S Degree of Hydration Capillary porosity C-S-H Calcium hydroxide AFt/AFm calcium sulfate C 4 AF C3A C 2 S C 3 S Relative Volume (%) C-S-H CH AFt/AFm Degree of Hydration Capillary porosity C-S-H Calcium hydroxide AFt/AFm calcium sulfate C 4 AF C3A C 2 S C 3 S 15

16 Relative Volume (%) Degree of Hydration Capillary porosity C-S-H Calcium hydroxide AFt/AFm calcium sulfate C 4 AF C3A C 2 S C 3 S Relative Volume (%) Degree of Hydration Capillary porosity C-S-H Calcium hydroxide AFt/AFm calcium sulfate C 4 AF C3A C 2 S C 3 S 16

17 Relative Volume (%) Degree of Hydration Capillary porosity C-S-H Calcium hydroxide AFt/AFm calcium sulfate C 4 AF C3A C 2 S C 3 S Capillary porosity results from the excess water used for economy and workability in the vast majority of concrete mixtures. Ca +2 SiO - OH - Al +3 Ca +2 Development of Microstructure Voids: Capillary Porosity Capillary Porosity (%) % Hydration Al +3 OH - Al +3 Ca +2 SiO - OH W/CM Young et al

18 Development of Microstructure Volume of capillary porosity in concrete is also related to degree of hydration, which is affected by curing (time, Temp, RH) 100 Degree of Hydration (%) 50 0 Degree of hydration Capillary porosity Curing time (days) Capillary Porosity (%) Young et al Voids The presence of voids affects Strength Stress distribution (concentrations) permeability freeze/thaw resistance Inverse relationship between strength (f c ) and porosity (p) f c =k(1-p) 3 k=strength of voidless mortar ~ 34,000 psi 18

19 Transport/Permeability Capillary porosity, which is a function of w/c, is an important factor in determining the permeability of the paste. Permeability = porosity Solids and Porosity Powers developed a simple model to estimate the amount of capillary porosity in a cement paste with varying degrees of hydration and at different waterto-cement ratios. Based on the assumption that 1cm 3 of cement produces 2cm 3 hydration product on full hydration. 19

20 Case A: Increasing Degree of Hydration Consider a paste with w/c of What is the capillary porosity at: 7 days assuming the cement is 50% hydrated? 28 days, 75% hydrated? 365 days, 100% hydrated? 20

21 Case B: Increasing w/c Assuming 100% hydration, what is the capillary porosity for w/c=0.70, 0.60, 0.50, and 0.40? Capillary Porosity Some other, simple models describing capillary porosity depend critically on the volume fractions of water-filled φ W (t) and total capillary porosity φ T (t) and unhydrated cement γ(t), as a function of time, t. Based on Power's model for cement hydration, for an ordinary portland cement paste, these quantities are given by: where (w/c) is the water-to-cement mass ratio α is the degree of hydration (reacted fraction) of the cement at time t, ρ cem is the specific gravity of cement, ƒ exp is the volumetric expansion coefficient for the "solid" cement hydration products relative to the cement reacted (often taken to be =1.15), CS is the chemical shrinkage per gram of cement (= 0.07 ml/g for sealed conditions and =0 for saturated conditions) T.C. Powers, T.L. Brownyard, Studies of the physical properties of hardened portland cement paste. Bulletin 22, Research Laboratories of the Portland Cement Association, Chicago, K.A. Snyder, D.P. Bentz, Cem Concr Res, 34 (11) (2004)

22 Microstructure 1-5 nm Ult. strength Adsorption <50nm Shrinkage Creep Durability C-S-H image credit: Dr. Eric Lachowski, S.Y. Hong, and F Glasser via Concrete Microscopy Library at UIUC Table and monosulfate and ettringite image credits: M&M Classification of Voids in the hcp C-S-H interlayer space nm in size; too small to adversely affect strength or permeability (independent of w/c) Capillary pores - space not take up by the cement and hydration products (dependent on w/c); nm in size in well-hydrated concrete - irregular in shape - size and amount is related to w/c and degree of hydration Micropores < 50 nm, more important for drying shrinkage and creep Macropores > 50 nm, more significant for strength, permeability Entrained air - spherical voids um in size; added for freeze/thaw resistance Entrapped air - irregular in shape; can be large Pores > 2.5nm may be filled with air, water (pore solution), or a mixture 22

23 Classification of Voids in the hcp Water in the hcp Ratio of mass of water to mass of cement in a mixture is the water-to-cement ratio or w/c When SCM s are used, this is the water-to-cementitious materials ratio or w/cm w/c or w/cm may range , but is typical Water is Introduced to the concrete during mixing Necessary for reaction of cement and SCMs Permeates the concrete during service Because the water in concrete contains ions, it is usually called pore solution and has a high ph 23

24 Pore Solution ion concentration (x10-3 mol/l) Na+ K+ Ca++ OH- SO3- Some model pore solutions: High alkali (ph ~ 13.8) 0.55M KOH M NaOH (Lawrence solution) Low alkali (ph ~ 13.5) 0.24M KOH M NaOH Saturated Ca(OH) M NaOH curing time (d) Based upon Page and Vennesland, Materiaux et Constructions V16:19-25, 1981 Pore Solution 24

25 Water in hcp Capillary water - water present in voids larger than 2.5nm - In capillaries >50nm, water exists as free water because its removal does not cause volume change - In capillaries nm, removal of water results in shrinkage because new bonds can form between C-S surfaces Water in hcp Adsorbed water - water physically adsorbed to the solid surfaces in C- S-H - can be removed on drying to RH ~ 30%, resulting in shrinkage Interlayer water- water associated with the C-S-H structure - can be removed only on strong drying to RH ~ 11%, resulting in shrinkage Chemically combined water - water that is an integral part of various hydration products - lost only on decomposition during heating 25

26 Nature of Composite Materials In virtually all composite materials, defects are present in greater density at the interface between the different constituents Often, the composite properties are governed by the nature of the interfaces electron.asp Image courtesy of Ben Mohr Effects of the 3rd Phase Mehta and Monteiro,

27 Interfacial Transition Zone The ITZ is the region 10-50um wide around coarse aggregate; characterized by: Higher local porosity Greater density of preexisting microcracks due to differential shrinkage and drying Larger CH crystals that tend to be oriented and more prone to cleavage. Interfacial Transition Zone: NIST models Agg Agg Cement particles (red) around a model square aggregate at w/c=0.47, before hydration After 77% hydration. Color key: unhydrated cement, C-S-H, CH, porosity 27

28 Interfacial Transition Zone The mechanism by which the transition zone is formed is associated with the development of water films around the aggregate in fresh concrete that, in effect, create a local region of with higher water-to-cement ratio. Wall effect packing effect; aggregate surface acts as a wall, making packing of the cement particles inefficient => high porosity region (more important) One-sided growth when no aggregate is present, hydration products grow in all directions. close to an aggregate, growth only occurs on the cement side, contributing to porosity. Wall Effect Porosity fraction near aggregate surface prior to hydration for two different cements. The median cement particle diameters of the two cements are: A1-28 µm, A7-11 µm.* Effects of bleeding ignored. *Bentz, D.P., Garboczi, E.J., and Stutzman, P.E., in Interfaces in Cementitious Composites, Ed. J.C. Maso (E & FN Spon, London, 1992) pp

29 Relative influence of 1-sided growth Wall effect and 1-sided growth 1-sided growth only (effect apparent only a few um from aggregate) Interfacial Transition Zone Stress-strain behavior for both the aggregate and cement paste alone are nearly linear elastic. But because of the ITZ, concrete displays some nonlinear and inelastic behavior in compression. 29

30 Interfacial Transition Zone Interconnectivity of microcracks and pores in TZ also increases permeability, durability suffers. Interfacial Transition Zone Interfacial Transition Zone By tailoring the concrete mixture to reduce the influence of the ITZ, strength, E, and impermeability are increased. lower w/c higher cement content use of SCMs smaller MSA reactive dolomitic aggregate lightweight aggregate extended moist curing 30

31 Interfacial Transition Zone The microstructural features and mechanical effects of the transition zone are the subject of some debate. Some researchers report the presence of a duplex film consisting of thin layer of CH adhering to the aggregate surface surrounded by thin layer of rod-like C-S-H [Hewlett, 1998] Others disagree finding C-S-H to be the solid phase most often in contact with the aggregate surface [Scrivener and Gartner, 1988] In addition, some researchers have come to believe that a weaker interfacial region does not always exist between the hydrated cement paste and the aggregate [Mindness and Diamond, 1992]. 31