Some Observations relating Kinetics, Chemistry, and Product - PowerPoint PPT Presentation

Some Observations relating Kinetics, Chemistry, and Product Structure of Hydrating Cement Paste – Reaction Mechanisms Hamlin M. Jennings MSE and CEE Northwestern University July 09 Kinetics Summit

Mechanism of reaction • Must explain kinetics – Divided into several periods • Must explain thermodynamics – Equilibrium • Must explain location of reaction • Must explain morphology

Required Steps in Reaction • Dissolution • Diffusion • Precipitation • Growth • Location, morphology, and rate of growth of product(s) must be considered

Rate of hydration (kinetics) Traditional view Kinetic description – 5 periods Deceleratory Acceleratory Period Period

Rate of hydration (kinetics) From a mechanistic view there are 3 periods: Nucleation And Rate of reaction Growth Diffusion Induction (dormant) 2 10 24 Time (hours)

There is an induction period under some conditions 40 (J/(h*g initial cement)) Rate of heat evolution 30 1% sugar 0% sugar 20 10 0 0 2 4 6 8 Time (days) Garci Juenger and Jennings CCR 2002

Hydration of C 3 S two products form C 3 S + (3 � x + y)H � C x � S � H y + (3 � x)CH X = 1.7 Y = 4 C 1.7 SH 4 + 1.3CH C 3 S + 5.3H 124 42.9 Phase volume: 72.5 95.4 166.9 Total: 167.9 - Volume of solids increases - Total volume decreases (Chemical shrinkage)

Why is there an induction period? Hypothesis #1 • Protective layer forms and is later disrupted – For • Physical evidence • Equilibrium and if so with what? – Against • Other Physical evidence • No obvious reason for disruption

Why is there an induction period? Induction period X C 3 S The concentration on 1000 C-S-H M is stable for weeks + Aqueous Phase M M µ 100 2 Equilibrium in C 3 S Both under-saturation [SiO S and over-saturation 10 return to phase line Implies equilibrium Aqueous Phase 1 with a protective layer After Jennings et al ICCC Sweden 1997 H.M. Jennings, “queous Solubility Relationships for Two Types 0.1 of Calcium Silicate Hydrate,” Journal of the American Ceramic 0 5 10 15 20 25 30 Society , 69 [8] 614 ‑ 618 (1986) . [CaO] (mM)

But is layer protective? SEM dry (two products form) 4 hrs 2 hrs 0.2 µ m 0.2 µ m

Layer • Layer prevents high concentrations in aqueous phase – Equilibrium is established quickly • Inconsistent physical evidence

Why is there an induction period? Hypothesis #2 • Delayed nucleation (Le Chatelier) – For • Ca ++ concentration • Kinetics – nucleation and growth – Against • What is the seed (CH or C-S-H)? nothing works

Why is there an induction period? Supersaturation in calcium at early times End of the Induction Period CH saturation [Ca] Time Retarders poison the precipitation

Phase diagram: equilibrium but some supersaturation Chen, J.J., J.J. Thomas, H.F.W. Taylor, and H.M. Jennings, Curves C, C', C" and A represent Cem. Concr. Res., 2004. 34 (9): p. 1499-1519. a spectrum of C-S-H structures Silicate polymerization is key to variations 1000 Jennite-like - long chain length A -more Ca-OH 100 C C' C'' SiO2 A 10 Tobermorite-like Ca(OH) 2 - short chain length - no Ca-OH 1 0 5 10 15 20 25 5 A 5 A C CaO (mM)

C-S-H nor CH accelerate much Gartner and Gaidis Materials Science of Concrete I (1989)

Acceleration period • Nucleation and growth has dominated modeling – Avrami: transformation throughout volume – Boundary: transformation starts at boundary Late period • Diffusion control -- very little argument

Location and Morphology • One valiant attempt in the 1979’s – Reverse silicate garden -- membrane forms and breaks from osmotic pressure resulting in the formation of needles • Double et al. • Birchall et al. • Otherwise not much, with confusion over morphologies such as Hadley grains

What is new? • C-S-H is colloid – Detailed model of density and pore structure – Packing and morphology change with time which explains many morphological variations • Kinetics described exactly by boundary N+G • Seed -- C-S-H can work well -- nucleates in volume

What is new: C-S-H is gel Small particles after Powers Wet TEM taken at Imperial College London (1980) 1 u m

Micrographs of shrinkage Huge deformation on drying Wet Dry 3 day .5 w/c

Surface area, density -- colloid model Fig 4

HYPOTHESIS: So what is the mechanism controlling early rate? • Layer, thermodynamically separating particle from aqueous exists • Normal hydration starts when nuclei form in the layer -- N+G controls rate • Autocatalytic growth of product into pore space -- both CH and C-S-H

Must explain kinetics Picture of self stimulation = kinetics (middle)

Must explain retarders: Sugar • Thomas and Birchall showed that sugar poisons C-S-H • Under normal conditions nucleation occurs in layer where some degree of supersaturation exists

Delayed addition of sugar greatly reduced effectiveness = nuclei formed within layer -- H.M. Jennings, H. Taleb, G. Frohnsdorff, and J.R. Clifton), Proceedings of the 8th International Congress on the Chemistry of Cement, Rio de Janeiro, Brazil , III 239 ‑ 243, (1986)

Must explain accelerators Seed from soluble salts - dispersed with active surface (Jeffrey J. Thomas, Hamlin Jennings, and Jeffrey J. Chen), Journal of Physical Chemistry C , 113 , 4327-4334 (2009).

Must explain location Schematic of N + G (Jeffrey J. Thomas, Hamlin Jennings, and Jeffrey J. Chen), Journal of Physical Chemistry C , 113 , 4327-4334 (2009).

First principles Compressive strength (kN/m 2 ) Surface area m easured by control of small angle neutron scattering 16 day old paste of ordinary (m 2 /cm 3 ), one year old paste of Portland cement paste , microstructure white Portland cement paste , w/c=0.5 (from [ 1]) w/c=0.5 No additives 123 35 Additive 143.5 40 [1] Millea, J. The Effects of Calcium Silicate Hydrate Seed on the Compressive Strength of Portland Cement Past e , Senior Thesis, Northwestern University, Evanston, 2006. Figure 3: SEM micrographs of hydrated paste made without C -S-H seed (left) and with 2% C -S-H seed by mass of C 3 S (right), after [ Error! Bookmark not defined. ]. Both pastes are 28 d old and were made at w/c = 0.5. Black is capillary porosity, grey is hydration product, and white is unreacted C 3 S. Note the much lower amount of capillary porosity in th e seeded paste at right. Thomas, Jennings, Chen, Physical Chemistry C, 2009

Must explain morphology Slow and fast drying = very open packing at early time 3 Day Old - rapid dry Fonsica and Jennings submitted 3 Day Old - 18 day dry

Seed activates Slag when soluabilized

Late Reaction • Possibly not diffusion control • Rate controlling step is difficulty in finding active sites or, equitantly the nucleation process just continues on slowly

Consumption of active sites on particles Not diffusion: D2O “Effects of Deuterium Oxide and Mixing on the Early Hydration Kinetics of Tricalcium Silicate, ” (J.J. Thomas and H.M. Jennings), Chemistry of Materials , 11 , 1907-1914 (1999).

Summary: Reaction kinetics / Mechnaism and control of microstructure • Nuclei must form from some degree of supersaturation -- normally within layer • Nuclei once formed stimulate new product when surface is available -- supersaturation not required • Formation of nuclei can be poisoned but active surface can not

Reaction Steps Particle dissolves If no seed If seed with active surface Supersaturation – likely in layer Stimulates new CH and C-S-H Nuclei form on product growth surface of cement in layer Growth continues until active sites exhaust

Overview • Dispersed seed accelerates - diffusion into pores does not seem to be rate limiting • Sugar retards -- poison formation of nuclei – Sugar does not prevent autocatalytic growth – Seed, if formed and dispersed, trumps sugar • Seed accelerates activated slag • But seed must have active surface -- prehydration does not work well because much surface is not available

C-S-H dry 2.85 g/cm 3 Globule Interlayer A B water Monolayer of IGP water C D Fig 1

Model of particles and pore CM: Colloid Deformation mapping Dry to 50% rh Dry to 5% rh Total shrinkage is sum of shrinking and restraining phases C.M. Neubauer and H.M. Jennings, J. Mater. Sci . 35 , 5741 (2000)

ESEM of Wet Samples 0.5 Hours 8 Hours

SANS, LOI, and N 2 LD early Surface area development and heat evolution * OPC Paste, 20ºC 300 160 Surface Area (m 2 /cc) Mikhail and Abo-El-Enein (1972) Heat Evolved (Joules) 120 200 80 Heat Evolved 100 Aging SANS Surface Area 40 125 days 0 0 0 10 20 30 40 50 60 70 Hydration Time (hours) * J.J. Thomas, H.M. Jennings and A.J. Allen, Cem. Concr. Res. 28 , pp. 897-905 (1998).

Two densities 1 u m

N+G in two areas • On surface of particles – See Jeff Thomas • In volume between particles