# Presentation: 524907

## CEMHYD3D: Overview and Current Status

- Dale P. Bentz
- Building and Fire Research Laboratory
- 6/13/00

## Model Characteristics

- Three-dimensional (100 μm on a side)
- Multi-size, multi-phase cement particles
- representative of real cement of interest
- silica fume, fly ash (future- slag, kaolin)
- gypsum, hemihydrate, and/or anhydrite sulfate sources
- simulates dissolution, diffusion, & reaction

## Inputs to CEMHYD3D

- Cement (mineral admixture) particle size distribution
- cement bulk composition and surface composition
- apparent activation energies for cement and mineral admixtures
- volume fraction of aggregates
- curing conditions
- isothermal/ adiabatic/ temperature profile
- maturity method principles
- saturated/ sealed/ drying
- self-desiccation and creation of “empty” porosity

## Outputs from CEMHYD3D

- Degree of hydration vs. time
- all phase volume fractions
- chemical shrinkage vs. time
- heat release vs. time
- adiabatic heat signature
- compressive strength (Power’s gel-space ratio)
- diffusivity (w/c, α, Vagg, MSF)
- percolation of porosity (CH) -- durability
- percolation of solids -- setting
- ITZ microstructure
- phase fractions vs. distance from aggregates

## Model Usage

- Academia (education and Ph. D. research)
- U.S. (ACBM, UC Berkeley, Georgia Tech, Tennessee Tech)
- France (Cachan)
- The Netherlands (Twente)
- Denmark (Technical University of Denmark)
- Japan (Tokyo Institute of Technology)
- China
- South Africa (Capetown)

## Model Usage

- Industry
- Germany (Dyckerhoff Zement)
- France (CSTB)
- The Netherlands
- Government
- Waterways Experiment Station

## Studies to Date Using Model

- Calibration/Prediction studies
- heat release and adiabatic heat signature
- degree of hydration
- chemical shrinkage
- compressive strength
- Effects of cement PSD on properties
- coarser cements for low w/c ratio concretes
- early age autogenous properties and cracking

## Heat of Hydration

- Determine heat of hydration by monitoring how much of each phase reacts/forms during each cycle
- Base results on heats of formation of each compound or on **measured heats of hydration of phases**
- _Compound_            _Heat of Hydration_
-     C3S                          517 J/g
-     C2S                          262 J/g
-     C3A                        1144 J/g
-    C4AF                         725 J/g
- Taylor, H.F.W., _Cement Chemistry_, 1992.
- Fukuhara et al., *Cem Concr Res*, **11**, 407-414, 1981.

## Heat of hydration

## Temperature Calculation for Adiabatic Conditions

- Update heat released and heat capacity after each cycle of hydration model and calculate T rise via:
-  ΔT = [H(I)-H(I-1)]/Cp(I)
- H(I) is heat released through cycle I
- H(I) is based on heats of reaction of clinker phases
- Cp(I) is heat capacity of concrete after cycle I
- heat capacity adjusted for imbibition of water into cement paste and conversion of free to bound water

## Predicted Adiabatic Heat Signature

## Predicted Adiabatic Heat Signature

## Prediction of Compressive Strength

- Use gel-space ratio theory of Powers and Brownyard
- X = (0.68 * α)/(0.32 * α + w/c)
- σc = A * Xn        (n=2.6 to 3.0)
- Calibrate A via measured 3-day compressive strength (assume n=2.6)
- Use hydration model to predict X vs. time and calculate 7-day and 28-day compressive strengths to compare to experiment

## Compressive Strength Prediction

## Cement PSD and Properties

- Cement finenesses have increased dramatically from the 1950’s (250-300 m2/kg) to present date (350-400 m2/kg)
- High performance concrete mixture proportions substantially different from conventional ones (lower w/c ratio, silica fume, etc.)
- Question: Have cements been optimized for HPC mixture proportions? --> CEMHYD3D

## Systems Examined

- Clinker ground to different finenesses
- <5 μm> 643 m2/kg     <10 μm> 520 m2/kg
- <15 μm> 387 m2/kg   <20 μm> 296 m2/kg
- <25 μm> 254 m2/kg   <30 μm> 212 m2/kg
- Composition: C3S- 56.3 %, C2S- 24.7 %, C3A- 0.6 %, C4AF- 13.5 %, Hemi- 4.6-5 %

## Slide 17

## Chemical Shrinkage

## Internal Relative Humidity

## Autogenous Shrinkage

## Conclusions

- In HPCs, coarser cements:
- reduce early hydration and heat release (and strength!)
- enhance “curability” by shifting depercolation of capillary porosity to lower values
- increase diffusivities at early times, but result in equivalent diffusivities at longer times (equivalent hydration)
- reduce internal RH reduction and autogenous strains and stresses

## What’s missing?

- Better modeling of influence of sulfate
- work in progress with Dyckerhoff Zement
- Modeling of influence of alkalies
- some work done for a Ph. D. in The Netherlands
- Modeling of slag, kaolin, others
- ongoing experimental studies to clarify mechanisms and reactions
- Modeling of influence of specific chemical admixtures
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