Numerical Model for Quantifying Degree of Hydration in Concrete Mixes with Reduced CO2 Footprint

The widespread application of innovative cementitious combinations in concrete raises the
need for more comprehensive investigation of the resulting concrete properties. Early age
behaviour is a major factor to be addressed, and tools are required for quantifying the
hydration state of concrete members, particularly at early-ages. Numerical models can
potentially be used in mass concrete construction to predict and prevent possible thermal
crack formation. They also provide an indirect means for characterizing development of the
hydration reaction in concrete. The latter can then be utilised in modelling and predicting
secondary concrete properties, such as diffusion coefficient. This is gaining increasing
importance as we harness the ability to develop innovative combinations.
The cement industry is estimated to be responsible for about 7% of the carbon dioxide
generated globally. As such, reducing the amount of CO2 emitted during cement production
is a key issue if the construction industry is to fully participate in sustainable development.
Under the terms of the Kyoto Protocol Emissions Trading Scheme it is also potentially
profitable for cement companies to reduce their CO2 emissions. By using blended cement
instead of ordinary Portland cement, it is possible to lower the share of clinker in cement,
resulting in reduced CO2 and energy emissions. In Ireland, CEM II now accounts for over
80% of the Irish cement production portfolio.
GGBS is a by-product of steel industry and a common replacement for cement. When
compared to Portland cement it has a reduced CO2 footprint and concretes containing GGBS
are less prone to deterioration due to aggressive chemical attacks. Its use has the potential
to produce more durable concrete with increased service life, lower maintenance costs and a
lower carbon footprint, increasing the sustainability of concrete construction.
The aim of the current study is to use numerical models to quantify the development of heat
of hydration when mixtures of CEM II and GGBS are utilised. Experiments were conducted
where the temperature profiles in 4 different mixes of concrete (CEM II with 0%, 30%, 50%
and 70% GGBS) are recorded. This was achieved by casting 6 identical concrete samples
from each mix, with thermocouples embedded to record the internal temperature of the mix
at regular time steps. Temperature changes of the mix are then used to quantify the heat
evolved, based on the principles of heat transfer. To account for the combined effect of time
and temperature on hydration development, activation energy of the mix is used, along with
the equivalent age maturity method. Total heat of hydration is determined based on the
composition and amount of cementitious materials.
It has long been accepted that the liberated heat of hydration, divided by the total available
heat of hydration is a good measure of the degree of hydration. The experimental data
describing hydration development with equivalent age are then used to calibrate the
exponential formulation presenting the S-shaped hydration curve. Values of β, τ, and αu (the
hydration parameters) are obtained for each mix, from the results of multivariate non-linear
regression analysis. Comments on the use of this method in quantifying concrete hydration
are then made.