Good morning everyone, today we are going
to discuss a next-generation of cement that
is Calcium Sulfoaluminate cement-based Binder.
We will look into its properties and applications.
As we all know that Concrete is the most used
man-made material in the world.
So it is obvious that we need to look into
the carbon footprint of Portland cement, which
is the binding phase of Concrete.
And we know that five to seven percent of
global CO2 emission is attributed to Portland
cement manufacturing.
Primarily it is due to the calcination of
limestone which is used to manufacture Portland
cement.
So, how can we reduce carbon dioxide footprint
of Portland cement?
And we know that most of it is arising from
the calcination of limestone.
There are conventional ways of reducing CO2
emission like increasing energy efficiency
by optimising processes and we can also increase
the cementitious additions into cement.
We have been doing that for a while, like
adding slag, fly ash, limestone.
Also we can think about substituting fuels,
we can think about more environmentally friendly
fuels.
But we need to think of non conventional ways
if we really want to significantly reduce
the CO2 emissions during the Portland cement
manufacturing.
So here is the graph which compares the CO2
emission of individual phases present in Portland
cement and compares it with, so let us look
into the CO2 emission of various phases does
not in Portland cement.
C3S has the highest CO2 emission followed
by C2S, C3A and C4AF.
Now if we compare it with calcium aluminate,right,
which is the main phase of calcium aluminate
cement, the carbon dioxide footprint is quite
low.
And C4A3S, which is calcium Sulfoaluminate
phase, main phase of CSA based cements is
the least among all these phases.
We know that very high kiln temperature is
used to manufacture Portland cement, roughly
around thousand and four fifty degrees Celsius
temperature is used.
But to make calcium Sulfoaluminate cement,
we need to go only to thousand and two fifty
degrees Celsius.
So we can cut down the kiln temperature by
two hundred degrees Celsius.
In addition to that we need less grinding
energy because the calcium Sulfoaluminate
cement clinker is more porous.
So all these advantages lead to reduction
in CO2 emissions and energy requirement.
So, what is calcium Sulfoaluminate Belite
cement?
It comprises of Ye'elimite, also called C4A3S,
Calcium Sulfoaluminate, Belite as you know
also present in Portland cement.
But it has a lot of Calcium Sulphate and Calcium
Aluminate Ferrite, C4AF.
So, this phase C4A3S is the main phase of
CSA cement, it was patented by Alexander Klein
in nineteen sixties at the University of California
Berkeley, but the goal was to achieve shrinkage
compensation in concrete.
At that time the focus was shrinkage compensation
by inducing expansion in concrete.
We will look into that in the later slides.
Also important thing to note is you don’t
need different kinds of raw materials.
It is the same what is used to make Portland
cement.
So limestone, clay, bauxite and gypsum is
used to make this Calcium Sulfoaluminate cement.
But the kiln temperature has to be around
thousand and two fifty degrees Celsius.
So if you look into cement, the CSA clinker
has roughly seventy five to ninety percent
in clinker, rest is calcium sulphate.
It is important to look into what exists.
So these are the existing CSA based products.
In USA, I think there are three companies
now which make this CSA cement Buzzi CSA cement,
Komponent by CTS company and the Royal White
cement company makes CSA cement.
Obviously the application is different, sometime
they sell it as a rapid set cement, sometimes
they sell it as zero shrinkage cement, that
depends.
In Europe see that this Primekss, a company
based in Latvia, it does not sell cement per
se but itself concrete products.
They sell it zero shrinkage concrete products.
In China this company has been manufacturing
for decades actually.
So, Oreworld Trade Co.
So these are the existing products on the
market, okay, it is good to know that there
is something which exists which is based on
CSA cement.
Now we will look into hydration of calcium
Sulfoaluminate based cement.
So we know that Ye'elimite, so I will use
Ye'elimite and calcium Sulfoaluminate at the
interchangeably, so Ye'elimite which is C4A3S
can hydrate just in presence of water and
gives monosulphate.
You have mono sulphate and aluminum hydroxide,
also called gypsite.
Now, when there is the presence of gypsum,
you have a formation of Ettringite, okay.
And again you will have aluminum hydroxide.
Now, when there is enough lime in the system,
which may be the case when you have C2S, enough
of C2S in CSA cement or it is used with Portland
cement, which we will see later in a few slides.
You have only Ettringite forming, so no aluminum
hydroxide, okay.
Now as you all know that C2S can react with
water and form CSH, right, calcium silicate
hydrate.
And the C2S can also react with this aluminum
hydroxide which is coming from these two reactions
and form stratlingite.
So these are the possible reactions which
may take place depending on obviously composition
of CSA cement.
Although C4AF is in minor amount but it can
also hydrate in presence of gypsum and form
Ettringite.
So these reactions are important to always
look into.
And we know that whenever we have formation
of Ettringite, the reaction can be expensive.
Okay, we will come down to this point later.
How can be leverage this expansion in making
concrete durable.
So, here is the study done by Zhang and Glasser
in early two thousands, shows like, if you
look into the hydration products, Y axis we
have hydration products and what happens when
you increase the amount of gypsum, right.
We know that when there is no gypsum, you
will have formation of monosulphate.
But as you increase the amount of gypsum,
you form a AFT phase or Ettringite, that is
what it is showing basically.
So with the increase in gypsum you see AFT
phase is increasing.
But beyond a point, like here in this case
we see this thirty percent limit, you will
have excess gypsum, okay.
Also we see formation of aluminum hydroxide.
So this study was done on this particular
type of cement, here is the phase composition,
you have forty eight percent of Ye'elimite,
twenty seven percent of C2S, very small amount
of Anhydrite, right.
Also I would like to point out that it depends,
what is a form of your calcium sulphate.
It could be in form of anhydrite, could be
in form of gypsum and dihydrite, could be
in form of hemi-hydrite.
So the dissolution kinetics of these individual
phrases will also influence the hydration
of Ye'elimite.
Again, if you look into now phase content,
right, so you start with Ye'elimite , particularly
Ye'elimite, suppose this clinker had anhydrite,
some Belite.
So, with that, as the reaction proceeds, you
see reaction of Ye'elimite, right.
And what is important to note is just in like
four days, hundred hours,right, roughly most
of the Ye'elimite is reacted.
So it tells us that the reaction is fast and
we can expect reaction to complete in seven
to ten days completely.
And as Ye'elimite is decreasing in amount,
we see increase in the amount of Ettringite.
Again that will depend upon the amount of
anhydrite or gypsum.
And depending on the C2S content, we can also
see a presence of stratlingite, as we have
seen earlier in one of the slides, C2S can
react with aluminum hydroxide and form stratlingite.
So here basically the data points, discrete
data points are the measurements and the solid
lines, solid and dashed lines the simulated
plot using thermodynamic modelling.
So, now we know that Gypsum plays a big role,
depending on the amount of gypsum we can either
form monosulphate or ettringite or both of
those, right.
So Chinese researchers have developed this
formula.
Basically what it says is like based on this
M, M is the molar ratio of gypsum to Ye'elimite,
we can develop different types of cements.
Rapid hardening cement, high-strength cement,
spans of cement, self stressing cement, right.
So basically if M is from one point five to
two point five, the cement will be expansive.
That means there will be increase in volume,
okay.
And self stressing system, you have significant
expansion that leads to significant compression,
we will come back to it.
But the important point to note is this M
plays a big role in governing the volume change,
okay.
So, this conventional way of looking into
the shrinkage cracking of the Portland cement
concrete, right.
What happens, you have a fresh concrete, dries
and due to restrain there is a tensile stresses
developed.
And that the stress exceeds strength, there
is cracking.
Now in CSA concrete, if you want to leverage
the expansion, right, suppose you are trying
to formulate mix which expands, what happens,
this expansion will lead to some compression,
right, and that compression can then counteract
the themselves stresses developed due to shrinkage.
So what I mean, like in self stressing system,
in earlier slides, you have the residual compression,
the level of compression will be higher than
just expansive system.
So, now if we look into this typical shrinkage
versus time plot, in Portland cement we see
some level of shrinkage, right.
It could be five hundred to six hundred microns,
micro strain depending on the drying conditions.
But in the expansive systems based on CSA,
because of this expansion at early-age, basically
we are reducing the net shrinkage, right.
So the net, there is a reduction in net shrinkage
when you CSA cement as an expansive agent.
So the point is, we know that there are advantages
in terms of reducing CO2 footprint, right,
less limestone is needed to make this cement,
kiln temperature is two hundred degrees Celsius
lower than what you use for Portland cement,
the grinding energy is low.
And also by leveraging this early age expansion,
we can make this cement shrinkage resistant,
right.
So it can be used to address both issues sustainability
and durability.
Now we will go over early age and hydration
properties, hardened properties of CSA based
binders.
We will look into fresh properties, how does
it compare with conventional concrete based
on Portland cement.
We will look into mechanical properties, also
we will look into early-age volume change.
We will also look into how do we model this
early-age volume change, okay.
Eventually we will see one application.
So here is like, if you make a concrete using
CSA cement because of the formation of Ettringite
crystals, there is a loss in slump.
And if you compare with OPC, right, when you
have a CSA in the system, there is a significant
loss in slump.
And obviously in this study no admixture was
used initially.
But we see that when you use a retarder, right
the CSA with the retarder, you can reduce
this slump loss, right.
And this is the conventional retarder which
people use for Portland cement.
And also I would like to point out, this study
was done in which like eighty, only fifteen
percent of OPC was replaced.
So it was OPC CSA blend.
And the CSA cement which was used in the study
had this phase composition twenty percent
Ye'elimite, thirty five percent C2S, Belite,
and calcium sulphate in all forms was around
forty percent.
So we see that there is a loss in workability
but can be reduced.
Now if you want to examine, we can use isothermal
calorimetry to look into this closely.
So here is the rate of heat evolution in three
different binders.
zero percent CSA corresponds to plain OPC,
right, now what happens when you add CSA,
fifteen percent CSA, thirty percent CSA?
Now if you zoom out this region, what happens
in early few hours, we see that in OPC we
see a dormant period, right, which is very
useful when you have to transfer the concrete
by some time.
But in the cement, when you have CSA, you
have a shorter dormant period, right.
Again, in the study, no chemical admixture
was used.
And as you increase the amount of CSA, we
can see the further reducing the dormant period.
This study was published in two thousand and
seven addressed this, addressed the issue
of this shorter dormant period by adding citric
acid, right.
Citric acid basically what you can see that
is a good retarder for CSA cement.
As you increase the amount of citric acid
in this system CSA, you are delaying the peak,
right.
Again, so this isothermal calorimetry is a
good tool to look into this early-age hydration
kinetics, right.
Now we know that, okay, in the system with
three percent retarder, we have delayed the
hydration by four to six hours, okay.
So this is only one example, there are other
examples also.
So there are ways you can retard the CSA,
the hydration of CSA cement.
Now, since Gypsum is integral part of this
the cement what happens sometimes, it is important
to calculate the optimal Gypsum.
We all know that optimal Gypsum for Portland
cement is roughly around three to five percent,
right.
So, again isothermal calorimetry can be used
to determine this optimal Gypsum.
So, you see 1st this peak, it is because of
Gypsum depletion.
When the Gypsum is depleted, this C4A3S or
Ye'elimite hydrates very fast and releases
a lot of heat.
Same phenomena happens in Portland cement
when you are run out of Gypsum, your C3A can
hydrate, right, very fast and their reaction
way, so you want to control that.
So you see that as we increase the amount
of Gypsum, right, we are basically delaying
this peak and basically the optimal Gypsum
is a content beyond which the isothermal calometry
curve does not change, right.
It is more clear in the later one, if you
see the right plot, so the peak, initial gypsum
depletion peak is here and when we add five
percent of Gypsum, you see it here, then beyond
that we do not see any change.
So it tells you that, okay, so the optimal
Gypsum for this cement is around eight percent,
okay.
Similar thing also can happen when you have
mineral admixtures, right.
So, here is the example, what happens, when
you have Portland cement, you have CSA cement
and class C fly ash and class F fly ash.
So depending on the chemistry you will see
very different behaviour.
So you notice that this shoulder occurring
only in the mix where you have class C fly
ash, right.
So as we saw earlier, usually any shoulder
after the main peak can be attributed to gypsum
depletion, okay.
Now if you want to verify, is it because of
only gypsum depletion, what you can do, you
can simply add additional Gypsum, that’s
what we saw here.
So, with five percent additional Gypsum, you
delay this peak and eventually add fifteen
percent, we do not even see that peak, okay.
So it was confirmed that this peak was due
to the depletion of Gypsum.
Now what caused that depletion?
So class C fly ash basically has C3A tricalcium
aluminate.
So that also competes for your Gypsum.
So obviously if you compare with the class
F fly ash which does not have C3A, you will
see early depletion of Gypsum in the mix with
class C fly ash.
So if we look into this ternary diagram, where
we put all sulphates in one corner, like in
the form of Ye'elimite and calcium sulphates,
you can combine C4A3S and calcium sulphate,
here is C2S, silicate phase, right and ferrite
phase, C4AF.
So, it gives a very good idea like where do
you want to be, right.
You do not want to be in this region because
you do not have enough C2S, you do not have
enough binding phase, right.
So if you are somewhere here, that means the
product you have has low durability.
Also if you are somewhere here, so very far
away from C4A3S corner, that means you have
a very small amount of C4A3S, the hardening
will be slow, right.
So this just gives you an idea about the sweet
spot you want to be in, right.
If you want normal hardening, you want to
be somewhere here, if you want rapid hardening,
you want to be somewhere here, right.
Now coming back to the strength, right, the
strength of CSA concrete, at least in the
study was comparable to Portland cement concrete,
and many researchers, many studies have reported
that.
So, here is the Portland cement concrete and
twenty eight days compressive strength, right.
When you just replace OPC with CSA, in this
case the replacement was fifteen percent,
small increase but I would say that we can
get comparable strength to Portland cement
concrete.
Further, you can also look into the effect
of these mineral admixtures, right.
What happens when you add class F Fly ash,
class C Fly ash and Silica fume.
Obviously these are the mineral admixtures,
take time to hydrate, right the pozzolanic
reaction is slow.
So this is twenty eight days strength, although
it is little bit lower, we expected to be
higher if we test it at later ages.
Again, now coming back to other mechanical
properties.
If you look into the tensile strength, right,
zero percent CSA again corresponds to plain
OPC, seven percent OPC means ninety three
percent OPC, seven percent CSA.
So with that age you see increase in strength,
right.
So, hydration is occurring and pores are being
filled, so you see continuous increase in
strength.
For OPC with CSA, the system also except in
the system when you have thirty percent CSA.
So what happens, when you add lot of CSA,
like in this case you got thirty percent CSA,
there was a cracking because there was a lot
of expansion which took place that led to
this drop, right.
But again the hydration is happening, right,
so your cracks are being filled and then you
see again strength increase, right.
Same behaviour we see when we try to monitor
the modulus, dynamic modulus, right.
So you can see increase in dynamic modulus
for all other systems, except thirty percent
CSA at early-age.
But then because of this continuous hydration,
which fills the pores, cracks, right, you
see increase in strength.
But in general, comparable strength, right,
will look into this, seven days comparable
strength of OPC CSA cement paste.
And here I would like to mention that this
study was done on paste and not concrete.
Also it is important to look into the pore
solution, right because the pH of pore solution
dictates a lot of things like corrosion for
an example.
CSA cement have lower pH than OPC, but you
can see that, at least in the study they reported
around twelve point six to seven pH, right.
I have not seen any study where it was lower
than twelve basically, right.
So, yeah it is lower than OPC but not very
low, right, to cause corrosion.
This was the study done when CSA was replaced,
right.
So you say a zero percent CSA is OPC, we see
like thirteen point three two four, the pH
in the range of thirteen point three two four
at the end of seven days, okay.
As you increase the amount of CSA, there is
a reduction in pH.
But even at thirty percent CSA, what we see
that around thirteen pH, okay.
So yes, reduction in pH with increasing CSA
cement but it is not too low.
Look into the microstructure, right, we know
that there is a formation of Ettringite and
the crystal.
And a lot of formation of Ettringite and we
see that there is a lot of crystal needle
extractions, right.
This was the, this was, these are the micrographs
seven-day old sample, right.
So, we see a lot of needle structures in both.
Now it is important to look into, because
this is it, this is the phase that is giving
you expansion, right.
If you have a lot of Ettringite, you will
see expansion.
And we will come, I will look into this, we
want expansion just enough to contract shrinkage,
right.
So it is important to look into factors which
influence early-age expansion, right.
How can we quantify?
How can we predict basically?
So we know that Gypsum to Ye'elimite ratio
is one of the factors governing early-age
expansion, right.
When you do not have enough Gypsum, you will
not have expansion.
Then amount of Ettringite, obvious choice,
right.
Degree of super saturation with respect to
Ettringite is also an important factor.
Okay, so it depends, this is, what is the
saturation level of your sulphates, right.
And these can be estimated, if we know the
composition of pore solution and using thermodynamic
modelling we can calculate the degree of saturation.
Apart from these chemical factors, we need
to also pay attention to stiffness of the
cementitious matrix, right.
It is like, if you compare a stone versus
rubber, same amount of expansion happening
in rubber will cause greater expansion, right,
compared to stone.
So we need to pay attention to the stiffness
of the cementitious matrix.
Now if you look into early-age expansion,
here again we are seeing the effect of CSA
addition, right.
It is also important to capture this expansion
very early on, a lot of conventional way of
measuring expansion or shrinkage is relying
on the one-day Datapoint.
You know, so you cast your sample demould
it after twenty four hours, right.
But we know that C4A3S starts reacting immediately
and you have formation of Ettringite, you
will see expansions occurring within twenty
four hours.
So it is very important to capture that.
For that we can use this corrugated tube,
right, which allows you to take measurements
immediately after casting.
So basically you fill your tube with cement
slurry and using this gauge you can monitor
the change in length, right.
And what is important is then you can cast
the parallel samples, the prisms which you
do and then add this data, okay.
But here is an evidence that the expansion
occurring twenty four within twenty four hours
is not insignificant, right.
So depending on your CSA content, you may
have significant expansion occurring within
twenty four hours.
Obviously as we increase the amount of CSA
cement, we see increase in expansion, right.
And order of magnitude difference, zero percent
CSA is OPC, plain OPC, when you add seven
percent CSA n order increase, fifteen percent
n order increase, right, thirty percent CSA,
the samples cracked, right.
Because we saw that earlier in mechanical
properties also, there was a reduction in
tensile strength and dynamic modulus and samples
cracked, okay.
So expansion within twenty four hours is not
insignificant, one thing to note.
And there are ways to capture that.
Now, we look into the hydration of Ye'elimite,
right, because that is what is causing the
expansion and that is what leads to the formation
of Ettringite, monosulphate and other phases.
So here again, comparing the four different
samples OPC and sample with CSA.
So you see presence of Ye'elimite, right,
why?
It gives you peak at around twenty three point
four degree in XRD.
And you can see that as you increase the amount
of CSA, this peak is more intense, right.
This is picture at one-day, what happens in
one day.
So you still have underrated Ye'elimite after
one day, right.
But now if you look at seven days, right,
basically Ye'elimite is consumed, there is
no Ye'elimite, right.
So all Ye'elimite has reacted and led to the
formation of Ettringite, right.
And in addition to Ettringite, you have other
phases like mono sulphate also, right.
And hemi-carbonate may form depending on the
conditions.
So important thing to note is that in seven
days, the hydration of Ye'elimite is complete.
Now, depending on the use of mineral admixtures.
We saw earlier that in isothermal calorimetry,
that class C fly ash behave differently, right
because there was the Gypsum depletion in
class C fly ash.
So, the effect of that on expansion is also,
is shown here basically.
So when you have class C fly ash in system,
the expansion was less compared to the plane
system.
But when you have class F fly ash, the expansion,
there was increase in expansion, right.
It again ties back to isothermal calorimetry
data.
So these other techniques can be used to understand
what is happening, what is the cause of this
expansion behaviour, right.
Again here we see that within twenty four
hours, the expansion of occurring twenty four
hours within twenty four hours is not insignificant.
So we all want to predict the expansion depending
on the phase completion.
We want to know like how much it system will
expand, right.
So this we have already seen when you had
class C, class F, this is a plot shown in
the earlier slide.
Now we know that class C fly ash, again going
back to isothermal calorimetry, we found that
when we added the Gypsum, the shoulder peak
disappear, right.
So, that led to again this increase in expansion.
When you had fifteen percent Gypsum, you see
increase in expansion, twenty percent, further
increase.
So this is one of the ways to mitigate this
loss in expansion.
Suppose you, your target is to achieve a level
of expansion, right, suppose you want to,
suppose you have class F fly ash, you want
your system to expand as class F fly ash but
you have class C fly ash, then you can add
Gypsum basically.
You know that there is going to be Gypsum
depletion in class C fly ash, so we can add
Gypsum and get that expansion back.
And when we try to calculate this calcium,
available calcium sulphate, right, we can
calculate that based on the stoichiometry,
we can calculate how much calcium sulphate
is available for the hydration of Ye'elimite,
right.
And we saw that it was very interesting to
that, it was a very nice correlation basically.
So the system which expanded most, as you
see has the highest amount of calcium sulphate
available for Ye'elimite.
These are the simple calculations you can
do to predict or compare two systems, right.
Because we know that Ye'elimite is reacting
with calcium sulphate and forming Ettringite,
that is causing expansion.
So this availability of calcium sulphate can
also give you an idea about the expansion.
Since we are talking about the expansion,
it is important to look into the mechanism,
right, what is that causing this expansion,
what is the origin of it.
So, long time ago, Taber did a very interesting
experiment in nineteen sixteen, where he had
a salt in its saturated solution, right and
you put weight on it.
Then what happens, when this, due to evaporation
yours salt your saturated solution will become
supersaturated.
And that led to the growth in Crystal and
that lifted the weight.
The simple experiment tells us that the super
saturation gives rise to crystallization stress.
This test was done long time back and we can
calculate this crystallization stress, okay.
So what does super saturation do?
Here are the two systems, the low and high
super saturation.
First of all we see that super saturation
can be linked to the curvature, right.
And we see when you have high super saturation,
it allows crystals to grow in smaller pores.
High super saturation means higher curvature,
higher curvature means lower radii, right.
So when you have high super saturation, that
means your crystals can form in smaller pores.
But when you have low super saturation, that
will not happen.
So these two schematics depict that, right.
In high we see the pores being filled, right,
the smaller pores being filled.
Now, this maximum crystallization stress can
be calculated, right, if we know the super
saturation.
This gives us the upper bound of the stress.
So, if we know the composition of pore solution,
and we know the concentration of various ionic
species, we can calculate the saturation index
for any phase.
In this case it was calculated for Ettringite,
which is the expansion causing phase.
So, you write the what is the ion activity
product.
Because the super saturation gives the ratio
of ion activity product and solubility product.
So, now if you know these concentrations,
right, using any thermodynamic modelling software,
you can calculate the ion activity, right.
And basically this GEMS is a software, relies
on the minimization of Gibbs free energy can
give you saturation index of individual face.
But the input is concentration, right.
And we see here now that what happens as you
increase the amounts of CSA, you see the increase
in super saturation.
So we saw that super saturation gives rise
to crystallization stress.
So when you have higher fraction of CSA, you
see increase in saturation index of Ettringite.
People have been relating expansion to just
Ettringite content.
But the correlation is very weak.
If you look into any problem related to sulphate
attack, delayed Ettringite formation, even
in this case when you have early-age expansion.
So you see these are the data points, each
data point is one mix, right.
But same Ettringite content if it is plotted
this way, like when you normalize it by capillary
porosity, you see better correlation right.
We will come back to this point why we did
this.
But same thing if we know the volume fraction
of Ettringite and we know capillary porosity,
that correlation improves.
So, here are some factors which influence
the expansion.
We saw this again, SC, I mean, this volume,
Ettringite volume fraction in capillary pores
can also be called SC and the relation is,
correlation is better.
And there is a good correlation between saturation
index and expansion.
Increase in saturation index leads to increased
in expansion.
What is interesting here is what we saw is
elastic modulus, right.
It is very important to pay attention to the
stiffness of the matrix.
A matrix which is less stiff will expand more,
right.
So as you increase the elastic modulus, you
see reduction in expansion.
Now, there are simple ways, obviously there
are a lot of assumptions like matrix being
isotropic and homogeneous, right, or elastic.
We can estimate the tensile stresses.
This is a case of cylindrical pores basically.
And for plain stress condition, what you can
calculate is average hydrostatic tensile stresses,
right.
Just to clarify this point, in cylindrical
pores we are assuming these are the crystals
right here causing these stresses, right.
So the Sigma C is the maximum crystallization
stress from thermodynamics, right.
Now, based on the volume fraction of crystals,
you can estimate this, average hydrostatic
tensile strength.
Using poromechanics also we can relate this
stress to this Sc factor.
We talked, there was, we saw also very like
better correlation of expansion with the SC
factor, we can calculate the tensile stress.
So, now what happens when we try to plot?
So, here is a graph compares the tensile stresses
for spherical pores, cylindrical pores, poromechanics,
like these are the three models used to predict
the tensile stresses.
And here is the tensile strength, right.
So basically the thirty percent CSA, right,
you can see that your, we are saying significant
amount of tensile stress, they exceed strength,
right.
So, these are simplistic models but can be
useful to predict, so, all models in this
case enables the prediction of failure, especially
at high dosage of CSA, that is what we also
verified experimentally, right, when we measure
the length change, we saw cracking in the
sample with thirty percent CSA.
Now, question comes, how to design shrinkage
compensating concrete.
If you want to design concrete, where you
want to leverage early-age expansion due to
the formation of Ettringite, how can you do
that?
So this is a guide, this is from American
concrete Institute 223R, guide for the use
of shrinkage compensating concrete exists.
And in that also they mention type K cement
basically, it is a blend of OPC, Ye'elimite
and calcium sulphate, right.
So, how do you do it?
Basically first we anticipate the shrinkage
of concrete, right, based on the experience
of the local area.
Depending on where you are, what are the conditions,
right, you know what is the member shrinkage,
right.
So you know suppose this is the shrinkage,
particular region, right.
Now this is a curve where you can get based
on the percentage of reinforcement, right,
based on the percentage of reinforcement you
can calculate how much maximum restraint,
how much expansion you want in the maximum
in the restraint concrete prism, right.
So, based on this area you can calculate the
maximum expansion in restraint concrete prism.
And how do you do this concrete prism test?
It is basically according to ASTM878, you
have this reinforcing bar, right.
You pour your concrete, right, and then measure
the early-age expansion.
So basically in this case we are seeing the
expansion from point zero percent point zero
six percent, right, depending on what you
have.
But then, again you can calculate it if you
are somewhere here and your percentage of
the enforcement is here, then you would like
to be here, so you want expansion, your expansion
of your concrete prism, restraint concrete
prism somewhere here.
So, this is a guide, right, you can use it.
Now will look into the small scale versus
large-scale.
Small-scale test, when you do this concrete
prism test, right, it compares, you have seen
similar plot before, so this is a conventional
concrete, when you do not have any expansion
agent, right.
There is a shrinkage of the order of six hundred
in this case six hundred to seven hundred
micro strain.
And you add the calcium Sulfoaluminate cement,
see this early-age expansion and the net shrinkage
is close to zero.
But because of this restraint, right, now
if you look into the strain level in slab,
the degree of restraint is different, it is
high, right.
Although we see some difference but we are
not seeing like zero net shrinkage, right.
There is a difference, like OPC is here, we
are able to reduce the net shrinkage.
So that is why it is important to pay attention
to the degree of restraint, level of, percentage
level of your reinforcement in the slab, when
you want to predict.
Now, coming back to the application of CSA
base binder, this was a field application
done using OPC CSA cement in Illinois, where
two decks were cast side-by-side.
You see normal deck with Portland cement and
when we replace Portland cement with CSA,
right.
And what was interesting to see that the deck
with CSA did not crack in early-age.
So, there was no cracking, no sign of shrinkage
cracking, right.
So it was a successful implementation of CSA
concrete.
This is the deck pour, the concrete was fluid,
workable, no issue.
So again this was a blend of OPC and CSA cement.
And there was no issue in terms of pouring
it in heavy reinforced areas, right.
So, in summary CSA cement can be formulated
as expansive or non-expansive cement system
based on users requirement.
If you want to reduce your shrinkage, then
you would, you will leverage the early-age
expansion, okay.
And successful field application of the OPC
CSA concrete demonstrates its role in enhancing
both sustainability and durability, right.
And the major applications of this cement
include in bridge decks, concrete slabs, self-levelling
floors, grouts, repair materials, pressure
pipes, etc.
So where we stand now in terms of research?
So, now we need to think about the manufacturing
of the CSA based cement using industrial byproducts,
because that will drastically reduce the cost,
okay.
Also durability of CSA based concrete against
carbonation, corrosion and acid attack is
an active area of research.
As we see the rheology is very important,
how can we come up with the new chemical admixtures
that can control the workability of the CSA
based cement.
So these are the future possible areas of
research.
That, thank you for your attention.