Zakharov S. A. «The advantages of high reactivity metakaolin application in concrete and dry construction mixtures»

1. Introduction

High reactivity metakaolin (HRM) is artificially produced
silicon-ground (puttsolan) additive possessing the
highest reactivity among active mineral additives available
in the market.

In particular, HRM is capable to bind up approximately
2,5 times more quick lime than micro-silicon (МS)
widely applied in building industry.

The basic characteristic and distinctive feature of HRM
as compared to MS is its chemical nature. Unlike MS, HRM is a
mixture of active silicon-ground and alumina almost in equal
proportions, i.e., it is not silicate but alumno-silicate puttsolan

By the way, initially, «puttsolana» — the one that gave
its name to modern active mineral additives and has been
used by ancient Romans as early as B.C., allowing them to
erect constructions that stood through millennia till our
time — was exactly alumno-silicate volcanic material.

2. Metakaolin characteritics

Therefore, HRM alongside with МS is highly active
puttsolan additive to concrete and solutions. At the same
time it is characterized by a number of specific features.
We shall consider them briefly.

2.1. HRM is thermally processed kaolinum. Thus, it
inherits from kaolinum many of its useful physical properties,
in particular:

•HRM has a light colour (usually from white to cream).
This property makes HRM practically unalternative
material for modifications of mixtures based on white
and coloured cements, with the purpose to increase
their weather-resistance, water-resistance and, as
the consequence, their durability

•In its form HRM represents lamellar particles of the average
size about 1-2 micron. These particles are approximately
several times smaller than particles of portland
cement, perfectly completing its granulemetrics.

Therefore, the metakaolin addition to cement concrete
allows:

•To increase plasticity and easiness of concrete mixtures
spreading.

•To facilitate furnishing of concrete surfaces through
the methods of rubbing in and smoothing down due
to the absence of concrete stickiness to the tool and
good tyxotropy.

•To considerably decrease the application of supersofteners
necessary to compensate the thickening effect
when thin dispersible addition is introduced to
cement. In particular, at rational selection of HRM and
softener dosage, the easiness of concrete spreading
with HRM may be even higher than the easiness of
concrete spreading with the same structure and with
the same quantity of softener, but without HRM.

Regarding the cement saving which recently became
especially urgent issue due to sharp prices increase,
HRM shows very remarkable synergetic properties with
softeners.

Fig. 1 below shows even-moving concrete mixtures
with equal water-binding ratio (i.e., practically evenly
strong).

Continuous curves show what saving of the cement
with no additions which is achieved by softeners at with
various dosage. Graph present the efficiency of naftalinsulfonatformaldegide
supersoftener «C-3» (Russia) and
polycarboxylat hypersoftener FOX-8H (China-Canada)
is presented.

Dashed lines indicate efficiency of softeners in a mixture
where 10% of PC is replaced with HRM.

The data in the graphs distinctly demonstrate that
cement partial replacement by metakaolin allows to lower
significantly the quantity of softener required to achieve
the necessary level of binding softener’s saving, or provides
greater saving of binding material at the same dosage
of softener.

Fig. 1. Dependence of cement economy on the dosage of softeners and HRM

If considering the same data from
the economic point of view it is obvious,
that combination of HRM with highly
effective polycarboxylat hypersoftener
provides improvement of concrerte
quality with simultaneous decrease in
its cost.

In Table 1 and on Diagram 1 the
cost of cement, sand, metakaolin and
softener per cubic meter of concrete
(that is, cost of cubic meter of concrete
without taking into account the cost of
rubble) is presented.

Therefore, the cost of concrete with
addition of HRM and FOX-8H is lower
than the cost of control mixture by
325 roubles/cubic meter.

At the same time the following
properties of concrete are improving as:

•Its tightness, chemical stability —
due to introduction of puttsolan
additive;

•Decrease in its shrinkable deformations,
creeping under pressure
and increase in resistance to cracking
— due to decrease of amount
of tightener.

Table 1. Comparison of concretes’ cost (roubles)

Here it is necessary to make a remark. Efficiency of
various softeners in mixtures containing HRM is different.
In particular, naftalinsulfonat supersofteners (such as
widely known C-3) considerably reduce their efficiency in
the presence of HRM. At the same time, other kinds of
softeners, in particular, lignosulfonat, melaminformaldegide
and polycarboxylat ones demonstrate high softening
ability in the mixtures with HRM application.

2.2. The high percentage of active alumina provides
for the following chemical features of this additive.

•Alumina is capable to bind several times more
quick lime compared to silicon-ground, that provides
higher puttsolan activity of HMR in comparison
with МS.

•Besides binding alkaline-ground metals (calcium
and magnesium hydroxides), metakaolin is capable
to reliably tie in the alkalis contained in portland
cement or introduced into concrete along with additives
(in particular, anti-froast ones) or from the
outside (for example, anti-icing mixtures). That
provides reliable protection of concrete constructions
against such problems as silicate-alkaline reaction
as well as discharge of alkalis on a surface
of products in the form of salty patches, worsening
appearance of products and constructions.

Diagramma 1. Comparison of concretes’ cost

Metakaolin is the basic component of new type of
binding substances under the general name «geopolymers
». These new ecologically clean binders
possess the whole complex of unique properties,
such as: high durability and resistance in various
environments, high impenetrability even for radio
nuclides, perfect adhesion to various materials,
perfect thermostability. The mechanism of reaction
of these binders is exactly based on ability of
aluminium silicate to enter into chemical interaction
with alkalis and alkaline silicates. Long before
the appearance of nice-sounding term «geopolymers
» those mixtures were called in Russia «alumnisilicates
activated by alkalis».

•Aluminate’s component of metakaolin is capable
to actively interact with the gypsum contained in
portland cement or added specially to cement mixtures.
The controllable formation of attringitte at
early stages of hardening of concrete and solutions
allows to significantly reduce shrinkable deformations
and even to get non-shrinkable and extending
mixtures.

3. The fields of HRM application

The listed above characteristics of metakaolin define
the following basic fields of its application

3.1. Heavy concretes, including high-strength and
self-condensed ones. Decrease in cement dosage at concrete
manufacturing has always been the target for the
task to optimize concrete mixture composition. It is important
both from the economic point of view and, what is especially actual in recent times, in connection with
intensive increase of prices on cement, and from the
point of view of increase of concrete characteristics, in
particular, diminishing their creeping and shrinkable
deformations.

In a combination with highly effective softeners application,
HRM allows to essentially lower quantity of cement
in compoundings of concrete, especially, in concrete
with higher requirements for water resistance and frost
resistance. Experience shows that even at a dosage of 2%
to PC metakaolin considerably increases water resistance
of the received mixtures.

Therefore in compounding of high-strength concrete
diminishing of the required quantity of metakaolin and
supersofteners compared to the mixtures containing
МS leads to additional decrease of the cost price of
compounding.

•Usual dosage of HRM — 3-5% to PC.

•Advantages:

•Increase of early and final solidness and impenetrability

•Increase of resistance to sedimentation

•Improvement of plasticity and pumpability.

3.2. Light concrete, including cellular concrete of the
lowered density

Adding HRM to compounding of pore-concrete mixtures
allows to increase durability of cellular concrete at
the preset density, or to decrease density at the preset
durability.

That is caused by two factors:

•Improving of foam structure due to thin dispenser
lamellar particles content.

•Increase in durability of inter-pores partitions due to
increase in durability of cement stone.

There is a positive experience of HRM application in
autoclave cellular concrete, too. In this case, HRM application
allows to facilitate the time of aerocrete hardening
allowing to ease the choice of time for monolith slicing into
blocks by means of a string.

3.3. Compositions on the basis of gypsum-cement
puttsolana levelers (plaster mixtures, floors flatteners).

For reliable prevention of uncontrollable formation
of attringitte in GTsPV mixtures, it is enough to add 10%
of HRM to the portland cement contained in GTsPV. It
leads to dosage of HRM as only 1-2% of the mixture compounding.
Such low dosages of HRM in a combination with
decrease in the required volume of supersoftener provide
good economic benefit in GTsPV compoundings.

In compounding of self-aligning mixtures HRM is
significantly improving the mixture resistance to stratification,
thus increasing its quality. It is also the case with
cement self-aligning mixtures.

3.4. Cement-limy mixtures

Cement-limy plasters are very popular due to high
plasticity and technological adaptivity. The replacement
of a part of quick lime (up to 50%) in these mixtures with
HRM allows to increase durability and weather-resistance
of those plasters without deterioration of their technological
adaptivity.

Thus, the received plaster mixtures possess with
higher protective properties in regards to underlying
layers, they are waterproof and have resistance to salt
formation. That allows forming highly effective mineral
decorative front plasters, including the ones based on
white cement.

3.5. Other cement mixtures

The high efficiency of HRM alongside with high
technological adaptivity of mixtures received with its
addition makes metakaolin the excellent modifier for
the materials, the surface of which is fixed by means of
rubbing and flattening down, in particular, for repair
concrete compounding and materials for creating concrete
floors covering (hardeners), in polishing mixtures
for inter-tile seams, etc.

3.6. Materials based on other types of binders

HRM demonstrates tremendous universality indeed
and allows to significantly increase materials water-resistance
based on other types of binders, including:

•Based on magnesia binder — HRM allows not only
to increase water-resistance of this binder but
also to solve partially the problem of instability
of the applied raw material mixture, binding
the quick lime, alkali contained in it as well as
magnesia hydroxide formed up at slow hydration
of «overburned» magnesit.

•Based on silicate binders — let’s remind of those
«geopolymers». HMR is able to interact with alkaline
silicate with formation of waterproof mixtures.

•HMR provides a possibility to increase the thermal
resistance of heat-resistant compositions based on
high-aluminous cement.

4. The known problems

The HRM application experience gathered by our
clients was not always the positive one. Sometimes the
clients were complaining that they could not achieve the
expected characteristics of their mixtures.

The analysis of faulty experiencies with HRM application
has enabled us to draw the follolwing conclusions:

•As well as MS, HMR, being thin dispersed powder
disposed to floculation, requires the compulsory adding
of an effective dispergator (softener) to the compounding
as well as applying the methods of mixing
up which provide even penetration of HMR
into the mixture and its dispergation.

•Despite many recommendations of the foreign manufacturers
to apply MR in dosage of up to 20% our
practice has proved that for this product the saying
«Porridge oil you will not spoil» is hardly true. That
is, overdosage of HMR may bring about concrete (solution)
durability diminishing and that may be explained
by two factors:

With the lack of quick lime and other materials in
the concrete (solution) with which HMR could interact
it becomes the inert staff that only reduces the mixture
durability.

The high dosage of HMR is significantly changing
the granulemetrics of the whole mixture. The surplus of
thin dispenser fraction also affects concretes and solutions
structure in the negative way and may decrease their durability
characteristics.


"I was surprised by the large number of participants in the Conference. It indicates that the cement industry is interested in the demand of cement, which very closely connected to the s...
"Well-organised, excellent speakers and high participation — a very beneficial event!"

Media partners