Alkali Activated Cements versus Geo polymers- Juniper Publishers
Juniper Publishers Journal of Civil Engineering
Opinion
Nowadays the scientific community of “cement people”
is involved into discussions: are the alkali activated cements and
geopolymers the same or not? In these discussions the scientists apply
different approaches based on chemistry of the processes that take
place, final phase composition of the resulted cement stone that is
formed, or types of raw materials that are used as cement components. In
order to put more light on the “very hot” topic of the above
discussions, I would like to start from the very beginning. The idea
taken as a basis for creation of a new class of cements
(alkali-activated cements) can be described as the following. The cement
and chemist specialists know that the processes of revealing hydraulic
activity by traditional aluminosilicate-based cements are attributed to a
possibility to transform and to fix aluminium-silicon-oxygen
tetrahedrons in a discrete state, at which their hydrates (sols) are
capable to polycondensation with the formation of gels-colloid solutions
and, finally, crystalline formations. In case of Portland cement this
process takes place due to blocking a discrete state of [SiOJ4' by Ca2+-ion (for example, C3S, C2S).
In this state, an autocatalytic destruction of the
above minerals when they are mixed with water with following
polycondensation of monomeric structures into layered CSH- structures
due to protonization of the Ca-0 bond takes place. When the CS-mineral
is mixed with water, the ionic force of water is not enough, because the
covalent phases are prevailing, and in order to break them high
temperatures or other activators that are capable to break the covalent
bonds of the silicon-oxygen framework of the structure with the largest
discretivity of [SiO4]4- are required. Just this
idea was put forward in 1957 by the Ukrainian (USSR) scientist Viktor
Glukhovsky (those times PhD, his DSc (Eng) degree and corresponding
title of professor were earned in 1966). He proposed to use compounds of
the elements of group 1 of the periodic table, which are known to
create in aqueous medium an alkaline reaction, these are, for example:
Na(K)2O CO3, Na(K)OH, N(K)2O•nSiO2•mH2O.
This idea was supported by deep analysis of the processes of rock-
mineral formation taking place in nature and a role played by alkali
metal compounds in these processes Palomo et al. [1],
As a result of these processes the alkaline or mixed alkaline-alkali
earth aluminosilicate hydrates-analogues of natural zeolites (for
example, Al2O3-SiO2+NaOH+H2O → Na2O-Al2O3-SiO2-H2O → CaO- Na2O-Al2O3-SiO2-H2O) are formed.
Taking into account that in normal conditions these
processes of synthesis flow very slowly and require rather high
temperatures Barrer RM [2],
in analogy to predominant processes taking place in nature of the mixed
type zeolite (Na- Ca) formation, Viktor Glukhovsky showed that elements
of group 2 of the periodic table could accelerate these processes (for
example, Ca2+). Viktor Glukhovsky proposed TWO new cement
types: alkaline and alkaline- alkali earth aluminosilicate hydrate
cements. Volyansky et al. [3] With account of similarity of two processes: Glukhovsky [4]
proposed to call these cements “soil silicates” or “soil cements”
(“gruntotsementy” or “gruntosilikaty” in Russian) in attempt to reflect
this similarity to natural minerals formations. Glukhovsky [5]
later, depending upon raw materials used as cement components and
composition of the resulted hydration products these cements were called
as listed below:
a. Geopolymers (Davidovich-1973)
b. Alkali-activated cements (Narang, Chopra-983)
c. F-cement (Forss-1983)
d. Gypsum-free portland cement (Odler, Skalny, Brunauer-1983)
e. SKJ-binder (Channgo-1991)
f. Geocement (Krivenko-1991)
g. Alkaline cements (Krivenko-1994)
However, in mechanizm of hardening and hydration
products their essence remains the same-to break an aluminium-silicon-
oxygen framework by alkaline activators with the formation of alkaline
(Na(K)2O-Al2O3-SiO2-H2O) or mixed alkaline- alkali earth hydration products (Na(K)2O-CaO-Al2O3-SiO2-H2O)
due to the processes of polycondensation (polymerization reaction). A
conclusion can be made that in their basic feature (chemical processes
of hydration and hardening) geopolymers are alkali activated materials.
Nowadays, with account of variety of raw materials (cement components)
which could be activated, the above mentioned phases can be formed in
parallel with the CSH- phases (so-called"hybrid cements"), thus giving a
possibility to use a variety of available raw materials to provide the
required properties (Shi C, Jimenez & Palomo [6] and Fernandez-Jimenez et al. [7] The first RILEM Technical Committee 224-AAM [8]
has proposed, with consideration of the above arguments, to call these
cementitious systems and materials based on them "the alkali activated
materials". Currently, the second RILEM Technical Committee 247- DTA
"Durability Testing of Alkali- Activated Materials" is dealing with
durability aspects of these materials.
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