18-05-2012, 03:51 PM
Concrete based on fly ash geopolymers
geopolymers.pdf (Size: 2.12 MB / Downloads: 206)
Introduction
The exposure of aluminosilicate bodies of the type cement clinker, slag, fly ash or
thermally activated substances to very alkaline environments (hydroxides, silicates)
gives rise to the formation of new materials – geopolymers characterized by a two- to
three-dimensional Si-O-Al structure. Substantial attention has been given to these new
materials, for instance, at symposia 1-7 or in papers mentioned in databases 8,9 . Such
contributions deal not only with the results obtained during the investigation into the
synthesis of geopolymers and their microstructure (by using predominantly SEM) but
also with their possible applications particularly in conjunction with the valorization of
inorganic rejects (first of all, fly ashes).
The geopolymers samples were also subjected to the tests consisting of alternating
freezing and defrosting in accordance with the CSN 72 2452 standard. The bodies with
the dimensions of 4 x 4 x 16 cm were kept at the laboratory temperature in the
environment with the relative humidity of 40 % for 28 days; their frost resistance was
then determined. Micro-reinforcement (dispersed fine steel wires) was added to the
concrete mix too. Also the effects of other types of reinforcement were studied (sized
fiberglass of various dimensions, fiberglass mats and various sizes of man-made fibers
and cords). The attention also focused on the verification of the function performed by
the concrete reinforcing steel rods especially as regards the adhesion and the interaction
in the contact zone with the geopolymers concrete.
Results and discussion
Rheological properties
Rheological properties of the fresh fly-composition are dependent on the fly ash content
in the mixture. In comparison with the fresh cement concretes, the mixtures containing
higher percentages of fly ash exhibit a different rheological behavior. Both the static and
dynamic viscosity of the geopolymers concrete are substantially higher. Therefore,
longer treatments by vibration or by other techniques are required in both cases and the
aggregate setup (ie the solid components of the mixture) must be selected with great
care. Despite the higher viscosity and a rather pronounced adhesion ability of the fresh
composition coarser and heavier grains of the aggregate start settling at higher values of
the water coefficient; this process is accompanied by the displacement of the mortar and
the paste towards the surface of the processed fresh concrete mix. This situation occurs
not only during an intense dynamic treatment by vibration but also during a simple free
deposition of the cast composition of the fresh geopolymers concrete.