Proceedings of the International rilem conference Materials, Systems and Structures in Civil Engineering 2016
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- VOLUME STABILITY OF ALKALI ACTIVATED PORTLAND CEMENT CONCRETES WITH ALKALI-SUSCEPTIBLE AGGREGATES Pavel Krivenko
- 1. Introduction
1 International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Service Life of Cement-Based Materials and Structures 22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
(1) Kyiv National University of Construction and Architecture, Scientific Research Institute for Binders and Materials, Ukraine
Abstract The paper presents the results of study on the influence of metakaolin additive on volume stability and strength characteristics of alkali activated portland cement concrete with alkali- susceptible aggregates with basalt as example. The structure- forming processes in an interfacial transition zone of the concrete "alkali activated portland cement – cast basalt bar" have been studied. The results of study allowed to reveal a positive effect arising from corrosion of the alkali- susceptible basalt bar in case of modification of the alkali- activated portland cement by a metakaolin additive. These conclusions are supported by data on interaction of structure forming processes in the interfacial transition zone and physico- mechanical characteristics of the concrete made with the alkali- susceptible aggregate (cast bar from basalt rock). A mechanism of AAR in case of alkali- susceptible aggregates in alkali- activated portland cement concretes modified by active Al 2 O
– containing additive (metakaolin) has been described and discussed in details.
The fact that aggregates for concrete containing amorphous silica in the form of opal can enter into reaction with the alkalis (Na 2 O, K
2 O) contained in the cement was first discovered and studied by STANTON [1]. As a result of this reaction a concrete expands in volume, cracks appear in it and its load carrying ability declines.
fundamentals of the mechanism of alkali- aggregate reaction (AAR):
a cement itself, concrete additives and external aggressive environment are a source of alkalis; 2 International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Service Life of Cement-Based Materials and Structures 22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
2 O equivalent (Na 2 O +
0.658 K 2 O) is restricted by a value 0.60%. In case of blended cements this value may reach 2%;
the expansion process is accompanied by osmotic pressure, which is created by a viscous- flow (plastic) gel of the alkali metal silicate formed as a result of reaction. The gel acts as a semi-permeable membrane, through which ions of , +
direction of a surface of the reaction- susceptible aggregate. First of all, the alkali metal silicate fills in the surrounding pore space, and only after a pressure of expansion occurs;
an important role in the above described process is played by a free ( ) 2 , present in the cement stone, which, first of all, enhances semi-permeable properties of the membrane, secondly, enables the formation of additional quantities of alkali as a result of exchange reactions between ( )
and alkali metal salts, which can be introduced into the concrete within various additives (plasticizers, accelerators of hardening, anti-freeze additives, etc.)[5];
minimization of the expansion effect is reached through an introduction into the cement composition of various active mineral additives of sedimentary and volcanic origin (limestone with
small amounts of amorphous silica, zeolites, perlite, tuff, pumice, etc.), as well as of man-made origin (fuel ashes, metallurgical slags, amorphous silica, etc.).
An effect of these additives is based, first of all, on their high reactivity with regard to alkali metal hydroxides. This promotes a homogeneous distribution of the reaction products in a concrete, thus preventing a harmful attack of alkalis of coarse alkali-susceptible aggregate. Secondly, pozzolana additives bind free 2+ - ions. This results in the decrease of CaO/SiO 2 ratio and stable binding of Na + and K
+ within the C-S-H phases.
The described views on the mechanism of AAR were accepted by scientists for many years as basic prerequisites for developing recommendations as to prevention of hazardous consequences of this reaction and did not allow to substantiate the application of new cements (alkali activated ash- and slag cements, geocements, geopolymers, etc.), within which the contents of alkalis are considerably higher than those of traditional cements.
Nevertheless, known in the art are attempts to explain corrosion processes in case of alkali- susceptible aggregates in the presence of alkalis not only from the point of view of quantitative contents of alkalis and free ( )
. So, MALEK and ROY [6] studied a role of Al 2 O 3 , being dissolved from a feldsparthoid stone and established that with increase in Al 2 O 3 /SiO
2 the AAR transforms from a destructive into constructive one. Later, the results of works reported in [7 11] showed that a possibility of formation in the interfacial transition zone of the alkaline or mixed alkaline – alkaline earth aluminosilicate hydrates depends not only upon aggregate type, but composition of the aluminosilicate component of the alkali activated cement. A conclusion was drawn that by regulation of the Al 2 O
-content within the aluminosilicate component of the cement by introduction of the active Al 2 O
-containing additives it became possible to prevent destructive consequences of the AAR in concretes even with high content of alkalis in the cements under study. This may be applicable in equal extent to the alkali activated portland cement.
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