Early age hydration of cement pastes with alkaline and alkali-free accelerators for sprayed concrete

Abstract

The objective of this study is to evaluate the early age hydration mechanisms of accelerated CEM I pastes. Liquid phase analysis, conductimetry, isothermal calorimetry, in situ XRD and SEM were performed on cement pastes produced with equivalent doses of an alkaline accelerator (sodium aluminate solution) and of an alkali-free accelerator (aluminum sulfate solution). Results showed that the addition of these chemicals first changes the ionic equilibria of the medium by the consumption of Ca2+ and SO42- ions from the liquid phase. The alkali-free accelerator contributes to the rapid formation of ettringite, as well as to a faster rate of alite dissolution and hydration. The aluminate accelerator leads to AFt and AFm formation, rapidly depleting gypsum and filling up the space, inhibiting further alite hydration. According to the results, variations in cement hydration mechanisms caused by accelerators with different chemical.compositions are explained.Peer ReviewedPostprint (author's final draft

Chemical aspects related to using recycled geopolymers as aggregates

“…Most literature findings describe the acceleration of hydration as a distinct effect of alkalis in the aqueous solution (Jawed and Skalny, 1978;Salvador et al, 2016). In this specific case though, no 'flash setting' took place but an acceleration of 6·4% of initial setting was observed.…”

Estudio de las reacciones de hidratación temprana en el hormigón proyectado

“…Existen algunos estudios centrados en las reacciones que ocurren durante la hidratación de sistemas acelerados [1][2][3][4][5]. En todos los casos la presencia del acelerador modifica las condiciones de hidratación, dependiendo los productos de hidratación que se forman del tipo y de la cantidad de acelerador que se utilice.…”

“…En concreto, Maltese y col. [6] encontraron que, para sistemas acelerados con acelerador sin álcalis basados en ácidos inorgánicos, cuanto menor es la velocidad de disolución instantánea del regulador de fraguado, mayor es la eficiencia del acelerador. Según Salvador y col. [3], los aceleradores sin álcalis basados en sulfato de aluminio además de fomentar la formación rápida de ettringita, promueven la disolución y rápida hidratación de la alita. Sin embargo, aceleradores alcalinos basados en aluminato sódico dan lugar a la formación de AFt y AFm, gastando el yeso disponible e inhibiendo la hidratación de la alita.…”

“…Estudio de las reacciones de hidratación temprana en el hormigón proyectado I. Galan (1) , A. Stauffacher (2) , F. Mittermayr (2) , M. Thumann (3) , W. Kusterle (3) , P. Juilland (4) , C. Stenger (4) y B. Lindlar (…”

Adaptation of the standard EN 196-1 for mortar with accelerator

“…In such situations, special admixtures known as accelerators may be added to the mixture with the aim of producing a rapid setting or an enhanced gain of strength at very early ages [6,7]. The behaviour of the concrete placed, the advance of the construction and the safety of the workers involved in the process depend on the dose of accelerator and on its compatibility with the cement or other admixtures present [8,9,10,11,12].…”