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2015SpringShotcreteEMag

Effect of Pozzolanic-Based Rheology Control Agent as a Replacement for Silica Fume on Wet-Mix Shotcrete Performance By Ezgi Yurdakul and Klaus-Alexander Rieder Tcrete containing a pozzolanic-based rhe- handling of SF may be challenging due to itsthat may cause unwanted side effects, such asdelays in setting time. Furthermore, the-he purpose of this article is to present theperformance evaluation of wet-mix shot ology control (RC) agent as a replacement for powder form. As a result of these limitations, silica fume (SF). The RC agent used in this there is a need to replace SF with another study is a liquid-based product that contains substance that can provide similar performance nanoscaled colloidal silica particles with high while avoiding the challenges associated with pozzolanic reactivity. In this experimental pro- the use of SF. gram, five different mixtures were evaluated. The pozzolanic-based RC agent is an alter- One mixture was prepared as a reference mix- native to SF, as it provides better (or at least ture incorporating 5% SF by mass of cement. equivalent) performance characteristics while Four mixtures were prepared with RC agent at avoiding the limitations mentioned previously. dosage rates of 0.67%, 0.80%, 1.30%, and The RC agent used in this study is a liquid- 1.60% by mass of cement. Slump, rebound, based, fully stable product that contains uni- early- and later-age strength, and the depth of formly distributed nanoscaled colloidal silica water penetration were tested. particles with a long shelf life. The specific Test results showed that the SF mixture and surface of colloidal silica is higher than that of mixtures containing RC agent provided similar conventional SF and it consists of more than 99% slump and they were all very efficient in reducing SiO2. Therefore, it has a very high purity and rebound to the range of 5 to 6%. The addition of pozzolanic reactivity compared to SF.1,2 When the RC agent increased the early-age strength added to a wet-mix shotcrete, the RC agent compared to the SF mixture and provided similar significantly increases the cohesiveness while strength at 7 and 28 days. The water penetration reducing bleeding and segregation.3 Therefore, depth of the mixtures containing the RC agent while also improving the sprayability and was slightly lower than that of the SF mixture, pumpability characteristics, it reduces rebound as desired. The RC agent was more dosage- and increases maximum thickness of buildup. efficient, as it required much lower addition In addition, many researchers4-8 stated that rate to provide equivalent performance to SF. colloidal silica fills the space between particles Overall, it was found that for the investigated of calcium silica hydrate (CSH) gel; hence, it mixtures, the RC agent was a suitable alternative acts as a filler to improve the microstructure. It to SF in wet-mix shotcrete applications. also reacts with calcium hydroxide, thus increasing the amount of CSH gel, which in turn Introduction increases the densification of the matrix and SF is commonly added to shotcrete mixtures improves durability. Furthermore, at sufficiently to improve strength and durability while high addition rates, colloidal silica can accelerate reducing rebound. However, there are a number the early-age hydration process of the shotcrete of limitations associated with its use. The first mixture, which reduces the time of setting and limitation is due to the variability of the purity increases early-age strength compared to of SiO2, which results in a significant variation shotcrete mixtures containing SF.9 in the performance of SF in shotcrete. In addition, This article presents a comparative study to SF is a by-product of silicon and ferrosilicon evaluate the performance of RC agents as alter- production. Therefore, it contains impurities natives to SF in wet-mix shotcrete applications. 26 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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