Effect of the aggregate type on the properties of alkali-activated slag subjected to high temperatures

Abstract

High temperatures present a risk of destruction for most silicate-based construction materials. Although these materials are not flammable, they lose their properties due to a thermal decomposition. In contrast to ordinary Portland-cement-based materials, alkali-activated slag exhibits a better thermal stability when exposed to temperatures up to 1200°C. Due to its different porosity it is less susceptible to spalling. However, the properties of the composites after a high-temperature treatment depend also on the stability of the aggregates. The effects of two different types of the aggregate (quartz and chamotte) on the residual mechanical properties and microstructure of the alkali-activated slag mortars exposed to 200-1200°C is presented in this study. The results showed an improved mechanical performance of the thermally stable chamotte aggregate at temperatures above 600°C. a-quartz is transformed to b-quartz at 573°C, causing a volume instability and, consequently, a strength deterioration. Although chamotte also contains some quartz phase, the reaction of mullite with the alkalis from the matrix leads to the formation of albite and anorthite, making the material tougher and, thus, compensating the negative effect of quartz. Keywords: alkali-activated slag, high temperatures, chamotte, quartz, strength, microstructure Visoke temperature so tveganje za razpad ve~ine gradbenih materialov, ki temeljijo na silikatih.^eprav ti materiali niso vnetljivi, izgubijo svoje lastnosti zaradi termi~ne razgradnje. Nasprotno od materialov, ki temeljijo na portland cementu, z alkalijo aktivirane`lindre izkazujejo bolj{o temperaturno obstojnost,~e so izpostavljene temperaturam do 1200°C. Zaradi razli~ne poroznosti so manj ob~utljive za pojav lu{~enja. Vendar pa so lastnosti kompozita po visokotemperaturni obdelavi odvisne tudi od stabilnosti agregatov. V {tudiji so predstavljeni vplivi dveh razli~nih vrst agregatov (kremen in {amota) na preostale mehanske lastnosti in mikrostrukturo malte iz z alkalijo aktivirane`lindre, izpostavljene temperaturam 200-1200°C. Rezultati so pokazali izbolj{ane mehanske lastnosti toplotno stabilnega {amotnega agregata pri temperaturah nad 600°C. a-kremen se transformira pri 573°C v b-kremen, kar povzro~i volumensko nestabilnost in posledi~no poslab{anje trdnosti.^eprav {amot vsebuje nekaj kremenove faze, reagiranje mulita z alkalijami iz osnove povzro~i nastanek albita in anortita, ki povzro~ita, da material postane bolj`ilav in s tem kompenzira negativni vpliv kremena.

Thermal Behaviour of Metakaolin/Fly Ash Geopolymers with Chamotte Aggregate

“…The motivation for the present work is to compare the thermal behaviour of geopolymers based on two different types of aluminosilicate, metakaolin and fly ash, and to investigate the influence of thermally-stable aggregate on the mechanical properties, shrinkage, and microstructure of both geopolymer composite materials. The positive effect of various artificially prepared aggregates on the engineering properties of thermally-treated alkali-activated slag composite was already presented in the author’s previous works [ 25 , 26 , 27 ]. Chamotte aggregate proved to have a very good stability in the alkaline environment of the slag matrix at very high temperatures.…”

“…A possible way of mitigating thermal shrinkage is through the utilization of thermally-stable aggregate. Quartz and granite have been proven to reduce shrinkage to 1% but the disruptive phase change of quartz at 573 °C limits the working temperature range of such geopolymer composites [ 23 , 24 , 25 ]. It is, therefore, worth using artificial aggregates such as chamotte, crushed porcelain, or cordierite, which are fabricated at a very high temperature and have a low thermal expansion coefficient.…”

Thermomechanical properties of particle-reinforced geopolymer composite with various aggregate gradation of fine ceramic filler

“…Most frequently granular fillers are used for this purpose, such as silica sand [24], calcite and dolomite sand [25], grinded electrical porcelain [26], grinded high-alumina refractory brick [27], aAl 2 O 3 powder [28], a-quartz sand and fine alumina powder [29], chamotte powder [30], kyanite (nesosilicate) aggregates [31], cordierite powder [32], burned clay [33], expanded clay [34], quartz fume [35]. Besides, fiber reinforcement was applied such as SiC fibers [36], basalt fibers [37], also steel or polymer fibers [38].…”

A Review on Durability of Alkali-activated System from Sustainable Construction Materials to Infrastructures

“…For instance, the use of expanded perlite as fine aggregate in the geopolymer mortars resulted in much higher thermal insulation and better fire resistance than silica sand 57 aggregate. A utilization of chamotte as an aggregate in the AAMs composites brought several benefits compared to quartz sand, including the improved mechanical parameters when exposed to elevated 56 temperatures. In addition, aggregate size governs the behaviors of AAMs at elevated temperatures.…”