Abstract
Alkali activated foams have been extensively studied in recent years, due to their high performance and low environmental footprint compared to foams produced via other methods. Three types of fly ash differing in chemical and mineralogical composition and specific surface, were used to synthesise alkali activated (AA) foams. Sodium Perborate Monohydrate (SPM) was added as a foaming agent and sodium dodecyl sulphate (SDS) as a stabilizing agent. Foams were characterized at room temperature and after exposure to an elevated temperature (1000 oC). Densities from 1.2 down to 0.3 g/cm3 were obtained, depending on the type of fly ash and quantity of foaming agent added. Correspondingly, compressive strength ranged from 1 to 6 MPa. After firing to 1000 oC, the density of samples prepared using fly ash with the highest SiO2 and Al2O3 content, i.e RI fly ash, remained approximately the same, while the compressive strength increased on average by 50 %. In the other two types of fly ash the density increased slightly after firing, due to significant shrinkage, and compressive strength increased by as much as 800 %. XRD analysis confirmed the occurrence of a crystallisation process after firing to 1000 oC, which resulted in newly formed crystal phases, including nepheline, sodalite, tridymite and gehlenite.
Highlights
Alkali-activated (AA) materials have been widely investigated in recent years (Provis and Bernal, 2014; Provis et al, 2015; Luukkonen et al, 2018; Provis, 2018) within the context of the construction and building industry
Results from other researchers to date have shown that the stability of foams at high temperatures is strongly affected by the composition of raw materials, when fly ash is used (Martin et al, 2015) and when other additives such as metakaolin and slag are present in the AA material (Mierzwinski et al, 2019; Tran et al, 2019)
Martin et al (2015) studied AA material from 25 to 600°C and their findings showed that the increase in temperature led to an increased compressive strength, increased pore size distribution, and the presence of hydroxysodalite, and that the dimensional stability of the material was affected at temperatures over 1,000°C
Summary
Alkali-activated (AA) materials have been widely investigated in recent years (Provis and Bernal, 2014; Provis et al, 2015; Luukkonen et al, 2018; Provis, 2018) within the context of the construction and building industry. Results from other researchers to date have shown that the stability of foams at high temperatures (especially with regard to shrinkage) is strongly affected by the composition of raw materials, when fly ash is used (Martin et al, 2015) and when other additives such as metakaolin and slag are present in the AA material (Mierzwinski et al, 2019; Tran et al, 2019). According to Chen-Yong et al (2017), the mechanical strength of foamed AA materials (class F fly ash with hydrogen peroxide) deteriorated to 3 MPa at 400°C but increased up to 11 MPa at 800°C, due to the formation of crystalline phases (nepheline, anorthite, and cristobalite) at higher temperatures, which potentially act as fillers to reinforce the matrix
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