Abstract
The key nanostructural changes occurring in a series of alkali-activated materials (AAM) based on blends of slag and fly ash precursors during exposure to temperatures up to 1000 °C are investigated. The main reaction product in each AAM is a crosslinked sodium- and aluminium-substituted calcium silicate hydrate (C-(N)-A-S-H)-type gel. Increased alkali content promotes the formation of an additional sodium aluminosilicate hydrate (N-A-S-(H)) gel reaction product due to the structural limitations on Al substitution within the C-(N)-A-S-H gel. Heating each AAM to 1000 °C results in the crystallisation of the disordered gels and formation of sodalite, nepheline and wollastonite. Increased formation of N-A-S-(H) reduces binder structural water content after thermal treatment and correlates closely with previous observations of improved strength retention and reduced microcracking in these AAM after heating to 1000 °C. This provides new insight into thermally induced changes to gel atomic structure and thermal durability of C-(N)-A-S-H/N-A-S-H gel blends which are fundamental for the development of new fire-resistant construction materials.
Highlights
Alkali-activated materials (AAM) have shown extensive promise as sustainable alternatives to Portland cement (PC), exhibiting desirable technical and environmental characteristics for a large array of applications [1]
The superior thermal performance of AAM is generally attributed to the differences in the structure of these materials when compared to that of hydrated PC, which rapidly loses strength when exposed to temperatures above 300 °C due to loss of structural water from hydrate phases [7]
Crystalline phases identified in the fly ash are quartz (PDF # 00-001-0649), mullite (PDF # 00-015-0776), hematite (PDF # 00-033-0664), magnetite (PDF # 04-007-1060) and gypsum (PDF # 00-033-0311), while the slag contains gypsum as a grinding aid, as well as an identifiable quantity of tricalcium silicate and tricalcium aluminate (PDF # 01-074-7039), likely to have been introduced as Portland cement contamination during blending, grinding and/or transport of the fly ash and slag
Summary
Alkali-activated materials (AAM) have shown extensive promise as sustainable alternatives to Portland cement (PC), exhibiting desirable technical and environmental characteristics for a large array of applications [1]. Blended slag/fly ash-based AAM can exhibit advantages, in terms of the balance between reaction kinetics and durability, over those produced from sole precursors [14,15,16], but the coexistence of C-(N)A-S-H and N-A-S-(H) gels results in complex thermodynamic and chemical interactions which dictate material properties and performance [17, 18] Coexistence of these gels has been observed in alkali-activated cement based on blast furnace slag [11, 19] and blends of low-Ca and high-Ca precursors [18, 20,21,22]. Despite the extensive focus on the mechanical properties and microstructure of AAM exposed to high temperatures and demanding thermal environments (i.e. those with rapid changes in temperature), little attempt has been made to examine thermally induced changes to gel nanostructure and chemistry These changes are of particular importance as they dictate the structural water content, pore network structure and the thermal performance of these materials. This provides new insight into thermally induced changes to gel atomic structure and thermal durability of C-(N)A-S-H/N-A-S-H gel blends
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