Influence of lime, calcium silicate and portlandite on alkali activation of calcined common clays

Felix Dathe,Vera Strelnikova,Nadja Werling,Katja Emmerich,Frank Dehn

doi:10.1016/j.oceram.2021.100152

Felix Dathe, Vera Strelnikova + Show 3 more

Open Access

https://doi.org/10.1016/j.oceram.2021.100152

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Abstract

The application of calcite rich common clays has marked impact on the formation of alkali-activated binders. Experiments have shown that the carbonate decomposition can be well controlled via the calcination procedure, whereby the majority of the decomposition and the subsequent CO2 release occurs above the calcination temperature of 750 °C. The decarbonation of the calcite can mostly be separated from the dehydroxylation of the layered silicates. Depending on the composition of the raw clay material, the CaCO3 decomposition leads either to the formation of lime or other Ca rich minerals. The mechanical properties of the alkali-activated binders were investigated and despite the very low amounts of layered silicates of the clay raw materials and the high content of unreactive minerals, compressive strengths of above 20 MPa of the mortars could be obtained. The presence of lime in calcined clays up to an adequate amount has a positive effect on post-solidification and the carbonation resistance of the mortars.

Highlights

Geopolymer (GP) and alkali-activated binders (AAB) have attracted a great deal of attention in research as alternative to conventional binding materials [1]
GPs are considered as Ca-poor AABs with a high Al and Si content and whereas products resulting from fly ash with more than 7 wt% of CaO or 10 wt% CaO, respectively, are classified as AAB, a lower calcium content is thought to result in the formation of a geopolymer
For Clay1 and Clay2 the formation of a high temperature phase could be observed at 902 C via differential scanning calorimetry and for the reference clay the formation of spinel phase at high temperatures consisting of Si and Al could be witnessed at 994 C

Summary

Introduction

Geopolymer (GP) and alkali-activated binders (AAB) have attracted a great deal of attention in research as alternative to conventional binding materials [1]. In contrast to the state-of-the-art construction material cement, the CO2 footprint can be tremendously reduced through the application of such alternative binders in the field of civil engineering Both geopolymers and alkali-activated binders can be obtained from condensation reactions of Si and Al-rich precursors in the presence of a base. The transition from a geopolymer to an alkali-activated binder depends strongly on the soluble Ca content of the raw material, since this is crucial for the formation of different amorphous phases, such as N-A-S-H (low Ca content), C-A-S-H, C-(N)-A-S-H, and C–S–H(I) (very high Ca and low Al content) [2]. Further classification criteria are based on the Ca content within the crystallographic structure of the reaction products (

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