Uptake and reversibility of aluminium in calcium silicate hydrates

Abstract

Produktion af Portland Cement (PC) er ansvarligt for en stor del af den menneskeskabte udledning af CO2, derfor er muligheder for at sænke udledningen, eftersøgt i stor stil. En af måderne udledning kan reduceres på, er at substituere dele af cementklinkeren med såkaldte "supplementary cementitious materials" (SCM). SCM indeholder generelt mere silica og aluminiumoxid, og mindre calcium end regulært PC. Dette studie forsøger at fastlægge, hvordan kinetikken og reversibiliteten af aluminium i calcium silikat hydrat (C-S-H) ændres, baseret på dennes sammensætning. Specifikt, så undersøger den hvordan Ca/Si molar forholdet af C-S-H ændrer kinetikken af aluminium optag og egenskaberne af det resulterende materiale, og om aluminium optaget er reversibelt. For at undersøge kinetikken af aluminium optag, skulle 240 prøver af forskellige Ca/Si og Al/Si molar forhold, med varierende hydrationstider, syntetiseres og analyseres. Resultaterne viste langsommere optag af aluminium i lav Ca/Si prøver, i forhold til høj Ca/Si, hvilket indikerer at den delvise substituering af PC klinker med SCM vil retardere optaget af aluminium i C-S-H’et af cement. Derudover øger inkluderingen af aluminium den gennemsnitlige kædelængde (MCL) af C-S-H’et ved at binde brobyggende tetraedre i C-S-H. I Ca/Si ≤ 1.0 og Al/Si ≤ 0.05, bliver alt aluminium optaget i C-S-H. Reversibiliteten af aluminium i C-S-H’et blev testet, ved at syntetisere prøver af calcium aluminium silikat hydrat (C-A-S-H) som blev filtreret og tilføjet til en opløsning uden aluminium. Resultaterne viser at aluminium er reversibelt i lav Ca/Si C-S-H, med en øgende stabilitet af Al for højere Ca/Si C-S-H. Disse resultater antyder at PC, som er delvist substitueret af SCM, vil have højere niveauer af MCL, på grund af inkluderingen af mere aluminium. Da PC systemer er komplekse, betyder det ikke nødvendigvis at aluminium vil være reversible i PC systemer, selvom det måske er muligt i lav Ca/Si C-S-H.

Production of Portland cement (PC) is responsible for a significant amount of anthropogenic CO2 emissions, and ways to lower emissions are highly sought after. One way of reducing emissions is by substituting part of the cement clinker with supplementary cementitious materials (SCMs). SCMs typically contain more silica and alumina and less calcium than ordinary PC, which could change the compositon of hydrated cement and C-S-H. This study aims to determine how the kinetics and reversibility of aluminium uptake in calcium silicate hydrates (C-S-H) changes, based on its composition. Specifically, it investigates how the Ca/Si molar ratio of C-S-H changes the kinetics of aluminium uptake and properties of the resulting solids, as well as whether the aluminium uptake is reversible. In order to investigate the kinetics of aluminium uptake, 240 samples of varying Ca/Si and Al/Si molar ratio with varying hydration time were synthesised and analysed. The results showed slower uptake of aluminium in low Ca/Si samples, compared to high Ca/Si. Additionally, the inclusion of aluminium increases the mean chain length (MCL) of the C-S-H by binding to bridging tetrahedra in C-S-H. In target Ca/Si ≤ 1.0 and Al/Si ≤ 0.05, all the Al goes to C-S-H. Furthermore, the reversibility of the aluminium uptake in C-S-H was tested by synthesising samples of calcium aluminium silicate hydrate (C-A-S-H) that were filtered and added to a solution without aluminium. The results showed that aluminium is reversible in low Ca/Si C-S-H short-term, with an increasing stability of Al in C-S-H with higher Ca/Si. These results suggest that PC which is partially substituted by SCMs will have increased MCL due to the incorporation of more aluminium. As PC systems are complex, the reversibility of aluminium uptake in C-S-H does not necessarily mean that aluminium will be equally reversible in PC systems.

A master thesis from Aalborg University

Uptake and reversibility of aluminium in calcium silicate hydrates