DEGRADATION RESISTANCE OF ALKALI-ACTIVATED BLAST FURNACE SLAG

conference proceedings

English

Alkali-activated blast furnace slag (AABFS) represents an environmentally friendly inorganic binder as an alternative to the Ordinary Portland cement (OPC). Beside the lower carbon footprint and financial expenses AABFS shows superior durability in aggressive environments contrary to the OPC. [1] Chemical degradation of AABFS can be realised by several reaction mechanisms: i) leaching of components of hydration products (e.g. decalcification), ii) formation of new reaction products and iii) substitution of calcium ions in C-A-S-H by various ions. During the service life of construction segments a physical or physical-mechanical degradation processes take place as well. These are realised through volume changes, changes in pore size and distribution or formation of micro or macro cracks. This can negatively influence the overall properties and durability of construction segments. [2, 3]. A higher resistance of AABFS compared to the OPC when exposed to acidic conditions is attributed to the absence of portlandite or incorporation of alumina units into the structure of C-A-S-H gel as a major hydration product. The overall durability is also improved by lower initial C/S molar ration in binder gel. On the other hand, the AABFS is more susceptible to formation of efflorescence due to the high content of alkalis [4] moreover it shows lower resistance to carbonation since the carbonation is realised firstly on portlandite in OPC to form calcium carbonate which block or at least partially blocks the internal pores to retard the further progress of carbonation reactions [5]. The sulphate resistance of the AABFS is significantly dependent on the type of sulphate used. While in the case of sodium sulphate there is no degradation of the material moreover thanks to the post-alkaline activation an increase in utility properties can be observed, in the contrary to the exposure to magnesium sulphate which results in the formation of expansion products, whose have negative impact on performance and overall durability of the system. [6, 7] This study deals with characterization of resistance of alkali-activated blast furnace slag (the 6% Na2O dose of Na2CO3 related to the slag weight was used as an alkaline activator) to various aggressive environments – 5% solutions of MgSO4 and Na2SO4 (sulphate corrosion), 6M NH4NO3 and pH = 3 CH3COOH (decalcification resistance/acid corrosion) and carbonation resistance (carbon dioxide chamber – 70% humidity, 1% CO2). The water storage of samples was used as a reference. Techniques like the determination of flexural and compressive strength, analysis of loss mass or specification of the area with modified pH using the phenolphthalein technique were used for characterization of degradation processes taking place. These tests were designed according to the [3] since there are no established standard testing procedures as for the OPC. No signs of deterioration of mechanical properties were observed for samples stored in carbon dioxide chamber, however the decrease of pH of samples was detected using the phenolphthalein technique. On the other hand, significant loss of residual compressive and flexural strength same as the loss of mass was observed when the AABFS was exposed to the ammonium nitrate or acetic acid. Exposure to the effect of sulphates did not cause almost any changes in the monitored parameters.

Alkali-activated materials, degradation, resistance

HRUBÝ, P.; KALINA, L.; BÍLEK, V.; TOPOLÁŘ, L.; MARKO, M.; HERČÍK, T.

24. 11. 2021

Slovenská chemická knižnica

Bratislava

978-80-8208-064-6

Chémia a technológie pre život

2

https://www.uiam.sk/~oravec/svk/e_zbornik_svk_2021.pdf