Acid degradation of alkali-activated materials

Acid degradation of alkali-activated materials

Acid corrosion of concrete sewerage pipes arouses both industry and academy concern in recent years due to the high cost of rehabilitation and replacement. Alkali activated materials, also named geopolymers, are recommended as potential alternative materials to Portland cement (PC)-based concrete in sewer environments due to their better resistance to the acid solution.

The reaction products, pore structure, sample curing conditions and acid environment conditions are the influencing parameters that determine the acid resistance of geopolymers. Particularly, the reaction products of geopolymers, i.e., calcium aluminosilicate hydrate (C-A-S-H) gel, calcium (alkali) aluminosilicate hydrate (C-(N)-A-S-H) gel and sodium aluminosilicate hydrate (N-A-S-(H)) gel determine the performance of geopolymers under acidic conditions. However, geopolymers use industrial wastes as raw materials; variations in chemical compositions could induce significant changes in the reaction products and microstructure of geopolymers, leading to different engineering properties and acid resistance behaviour.

The ultimate goal of this research is to investigate the acid degradation mechanism of alkali-activated materials. This lies on a basis of a full understanding of the degradation mechanism of various geopolymers phases under acidic conditions. In this thesis study, instead of using industrial by-products as raw materials, pure precursors were synthesized by organic polymeric steric entrapment solution-polymerisation route. The obtained stoichiometrically controlled powders were activated by alkaline activators to produce ‘pure’ reaction productions, e.g., N-A-S-(H) gel and C-(N)-A-S-H gel, or blends of C-(N)-A-S-H and N-A-S-(H) gels, with designed Al/Si, Na/Al and Ca/Si ratios. The acid resistance of different reaction products based on the pure precursors was evaluated. The acid degradation of alkali-activated materials derived from fly ash and blast furnace slag were investigated. The innovation and significant contribution to geopolymer chemistry can be summarized as following:
(1) The synthesized precursors used stoichiometrically controlled routine showed similar chemical properties as the industrial precursors from the TGA, XRD and FTIR results. The non-calcium precursors were mainly amorphous with a higher surface area and higher hygroscopicity. The calcium promoted the formation of crystalline phases. The synthesized routines could be an effective method for the intrinsic investigation of the acid degradation of AAMs.
(2) The dealumination dominated the microstructural change of N-A-S-H gel under the sulphuric acid attack, and the dealumination had minor effect on the bulk structure integrity. The C-(N)-A-S-H gel showed both dealumination and decalcification, together with significant and rapid loss of sodium and slow loss of silicon. The dealumination and the loss of sodium continued after decalcification and desiliconization. The Si/Al ratio of non-crosslinked C-(N)-A-S-H gel decreased, while the Si/Al ratio of N-A-S-H gel increased after exposure to sulphuric acid. The silicate polymerization degree (Q4 concentration) increased, especially for C-(N)-A-S-H gel.
(3) Low calcium samples showed a higher leaching amount of aluminium than high calcium samples, indicating that the calcium content in the mixture promoted the dealumination resistance when exposed to sulphuric acid. The formation of additional calcium-containing products in Ca-rich, Al-rich sample was decomposed prior to C-(N)-A-S-H/N-A-S-H gels. The Si-O bond strength in N-A-S-H gel appeared weaker than that in the C-(N)-A-S-H gel, due to the calcium prevented the leaching of silicon from samples. The silicon chains length of the C-(N)-A-S-H gel increased with increased Si/Al ratio. However, the chain length of the N-A-S-H gel increased with reduced Si/Al ratio.
(4) The AAMs derived from fly ash shows more obvious mass loss and volume loss than that derived from slag under sulphuric acid attack, and the lower initial compressive strength was difficult to meet the practical engineering needs. For slag based AAMs, the increased SiO2 content could greatly improve the initial compressive strength but had less effect on the residual strength under sulphuric acid attack. The hydration products from the paste reacted with sulphuric acid to form expansive calcium sulphates, mainly existing in the weak area between the slurry and fine aggregate, and the expansion stress caused flaking. For low-Ca AAMs derived from fly ash, minor sulphur intruded into the sample, while the high-Ca AAMs derived from slag attracted more sulphur. The higher calcium content in AAMs attracts more sulphur into the sample due to the high ionic force between Ca2+ and SO42-.

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