Tasmim, R 2017, 'Acid sulfate soil resistance of mortar and concretes using modified bauxite refinery residue', MSc thesis, Southern Cross University, Lismore, NSW.
Copyright R Tasmim 2017
Acid sulfate soils (ASS) occur naturally and are soils and sediments containing commonly iron sulfides, mostly pyrite. ASS occur mostly, but not always in low-lying coastal areas (≤5m of sea level) where significant concrete infrastructure such as bridges, roads and port facilities exists. The sulfates produced (particularly sulphuric acid) as disturbed ASS oxidise on from air exposure, reacts with concrete often causing premature failures and reduced service life through chemical expansion, fracturing and spalling. In this study Modified Bauxite Refinery Residue (MBRR) are proposed as a part natural sand replacement in mortar and concrete mixes to improve acid sulfate resistance; observed through physical and strength changes under different sulfate solutions.
Mortar and concrete blocks were cast having 0%, 5%, 10%, 15% and 20% MBRR (by cement wt.) as a sand replacement. Compressive strength tests at 0, 56 and 180days (after 28days curing) under eight different sulfate solutions of: FeSO4, Al2(SO4)3, FeSO4+H2SO4, Al2(SO4)3+H2SO4, MgSO4+FeSO4, MgSO4+Al2(SO4)3, Na2SO4+FeSO4 & Na2SO4+Al2(SO4)3, and H2SO4, Na2SO4, and MgSO4 solutions from the literature. The findings of this study confirm that addition of MBRR in mortars substantially increased test-block compressive strengths in sulfate solutions compared to controls. Compressive strength Increases ranged from 5%-92% for mortar over control specimens, and in addition indicates a descending order of solution aggression of FeSO4+H2SO4, >Al2(SO4)3+MgSO4, >1%H2SO4, >10%Na2SO4, >FeSO4, >FeSO4+MgSO4, ≈Al2(SO4)3+Na2SO4, ≈5% Na2SO4, >Al2(SO4)3+H2SO4, >MgSO4, ≈Al2(SO4)3, and ≈FeSO4+Na2SO4. Moreover, data suggest that an addition rate between 10-15% is most effective, although at times 20% additions were preferable. Furthermore, neutron imaging, and SEM analyses indicate that the main mode of action and increased sulphate resistance is generated through a reduced permeability, thereby reducing sulphate penetration within the matrix. Laser ablation-ICP-MS data also indicate that with increased MBRR sand replacement percentages had reduced sulphate penetration depths. Concrete specimens, showed similar (if somewhat reduced results) to the mortar specimens, with between a 4% to 22% improvement in compressive strengths (except 1 example) compared to 180-days sulphate-solution exposed controls.