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Fly ash (FA) and Basic oxygen furnace (BOF) slag were used to as additives in the geopolymerisation of gold mine tailings (GMT).The aim of the research was to determine the effects of the two additives on the strength formation and mechanism of metal immobilisation by modified GMT geopolymers. GMT, FA and BOF were mixed, respectively, and made into a paste with the addition of potassium hydroxide (KOH) before curing at various conditions. 50% replacement of GMT in the starting materials gave the highest unconfined compressive strength (UCS). The UCS for BOF-based geopolymer was 21.44 Mega Pascals (MPa), whilst the one for FA-based geopolymer was 12.98 MPa. The BOF-based geopolymer cured at lower temperature (70 °C) as compared to the FA-based geopolymer (90 °C). The optimum KOH concentration was 10 and 15 M for BOF-and FA-based geopolymers, respectively. BOF-based geopolymers resulted in the formation of calcium silicate hydrate (CSH) phases which contributed to higher strength; whereas in FA-based geopolymers, no new structures were formed. BOF-based geopolymers resulted in over 94% iron (Fe) immobilisation, whereas FA-based geopolymers had 76% Fe immobilisation. Fe immobilisation was via incorporation into the CSH or geopolymer structure, whilst other metal immobilisations were thought to be via encapsulation. 12-month static leaching tests showed that the synthesised geopolymers posed insignificant environmental pollution threat for long-term use.

Attapulgite calcined at 973.15K was characterized and utilized as an adsorbent for the removal of heavy metals and neutralization of acid mine drainage (AMD) from a gold mine. Batch adsorption experiments were carried out using a thermostatic shaker. Activated attapulgite showed that it can neutralize AMD as it raised the pH from 2.6 to 7.3 after a residence time of 2 h. Metal ion removal after 2 h was 100% for Cu (II), 99.46% for Fe (II), 96.20% for Co (II), 86.92% for Ni (II) and 71.52% for Mn (II) using a 2.5% w/v activated attapulgite loading. The adsorption best fit the Langmuir isotherm; however, Cu (II), Co (II), and Fe (II) data fit the Freundlich isotherm as well. Calcination at 973.15 K resulted in the reduction of the equilibrium residence time from 4 to 2 h, solid loading reduction from 10 to 2.5% m/v and an increase in maximum adsorption capacity compared with unactivated attapulgite.

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In this study, acidic gold mine tailings were alkali activated using potassium hydroxide (KOH). The effect of potassium silicate (KS) and potassium aluminate (KA) on the strength and durability of the synthesised geopolymers was investigated. An optimisation experimental programme of various conditions was used to get the best geopolymers in terms of strength and metal leachability. A 1·1 mass/mass (m/m) potassium silicate : potassium hydroxide activation solution yielded a geopolymer with an unconfined compressive strength (UCS) of 9·94 MPa after curing for 5 d at 80ºC. A 2·8 m/m potassium aluminate : potassium hydroxide activation solution yielded a geopolymer with a UCS of 18·10 MPa after curing for 5 d at 100ºC. The potassium aluminate-based geopolymer was more durable for use as a backfill material as, after 10 wet and dry cycles, it had a UCS of 4·8 MPa whereas the potassium silicate-based geopolymer had a UCS of 1·23 MPa. The potassium aluminate-based geopolymer resulted in over 85% reduction in leachability of heavy metals from the cured geopolymer. This study showed that potassium aluminate activation of acidic gold mine tailings is an attractive route to stabilise/solidify hazardous material. The use of elevated temperature to achieve high strength for the geopolymer pales in comparison to the energy requirements of cement production.

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