Elaboración de instrumentos

Sulphidic cemented paste backfill containing Type GU cement as a sole binder has been established to be vulnerable to internal sulphate attack and subsequent progressive loss of structural integrity under certain conditions. In internal sulphate attack, the deterioration of hardened mortars and concrete is brought about by the action of sulphates present in the original mix in excessive amount, or in those formed from sulphur compounds other than sulphates, also present in the starting material (Skalny et al. 2002).

Several studies (Benzaazoua et al. 1999 and 2002, Hassani et al. 2001, Fall and Benzaazoua 2005, Kesimal et al. 2005, Ouellet et al. 2006, Ercikdi et al. 2009b and 2010a, Ouellet et al. 1998, De Souza et al. 1997) conducted on cemented paste tailings have confirmed that sulphates present in tailings water and those produced by the oxidation of pyrite FeS2(in a basic pH medium created by cement hydration) can react

with free calcium ions produced by the dissolution of unstable portlandite [Ca(OH)2],

giving rise to the precipitation of swelling secondary gypsum (CaSO4•2H2O) and highly expansive ettringite (3CaSO4•3CaO•Al2O3•32H2O). Researchers have confirmed the phenomenon of internal sulphate attack in ordinary Portland cement based sulphidic tailings cemented paste and have observed secondary gypsum as an ubiquitous phase in such matrices. This secondary gypsum can create softening and swelling effects in the cement solidified matrices such as mortars and concretes and can be a possible source of backfill internal cracking and subsequent strength deterioration over time (Cohen and Mather 1991, Tian and Cohen 2000, Mehta 1983).

26 Fall and Benzaazoua (2005) argued that sulphate present in cemented paste tailings significantly influences strength parameters. This effect is intimately related to the sulphate concentration, curing time and the amount and chemical composition of the cement. Bellmann et al. (2006) analyzed the influence of sulphate solution concentration on the formation of gypsum in concretes. The results revealed that portlandite reacted to form gypsum at a minimal sulphate concentration of approximately 1400 ppm (pH~12.5). As well, at moderate sulphate concentrations (up to 1500-3000 ppm), gypsum formation is either not possible or cannot lead to the damage of the cementitious matrix because of the very low super-saturation and swelling pressure. Portland cement alone produces a relatively porous treated waste in stabilization processes (Means et al. 1995). The undesired oxidation of sulphide minerals within tailings before solidification using Portland cement and/or the high porosity within the solidified matrices, such as cemented paste can produce acidity (low pH conditions), metal remobilization, sulphate ions release and dissolution of formed hydrates (Fall and Benzaazoua 2005). At a pH <12, partial or total dissolution of portlandite and decalcification of C-S-H phases giving rise to calcium release from hydrates can be expected. This situation in turn increases the micro and meso-porosity of the cementitious matrix (Belem and Benzaazoua 2008).

Sulphates present in mine tailings pastes perform various functions depending on their concentrations. Such sulphates can contribute to early strength gain by virtue of filling pores with hydrated cement products and through the precipitation of hydrated sulphates along with modification of pore-structure below a concentration of 2000 ppm, However, depending on sulphate concentration, cement proportion, and curing time, the positive

27 effect of strength acquisition due to the presence of sulphates can have a negative effect (Fall and Benzaazoua 2005). According to Benzaazoua et al. (2004a), the inhibition stage of cemented paste likely occurs at a sulphate concentration range of 200-8000 ppm. For sulphate concentration of 8000 to 10,000 ppm, the precipitation of gypsum is possible and can contribute to the strength development within paste matrix. However, if the sulphate contents exceed 10,000 ppm, massive and detrimental precipitation of swelling secondary gypsum may be expected, which can be a source for an internal sulphate attack in cementitious matrix (Belem and Benzaazoua 2008). The generated volume of hydration products can be far in excess of the available pore volume, thereby creating internal stresses that can lead to expansion and, subsequently micro-cracking. According to Divet (1996), the expansive products resulting from the undesired chemical reactions between sulphates and Portland cement hydration can generate internal stresses of 70 to 200 MPadue to crystallization pressure, which can be a source of drastic deterioration in the cemented matrix. These phenomena can culminate in the loss of initially developed strength within cemented paste tailings.

Fall and Pokheral (2010) investigated the coupled effect of sulphate and temperature on the strength development of cemented paste backfill and argued that initial sulphate contents (>15,000 ppm) led to the absorption of a larger amount of sulphate ions by the C-S-H. The experimental results provided strong indications that binding of sulphate to C-S-H can lead to the formation of lower quality C-S-H, which, in turn, results in the decrease in the strength of Portland cement based paste backfill.

28 Additional water is normally required in paste backfill design in order to meet workability and pumpability requirements, therefore, the volumetric water content of paste backfill is always far in excess of the binder hydration requirements (Belem and Benzaazoua 2008). The additional water in paste backfill design can be either tailings process water, lake water, or municipal water. The sulphate salts content and pH of process water within tailings and additional water required to make desired consistency are important for paste backfill strength acquisition, durability and stability in the long- term. Acidic water and sulphate salts can also attack cementitious bonds within the paste backfill composite, leading to loss of strength, durability, and stability (Lawrence 1992, Wang and Villaescusa 2001, Benzaazoua et al. 2002 and 2004a, Fall and Benzaazoua 2005, Fall and Pokharel 2010). This phenomenon can be particularly more pronounced in cemented paste tailings due to its low cement content and high concentration of sulphates. The relatively low cement content in mine tailings paste containing high sulphur concentration causes significant dilution of the binder within cemented paste matrix, which may lead to (a) vulnerable pH buffering initially fixed by portlandite (~12.5), and (b) development of cohesion in the cemented matrix due to entanglement of hydrated phases as a result of grain residue in the tailings-binder mixture. Such conditions can result in progressive decrease in pH accompanied by dissolution of hydrates, i.e. release of OH- ions and further formation of metal hydroxides (Benzaazoua

et al. 1999).

The mining industry faces the need to appraise the cost benefits of binding agent addition to upgrade the mechanical properties of cemented backfill in underground mining

29 operations to cope with ground stability issues and to enhance productivity (Scoble and Piciacchia 1986). Consequently, there exists sufficient incentive to curtail Portland cement consumption without impairing the mine tailings paste performance. The high cost of portland cement as well as its alleged poor performance as a sole binder in the long term and today’s robust engineering design requirements for cemented paste tailings has prompted users to appraise less expensive and technically efficient subsitutes for mine tailings paste formulations.