Doctor of Philosophy
Civil and Environmental Engineering
DR. ERNEST K. YANFUL
Many researchers have validated the phenomenon of internal sulphate attack in ordinary Portland cement (CSA Type GU) based cemented paste tailings containing sulphide minerals and have observed secondary gypsum as an ubiquitous phase in such matrices. The secondary gypsum, due to its soft nature and swelling and expansive properties, is principally considered to be the sole culprit responsible for subsequent internal cracking and strength degradation of sulphidic paste at later stages. High Portland cement cost and its apparent poor performance in the long-term in sulphate rich environments advocate for the formulation of resilient cemented paste tailings using economical but efficient binders.
This study demonstrates the effectual utilization of selective cement kiln dusts (CKDs) in composite binder systems containing ground granulated blast furnace slag (SL), Type C fly ash (FC) and a small amount of Portland cement. The addition of calcium and alkali rich cement kiln dust in Portland cement-slag and Portland cement-fly ash binder systems functioned as an excellent alkaline activator and accelerator in promoting the hydration mechanisms within cemented paste tailings formulations. High alkaline pore solution created by free calcium rich cement kiln dust (CKD) is capable of disintegrating the solid glassy network of supplementary cementing materials to produce reactive silicate and aluminate compounds. Comparative assessment of a range of binder combinations has been carried out for cemented paste tailings formulations using performance assessment indicators such as unconfined compressive strength, saturated hydraulic conductivity, micro-structure, and contaminant containment capability (leachability).
The short-and long-term performance evaluation of the unconfined compressive strength of blended cements in cemented pastes of sulphidic tailings was carried out over a curing period of 480 days. Cement kiln dusts (DA and DH) containing high CaO content (52.4% and 57.8%, respectively) and low Loss on Ignition (LOI), i.e., 5.1% and 4.8%, respectively, performed best in making cemented paste tailings, whereas the paste mixtures containing DB and DL (with very low free lime and very high LOI) as single binders in combination with mine tailings (MT) never hardened. The addition of selective CKD, and SL, and/or FC as partial replacement for Portland cement can improve the performance of the cemented paste tailings containing sulphide minerals and alleviate the strength loss associated with Type GU cement based paste. The behavior is attributed to the latent strength acquisition of SL and FC in the blended cements containing DA and DH as a result of proliferating hydration and augmented pozzolanic reactivity. Previous studies carried out by other researchers on hardened cemented paste tailings specimens containing Portland cement-pozzolans binders confirmed the possibility of the presence of un-reacted or un-hydrated particles of slag and fly ash in such matrices. In the present study, SEM was carried out on cemented paste tailings samples to investigate the effect of using calcium and alkali rich CKD on pozzolanic reactivity of binders containing SL and FC. The effect of incorporation of DA or DH in GU/HS-SL and GU/HS-FC binders was analyzed in high magnification modes. No un-reacted grains of SL and/or FC were found within the crushed surfaces of the matrices. The glassy SL was completely disintegrated by the highly alkaline pore solution when attacked by OH- ions. As well, all the amount of FC used in composite binder system was completely exhausted by the accelerated pozzolanic activity. Concomitantly, the observed microstructure delineation of selected cemented paste tailings samples was found to be well acquiescent with their respective mechanical performance (based on the unconfined compressive strength).
The precipitation of calcium hydroxide in Type GU based cemented paste tailings can be inadequate for total blocking of pores within cemented paste tailings matrix. The enhanced pozzolanic reactivity of SL and FC with additional calcium hydroxide produced by the incorporation of high calcium and alkali rich CKD in cemented tailings matrices generated additional hydration products, favoring enhanced pore refinement and densification, which, in turn, reduced the transportability of fluid through the hardened matrices.
Meticulous geochemical characterization of cemented paste tailings formulations is imperative, particularly when binders replace the traditional Portland cement. The leaching behavior of metals in cemented paste tailings formulations was studied using field mimicking leaching protocols to obtain information on the fixation involving both chemical stabilization and physical encapsulation mechanisms. Leaching test results based on the Synthetic Precipitation Leaching Procedure (US EPA Method 1312) showed that cemented paste tailings formulated with Type GU cement alone are susceptible to contaminant leaching and, therefore, do not eliminate interaction between the monolith and contact waters. On the other hand, optimized cementing blends containing a small amount of Type GU cement and SL activated by DH offer more efficient contaminant fixation capabilities for tailings pastes. Low calcium cement kiln dust (DL) promoted chemical stabilization only within the resultant composites due to lack of cementitious properties and is, therefore, incapable of providing physical encapsulation of the contaminants.
The results advocate the technically beneficial synergy of composite binders containing selective CKDs for paste preparation in contrast with Portland cement alone, Portland cement-slag, and Portland cement-fly ash binder systems in sulphidic mine tailings management. An optimized mix proportion is however required to achieve optimal performance.
Tariq, Amajd, "Synergistic and Environmental Benefits of Using Cement Kiln Dust With Slag and Fly Ash in Cemented Paste Tailings" (2012). Electronic Thesis and Dissertation Repository. 560.