Strength and mineralogical developments in magnesia-GGBS stabilised biochar-sequestered acid sulphate soils
Creators: name: Le, Xue
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: Acid sulphate soil (ASS) which is extensively distributed along the coastal areas of Australia, is considered problematic in the context of geotechnical engineering, primarily due to the low shear strength and potential of sulphate attack. A search of literature shows that there is no cost-effective and environmentally-friendly ground improvement technique for treating ASS. Recently, it has been reported that soil mixing with reactive magnesia-activated ground granulated blastfurnace slag (GGBS) improves both strength and sulphate resistance of non-sulphate-bearing soils. This finding is of considerable environmental significance as both MgO and GGBS are carbon-efficient materials, but their effectiveness on stabilising ASS remains unknown. Moreover, the concept of imparting carbon sequestration with biochar into urban soils has to be considered seriously, which opens a possibility to significantly offset the construction-induced CO2 emissions. Nevertheless, the viability of incorporating biochar sequestration into geotechnical engineering has not been investigated yet. The primary objectives of this study are to assess the effectiveness of using MgO-activated GGBS to stabilise ASS and to investigate, for the first time, the feasibility of sequestering biochar into chemically stabilised ASS. <br> <br> To fulfil the objectives of this study, the experimental program was divided into three stages. In Stage 1, two categories of ASS (i.e. passive ASS, PASS, and active ASS, AASS) were treated with reactive MgO at 5 to 30% and GGBS at 15 or 20% (% by the weight of dry soils) to primarily assess the influence of the category of ASS on the strength development. In Stage 2, PASS was mixed with MgO at 5 to 15% and GGBS at 10 to 20% to determine the optimum MgO and GGBS contents for the strength development. In Stage 3, PASS was mixed with MgO or lime at 5 to 15% and 20% GGBS to compare the performance of GGBS activation with different alkalis (i.e. MgO and lime). To investigate the feasibility of incorporating biochar sequestration into soil mixing, a biochar derived from timber railway sleepers was added into PASS at 10% in Stage 2 and 3. The treated ASS was cured in a humidity chamber for up to 12 months. A range of tests, including pH test, particle density test, unconfined compression test, scanning electron microscopy and X-ray diffraction analysis, were carried out to investigate the engineering properties, mechanical properties, mineralogical evolvement and microstructural development of the MgO/lime-GGBS treated biochar-sequestered ASS. <br> <br> The results of this study indicated that reactive MgO-activated GGBS could effectively improve the strength and stiffness of PASS; however, it had limited efficacy for treating AASS, owing to the moderate alkalinity of MgO and strong acidity of AASS. It was also found that the optimum MgO-to-GGBS ratio for improving the mechanical properties of PASS was 1:4. Increasing this ratio by either elevating the MgO content or decreasing the GGBS content tended to result in a decrease in the strength and stiffness. In addition, an empirical relationship was developed which may be used to quantify the effects of curing time, MgO content and GGBS content on the strength and stiffness of the treated PASS. More specifically, the unconfined compressive strength (UCS) of the MgO-GGBS treated PASS can be estimated with the empirical relationship, q<sub>c</sub>(kPa)=t/(mt+c) , where q<sub>c</sub> is the UCS, t is the curing period in days, and m and c are constants that characterise the strength development. The constants m and c are governed by the GGBS and MgO content, the relationship of which can be expressed as m=0.007931×(5.849×10<sup>-6</sup>)<sup>S</sup> and c=0.2913×223285<sup>M</sup>×(4.8935×10<sup>-9</sup>)<sup>S</sup>, where M and S are the MgO content and GGBS content, respectively. Similarly, the stiffness, E<sub>50</sub>, of the MgO-GGBS treated PASS can be estimated with the empirical function, E<sub>50</sub>(MPa)=t/(nt+d) , where t is the curing period in days, and n and d are stiffness characteristic constants. The constants n and d are also governed by the GGBS and MgO content. As such, they can be characterised with the empirical functions, n=0.023538×(5.185×10<sup>-4</sup>)<sup>S</sup> and d=2.5126×19773<sup>M</sup>×(7.7506×10<sup>-9</sup>)<sup>S</sup>, where M and S are the MgO content and GGBS content, respectively. <br> <br> In terms of the performance of MgO- or lime-activated GGBS, it was found that, at a low alkali content (5%), MgO-GGBS treated PASS yielded higher 28-day strength while at higher alkali contents (10 to 15%), lime-GGBS stabilised PASS showed greater strength in both short and long terms. The primary hydration product in both MgO- and lime-GGBS treated PASS was calcium silicate hydrate-like phases; the minor hydration products in MgO-GGBS treated PASS were ettringite and hydrotalcite while the minor hydration products in lime-GGBS treated PASS were ettringite and a hydrocalumite-like phase, C<sub>4</sub>AH<sub>13</sub>. The experimental results also demonstrated that inclusion of biochar in ASS decreased both the strength and stiffness; however, this adverse impact may be compensated by the environmental benefits brought by biochar.