1 Introduction

Several world regions, especially arid and semi-arid regions, are especially prone to comparatively large percentages of expansive soil compositions (Schanz and Elsawy 2015). The distinguishing feature of expansive soils is their ability to swell and shrink cyclically due to changes in the soil moisture content caused by changing environmental conditions. The expansion or shrinkage ability of the soil depends upon the clay mineralogy (Grim 1968; Cherian and Arnepalli 2015), particle composition and arrangement (Snethen et al. 1977), moisture content (MC), reduction of overburden stress and the presence of cations such as Na+, Ca2+, Mg2+ and K+, which can result in the accumulation of large quantities of water in between the clay particles, causing the closely packed clay particles to drift apart (Mitchell and Raad 1973; Zhao et al. 2015).

There are various methods available to identify the shrink-swell potential of any specific type of soil, and the validity of the test results is largely based on the efficiency of the testing method. Similarly, there are several soil stabilisation approaches for countering the hazards presented by these soils, such as chemical, electrical, mechanical and thermal stabilisation (Hausmann 1990; Nicholson 2014). Conventional methods for the improvement of soil texture, plasticity and strength incorporate mechanical stabilisation through mixing of distinctive soils, surcharge loading, compaction-controlled replacement of soil and pre-wetting. Another technique is chemical stabilisation through the induction of particular proportions of additives like lime, fly ash, slag, cement and/or gypsum.

Chemical or additive stabilisation is one of the most commonly applied soil stabilisation methods. The treatment can be aimed at increasing soil particle size, reducing the plasticity index, enhancing strength, and decreasing the shrink-swell potential or cementation. It involves introducing one or more chemical compounds into the soil mass. Conventional compounds like cement and unconventional or recycled compounds such as slag, fly ash and lime have been effectively utilised for soil stabilisation (Phani Kumar and Sharma 2004; Al-Rawas et al. 2005; Neeraja and Rao Narsimha 2010; Yadu and Tripathi 2013; Khemissa and Mahamedi 2014; Ortega-López et al. 2014; Thyagaraj and Zodinsanga 2014).

The modernisation of architectural practices and industrialisation has led to large quantities of debris generated from the demolition of structures. This construction waste (CW) forms a major component of the solid waste in many countries, with only marginal quantities being utilised in backfilling on construction sites and the rest being mostly dumped on already scarce landfill sites. Therefore, construction waste causes land, resource and material depletion and deterioration (Behera et al. 2014; Wang et al. 2014). The Australian construction industry alone produces approximately 38 % waste for landfills each year (Schanz and Elsawy 2015).

Furthermore, many countries have problems with illegal or environmentally unsafe disposal of industrial by-products, for instance blast furnace slag. Globally growing concern over environmental safety has driven researchers worldwide to find more sustainable solutions to these problems and reuse waste materials. Similarly, researchers have also worked to find alternative materials for achieving soil stabilisation utilising several industrial by-products (Al-Malack et al. 2014; Amu et al. 2005; McCarthy et al. 2014).

Considering the huge imprint of construction activities on the environment (Horvath 2004), this research work is focused on the use of recycled materials for soil stabilisation. The techniques of both chemical/additive stabilisation and mechanical composite stabilisation have been employed with GGBFS and construction waste as the soil stabilisers. Recycled construction waste (CW) has been preferred as the production of new aggregates increases CO2 imprint (Limbachiya et al. 2012). These materials have already been used for reinforcement in various developed countries around Europe (Kasai 1989; Hansen 1986; Khalaf and DeVenny 2004; Poon and Chan 2007; Chakradhara Rao et al. 2011). The use of CW can not only reduce the carbon footprint of soil stabilisation but can produce better results in improving the bearing capacity of soft soils. Furthermore, the cost of procuring recycled, screened and sieved construction waste is extremely low in developed countries like Australia, for example in Western Australia, the screened construction waste only costs 8–10 AUD per tonne, and Commonwealth government provides incentives for greener construction practices like reuse of construction and demolition waste (Hyder Consulting 2011). This study had, therefore, been conducted to investigate the feasibility of utilising construction waste and GGBFS for the stabilisation of clay.

2 Materials and Methodology

2.1 Test Design

This study employed the techniques of both mechanical stabilisation through the induction of recycled construction waste, and chemical stabilisation by introducing GGBFS, in controlled proportions. The flowchart of the research work is presented in Fig. 1 and illustrates the soil treatment process.

Fig. 1
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Outline of research tests

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2.2 Material and Sample Preparation

2.2.1 Clay

The clay selected for the purpose of this test was commercially manufactured bentonite procured from Sibelco Australia Limited. Other researchers have also used sodium bentonite in their investigation of expansive soils (Abdelrahman et al. 2013). The bentonite used was off-white in colour and had a fine powdered composition, as illustrated by Fig. 2a.

Fig. 2
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Test material images a bentonite clay, b construction waste, and c GGBFS

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Bentonite has a specific gravity of 2.75 and is predominately composed of the smectite group of clay minerals (more than 74 %). Bentonite contains primarily fine Na-montmorillonite particles (surface area of 800 m2/kg) (National Research Council 2006). The particle size distribution of the bentonite is depicted in Fig. 3.

Fig. 3
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Particle size distribution of the materials

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2.2.2 Construction and Demolition Waste

Construction waste (CW) was obtained from Capital Recycling Australia and mainly consisted of crushed concrete, tiles and masonry works that had been sieved by the supplier to remove the wood and glass residues. The CW was stored in a reinforced plastic container. It was oven dried for 24 h at a temperature of 105 °C and sieved. The particles passing through sieve 4.75 mm and retained on sieve 2.36 mm were selected for the reason of this study. The particle size distribution of construction waste is presented in Fig. 3. The texture and element of the construction waste varied widely due to variations in the source materials, as shown in Fig. 2b.

2.2.3 Slag

The slag utilised for this study was ground granulated blast furnace slag (GGBFS) provided commercially by BGC Cement Australia Pty Limited. It generally has a glassy, fine and crystal-like texture as illustrated in Fig. 2c. The surface area of GGBFS is typically >350 m2/kg (Siddique and Khan 2011; Al-Rawas et al. 2005). The particle size distribution of GGBFS is shown in Fig. 3.

GGBFS has mildly cementitious properties and is used with cement or lime (Higgins 2005). However, due to the hydraulic nature of the slag and the considerable presence of calcium, it may harden and hydrate without any additional material in the presence of water, even though the hydration process is considerably slower and may take considerable time to complete (Song et al. 2000). The chemical composition of GGBFS is provided in Fig. 4.

Fig. 4
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Chemical composition of GGBFS provided by Cement Australia Pty Limited

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2.2.4 Sample Preparation

Different percentages of the two additives, GGBFS and CW, were used to separately investigate the influence of each additive on the UCS over a number of different curing periods. The sample combinations and the identified curing period are listed in detail in Table 1.

Table 1 Test specimen matrix
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It should be noted at this stage that this research is part of an ongoing research on use of recycled materials for soil stabilisation and is intended to serve as foundation for further research using higher percentages of both additives. Lower CW proportions were chosen to prevent segregation of CW particles within the specimen matrix as the research focuses on the interlocking of construction grains with bentonite-GBBFS particles instead of other CW particles as the irregular shape may cause formation of small waterways within the specimen, thereby causing uneven water distribution. The optimum moisture content (OMC) by weight of the dry specimen was used for all the specimens to deduce comparative data for assessing the effects of the aforementioned two controlling factors. A manual mixing technique was adapted to mix the additives into the soil mass. The mixtures were then prepared in a large tray by constantly spraying water at amounts calculated for the OMC through a spray bottle and mixing with the help of spatula till a homogeneous appearance was attained. Then the stabilised clay mixes were compacted in standard cylindrical steel moulds to produce specimens with dimensions of 10 cm × 20 cm under the standard compaction effort following Standards Australia AS 5101.4 (2008). The extracted specimens were then wrapped with thick plastic sheets and placed in a storage room for the respective curing periods to limit the effects of any external factors on the specimens. Bearing in mind that specimens were prepared by tamping at OMC; therefore the specimens were not saturated during UCS testing. Moreover, the UCS test has a limitation dealing with a fully saturated sample.

2.3 Laboratory Tests

2.3.1 Compressive Strength

The laboratory tests involved the investigation of the effects of stabilisers, GGBFS and construction waste, on the engineering behaviour of stabilised bentonite clay; for example, the unconfined compressive strength (UCS) (see Fig. 5). The UCS tests were performed as per the Standards Australia AS 5101.4 (2008) specifications. The device used for this purpose is the GCTS Stress-Path Soil Triaxial System (STX-300) as shown in Fig. 5d. It has dedicated software in the attached computer interface, enabling control of the test parameters and results generation. An axial load was applied at the continuous rate of 1.0 ± 0.1 mm/min and the testing was stopped once the deformation had been recorded.

Fig. 5
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UCS testing a prepared stabilised mixture, b specimen compaction, c stabilised specimen, and d STX-300 UCS test device

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Figure 6 illustrates the unconfined compression of a typical test specimen. Once the load was applied, the specimen underwent deformation. A reduction in the length of the specimen was observed, whereas the cross-sectional area increased from ‘A’ to ‘A/’ under the influence of the applied compressive loading and the length changes from ‘L’ to ‘Lo’ after the ΔL change. The compressive strength of the specimen is associated with two parameters; the applied normal or axial compressive loading, ‘P’, and the surface area of the specimen where the load is being applied, which is dependent upon the diameter of the specimen.

Fig. 6
figure 6

Unconfined compression testing of soil specimen

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2.3.2 Microanalysis

Microanalysis was performed to identify any changes in the elemental composition and physical appearance of the bentonite after the action of the stabilisers. X-ray diffraction (XRD) graphs and scanning electron microscope (SEM) images were developed. The elemental distribution, morphology and arrangement at the microscale were analysed using a Zeiss Evo 40XVP SEM analysis device. This is also capable of performing energy dispersive spectrometer (EDS) analysis, among other functions. It is provided with a dedicated computer and Oxford X-ray system controlled by INCA software. An electron beam with an acceleration voltage of 20 kV and a 100 µA beam current was used with a 450 nm spot size and 8.5 mm working distance. X-ray diffraction quantitative analyses were carried out to determine the mineralogical characteristics of the test specimens using a Bruker-AXS D8 Advance Powder Diffractometer with Cu-Ka X-ray radiation beam (λ = 0.15404 nm, voltage = 40 kV, current = 40 mA). 0.02◦/s was set as the scanning rate whereas the 2-theta data was collected in the 7.5°–90° area.

3 Results and Discussion

3.1 Material Microstructural and EDS Characterisation

3.1.1 Bentonite Clay

Figure 7 shows the SEM micromorphology of pure bentonite clay.

Fig. 7
figure 7

Bentonite clay a SEM micrograph, and b EDS spectra

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The bentonite particles have amorphous crystallography as illustrated in Fig. 7a. The EDS spectra and results of the quantitative analysis of the bentonite particles are shown in Fig. 7b and Table 2.

Table 2 Quantitative chemical composition of bentonite clay
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Silicon and oxygen were observed to be the most dominant elements, followed by aluminium, sodium, titanium, magnesium, iron and calcium. The results indicate the presence of the montmorillonite-beidellite series (Deer et al. 2013).

3.1.2 GGBFS

Figure 8 displays the SEM micrograph and EDS spectra of the GGBFS used in this study.

Fig. 8
figure 8

GGBFS a SEM micrograph, and b EDS spectra

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As can be observed from the figure, the particles showed a definitive crystalline morphology. However, there was little uniformity in the shapes of the crystals. The observed particle sizes showed a varied size range of 30–45 μm.

The EDS spectra and results of the quantitative analysis of the GGBFS particles are summarised in Fig. 8b and Table 3. The results indicate a strong presence of oxygen, with SiO2 and MgO as the leading chemical compounds followed by aluminium, ferric and calcium oxides.

Table 3 Quantitative chemical composition of GGBFS
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3.1.3 Construction Waste

Figure 9 illustrates the micromorphology of the construction waste particles. As can be seen, the construction waste particles have irregular or lenticular shapes (Fig. 9a). Fissures or vesicles and cracks were often found on the surfaces as is quite common with gravels and building aggregates, and this may have aided in the interlocking mechanism between the bentonite and construction waste particles.

Fig. 9
figure 9

Construction waste a SEM micrograph, and b EDS spectra

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The EDS spectra and quantitative data of the selected construction waste grains are displayed in Fig. 9b and Table 4. Since the construction waste contains a variety of materials such as crushed bricks, rubble, mortar etc., the elemental distribution may vary between the grains.

Table 4 Quantitative chemical composition of construction waste
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As the source of the grain is most likely crushed bricks, the quantitative chemical distribution indicates that these grains may belong to the large mineral family of plagioclase feldspars (Deer et al. 2013).

3.2 Standard Compaction Test

Bentonite was subjected to a standard compaction effort of 596 kJ/m3 as per Standards Australia AS 1289.5.4.2 (2007) to calculate the optimum moisture content (OMC) and the corresponding maximum dry density (\(\gamma_{{d_{max} }}\)). A compaction curve was plotted after a series of tests and the OMC and \(\gamma_{{d_{max} }}\) were respectively obtained as 43 % and 1.08 g/cm3.

3.3 Unconfined Compressive Strength

Unconfined compressive strength is essentially a parameter of stabilisation process efficiency which is defined as the maximum unit stress calculated after subjecting the stabilised specimens to a monotonic axial load. The deformation was mainly observed through the formation of minor cracks on the outer shell of the specimen which widened until specimen failure was observed. Figure 10 shows the failure mechanism of the test specimens. This phenomenon is a common failure mechanism of ductile materials during unconfined compression.

Fig. 10
figure 10

Specimen failure mechanism a undeformed specimen, b deformed specimen

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A strength development index (SDI) was introduced as a non-dimensional parameter to easily quantify the effect of each of the controlling factors, i.e. the effect of the percentage of each additive and the curing time on the UCS of the stabilised soil. It is defined as:

$$SDI_{{n_{curing, \,\% \,of\,additives} }} = \frac{UCS \,of\,the\,stabilised\,soil\,specimen}{UCS\,of\,the\,unstabilised\,soil\,specimen} \ge 1$$

where ‘n’ corresponds to either of the controlling factors, representing either the percentage of the two stabilising additives in the mixture or the number of days the specimen has been subjected to the curing conditions. The lower limit of ‘1’ indicates that the addition of any admixture should not result in a decrement in the UCS. A similar parameter has also been proposed by Saride et al. (2013).

3.3.1 Initial Strength of the Untreated Specimen

The stress–strain relationship of pure bentonite is illustrated in Fig. 11 for the different curing periods investigated in this research. Test results suggest that although both specimens showed a slight variation in the failure strain, little or no strength development was observed for the unstabilised soil specimens for different curing periods. The maximum compressive strength of the untreated bentonite was found to be 76.71 kPa.

Fig. 11
figure 11

Stress-strain curves of pure bentonite for 7 and 28 day curing periods

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3.3.2 Effect of Curing Time on Stabilised Compacted Soil

The curing time of the additive-stabilised bentonite specimens was among one of the primary control factors. The evolution of the unconfined strength of the stabilised specimens throughout the final curing period of 28 days is illustrated in Figs. 12, 13 and 14 for different proportions of both additives.

Fig. 12
figure 12

Evolution of UCS with curing time for 2–5 % slag and 10 % construction waste

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Fig. 13
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Evolution of UCS with curing time for 2–5 % slag and 15 % construction waste

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Fig. 14
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Evolution of UCS with curing time for 2–5 % slag and 20 % construction waste

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The UCS results for the construction waste and bentonite blended specimens with increasing percentages of slag show the development of strength with curing. The effect was more noticeable in the initial periods of curing with all of the specimens gaining at least 75 % of the strength increment by the 14th day of curing. Although further curing resulted in an increase in strength values, the effect was not as prominent as for the 14 day curing period. The effect of curing became more noticeable as the percentage of additives, specifically GGBFS, was increased. This might have been due to the cementitious nature of the slag and its slower hydration reaction. Even though GGBFS reacts with water to show hydration or pozzolanic behaviour, the process is fairly negligible at the lower slag percentages. For the specimen S1G2, the UCS only showed an increment of approximately 2.79 % between the 1st and 3rd day of sample curing. However, specimen S1G4 exhibited strength improvement of 3.97 % and S1G5 corresponded to UCS development of 5.03 %, between the same curing days. Further curing resulted in the improvement of the UCS value and optimum values were reached after 28 days of curing for all specimen, therefore, showing inclination in the UCS versus curing period graphs for all test specimens. As shown in Fig. 12, for specimen S2G3, the 28th day cured specimen showed a UCS value of approximately 160.65 kPa which was 46.98 % higher than that noted after 1 day of specimen curing. Although the hydration reaction of the stabilising GGBFS might have continued after the 28th day of curing (Kim et al. 2011), the construction waste grains tend to produce little effect with each successive curing day, and may have showed a negligible increase in the compressive strength of the stabilised mixture over time.

3.3.3 Effect of Additives

Each of the additives produced different effects on the UCS of the stabilised mix for the same curing period. The effect of each additive is discussed separately.

3.3.3.1 Effect of Slag

It was observed that for all combinations of additives, the maximum strength was achieved in the 28 day curing period. This finding shows that the cementitious nature of GGBFS plays a vital role in the development of compressive strength. It can be seen in Fig. 15 that the addition of GGBFS alone has shown increasing trends of the compressive strength of the bentonite.

Fig. 15
figure 15

Evolution of UCS with GGBFS addition with different construction waste percentages over a 28 day curing period

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It was also observed that the increase in UCS value with the increment in the proportion of GGBFS only grew by 15 % when the percentage of GGBFS was increased from 2 to 3 % for 28 days of curing, SDI28 of 1.92 to SDI28 of 2.07 respectively for the 10 % proportion of the construction waste. Conversely, a higher increment in the strength was observed when the percentage of slag was further increased. The SDI28 value rose by a factor of 0.19, corresponding to a 19.27 % rise when the GGBFS percentage was increased from 3 to 4 % for a construction waste content of 10 %. Furthermore, an increase of 0.58 was documented in SDI28, being a sharp 58.16 % rise when the GGBFS proportion was further increased to 5 % for the same percentage of construction waste. One of the reasons behind this observation could be the unavailability of sufficient slag particles within the specimen matrices for the purpose of hydration to form stronger structural bonds in form of silicates or cementitious hydration compounds such as portlandite. The maximum increase after 1 day of curing was observed for the specimen G5S3 which had an SDI1 of 1.71. However, further curing resulted in a sharper increase in the compressive strength, where a maximum strength of 236.93 kPa was observed for the specimen G5S3 (SDI28 = 3.09).

3.3.3.2 Effect of Construction Waste

Figure 16 shows the development of compressive strength with increasing percentages of the selected gradation of construction waste over a 28 day curing period.

Fig. 16
figure 16

Evolution of the UCS of construction waste for different GGBFS concentrations after 28 days of curing

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The increase in the UCS of the stabilised bentonite clay specimens was observed to be directly correlated with the proportion of the demolition and construction waste in the mixture. After the first day of curing (Fig. 13), the UCS of the stabilised specimen with the least amount of slag, i.e. 2 %, was seen to grow by an SDI factor of 0.03 from SDI2 % GGBFS + 10 % CW = 1.31, corresponding to a strength of 100.82 kPa, to SDI2 % GGBFS + 15 % CW = 1.34 (UCS = 103.07 kPa), with a 5 % increment in the amount of inducted construction waste. A further increase of 5 % yielded an increase in the SDI by a factor of 0.04 which produced a UCS value of 106 kPa. Further strength increment was observed with the lengthening curing periods, because of the GGBFS cementitious nature, and the effects of the increased percentage of construction waste in the stabilised mixture were more pronounced with the high percentages of GGBFS in the treated specimens. This statement was reconfirmed, as after the curing of the stabilised specimens for 28 days (Fig. 16), the UCS value was found to have increased from 147.93 to 150.67 kPa, which shows an SDI increment from SDI2 % GGBFS + 10 % CW = 1.93 to SDI2 % GGBFS + 15 % CW = 1.96. Upon addition of another 5 % of construction waste, the UCS value rose slightly to 153.09 kPa, corresponding to SDI2 % GGBFS + 20 % CW = 1.99. However, the contribution of the construction waste alone without any GGBFS addition seems to have a minor effect on the UCS value of the bentonite.

3.3.4 Optimum Strength Improvement

Figure 17 shows the strength development indices of all of the treated specimens for the 28 day curing period, as the maximum strength for all admixture-stabilised bentonite specimens was observed after the specimens had been cured for 28 days. The maximum strength after the 28 day curing period was observed for specimen G5S3, which contained 5 % slag and 20 % construction waste, being SDI5 % GGBFS + 20 % CW = 3.09. This shows that stabilised soil strength grew by almost 209 %, while the least strength development was observed for specimen G2S1 which contained the lowest percentages of additives.

Fig. 17
figure 17

SDI for 28 days curing (SDI28)

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Since all admixture-stabilised bentonite specimen was prepared by tamping at OMC therefore the effect of saturation condition was not targeted in this study. In addition, this study is based on the certain period of curing time, considering possibility of stabilized clay becomes saturated after long time of submergence, further investigation is required in order to generalise the UCS result.

3.4 Microanalysis of Materials

3.4.1 SEM Microstructural and EDS Characterisation of Post-28 Days G5S3

Figure 18 shows the SEM micrographs of the stabilised specimen G5S3. This specimen contains 5 % GGBFS and 20 % construction waste added to the bentonite clay.

Fig. 18
figure 18

SEM micrograph of a a completely covered, and b partially covered specimen from G5S3 after 28 days of curing

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In general, the bentonite grains and the GGBFS crystals occupied the fissures and cavities of the construction waste particles as illustrated in Fig. 18.

Figure 19 summarises the results of the quantitative analysis and the EDS spectra of the stabilised specimen G5S3. Figure 19a shows silicon and oxygen to be the most dominant elements, followed by magnesium, iron, calcium and aluminium. The cations were observed to be bonded with their oxides. These results are quite similar to the SEM and EDS results for GGBFS. Figure 19b EDS and SEM results are consistent with the chemical composition and crystallography results for bentonite which indicated a higher percentage of montmorillonite. The results shown in Fig. 19c are quite similar to those for the quantitative analysis of the construction waste grains as shown in Fig. 9 and Table 4.

Fig. 19
figure 19

EDS spectra and quantitative data for S3G5 after 28 days of curing with a GGBFS, b bentonite, and c construction waste grains

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3.4.2 Mineralogical XRD Analysis

In addition to the SEM and EDS analyses on the material morphology and mineralogy, the diffraction bands of the untreated bentonite clay were used to understand the presence of different minerals and to obtain benchmark data for comparing the effects of the two stabilising agents on the virgin clay. The results of the XRD test on untreated bentonite clay are presented in Fig. 20b.

Fig. 20
figure 20

X-ray diffraction spectrum of a stabilised soil specimen (G5S3) after 28 days of curing, and b untreated bentonite clay (S smectite, M mica, K kaolinite, Q quartz, F feldspar, I illite, CC calcium carbonate, P plagioclase, CH calcium hydroxide, C calcite)

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The XRD characterisation results illustrated the dominance of clay minerals such as quartz [SiO 2], illites [K 1.5–1.0 Al 4[Si 6.5–7.0 Al 1.5–1.0 O 20](OH)4], alkali feldspars [(K, Na)[AlSi3O8]], mica [AlSi2O6(OH)2] and kaolinites [Al4[Si4O10](OH)8] (Deer et al. 2013) This confirms the findings from the EDS tests which also showed SiO2 as the major component of the bentonite clay. These XRD test results also show a similarity to the results of tests previously conducted on bentonite clay from a similar source (Amiralian et al. 2015).

Figure 20 illustrates the X-ray diffraction (XRD) spectrum for the stabilised soil specimen G5S3 after 28 days of curing alongside the XRD for untreated bentonite clay, so as to gain further insight into the crystallography and mineralogy of the treated and untreated specimens. The specimen was selected based upon the SDI criterion as it showed a maximum SDI value (SDI28 = 3.09), indicating that the maximum strength had been achieved for this specimen. The peaks of the XRD diffraction bands indicate that the tested stabilised mixture showed similar peaks overall, with slight variation resulting from the formation of cementitious compounds (Fig. 20a). Calcium hydroxide Ca(OH)2 was found, which might have formed as a result of the hydration of calcium oxide (Birss and Thorvaldson 1955).

4 Conclusion

A series of unconfined compressive strength (UCS) tests was performed to investigate the effect GGBFS and construction and demolition waste for stabilisation of bentonite clay. The results showed that both additives have diverse contribution for the stabilisation purposes. The UCS values for the treated specimens insignificantly increased from 124.77 to 127 kPa when the CW proportion was increased from 10 to 15 % for the 1 day curing period. Another 5 % addition of the construction waste also led to very little increment of UCS value of 131.42 kPa. However, the evolution of the UCS with increasing stabiliser proportions was influenced by the curing periods. The optimum results for all combinations were obtained after the curing period of 28 days. It was observed that the UCS value for the bentonite clay specimen stabilised with 5 % slag and 20 % construction waste grew from 131.42 kPa after 1 day of curing, to 236.93 kPa after 28 days of curing. This corresponds to the maximum strength improvement observed in the treated specimens and showed a 209.09 % increase from the initial untreated specimen strength. Meanwhile, the XRD results indicated that the presence of moisture in the mixture resulted in changes in the mineralogical composition of the stabilised mixtures with Ca(OH)2 formation due to the hydration of CaO. The outcome of the experimental study showed that the compressive strength of the bentonite increases along with GGBFS addition. However, the addition of construction waste alone did not significantly increase the compressive strength of the bentonite clay.