Bases teóricas científicas

The aim of physicochemical properties identification for the WPSA and the other candidate materials in this research study was to evaluate their suitability and potential for the development of cement-free blends to be used as a fully cement replacement in soft soil stabilisation. Therefore, a comparison was carried out to select the most suitable materials that have the potential to produce the new binder. This comparison was evaluated using chemical properties such as the oxides contents achieved from XRF analyses and pH values measured for each candidate, together with physical properties, such as PSD and LOI of the candidate materials. Since the identified peaks of free lime by XRD analysis for F-WPSA were more than those for C-WPSA in addition to the finer grade of F-WPSA, F-WPSA was considered in the comparison as well as in later experimental works for this research project. The chemical properties of all candidate materials along with those for the reference OPC are summarised in Table 4.11. From this table, it can be seen that the principal oxides of WPSA are very comparable to those for OPC in addition to the free lime that was identified in the XRD patterns of WPSA. This free lime is expected to cause true hydration behaviour. However, the real hydraulic properties of WPSA have not been reported by previous studies as they indicated that the primary mineral content was gehlenite which behaves as an inert material in the presence of water (Segui et al., 2012; Spathi et al., 2015).

Chapter 4 Results of Materials Identification

Additionally, by the definitions of fly ash for concrete requirements specified in the British standard BS EN 450:1995 (British Standard, 1999c), the value of LOI% should not exceed 5.0% by mass. Therefore, PFA2 was excluded due to its high LOI (9.64%). Table 4.11 also shows that significant silica contents were revealed by the XRF analysis of SF and RHA which could be good pozzolanic activators to boost the pozzolanic reaction. The chemical analysis also showed that a high value of pH (13.04) was indicated for POFA along with an acceptable amount of pozzolanic oxides (SiO2, Al2O3, and Fe2O3) which reached to 70.41% of the total oxides content of POFA. These chemical properties could be useful for both providing the hydration reaction with the pozzolanic materials and maintaining an adequate level of alkalinity for the hydration environment, which is required for the hydration reaction continuity (Habert, 2014).

Table 4.11 Comparative chemical properties of candidate materials.

Item pH CaO % SiO2 % Al2O3 % MgO % Fe2O 3 % K2O % SO3 % Na2O % TiO2 % Ref. OPC 13.04 65.21 24.56 1.7 1.3 1.64 0.82 2.62 1.34 -- F- WPSA 12.86 66.76 25.12 2.38 2.57 0.03 0.31 0.26 1.72 0.41 POFA 13.04 10.47 61.36 7.51 1.54 5.64 7.53 2.93 1.73 0.63 RHA 8.98 0.49 90.2 4.03 0.61 0.183 1.36 -- 0.897 0.067 PFA1 10.68 4.47 57.05 9.56 8.25 9.29 3.29 1.15 1.74 1.57 GGBS 11.65 38.67 37.87 4.714 3.73 -- 0.634 1.176 2.868 0.903 SF 8.81 -- 96.89 0.5 0.51 0.131 0.63 -- 0.891 0.034 FGD 12.3 35.89 14.3 -- 0.54 -- -- 34.64 1.23 --

Chapter 4 Results of Materials Identification

Based on BS EN 197-1:2000 (European Committee for Standardisation, 2004), the clinker in Portland cement shall contain not less than two-thirds by mass calcium silicates (C3S: (CaO)3. SiO2 and C2S: (CaO)2. SiO2). The remainder containing the other oxide compounds like Aluminium oxide (Al2O3) and iron oxide (Fe2O3). The specification also states that the ratio by mass of (CaO/ SiO2) shall be equal to or be higher than 2.0, and magnesium oxide content (MgO) shall not exceed 5.0%. Concerning the pozzolanicity, the specifications identify that the reactive silicon dioxide shall be not less than 25.0 % by mass as illustrated in Table 4.12. From this table, it can be seen that only WPSA met all the requirement of the aforementioned standard. However, the other materials would not affect the requirements if they were mixed in small portions with WPSA to produce a new cementitious binder. Figure 4.31 shows the percentages of principal oxide components for each of the candidate materials that were to be taken into consideration with those for OPC. From Tables 4.11 and 4.12, and Figure 4.31 indicate the quantitative oxide compositions of all candidate waste materials and revealed that WPSA has comparable principal oxide composition to OPC and encouraging figures in terms of CaO+SiO2, SiO2+Al2O3+Fe2O3, and CaO/SiO2. The other candidates have oxide components more than OPC when excluding CaO, which means silica, aluminium, and iron oxides, which play an important role in the pozzolanic reaction.

Table 4.12 Comparative chemical properties of candidate materials with the requirements of the British Standard

Item OPC WPSA POFA RHA PFA1 GGBS SF

CaO+SiO2 ≥ 67 % 89.77 91.88 71.83 90.69 61.52* 76.54 96.89 (SiO2+Al2O3+Fe2O3) % 27.9 27.53 74.51 94.413 75.9 42.584 97.521

CaO/SiO2 ≥ 2 2.65 2.66 0.17* 0.01* 0.08* 1.02* 0.01*

MgO % 1.3 2.57 1.54 0.61 8.25* 3.73 0.51

Chapter 4 Results of Materials Identification

For the GGBS, most of the principal oxides (Ca, Si, Al, and Mg) were found to have significant proportions. However, a low CaO/SiO2 ratio was observed (1.02); thus, WPSA has been considered as a more promising material for use as the calcium-base material in the experimental works later on in this study. As per the cement manufacturing, gypsum is added to the cement clinker as a retarder material as well as a grinding agent to prevent the agglomeration of cement particles during the grinding process (Marchon and Flatt, 2016b). Additionally, the gypsum is required to provide the sulphate which reacts with hydrated lime and alumina and forms the calcium sulphoaluminate which is known as ettringite; the ettringite contributes to the development of early age strength (Puppala et al., 2015; Aïtcin, 2016a). Consequently, the FGD identified in this study is composed mainly of gypsum hemihydrate thus, it is promising for use as a sulphate activator along with aiding the grinding activation for WPSA.

The specific surface area and the particle size distribution of OPC, as well as the candidate materials, have an undeniable effect on the compressive strength of stabilised soil. It was found that the finer the particles of fly ashes used in concrete with cement, the higher the compressive strength obtained (Kumar et al., 2008; Zhao

et al., 2016). From the overall particle size distribution of all candidate materials as

illustrated in Figure 4.32 and their volume statistics as listed in Table 4.13, it can be seen that the finer material is GGBS, while SF is the coarser amongst the candidate

0 10 20 30 40 50 60 70 80 90 100

OPC WPSA PFA1 GGBS SF RHA POFA

O xi de C ont ent % Candidate Material CaO SiO2 Al2O3 Fe2O3 MgO

Chapter 4 Results of Materials Identification

materials. OPC and RHA are nearer to each other with slightly finer results for OPC. POFA and PFA1 have smaller particles than WPSA to some extent. Therefore, a further grinding activation may be essential to increase the pozzolanic reactivity of WPSA. Additionally, a fine material could be produced during the mixing design by mixing portions of POFA and RHA (finer materials) with the ground activated WPSA.

Table 4.13 Volume statistics of candidate materials with reference OPC.

Item OPC PFA1 WPSA GGBS SF RHA POFA

d10 (µm) 0.964 1.012 2.378 0.62 54.29 5.011 4.945 d50 (µm) Median 11.78 10.67 24.64 7.565 214.1 24.83 29.29 d90 (µm) 44.27 67.45 166.2 27.12 1412 54.88 77.6 Mean (µm) 17.83 23.83 57.87 11.19 364.2 27.69 36.17 Figure 4.32 Comparative PSD of candidate materials.

Chapter 4 Results of Materials Identification

4.5 SUMMARY

This chapter presented the identification of the physical, geotechnical and chemical properties of the soft soil used in this study as well as the determination of the physicochemical properties of specific types of waste materials along with the reference binder which was a specific type of OPC. The first aim of this chapter was to classify the virgin soil and characterise its physical and geotechnical properties to be used to evaluate the levels of improvement after treating with different mixtures. Additionally, this chapter aimed to select a group of the most promising waste materials dependent on the comparative chemical and physical properties of the candidate materials. The latter are devoted to be employed in the development of many cement-free mixtures as new binders in soft soil stabilisation. As explained in the previous section, the materials that were adopted in this study were selected after conducting a comparison of physicochemical properties of all candidate materials with those of the reference OPC. The output of this chapter can be summarised as follows.  The classification experiments revealed that the virgin soil used in this study was an intermediate plasticity silty clay with medium organic content and its symbol was CI.

 Based on the comparative physicochemical properties of the candidate materials, the selected materials used in the mixing design at the later stages were WPSA as the main calcium-based material, POFA as an alkaline activator due to its high pH value in addition to its acceptable pozzolanic compounds, and RHA due to its significant content of amorphous silica and general fineness. Additionally, the FGD gypsum was selected as a grinder as well as sulphate activator to help in the breaking of the glassy phase of non- amorphous silica, and in turn to increase the production of the cementitious gels C-S-H and C-A-H.

 The coarser particles of WPSA as well as the agglomeration could affect and delay its pozzolanic reactivity during the hydration reaction, therefore it would exhibit better reactivity if it was subjected to mechanical activation such as a low energy grinding.

Chapter 5 Mixing Design for the New Binder Mixtures Optimisation

CHAPTER 5

MIXING DESIGN FOR THE NEW BINDER MIXTURES

OPTIMISATION

5.1 INTRODUCTION

This chapter presents the results and discussion of a set of laboratory experiments carried out on the soft soil treated with different types of mixtures consisting of cement-free blending of waste materials fly ashes. The mixtures were prepared using the waste materials selected from the previous chapter by mixing them using different proportions to produce unary, binary and ternary mixtures. Therefore, this chapter contains three stages of optimisation to address the most effective mixtures by examining the performance of different blends in soft soil stabilisation. The behaviour of these mixtures was evaluated dependent on the results obtained from the Atterberg limits, compaction parameters and unconfined compressive strength (UCS) tests. The aforementioned results were compared with those for untreated soil only, at this stage of the research project. However, the results of soil treated with the reference binder were referred to on some occasions in this chapter.