Effect of Fly Ash on microstructural and resistance characteristics of dredged sediment stabilized with lime and cement To

: Contribution of fly ash (FA) as a complementary additive for dredged sediment (DS) stabilization was studied. The study is focused on definition of an efficient combination(s) to raise the DS properties. FA with lime and cement with both was used. Then, micro and macroscale investigations was performed. Test results demonstrate that FA additions promoted nucleation, formation of cementitious compounds and fabric modifications in sediment. DS stabilized using FA component show lower shrinkage and higher mechanical resistance than that stabilized using conventional binders. FA is found to be efficient in sediment-lime and sediment-cement mixtures since it accelerates cementation and strength gain. Abstract 21 This study deals with the definition of an efficient combination of fly ash (FA) with lime or cement and 22 with both, to improve the dredged sediment (DS) properties. At early age, filler and nucleation effects 23 of FA lead to a refinement of the microstructure in addition to the macro porosity reduction induced 24 by lime and cement. At long term, the microstructure becomes denser due to the pozzolanic property 25 of FA. At macroscale, DS stabilized using FA show lower shrinkage and higher mechanical resistance 26 than that stabilized without FA, with more pronounced effects when FA is mixed with cement. MULTISCALE ANALYSIS using

which occur at different levels, in order to determine how the action mechanisms of different additives 35 affect soil resistance. 36 Multiscale analysis leads to a comprehensive knowledge of improvement of stabilized soil using 37 hydraulic binders. In such a way that microscale analysis provides evidences of pozzolanic reactions, 38 cementing material formation and evolution of crystalline phases, and macroscale analysis explains 39 enhances in mechanical properties resulted from previous interaction highlighted at microscale level 40 It is known that the hydration of clinker constituents produces cementitious compounds such as 52 calcium silicate hydrates (C-S-H), calcium hydroxide (CH). In pre induction of hydration, cement reacts 53 with the available water and the most active phase C3S produces calcium ions and OH -, SO 2-4, K + , Na + . 54 In dormant stage, C3S hydration continues and C2S begins to be hydrated. CH separates from hydrolysis 55 of C3S and C2S and may precipitate into empty voids. Ettringite (Ett) also forms due to the reaction of 56 gypsum with C3S and C4AF [8]. 57 Over time (at early stage), C-S-H crystalline phases form an acicular morphology which branch, forming 58 a honeycomb-shape structure. CH crystallizes in large crystals (~40μm), presenting hexagonal plate-59 shape, depending on the produced lime amount in early stages and the available free space. Ettringite 60 crystalizes in a needle-shape, with length up to 10μm and diameter of 0.25 μm, which does not branch. 61 All aforementioned phases and processes have been observed in cement-based materials, included in 62 soil-cement mixtures [3][7] [9]. 63 At microscale level, when cement is mixed with soil, the soil-cement mixture presents a significant gain 64 of mechanical strength. Soil-cement strength results from cementation bonds and pore space 65 reduction [10][11]. Accordingly, cementitious compounds fill up the pores and connect cement grains 66 resulting in the increase in intra-aggregate pore volume [12]. 67 When lime is used in soil stabilization, soil fabric changes since lime addition induces cation exchanges 68 and generates flocculation/agglomeration mechanism, in short term. In other words, water dissolves 69 some constituents of lime (CaO, CaSO4, MgO and quartz) that react with soil and reduce double diffuse 70 layer (DDL), resulting in flocculation (agglomeration). This process reduces soil plasticity and improves 71 workability [13]. Over time, pozzolanic reactions take place at alkaline environment (pH=12), forming 72 cementitious compounds i.e. C-S-H and C-A-S-H, that are responsible for increasing of long-term 73 strength [3] [10]. 74 At microscale level, soil-lime strength gain is also explained based on the formation of cementitious 75 compounds from pozzolanic reactions. However, the increase of soil resistance generated by lime 76 addition is less significant than that observed in mixtures using cement; and the use of lime is 77 preferably recommended for clayey soils, in order to improve soil fabric, plasticity and workability [1] 78 [2] [13]. It is also worth noting that the mechanical properties of lime-treated soils are affected by the 79 curing temperature. By studying the stiffness evolution of a silt soil stabilized with quicklime cured at 80 30 °C, Silva et al. [14] observed two different stages on the stiffness evolution suggesting the existence 81 of two different chemical phenomena involved. Evolution in the first stage seems to be mostly related 82 to the formation of calcium aluminate hydrates (CAH). However, the evolution in the second stage can 83 be more related to a structural rearrangement of CAH and the formation of calcium silicate hydrates 84 (CSH). These two distinct stages involved in the evolution of elastic modulus (E) with time suggest the 85 existence of two apparent activation energies (one for each process). 86 To maximize the benefits of soil stabilization, a binary or ternary combination of hydraulic binder is 87 proposed. Most of the time, lime and cement combination is recommended. Lime-cement stabilization 88 combines workability enhancement from lime addition and resistance gain from cement addition. Both 89 additives may be mixed without disturbing their own action mechanism. However, multicomponent 90 mixed materials show differences in physical and chemical properties due to coexistence of cement 91 hydration and mineral admixtures, changing hydration kinetics process and microstructure formation 92 mechanisms [15]. According to [16] 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64 FA is a waste from coal-fired electrical power plants that is being studied as admixture in soil 110 In concrete, FA addition stimulates reaction rate of cement hydration, promotes nucleation and 116 growth of cementitious compounds. However, FA may also retard the onset of acceleration (Stage III 117 of hydration) because of (i) inhibition of CH precipitation due to the formation of water containing 118 organic species and (b) slows formation of Ca rich surface layers on clinker phases, in the case of 119 aluminum rich FA [26]. 120 The effects of FA action on soils are then physical and chemical. At physical point of view, finer particles 121 of FA would fill voids of soil particles. Chemically, pozzolanic products induced by FA presence would 122 fill pores. Both effects reduce porosity so that microstructure becomes denser. As results, strength and 123 stiffness are increased and compressibility is reduced [27]. 124 Research findings suggest that FA disperses clusters of soil and strength development is controlled by 125 FA hydration [20] [28][29], leading to an analogy with the aforementioned processes in cement pastes. 126 It is worth noting that water content of the fly ash stabilized soil mixture affects the strength [30]. The 127 maximum strength reached in soil-fly ash mixtures generally occurs at moisture contents below 128 optimum moisture content for density. For silt and clay soils the optimum moisture content for 129 strength is generally four to eight percent below optimum for maximum density while for granular 130 soils the optimum moisture content for maximum strength is generally one to three percent below 131 optimum moisture for density. Therefore, it is crucial that moisture content be controlled during 132 construction. Initial water content significantly affects the efficiency of soil stabilization. 133 In soil stabilization, FA addition alone is not sufficient to significantly increase strength to the design 134 allowable levels, then a combination with other additives is required [29]. Particular attention must be 135 paid when FA is added to soil-cement mixture. FA may compete with cement for the water available 136 in the soil. This competition may be detrimental to cement hydration and it is more problematic in soil-137 cement because of the typical low water and cement ratio (w/c).    In this study, dredged sediment from La Baule Le Pouliguen Harbor, France, and two cementitious 177 additives were used. Fly ash was added to mixtures containing lime, cement and lime and cement. 178 Only chemical stabilization was made (so without gradation correction) in order to find the best 179 combination of additives to improve the characteristics of this dredged soil considered as waste, 180 regarding economic and environmental aspects. 181 182   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64 The geotechnical properties of the sediment were carried out in accordance with GTR guide [32] 183 dedicated for embankment and pavement applications. Table 1 shows results of soil characterization 184 based on results of maximum specific gravity, and organic material, pH and carbonates contents. 185 Portland cement was a CEM II/B-LL 32,5R CE CP2 (French Standard) whose short-term resistance is 191 32.5 MPa. Clinker content is between 65 and 79%, being its chemical constituents: tricalcium silicate 192 (66%), dicalcium silicate (10%) and tricalcium aluminate (7%). Limestone is the main natural 193 component of this cement, presenting a total organic material less than 0.20% in mass. The cement 194 content added to soil was 7% of the dry mass of the sediment. 195

Results
Regarding cementitious compounds, one may observe that all treated samples had produced them at 296 In other words, since C-S-H are amorphous and/or poorly crystallized, it may be advisable to be careful 306 when reading XRD patterns because of the disturbing factors (producing a heterogeneous sample) and 307 to combine other microstructural data sources to help to support the results analyses. 308

Macroscale analysis 386
Macroscale analysis was done in two steps. The first consisted of immediate property assessment of 387 the mixtures based on Proctor, shrinkage, and UCS test results. The latter was based on UCS results 388 throughout curing time. First and second steps focused on distinguishing the filler effect from curing 389 effect, respectively, in an attempt to remark the most adequate stabilization process, in terms of 390 additive content and strength gain. 391

Design parameters and immediate strength 392
Binder addition and content may change soil gradation. Since additives are finer than the soil ( Figure  393 1), they shall fill partially empty voids of soil and increase the initial strength values, due to the filler 394 effect. Table 2 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  To illustrate filler effect, Figure 6 shows correlations between Proctor and shrinkage parameters, and 402 immediate UCS values in function of additive content. Figure 6(a) shows that the reduction of ρdmax is 403 a result of the increase of binder content, but this behavior depends on the type of binder considered. 404 It is observed that the evolution of the ρdmax follow a linear regression law with high degree of 405 accuracy (R 2 ). R 2 values for mixtures containing lime (L) and cement (C) are 0.81 and 0.88, respectively. 406 FA addition reduced ρdmax in mixtures with cement. In mixtures with lime this behavior was not 407 observed. 408 Linear law demonstrates that cement addition reduces maximum dry density less than lime addition. 409 Both curves converge to the ρdmax value of the SLCFA mixture, which was the minimum one. 410 Volumetric deformation is often consequence of shrinkage which occurs in cemented materials due to 411 the water consumption for clinker component, generating fissures. Shrinkage test reports the variation 412 of volume (dV) that a soil-cement may present and gives an idea about material durability in dosage 413 phase. Figure 6(b) shows a strong correlation between dV and additive content (R 2 =0.93), indicating 414 that dV decreases with increasing additive content. In other words, partial filling of the voids would 415 inhibit shrinkage. 416 417   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62 63 64 It would be reasonable to indicate the addition of inactive fillers in order to reduce shrinkage in soil-419 cement type mixtures. Figure 6(b) led to infer that FA plays this role in this phase, because its addition 420 in the mixtures further reduced dV. In mixtures without FA, the reduction of dV was approximately 421 equals to 30% in relation to untreated soil. When FA was added, this reduction was about 40% for SL 422 and SC mixtures, and 55% for SLC mixture. 423 Regarding the filler effect on immediate strength, Figure 6(c) shows a good correlation (R 2 =0.79) 424 between additive content and UCS of the mixtures, where the higher the additive content the greater 425 the UCS0d. It is important to point out that immediate UCS0d of SL mixture was lower than the 426 untreated sediment. This behavior is attributed to flocculation reactions of lime that increase of voids 427 in the mixture, decreasing ρdmax and increasing water content (as seen in Table 2). 428 It is worth emphasizing that the combination of additives changes gradation of untreated sediment. 429 As additives are finer than the sediment soil, they may fill empty voids, increasing the sediment 430 strength. As a rule, the initial strength values gradually improve as binder percentages increase. 431 In spite of the good correlation between immediate UCS and additive content, the strength gain may 432 be also related to the type of additive, or combination between them, since the greatest immediate 433 UCS were observed in mixtures SLC, SCFA and SLCFA. The common point of these mixtures is the 434 presence of cement associated to other(s) additive(s). It seems to be a contribution of additive 435 gradations to the particle size arrangement of mixtures, resulting in the increase of their immediate 436 UCS. Immediate UCS is a reference point that might lead to dosage optimization attempts. Accordingly, 437 from this point on, it shall be observed the strength gain over time due to chemical reactions. 438

UCS and water content results over time 439
In order to highlight the effects of additives on the mechanical resistance of mixtures, Figure 7 (Figures 2 to 4). 477 Assuming that the gain of resistance over time results from the cementing products of the additive 478 components [28], it is also necessary to understand how the hydric state of the mixtures evolves. 479 In early age (7 days), water contents change slightly, maintaining practically the initial values, i.e. close 486 to the wopt (Table 4). However, for intermediary age (28 days), these changes evidence the reduction 487 of water content, the average reduction was 5%. For longer period (90 days), the average reduction of 488 water content is 12%, except for SLC, which was 31%. 489 FA addition also alters hydric conditions of stabilized mixtures over time but at lower intensity. As a 490 rule, one may suppose that the greater the decrease in moisture content the greater the gain in 491 resistance. This hypothesis is true since water is consumed in formation and/or crystallization of 492 cementing products, becoming the structure denser and more resistant than the original soil. 493

Discussion 494
The decision for a given product takes into account technical, economic and environmental 495 parameters. This study has shown that combining FA with stabilized soil with lime and cement (or both) 496 is promising because of the FA action mechanisms promoted an acceleration of strength gain 497 regardless of the curing time. Besides, mixtures using FA have always shown higher resistance, either 498 through physical interactions or chemical reactions. These findings support technically the choice for 499 combining FA with chemical soil stabilization using conventional binders. 500 For instance, UCS value at 90 days of mixture using cement (S7C) is compatible with that of the mixture 501 using cement and FA (S7C9FA) at 28 days, or with mixture using lime and FA (S2L9FA) at 90 days. These 502 mechanical compatibilities indicate alternative material options that might save time and economy, 503 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64 since it is possible to select mixtures that present greater resistance ahead of time, use cheaper 504 additives or smaller quantities of conventional binders. 505 These benefits have a positive impact on environmental indicators because they promote the rational 506 use of waste and byproducts (marine sediment and fly ash). In this sense, it is worth mentioning that 507 the mixture with lime, cement and FA (S2L7C9FA) presented resistance compatible with the mixture 508 without FA (SLC), leading to question the advantages of this mixture (S2L7C9FA) as well as to think 509 alternatively about the possibility of reducing the cement content in order to optimize the mixture 510 design in terms of economic and environmental. 511

Fundamental assumption
Chemical stabilization induces physicochemical changes of soil characteristics and improves mechanical behavior of the soil

Experimental program
To relate microstructural characteristics to mechanical properties of a stabilized soil with and without FA

Conclusions
FA in soil-lime and soil-cement is beneficial since FA accelerates cementation and strength gain

Findings
Hydraulic binders act as filer, in short term and cementation controls the resistance gain, in long term