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Use of uncontaminated marine sediments in mortar and concrete by partial substitution of cement
Cement and Concrete Composites 93 (2018) 155–162
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Use of uncontaminated marine sediments in mortar and concrete by partial substitution of cement
T
Zengfeng Zhaoa,b,∗, Mahfoud Benzerzoura, Nor-Edine Abriaka, Denis Damidota, Luc Courardb, Dongxing Wangc a
IMT Lille Douai, Univ. Lille, EA 4515 - LGCgE, Civil and Environmental Engineering Department, F-59000, Lille, France Department of ArGEnCo, GeMMe Building Materials, Urban and Environment Research Unit, University of Liège, Liège, Belgium c School of Civil Engineering, Wuhan University, Wuhan, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Sediments Mortar Substitution of cement Mechanical properties Porosity Concrete
The disposal of dredged marine sediments has become a major economic and environmental issue in the world. In this study, uncontaminated marine sediments dredged in the harbor of Dunkirk (France) were dried and ground and then used in partial substitution of cement in the manufacture of mortars and concretes. A given volume of cement has been replaced by the same volume of sediment for three substitution contents (10%, 20%, 30%) of a Portland cement CEM I 52.5. The flexural and compressive strengths of mortars decreased when the sediment replacement content increased. However, the mechanical properties of the mortar with 20% replacement of cement with sediments were better than those of a mortar made from cement CEM II/A-LL 32.5 containing a proportion of limestone similar to the sediment substitution. The total porosity measured by mercury intrusion porosimetry of different types of mortars showed that the porosity increased as the sediment substitution content increased but the pore size distribution was shifted toward smaller pores. Finally, it was demonstrated that concrete C30/37 could be designed with 20% cement replaced by sediment without the use of admixture. Additionally, this concrete fulfilled the standards with respect to the total chloride content required for unreinforced concrete. As a conclusion, dried and finely ground uncontaminated sediments appeared to be a very interesting constituent for partially substituting up to 20% of cement as its efficiency overpass limestone filler.
1. Introduction A large amount of sediment is dredged for navigation every year. In France, about 50 million m3 of sediment are produced from harbors. While in China, about 400 million m3 of sediment are dredged annually [1]. Thus, the disposal of dredged sediment has become a major economic and environmental issue in the world [2–4]. The sediment can contain a variable amount of organic pollutants (PAHs: polycyclic aromatic hydrocarbons, PCBs: polychlorinated biphenyls, TBT: tributyltin and dioxins) and different levels of inorganic contaminants (heavy metals such as As, Cd, Pb, Cr, Cu, Ni, Hg and Zn). Polluted sediments can be treated for example by Novosol process which consists of two major phases: phosphatation (2–3.5% of phosphoric acid H3PO4) and calcinations at ≥ 650 °C [5]. Treated sediments have been successfully incorporated into the brick production as a raw material [6,7]. The industrial-scale experiment showed that bricks made of sediments would have no environmental impact restricting
∗
their application [8]. So far, a lot of research are devoted to reuse marine dredged sediments as a new material resource as foundation and base layers for road construction [9–14]. The results showed that the solidification/stabilization of sediment by using cement, lime and other additives such as fly ash and slag can satisfy the mechanical properties specification and the prescribed thresholds for environmental impact [10,13]. However, there are fewer studies on the reuse of sediments in mortar or concrete comparing to the reuse of sediment in the road construction [15–18]. Concrete is the first material used in the world which needs very large amounts of cement, aggregate and water [19,20]. In the last decades, the interest of reusing recycled aggregate and sand or recycled filler in the concrete industry was demonstrated [21–26]. Following this trend, it thus appears that the use the sediment in the manufacture of mortars and concretes is very promising [27,28]. Some authors reported the production of lightweight coarse aggregate (LWA) at the temperatures of 1100–1200 °C by using sediments for the
Corresponding author. IMT Lille Douai, Univ. Lille, EA 4515 - LGCgE, Civil and Environmental Engineering Department, F-59000, Lille, France. E-mail address: [email protected] (Z. Zhao).
https://doi.org/10.1016/j.cemconcomp.2018.07.010 Received 8 November 2017; Received in revised form 1 June 2018; Accepted 17 July 2018 Available online 18 July 2018 0958-9465/ © 2018 Elsevier Ltd. All rights reserved.
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masonry and concrete [29,30]. The results showed that the LWA ranged from 1010 to 1380 kg/m3 for the particle density, and met the requirement for the lightweight aggregate. The concrete made from sedimentary LWA ranged from 19.8 to 34.7 MPa, can satisfy the requirement for structural lightweight concrete [30]. Some authors investigated the possibility of sediments as sand in the cement-based materials [2,18,31]. Agostini et al. replaced sand by sediment treated by Novosol process at substitution levels of 33%, 66% and 100% [31]. Despite that the fine fraction of sediment had a high porosity leading to a higher amount of absorbed water and greater drying shrinkage of mortar, mortars made with 33% of sediment showed significant compression strength improvement (up to 20%). The formation of a denser interfacial transition zone in the presence of treated sediment was expected to be at the origin the reported mechanical strength improvement [2,31]. Couvidat et al. also indicated that the use of the coarse fraction marine sediment offered an interesting valorization potential as sand in the cement mortars for non-structural applications [18]. Some authors studied the use of contaminated sediment to replace a portion of raw materials in the production of Portland cement clinker [32,33]. Aouad et al. showed that the laboratory manufactured cement based on the sediment produced the equivalent compressive strengths to those commercially produced Portland cement [32]. Dang et al. investigated the new blended cements made of a mixture of Portand cement and 8%, 16% and 33% of thermally treated sediment at 650 °C and 850 °C. The blend cement based on the sediment treated at 650 °C involved higher compressive strength than the one based on the classical calcareous filler [34]. Rozière et al. studied the use of treated sediments at 650 °C in self-consolidating concrete as a replacement of limestone filler and aggregates. The compressive strength of sedimentbased concrete specimen was comparable to that of reference concrete [35]. However, fewer studies were devoted to the valorization of uncontaminated marine sediment just subjected to drying at ambient temperature for long period or at moderated temperature if the process has to be speed up. This study is thus reporting the use of marine sediments, dried at 40 °C and then ground, as partial substitution of cement in the manufacture of mortars and concretes. Three contents of sediments were used as cement CEM I 52.5 substitution (10%, 20%, 30%) to produce mortars. The fresh properties such as fresh density, slump and the mechanical properties of mortars were measured and the microstructural properties of mortars were also studied. Finally concretes were designed from the results obtained on mortars.
Table 1 Chemical composition of sediment. Element symbol
Element name
Percentage (%)
O Na Mg Al Si P S Cl K Ca Ti Fe Pb C Mn Cr Cu Zn Sr Zr V
Oxygen Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Potassium Calcium Titanium Iron Lead Carbon Manganese Chromium Copper Zinc Strontium Zirconium Vanadium
50.2 1.8 1.1 4.6 16.5 0.1 1.5 2.1 1.5 16.2 0.3 3.7 0.1 Present Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1
Table 2 Leaching test results and limiting values of classification of wastes from 2003/ 33/CE.
2. Materials and methods 2.1. Sediments characterization
Tests
ICP (Inductively Coupled Plasma) values
2003/33/ CE Class III: inert waste
2003/33/CE Class II: nonhazardous waste
2003/33/CE Class I: hazardous waste
pH As (mg/kg) Ba (mg/kg) Cd (mg/kg) Cr (mg/kg) Cu (mg/kg) Mo (mg/kg) Ni (mg/kg) Pb (mg/kg) Sb (mg/kg) Se (mg/kg) Zn (mg/kg) Chlorides (mg/kg) Sulfates (mg/kg)
8.74 0.117 0.909 0.009 0.043 0.485 1.101 0.076 < 0.082 0.118 0.135 0.125 20555
– < < < < < < < < < < < <
– < < < < < < < < < < < <
– < < < < < < < < < < < <
3485
< 1000
0.5 20 0.04 0.5 2 0.5 0.4 0.5 0.06 0.1 4 800
2 100 1 10 50 10 10 10 0.7 0.5 50 15000
< 20000
25 300 5 70 100 30 40 50 5 7 200 25000
< 50000
12457–2 [38]. Metallic elements and ions such as chlorides and sulphates were analyzed. According to the prescribed limits criteria and procedure for the acceptance of waste at landfills in European directive 2003/33/CE [39], all the heavy elements were in the limit of Class III (inert waste) except for the Mo and Se in the Class II (non-hazardous waste). Thus the sediments were not contaminated with heavy metals but they contained a high chloride content that impacts the mix design of concrete. Indeed, chloride ions can catalyze the corrosion of steel rebar contained in reinforced concrete. For these reasons, limits of the amount of chloride in cement and thus in the present case, of the amount of marine sediment, will depend on the type of concrete and its usage as defined in the standard EN 206–1 [40] (Table 3). The aim was to use the maximum quantity of sediment in the concrete mix design, thus unreinforced concrete was targeted in order to keep the chloride content at value less than 1% relative to the cement.
The used marine sediments were dredged in the port of Dunkirk (France). The measured initial water content of sediment was about 95%. The clay fraction activity was low (3.2% measured by the methylene blue test). Then they were dried at 40 °C until constant weight and then ground in a laboratory mill (The treatment of 40 °C consumes less energy comparing to the treatment at 650 °C. Here, grinding is necessary in order to use to use all fractions of sediment). In this study, the fraction 0/80 μm was used for partially replacing cement in the manufacture of mortars. The organic matter content was measured as 13.8% according to the standard XP P94-047 [36] by calcination in oven at 450 °C for 3 h. The true density was 2.48 g/cm3 measured by helium pycnometer, which is lower than Portland cement. Table 1 shows the chemical composition of sediment determined by X-ray fluorescence. The major chemical elements of the sediment were oxygen, silicon, calcium, aluminum and iron. PAHs (polycyclic aromatic hydrocarbons) value was 0.29 mg/kg and PCBs (polychlorinated biphenyls) value was 6.34 mg/kg, which were under the limited value according to AMATR [37]. Table 2 shows the average values of leaching test results with a liquid to solid ratio of ten according to standard EN
2.2. Mix design of mortars Portland cement CEM I 52.5 and CEM II/A-LL 32.5 were used to produce reference mortars. The density of these two cements measured 156
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Table 3 Maximum chloride content of concrete according to EN 206–1. Concrete use
Chloride content class
Not containing steel reinforcement or other embedded metal Containing steel reinforcement or other embedded metal Containing pre-stressing steel reinforcement or other embedded metal
Maximum Chloride content by mass of cement
Cl 1.0
1.00%
Cl Cl Cl Cl
0.20% 0.40% 0.10% 0.20%
0.20 0.40 0.10 0.20
Table 5 Physical properties of cements and sediment.
2
Specific surface area Blaine (m /g) Specific surface area BET (m2/g) Median particle size (μm) Mean particle size (μm)
CEM II/A-LL 32.5
Sediment
0.33 0.80 15.71 12.33
0.29 1.10 22.47 14.32
0.82 9.10 14.32 7.19
Table 6 Compositions of mortars.
Sand (g) Cement (g) Sediment (g) Efficient water (g) Eeff/(C + S) Content of Cl− to cement (%)
Table 4 Mineralogical composition of cements determined by XRD-Rietveld method.
CEM I 52.5 CEM II/ALL 32.5
CEM I 52.5
C3S
C2S
C3A
C4AF
Anhydrite
Gypsum
Quartz
Calcite
68.11
22.62
0.68
7.81
0.05
0.53
0.2
0
52.93
13.91
7.42
–
0.74
2.63
2.31
17.31
M0-52.5
M10–52.5
M20–52.5
M30–52.5
M0-32.5
1350 450 0 225 0.5 0.00
1350 405 35.77 225 0.51 0.18
1350 360 71.54 225 0.52 0.40
1350 315 107.31 225 0.53 0.68
1350 450 0 225 0.5 0
was followed according standard EN 196–1 [43]. The highest content of chloride was 0.68% for the mortar M30–52.5, which was lower than the limiting value for unreinforced mortar or concrete.
by helium pycnometer was 3.12 and 2.99 g/cm3 respectively. Table 4 reports the mineralogical composition of these two cements determined by XRD using Rietveld method. Fig. 1 presents laser granular analysis of the two cements and the sediment used as constituents in the mortars. The sediment contained a higher proportion of fine particles (1 μm–20 μm) than cements. Table 5 reports the specific surface determined by Blaine according to standard EN 196–6 [41] and by the N2 BET method, and the mean and median diameters of cements and sediment. The specific surface area of sediment was three times larger than that of the cements according to the Blaine method and ten times larger than the cements according to the BET method. These differences are due to the different methods of measurement knowing the value given by BET method is more accurate but Blaine method is used in the standards. Crushed calcareous natural sand sourced from Tournai (provided by Holcim France Benelux) was used for the manufacture of mortars. This sand had a density of 2.66 g/cm3and water absorption of 1.05% according to standard EN 1097–6 [42]. Four mortars were made with CEM I 52.5 being replaced by the same volume of sediment at replacement levels of 0%, 10%, 20% and 30% (noted M0-52.5, M10–52.5, M20–52.5, M30–52.5 respectively). A mortar was also made with cement CEM II/A-LL 32.5 (noted M0-32.5) to compare the effect of sediment relative to limestone. Table 6 shows the compositions of the studied mortars. A precise mixing procedure
2.3. Experiments on mortar After mixing, the slump of mortar was measured with Abrams' minicone (h = 150 mm, D = 100 mm, d = 50 mm). This cone is used to determine the mix-design of a mortar having rheological properties correlated with those of a concrete made with similar constituents at the exception of the aggregates. The preparation of specimens (40 mm × 40 mm × 160 mm) for mechanical strength tests was followed in accordance with standard EN 196–1. The flexural and compressive strengths of hardened mortar were determined in accordance with standard EN 196–1. These two mechanical tests were carried out with an INSTRON 5500 R 4206–006 (loading capacity of 1500 KN) after being cured 7 and 28 days in water. The porosity of mortar was tested by using Mercury Intrusion Porosimetry (MIP) technique (Micromeritics Autopore IV). The microstructure of mortar was also observed by Scanning Electron Microscope (SEM) with Energy Dispersive x-ray Spectroscopy (EDS). After 28 days curing in the water, pieces of hydrated samples of 10 mm × 10 mm × 10 mm were cut from the core of the mortar samples. After the solvent exchange to stop hydration, the samples were passed to vacuum to remove solvent and stored in a desiccator. Then they were impregnated in the resin and polished down to 0.25 μm by using diamond pastes. The samples were coated with carbon and examined by using machine HITACHI S4300SE/N. A voltage of 15 kV and the model of “backscattered electrons diffraction” were used for the observation. 2.4. Mix design of concretes Three concretes (noted C-0S, C-10 S, C-20 S) were made from Portland cement CEM I 52.5, which corresponded respectively to 0%, 10% and 20% of cement being replaced by the same volume of sediment. The used cement and sediment were the same as used for mortar manufacture. The concretes were made with a crushed calcareous natural sand (the same as for the manufacture of mortars) and calcareous coarse aggregate sourced from Tournai (provided by Holcim France Benelux). Table 7 shows the detailed compositions of concrete. The highest content of chloride was 0.40% for the concrete C-20 S, which was lower than the limiting value for non-steel reinforcement concrete and equal to the maximum limiting value for steel reinforced (Table 3). The water absorption of the crushed calcareous coarse aggregate was 0.8% according to standard EN 1097–6. The air dried
Fig. 1. Particle size distributions of cements and sediment used in mortars. 157
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Table 7 Compositions of concrete (1 m3).
Coarse aggregate (kg) Sand (kg) Cement (kg) Sediment (kg) Efficient water (kg) Absorbed water (kg) Weff/(C + S) Content of Cl− to cement (%)
C-0S
C-10 S
C-20 S
1061.45 768.63 335 0 208.37 16.56 0.62 0.00
1061.45 768.63 301.50 26.63 204.10 16.56 0.62 0.18
1061.45 768.63 268 53.26 199.82 16.56 0.62 0.40
natural sand and coarse aggregate were used for all concretes, and water compensation was adjusted according to the water content and water absorption of natural sand and coarse aggregate during concrete batching.
Fig. 3. Flexural strength as a function of mortars.
M20–52.5, M30–52.5 respectively) slightly decreased as the substitution of cement by sediment increased, which is certainly due to the lower density of sediment compared to cement.
2.5. Experiments on concrete After mixing, the workability of fresh concrete was measured by slump test using the Abrams cone according to EN 12350–2 [44]. For each concrete mix, nine cylindrical specimens (Ø110 mm x H220 mm) were cast to determine the mechanical properties. All specimens were cast in two layers and compacted on a vibrating table until no more air bubbles appeared. They were covered with a plastic sheet and left in the mould in the laboratory at 20 ± 2 °C for 24 h. After that, all the cylinders were cured in water until ages of 28 days. The compressive strength was measured according to standard EN 12390–3 [45] using a compression machine. The loading rates used were 0.6 ± 0.2 MPa/s for the compressive test. The splitting tensile strength was determined according to standard EN 12390–6 [46]. The loading rates used were 0.05 ± 0.01 MPa/s for the splitting tensile strength. Modulus of elasticity was measured by three loading cycles according to standard EN 12390–13 [47].
3.1.1. Fresh properties of mortars The slump value of mortar decreased when the substitution of sediment increased (Fig. 2). This trend could be due to the higher specific surface area of sediment compared to cement. Thus, part of the mixing water was expected to be adsorbed by sediment and thereby the free water quantity decreased, leading to a significant loss of workability. The slump value of mortar made with CEM II/A-LL 32.5 was similar than the reference mortar made with CEM I 52.5 indicating that limestone filler did not modify the amount of free water. The fresh density of mortar (2.37, 2.371, 2.346, 2.329 g/cm3 for M0-52.5, M10–52.5,
3.1.2. Mechanical properties of mortars Fig. 3 and Fig. 4 present the flexural and compressive strengths of mortars (average values obtained by three measurements for flexural strength and six measurements for compressive strength). Both flexural and compressive strengths of mortars after 28 days decreased when the substitution of cement by sediment increased (Figs. 3 and 4). This result is in agreement with the dilution of cement when part of the cement is substituted by unreactive or slightly reactive compounds. Nevertheless after 7 days, the compressive strength of M10–52.5 was slightly higher than the reference mortar without sediment indicating that ground sediment was not inert. The efficiency of sediment can be refined by drawing the percentage of mechanical strength of sediment mortar with respect to the unsubstituted reference mortar as a function of the amount of cement substituted by sediment (Fig. 5). First, the slope of all curves is not monotonous indicating that the efficiency of sediment varies not linearly with the amount of cement that is substituted by sediment. Substituting more than 10% of cement by sediment is not recommended with respect to compressive strength whereas amounts up to 20% are still acceptable with respect to flexural strength. The flexural strength of mortar M20–52.5 after 28 days was about 10.7 MPa, which is slightly lower than that of mortar M0-52.5 (12.1 MPa). If 30% of cement is substituted by sediment, both flexural and compressive strengths are close to the values obtained theoretically by considering the diluting effect of the cement by inert filler (For the calculation of the theoretical value (dashed line in Fig. 5), we assume that the mechanical properties of mortars decrease linearly with the amount of substitution of cement by inert filler. The theoretical values of the mechanical strength of mortars M0-52.5, M10–52.5, M20–52.5, M30–52.5 are 100%, 90%, 80%, 70% respectively comparing to the
Fig. 2. Slump as a function of mortars.
Fig. 4. Compressive strength as a function of mortars.
3. Results and discussions 3.1. Mortars
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Fig. 5. Strength percentage as a function of replacement percentage of sediments.
Fig. 8. SEM micrographs of mortar M0-52.5. Magnification: 400×.
Fig. 6. Total porosity of mortars measured by MIP.
Fig. 9. SEM micrographs of sediment mortar M30–52.5. Magnification: 400×.
3.1.3. Microstructural properties of mortars The microstructure of mortars containing sediments was investigated in order to determine which parameters could explain the positive effect of sediments indirectly reported by measuring the mechanical strength. Fig. 6 shows the total porosity of mortar after 7 and 28 days curing in water measured by mercury intrusion porosimetry. The total porosity of mortar increased as the substitution of cement by sediment increased. This can be linked to lower volumes of hydrates generated by cement hydration as the amount of cement decreased. Nevertheless, sediment impacts positively cement hydration as it can be emphasized by comparing the results of M20–52.5 with respect to the mortar made with cement CEM II/A-LL 32.5 (M0-32.5). The total porosity is less for the mortar containing sediment (M20–52.5). Indeed, values of total porosity of M0-32.5 are more comparable with M30–52.5 that contains 30% of sediment. As sediments did not appear to significantly reduce the initial compactness, the positive effect of sediment would be associated to the increase of cement hydration degree lead to compensate partly the volume loss of hydrates due to the decrease of cement content. The pore size distribution of the cement paste of mortars measured by MIP can lead to further information (Fig. 7). Indeed, the presence of sediment lead to an increase of the pore volume between 0.006 and 0.05 μm associated to a decrease of the pore volume between 0.06 and
Fig. 7. Pore size distribution of mortar measured by MIP.
reference mortar). Nevertheless, the mechanical strength of mortar M20–52.5 with 16.6% replacement of cement by sediment by mass was higher than that of mortar made with cement CEM II/A-LL 32.5 containing about 17.3% of limestone. Limestone filler is known to have a beneficial physico-chemical effect on the cement hydration [34,35,48,49]. Thus, it could be hypothesized that fine marine sediment may also have a positive impact on cement hydration and thus on the microstructure of the cement paste leading to an improvement of the mechanical strength of mortars.
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Fig. 10. X-Ray maps of sediment mortar M30–52.5. Magnification: 400×. (The value above each figure means the pixel intensity, which relates to element concentration).
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concrete after 28 days curing in water. The splitting tensile strength of concrete (Rt) and compressive strength of concrete (Rc) decreased as the substitution of cement by sediment increased. The compressive strength of concrete was 93.7% when the substitution of sediment was 10%, which was higher than 90% considered sediment as inert filler. However, when the substitution rate increased to 20%, the compressive strength was 73.3% compared to the reference concrete, which was lower than 80% considered sediment as inert filler. As mentioned in the mortar part, the ground sediment thus had a beneficial effect on the hydration of the cement, but a high substitution rate led to minimize the beneficial impact of the sediment as the solid volume gain was not large enough due to the decrease of cement content. This parameter is all the more critical that the total porosity is high especially due to a worse initial compactness which is the case in a concrete compared to a mortar. Nevertheless, the compressive strength of concrete made with 20% sediment could reach 30 MPa after 28 days (38.4 MPa for concrete made with 10% sediment) that makes this mix design sufficient for numerous cases requiring unreinforced concrete.
Table 8 Properties of hardened concrete after 28 days curing in water.
3
Saturated Density (g/cm ) Rc (MPa) Rc percentage with reference concrete (%) Rt (MPa) Rt percentage with reference concrete (%) Ratio of Rt/Rc E (GPa) E percentage with reference concrete (%)
C-0S
C-10 S
C-20 S
2.585 41.0 100 4.6 100 0.112 35.8 100
2.565 38.4 93.7 3.9 84.5 0.102 33.4 93.3
2.549 30.0 73.3 3.6 78.2 0.120 28.1 78.4
0.1 μm. Sediments allowed for a finer distribution of porosity, mostly associated to a better texture of C-S-H which is beneficial for the mechanical strength but also for durability. Backscattered electron was performed on flat polished section in order to depict some microstructural modifications that could be induced by sediment despite that the direct observation of C-S-H texture requires higher magnification associated to transmission electron microscopy observation. Micrographs of the cement paste of mortars M0-52.5 and M30–52.5 are presented in Fig. 8 and Fig. 9 respectively. Sediment could be evidenced in areas rich in Al and Si containing alkalis (Na and K) and having low Ca content (Fig. 10) [35]. This specific chemical composition allowed us to locate the sediment grains in the cement paste which had a high Ca content but lower Si, Al and alkali contents (Fig. 10) [50,51]. The X-ray maps actually revealed that sediment was homogeneously distributed in the cement paste and thus engulfed by hydrates (Fig. 10). Moreover, only a few anhydrous cement particles were present in the microstructure of mortar M30–52.5 containing sediment compared to the reference mortar, indicating that the percentage of reaction of cement hydration was greater in the presence of sediment. Thus, it could be assumed that sediment acts as nucleating surface for CS-H such as limestone but even more efficiently. Indeed, nucleating C-SH on additional surface than clinker grains, lead to reach more rapidly higher percentage of reaction before the hydration kinetics is restricted by the hydrate layer formed around clinker grains.
4. Conclusions In this study, uncontaminated marine dredged sediments, were dried and ground and subsequently used in partial cement replacement in the manufacture of mortars and concrete. A given volume of cement has been replaced by the same volume of sediment for three substitution contents (10%, 20%, 30%) of a Portland cement CEM I 52.5. As expected, the flexural and compressive strengths of both mortars and concretes decreased when part of the cement was substituted by sediment. However, the mechanical properties of the mortar with 20% replacement of cement with sediments were better than those of a mortar made from cement CEM II/A-LL 32.5 containing a proportion of limestone similar to the sediment substitution. The total porosity measured by mercury intrusion porosimetry of different types of mortars showed that the porosity increased as the sediment substitution content increased but the pore size distribution was shifted toward smaller pores. Sediment appeared to have a positive impact on cement hydration leading to higher percentage of reaction and thus compensating partly the reduction of cement content due to its substitution by sediment. Concrete C30/37 could be designed with 20% cement replaced by sediment without the use of admixture. Nevertheless, the beneficial effect of sediment is more efficient on mortars than on concretes with respect to their mechanical properties (as demonstrated in Fig. 4 and Table 8). Moreover, it appeared that sediments positively influenced the cement hydration even more effectively than limestone fillers. Thus, uncontaminated sediments could be a very promising for producing concrete for works requiring unreinforced concrete that may contain up to 1% of chloride with respect to the cement content. Indeed, even reasonable substitution amounts of sediments (< 20% in volume) could efficiently consume large amounts of uncontaminated dredged marine sediments. Supplementary experiments would be needed to assess the durability of concrete containing sediments but also the rebar corrosion in order to extend their use to reinforced concrete.
3.2. Concrete 3.2.1. Properties of fresh concrete The slump of concrete was 12.5 cm, 9.5 cm and 4 cm for concretes C-0S, C-10 S and C-20 S respectively. The slump decreased more markedly with larger substitution of cement by sediment as observed for the mortar. Similarly to mortars, the higher specific surface area and porous nature of sediment lead to water adsorption by sediment and thereby the free water quantity decreased, leading to a significant loss of workability. The fresh density of concrete (2.585, 2.562, 2.548 g/cm3 for C-0S, C-10 S, C-20 S respectively) slightly decreased as the substitution of cement by sediment increased, which is certainly due to the lower density of sediment compared to cement.
Acknowledgements
3.2.2. Mechanical properties of concrete The measurement of the elastic modulus was obtained on cylinders using three LVDTs (linear variable differential transducer). The elastic modulus E of concrete was obtained from the slope of the secant line between initial and 30% of the ultimate compressive strength on the stress-strain curve. The mean values of the elastic modulus are given in Table 8. The elastic modulus decreased with the increase of cement substitution by sediment. The elastic modulus of concrete C-10 S was 93.3% (78.4% for concrete C-20 S) when the substitution of sediment was 10%. Compressive strength is the mostly widely used requirement for hardened concrete, and moreover it is used as a general durability indicator. Table 8 shows the average mechanical strength of hardened
Authors would like to thank the Dunkirk port for providing us sediment and Lafarge Company for providing us cement. Authors would also like to thank the Holcim Company for supplying natural aggregates. Finally, authors would also like to thank Sustainable Environmental Treatment and Reuse of Marine Sediment (SETARMS) project for its financial support. References [1] D. Wang, Solidification et valorization de sediments du port de Dunkerque en travaux routiers, Ph.D. thesis Ecole des Mines de Douai, 2011.
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Cement and Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp
Use of uncontaminated marine sediments in mortar and concrete by partial substitution of cement
T
Zengfeng Zhaoa,b,∗, Mahfoud Benzerzoura, Nor-Edine Abriaka, Denis Damidota, Luc Courardb, Dongxing Wangc a
IMT Lille Douai, Univ. Lille, EA 4515 - LGCgE, Civil and Environmental Engineering Department, F-59000, Lille, France Department of ArGEnCo, GeMMe Building Materials, Urban and Environment Research Unit, University of Liège, Liège, Belgium c School of Civil Engineering, Wuhan University, Wuhan, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Sediments Mortar Substitution of cement Mechanical properties Porosity Concrete
The disposal of dredged marine sediments has become a major economic and environmental issue in the world. In this study, uncontaminated marine sediments dredged in the harbor of Dunkirk (France) were dried and ground and then used in partial substitution of cement in the manufacture of mortars and concretes. A given volume of cement has been replaced by the same volume of sediment for three substitution contents (10%, 20%, 30%) of a Portland cement CEM I 52.5. The flexural and compressive strengths of mortars decreased when the sediment replacement content increased. However, the mechanical properties of the mortar with 20% replacement of cement with sediments were better than those of a mortar made from cement CEM II/A-LL 32.5 containing a proportion of limestone similar to the sediment substitution. The total porosity measured by mercury intrusion porosimetry of different types of mortars showed that the porosity increased as the sediment substitution content increased but the pore size distribution was shifted toward smaller pores. Finally, it was demonstrated that concrete C30/37 could be designed with 20% cement replaced by sediment without the use of admixture. Additionally, this concrete fulfilled the standards with respect to the total chloride content required for unreinforced concrete. As a conclusion, dried and finely ground uncontaminated sediments appeared to be a very interesting constituent for partially substituting up to 20% of cement as its efficiency overpass limestone filler.
1. Introduction A large amount of sediment is dredged for navigation every year. In France, about 50 million m3 of sediment are produced from harbors. While in China, about 400 million m3 of sediment are dredged annually [1]. Thus, the disposal of dredged sediment has become a major economic and environmental issue in the world [2–4]. The sediment can contain a variable amount of organic pollutants (PAHs: polycyclic aromatic hydrocarbons, PCBs: polychlorinated biphenyls, TBT: tributyltin and dioxins) and different levels of inorganic contaminants (heavy metals such as As, Cd, Pb, Cr, Cu, Ni, Hg and Zn). Polluted sediments can be treated for example by Novosol process which consists of two major phases: phosphatation (2–3.5% of phosphoric acid H3PO4) and calcinations at ≥ 650 °C [5]. Treated sediments have been successfully incorporated into the brick production as a raw material [6,7]. The industrial-scale experiment showed that bricks made of sediments would have no environmental impact restricting
∗
their application [8]. So far, a lot of research are devoted to reuse marine dredged sediments as a new material resource as foundation and base layers for road construction [9–14]. The results showed that the solidification/stabilization of sediment by using cement, lime and other additives such as fly ash and slag can satisfy the mechanical properties specification and the prescribed thresholds for environmental impact [10,13]. However, there are fewer studies on the reuse of sediments in mortar or concrete comparing to the reuse of sediment in the road construction [15–18]. Concrete is the first material used in the world which needs very large amounts of cement, aggregate and water [19,20]. In the last decades, the interest of reusing recycled aggregate and sand or recycled filler in the concrete industry was demonstrated [21–26]. Following this trend, it thus appears that the use the sediment in the manufacture of mortars and concretes is very promising [27,28]. Some authors reported the production of lightweight coarse aggregate (LWA) at the temperatures of 1100–1200 °C by using sediments for the
Corresponding author. IMT Lille Douai, Univ. Lille, EA 4515 - LGCgE, Civil and Environmental Engineering Department, F-59000, Lille, France. E-mail address: [email protected] (Z. Zhao).
https://doi.org/10.1016/j.cemconcomp.2018.07.010 Received 8 November 2017; Received in revised form 1 June 2018; Accepted 17 July 2018 Available online 18 July 2018 0958-9465/ © 2018 Elsevier Ltd. All rights reserved.
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masonry and concrete [29,30]. The results showed that the LWA ranged from 1010 to 1380 kg/m3 for the particle density, and met the requirement for the lightweight aggregate. The concrete made from sedimentary LWA ranged from 19.8 to 34.7 MPa, can satisfy the requirement for structural lightweight concrete [30]. Some authors investigated the possibility of sediments as sand in the cement-based materials [2,18,31]. Agostini et al. replaced sand by sediment treated by Novosol process at substitution levels of 33%, 66% and 100% [31]. Despite that the fine fraction of sediment had a high porosity leading to a higher amount of absorbed water and greater drying shrinkage of mortar, mortars made with 33% of sediment showed significant compression strength improvement (up to 20%). The formation of a denser interfacial transition zone in the presence of treated sediment was expected to be at the origin the reported mechanical strength improvement [2,31]. Couvidat et al. also indicated that the use of the coarse fraction marine sediment offered an interesting valorization potential as sand in the cement mortars for non-structural applications [18]. Some authors studied the use of contaminated sediment to replace a portion of raw materials in the production of Portland cement clinker [32,33]. Aouad et al. showed that the laboratory manufactured cement based on the sediment produced the equivalent compressive strengths to those commercially produced Portland cement [32]. Dang et al. investigated the new blended cements made of a mixture of Portand cement and 8%, 16% and 33% of thermally treated sediment at 650 °C and 850 °C. The blend cement based on the sediment treated at 650 °C involved higher compressive strength than the one based on the classical calcareous filler [34]. Rozière et al. studied the use of treated sediments at 650 °C in self-consolidating concrete as a replacement of limestone filler and aggregates. The compressive strength of sedimentbased concrete specimen was comparable to that of reference concrete [35]. However, fewer studies were devoted to the valorization of uncontaminated marine sediment just subjected to drying at ambient temperature for long period or at moderated temperature if the process has to be speed up. This study is thus reporting the use of marine sediments, dried at 40 °C and then ground, as partial substitution of cement in the manufacture of mortars and concretes. Three contents of sediments were used as cement CEM I 52.5 substitution (10%, 20%, 30%) to produce mortars. The fresh properties such as fresh density, slump and the mechanical properties of mortars were measured and the microstructural properties of mortars were also studied. Finally concretes were designed from the results obtained on mortars.
Table 1 Chemical composition of sediment. Element symbol
Element name
Percentage (%)
O Na Mg Al Si P S Cl K Ca Ti Fe Pb C Mn Cr Cu Zn Sr Zr V
Oxygen Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Potassium Calcium Titanium Iron Lead Carbon Manganese Chromium Copper Zinc Strontium Zirconium Vanadium
50.2 1.8 1.1 4.6 16.5 0.1 1.5 2.1 1.5 16.2 0.3 3.7 0.1 Present Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1 Traces < 0.1
Table 2 Leaching test results and limiting values of classification of wastes from 2003/ 33/CE.
2. Materials and methods 2.1. Sediments characterization
Tests
ICP (Inductively Coupled Plasma) values
2003/33/ CE Class III: inert waste
2003/33/CE Class II: nonhazardous waste
2003/33/CE Class I: hazardous waste
pH As (mg/kg) Ba (mg/kg) Cd (mg/kg) Cr (mg/kg) Cu (mg/kg) Mo (mg/kg) Ni (mg/kg) Pb (mg/kg) Sb (mg/kg) Se (mg/kg) Zn (mg/kg) Chlorides (mg/kg) Sulfates (mg/kg)
8.74 0.117 0.909 0.009 0.043 0.485 1.101 0.076 < 0.082 0.118 0.135 0.125 20555
– < < < < < < < < < < < <
– < < < < < < < < < < < <
– < < < < < < < < < < < <
3485
< 1000
0.5 20 0.04 0.5 2 0.5 0.4 0.5 0.06 0.1 4 800
2 100 1 10 50 10 10 10 0.7 0.5 50 15000
< 20000
25 300 5 70 100 30 40 50 5 7 200 25000
< 50000
12457–2 [38]. Metallic elements and ions such as chlorides and sulphates were analyzed. According to the prescribed limits criteria and procedure for the acceptance of waste at landfills in European directive 2003/33/CE [39], all the heavy elements were in the limit of Class III (inert waste) except for the Mo and Se in the Class II (non-hazardous waste). Thus the sediments were not contaminated with heavy metals but they contained a high chloride content that impacts the mix design of concrete. Indeed, chloride ions can catalyze the corrosion of steel rebar contained in reinforced concrete. For these reasons, limits of the amount of chloride in cement and thus in the present case, of the amount of marine sediment, will depend on the type of concrete and its usage as defined in the standard EN 206–1 [40] (Table 3). The aim was to use the maximum quantity of sediment in the concrete mix design, thus unreinforced concrete was targeted in order to keep the chloride content at value less than 1% relative to the cement.
The used marine sediments were dredged in the port of Dunkirk (France). The measured initial water content of sediment was about 95%. The clay fraction activity was low (3.2% measured by the methylene blue test). Then they were dried at 40 °C until constant weight and then ground in a laboratory mill (The treatment of 40 °C consumes less energy comparing to the treatment at 650 °C. Here, grinding is necessary in order to use to use all fractions of sediment). In this study, the fraction 0/80 μm was used for partially replacing cement in the manufacture of mortars. The organic matter content was measured as 13.8% according to the standard XP P94-047 [36] by calcination in oven at 450 °C for 3 h. The true density was 2.48 g/cm3 measured by helium pycnometer, which is lower than Portland cement. Table 1 shows the chemical composition of sediment determined by X-ray fluorescence. The major chemical elements of the sediment were oxygen, silicon, calcium, aluminum and iron. PAHs (polycyclic aromatic hydrocarbons) value was 0.29 mg/kg and PCBs (polychlorinated biphenyls) value was 6.34 mg/kg, which were under the limited value according to AMATR [37]. Table 2 shows the average values of leaching test results with a liquid to solid ratio of ten according to standard EN
2.2. Mix design of mortars Portland cement CEM I 52.5 and CEM II/A-LL 32.5 were used to produce reference mortars. The density of these two cements measured 156
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Table 3 Maximum chloride content of concrete according to EN 206–1. Concrete use
Chloride content class
Not containing steel reinforcement or other embedded metal Containing steel reinforcement or other embedded metal Containing pre-stressing steel reinforcement or other embedded metal
Maximum Chloride content by mass of cement
Cl 1.0
1.00%
Cl Cl Cl Cl
0.20% 0.40% 0.10% 0.20%
0.20 0.40 0.10 0.20
Table 5 Physical properties of cements and sediment.
2
Specific surface area Blaine (m /g) Specific surface area BET (m2/g) Median particle size (μm) Mean particle size (μm)
CEM II/A-LL 32.5
Sediment
0.33 0.80 15.71 12.33
0.29 1.10 22.47 14.32
0.82 9.10 14.32 7.19
Table 6 Compositions of mortars.
Sand (g) Cement (g) Sediment (g) Efficient water (g) Eeff/(C + S) Content of Cl− to cement (%)
Table 4 Mineralogical composition of cements determined by XRD-Rietveld method.
CEM I 52.5 CEM II/ALL 32.5
CEM I 52.5
C3S
C2S
C3A
C4AF
Anhydrite
Gypsum
Quartz
Calcite
68.11
22.62
0.68
7.81
0.05
0.53
0.2
0
52.93
13.91
7.42
–
0.74
2.63
2.31
17.31
M0-52.5
M10–52.5
M20–52.5
M30–52.5
M0-32.5
1350 450 0 225 0.5 0.00
1350 405 35.77 225 0.51 0.18
1350 360 71.54 225 0.52 0.40
1350 315 107.31 225 0.53 0.68
1350 450 0 225 0.5 0
was followed according standard EN 196–1 [43]. The highest content of chloride was 0.68% for the mortar M30–52.5, which was lower than the limiting value for unreinforced mortar or concrete.
by helium pycnometer was 3.12 and 2.99 g/cm3 respectively. Table 4 reports the mineralogical composition of these two cements determined by XRD using Rietveld method. Fig. 1 presents laser granular analysis of the two cements and the sediment used as constituents in the mortars. The sediment contained a higher proportion of fine particles (1 μm–20 μm) than cements. Table 5 reports the specific surface determined by Blaine according to standard EN 196–6 [41] and by the N2 BET method, and the mean and median diameters of cements and sediment. The specific surface area of sediment was three times larger than that of the cements according to the Blaine method and ten times larger than the cements according to the BET method. These differences are due to the different methods of measurement knowing the value given by BET method is more accurate but Blaine method is used in the standards. Crushed calcareous natural sand sourced from Tournai (provided by Holcim France Benelux) was used for the manufacture of mortars. This sand had a density of 2.66 g/cm3and water absorption of 1.05% according to standard EN 1097–6 [42]. Four mortars were made with CEM I 52.5 being replaced by the same volume of sediment at replacement levels of 0%, 10%, 20% and 30% (noted M0-52.5, M10–52.5, M20–52.5, M30–52.5 respectively). A mortar was also made with cement CEM II/A-LL 32.5 (noted M0-32.5) to compare the effect of sediment relative to limestone. Table 6 shows the compositions of the studied mortars. A precise mixing procedure
2.3. Experiments on mortar After mixing, the slump of mortar was measured with Abrams' minicone (h = 150 mm, D = 100 mm, d = 50 mm). This cone is used to determine the mix-design of a mortar having rheological properties correlated with those of a concrete made with similar constituents at the exception of the aggregates. The preparation of specimens (40 mm × 40 mm × 160 mm) for mechanical strength tests was followed in accordance with standard EN 196–1. The flexural and compressive strengths of hardened mortar were determined in accordance with standard EN 196–1. These two mechanical tests were carried out with an INSTRON 5500 R 4206–006 (loading capacity of 1500 KN) after being cured 7 and 28 days in water. The porosity of mortar was tested by using Mercury Intrusion Porosimetry (MIP) technique (Micromeritics Autopore IV). The microstructure of mortar was also observed by Scanning Electron Microscope (SEM) with Energy Dispersive x-ray Spectroscopy (EDS). After 28 days curing in the water, pieces of hydrated samples of 10 mm × 10 mm × 10 mm were cut from the core of the mortar samples. After the solvent exchange to stop hydration, the samples were passed to vacuum to remove solvent and stored in a desiccator. Then they were impregnated in the resin and polished down to 0.25 μm by using diamond pastes. The samples were coated with carbon and examined by using machine HITACHI S4300SE/N. A voltage of 15 kV and the model of “backscattered electrons diffraction” were used for the observation. 2.4. Mix design of concretes Three concretes (noted C-0S, C-10 S, C-20 S) were made from Portland cement CEM I 52.5, which corresponded respectively to 0%, 10% and 20% of cement being replaced by the same volume of sediment. The used cement and sediment were the same as used for mortar manufacture. The concretes were made with a crushed calcareous natural sand (the same as for the manufacture of mortars) and calcareous coarse aggregate sourced from Tournai (provided by Holcim France Benelux). Table 7 shows the detailed compositions of concrete. The highest content of chloride was 0.40% for the concrete C-20 S, which was lower than the limiting value for non-steel reinforcement concrete and equal to the maximum limiting value for steel reinforced (Table 3). The water absorption of the crushed calcareous coarse aggregate was 0.8% according to standard EN 1097–6. The air dried
Fig. 1. Particle size distributions of cements and sediment used in mortars. 157
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Table 7 Compositions of concrete (1 m3).
Coarse aggregate (kg) Sand (kg) Cement (kg) Sediment (kg) Efficient water (kg) Absorbed water (kg) Weff/(C + S) Content of Cl− to cement (%)
C-0S
C-10 S
C-20 S
1061.45 768.63 335 0 208.37 16.56 0.62 0.00
1061.45 768.63 301.50 26.63 204.10 16.56 0.62 0.18
1061.45 768.63 268 53.26 199.82 16.56 0.62 0.40
natural sand and coarse aggregate were used for all concretes, and water compensation was adjusted according to the water content and water absorption of natural sand and coarse aggregate during concrete batching.
Fig. 3. Flexural strength as a function of mortars.
M20–52.5, M30–52.5 respectively) slightly decreased as the substitution of cement by sediment increased, which is certainly due to the lower density of sediment compared to cement.
2.5. Experiments on concrete After mixing, the workability of fresh concrete was measured by slump test using the Abrams cone according to EN 12350–2 [44]. For each concrete mix, nine cylindrical specimens (Ø110 mm x H220 mm) were cast to determine the mechanical properties. All specimens were cast in two layers and compacted on a vibrating table until no more air bubbles appeared. They were covered with a plastic sheet and left in the mould in the laboratory at 20 ± 2 °C for 24 h. After that, all the cylinders were cured in water until ages of 28 days. The compressive strength was measured according to standard EN 12390–3 [45] using a compression machine. The loading rates used were 0.6 ± 0.2 MPa/s for the compressive test. The splitting tensile strength was determined according to standard EN 12390–6 [46]. The loading rates used were 0.05 ± 0.01 MPa/s for the splitting tensile strength. Modulus of elasticity was measured by three loading cycles according to standard EN 12390–13 [47].
3.1.1. Fresh properties of mortars The slump value of mortar decreased when the substitution of sediment increased (Fig. 2). This trend could be due to the higher specific surface area of sediment compared to cement. Thus, part of the mixing water was expected to be adsorbed by sediment and thereby the free water quantity decreased, leading to a significant loss of workability. The slump value of mortar made with CEM II/A-LL 32.5 was similar than the reference mortar made with CEM I 52.5 indicating that limestone filler did not modify the amount of free water. The fresh density of mortar (2.37, 2.371, 2.346, 2.329 g/cm3 for M0-52.5, M10–52.5,
3.1.2. Mechanical properties of mortars Fig. 3 and Fig. 4 present the flexural and compressive strengths of mortars (average values obtained by three measurements for flexural strength and six measurements for compressive strength). Both flexural and compressive strengths of mortars after 28 days decreased when the substitution of cement by sediment increased (Figs. 3 and 4). This result is in agreement with the dilution of cement when part of the cement is substituted by unreactive or slightly reactive compounds. Nevertheless after 7 days, the compressive strength of M10–52.5 was slightly higher than the reference mortar without sediment indicating that ground sediment was not inert. The efficiency of sediment can be refined by drawing the percentage of mechanical strength of sediment mortar with respect to the unsubstituted reference mortar as a function of the amount of cement substituted by sediment (Fig. 5). First, the slope of all curves is not monotonous indicating that the efficiency of sediment varies not linearly with the amount of cement that is substituted by sediment. Substituting more than 10% of cement by sediment is not recommended with respect to compressive strength whereas amounts up to 20% are still acceptable with respect to flexural strength. The flexural strength of mortar M20–52.5 after 28 days was about 10.7 MPa, which is slightly lower than that of mortar M0-52.5 (12.1 MPa). If 30% of cement is substituted by sediment, both flexural and compressive strengths are close to the values obtained theoretically by considering the diluting effect of the cement by inert filler (For the calculation of the theoretical value (dashed line in Fig. 5), we assume that the mechanical properties of mortars decrease linearly with the amount of substitution of cement by inert filler. The theoretical values of the mechanical strength of mortars M0-52.5, M10–52.5, M20–52.5, M30–52.5 are 100%, 90%, 80%, 70% respectively comparing to the
Fig. 2. Slump as a function of mortars.
Fig. 4. Compressive strength as a function of mortars.
3. Results and discussions 3.1. Mortars
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Fig. 5. Strength percentage as a function of replacement percentage of sediments.
Fig. 8. SEM micrographs of mortar M0-52.5. Magnification: 400×.
Fig. 6. Total porosity of mortars measured by MIP.
Fig. 9. SEM micrographs of sediment mortar M30–52.5. Magnification: 400×.
3.1.3. Microstructural properties of mortars The microstructure of mortars containing sediments was investigated in order to determine which parameters could explain the positive effect of sediments indirectly reported by measuring the mechanical strength. Fig. 6 shows the total porosity of mortar after 7 and 28 days curing in water measured by mercury intrusion porosimetry. The total porosity of mortar increased as the substitution of cement by sediment increased. This can be linked to lower volumes of hydrates generated by cement hydration as the amount of cement decreased. Nevertheless, sediment impacts positively cement hydration as it can be emphasized by comparing the results of M20–52.5 with respect to the mortar made with cement CEM II/A-LL 32.5 (M0-32.5). The total porosity is less for the mortar containing sediment (M20–52.5). Indeed, values of total porosity of M0-32.5 are more comparable with M30–52.5 that contains 30% of sediment. As sediments did not appear to significantly reduce the initial compactness, the positive effect of sediment would be associated to the increase of cement hydration degree lead to compensate partly the volume loss of hydrates due to the decrease of cement content. The pore size distribution of the cement paste of mortars measured by MIP can lead to further information (Fig. 7). Indeed, the presence of sediment lead to an increase of the pore volume between 0.006 and 0.05 μm associated to a decrease of the pore volume between 0.06 and
Fig. 7. Pore size distribution of mortar measured by MIP.
reference mortar). Nevertheless, the mechanical strength of mortar M20–52.5 with 16.6% replacement of cement by sediment by mass was higher than that of mortar made with cement CEM II/A-LL 32.5 containing about 17.3% of limestone. Limestone filler is known to have a beneficial physico-chemical effect on the cement hydration [34,35,48,49]. Thus, it could be hypothesized that fine marine sediment may also have a positive impact on cement hydration and thus on the microstructure of the cement paste leading to an improvement of the mechanical strength of mortars.
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Fig. 10. X-Ray maps of sediment mortar M30–52.5. Magnification: 400×. (The value above each figure means the pixel intensity, which relates to element concentration).
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concrete after 28 days curing in water. The splitting tensile strength of concrete (Rt) and compressive strength of concrete (Rc) decreased as the substitution of cement by sediment increased. The compressive strength of concrete was 93.7% when the substitution of sediment was 10%, which was higher than 90% considered sediment as inert filler. However, when the substitution rate increased to 20%, the compressive strength was 73.3% compared to the reference concrete, which was lower than 80% considered sediment as inert filler. As mentioned in the mortar part, the ground sediment thus had a beneficial effect on the hydration of the cement, but a high substitution rate led to minimize the beneficial impact of the sediment as the solid volume gain was not large enough due to the decrease of cement content. This parameter is all the more critical that the total porosity is high especially due to a worse initial compactness which is the case in a concrete compared to a mortar. Nevertheless, the compressive strength of concrete made with 20% sediment could reach 30 MPa after 28 days (38.4 MPa for concrete made with 10% sediment) that makes this mix design sufficient for numerous cases requiring unreinforced concrete.
Table 8 Properties of hardened concrete after 28 days curing in water.
3
Saturated Density (g/cm ) Rc (MPa) Rc percentage with reference concrete (%) Rt (MPa) Rt percentage with reference concrete (%) Ratio of Rt/Rc E (GPa) E percentage with reference concrete (%)
C-0S
C-10 S
C-20 S
2.585 41.0 100 4.6 100 0.112 35.8 100
2.565 38.4 93.7 3.9 84.5 0.102 33.4 93.3
2.549 30.0 73.3 3.6 78.2 0.120 28.1 78.4
0.1 μm. Sediments allowed for a finer distribution of porosity, mostly associated to a better texture of C-S-H which is beneficial for the mechanical strength but also for durability. Backscattered electron was performed on flat polished section in order to depict some microstructural modifications that could be induced by sediment despite that the direct observation of C-S-H texture requires higher magnification associated to transmission electron microscopy observation. Micrographs of the cement paste of mortars M0-52.5 and M30–52.5 are presented in Fig. 8 and Fig. 9 respectively. Sediment could be evidenced in areas rich in Al and Si containing alkalis (Na and K) and having low Ca content (Fig. 10) [35]. This specific chemical composition allowed us to locate the sediment grains in the cement paste which had a high Ca content but lower Si, Al and alkali contents (Fig. 10) [50,51]. The X-ray maps actually revealed that sediment was homogeneously distributed in the cement paste and thus engulfed by hydrates (Fig. 10). Moreover, only a few anhydrous cement particles were present in the microstructure of mortar M30–52.5 containing sediment compared to the reference mortar, indicating that the percentage of reaction of cement hydration was greater in the presence of sediment. Thus, it could be assumed that sediment acts as nucleating surface for CS-H such as limestone but even more efficiently. Indeed, nucleating C-SH on additional surface than clinker grains, lead to reach more rapidly higher percentage of reaction before the hydration kinetics is restricted by the hydrate layer formed around clinker grains.
4. Conclusions In this study, uncontaminated marine dredged sediments, were dried and ground and subsequently used in partial cement replacement in the manufacture of mortars and concrete. A given volume of cement has been replaced by the same volume of sediment for three substitution contents (10%, 20%, 30%) of a Portland cement CEM I 52.5. As expected, the flexural and compressive strengths of both mortars and concretes decreased when part of the cement was substituted by sediment. However, the mechanical properties of the mortar with 20% replacement of cement with sediments were better than those of a mortar made from cement CEM II/A-LL 32.5 containing a proportion of limestone similar to the sediment substitution. The total porosity measured by mercury intrusion porosimetry of different types of mortars showed that the porosity increased as the sediment substitution content increased but the pore size distribution was shifted toward smaller pores. Sediment appeared to have a positive impact on cement hydration leading to higher percentage of reaction and thus compensating partly the reduction of cement content due to its substitution by sediment. Concrete C30/37 could be designed with 20% cement replaced by sediment without the use of admixture. Nevertheless, the beneficial effect of sediment is more efficient on mortars than on concretes with respect to their mechanical properties (as demonstrated in Fig. 4 and Table 8). Moreover, it appeared that sediments positively influenced the cement hydration even more effectively than limestone fillers. Thus, uncontaminated sediments could be a very promising for producing concrete for works requiring unreinforced concrete that may contain up to 1% of chloride with respect to the cement content. Indeed, even reasonable substitution amounts of sediments (< 20% in volume) could efficiently consume large amounts of uncontaminated dredged marine sediments. Supplementary experiments would be needed to assess the durability of concrete containing sediments but also the rebar corrosion in order to extend their use to reinforced concrete.
3.2. Concrete 3.2.1. Properties of fresh concrete The slump of concrete was 12.5 cm, 9.5 cm and 4 cm for concretes C-0S, C-10 S and C-20 S respectively. The slump decreased more markedly with larger substitution of cement by sediment as observed for the mortar. Similarly to mortars, the higher specific surface area and porous nature of sediment lead to water adsorption by sediment and thereby the free water quantity decreased, leading to a significant loss of workability. The fresh density of concrete (2.585, 2.562, 2.548 g/cm3 for C-0S, C-10 S, C-20 S respectively) slightly decreased as the substitution of cement by sediment increased, which is certainly due to the lower density of sediment compared to cement.
Acknowledgements
3.2.2. Mechanical properties of concrete The measurement of the elastic modulus was obtained on cylinders using three LVDTs (linear variable differential transducer). The elastic modulus E of concrete was obtained from the slope of the secant line between initial and 30% of the ultimate compressive strength on the stress-strain curve. The mean values of the elastic modulus are given in Table 8. The elastic modulus decreased with the increase of cement substitution by sediment. The elastic modulus of concrete C-10 S was 93.3% (78.4% for concrete C-20 S) when the substitution of sediment was 10%. Compressive strength is the mostly widely used requirement for hardened concrete, and moreover it is used as a general durability indicator. Table 8 shows the average mechanical strength of hardened
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