1 Introduction
2 Experimental Investigation
2.1 Materials
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Calcined silt (CS): silt used in this study was taken from stockpiles of dredged material (Dam of CHOURFA in the North–West Algeria). It was calcined at a temperature of 750 °C for 5 h. The calcined silt (CS) are crushed and then further pulverized in a ball mill until the particles could pass through an 80 μm.
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Silica fume (SF) named MEDAPLAST HP from the Granitex Company (Algeria), containing 94 % SiO2, it is a pozzolanic addition with gray color, used in the formulation of high performance concrete (HPC).
Portland cement (PC) | Silica fume (SF) | Calcined silt (CS) | |
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SiO2 (%) | 21.1 | 94.30 | 51.36 |
Al2O3 (%) | 4.2 | 0.69 | 12.78 |
Fe2O3 (%) | 5.32 | 0.14 | 5.18 |
CaO (%) | 61.8 | 1.17 | 17.08 |
MgO (%) | 2.3 | 0.29 | 2.29 |
SO3 (%) | 2.0 | 0.00 | 0.00 |
Na2O + K2O (%) | 0.6 | 1.67 | 2.42 |
LOI (%) | 2.28 | – | 7.06 |
[SiO2 + Al2O3 + Fe2O3] (%) | 30.62 | 95.13 | 69.32 |
∑ (%) | 99.64 | 98.26 | 98.17 |
Specific gravity | 3.1 | 2.71 | 2.38 |
2.1.1 Preparation of Metakaolin
2.2 Mixture Proportions
Notation | Description | Portland cement (PC) | Water (W) | Sand (S) | Calcined silt (CS) | Silica fume (SF) |
---|---|---|---|---|---|---|
OM | Ordinary mortar | 586 | 293 | 1,758 | 0 | 0 |
MCS1 | Mortar with 5 % calcined silt | 556.7 | 29.3 | 0 | ||
MCS2 | Mortar with 10 % calcined silt | 527.4 | 58.6 | 0 | ||
MCS3 | Mortar with 15 % calcined silt | 498.1 | 87.9 | 0 | ||
MSF | Mortar with 10 % silica fume | 527.4 | 0 | 58.6 |
2.3 Casting, Curing and Testing Specimens
3 Results and Discussions
3.1 Pozzolanic Reactivity of Additions Used
additions | Concentration of CaO (g/L) | Fixed amount of CaO | |
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Concentration (g/L) | Percentage (%) | ||
SF | 0.06 | 1.12 | 95 |
CS | 0.3 | 0.88 | 74.5 |
3.2 Mechanical Strength
Mortars | Compressive strength at 28 days (MPa) | Strength activity index (%) |
---|---|---|
OM | 25.5 | 100 |
MSF | 26.7 | 104.7 |
MCS1 | 28.6 | 112.15 |
MCS2 | 30 | 117.64 |
MCS3 | 25.9 | 101.56 |
3.3 Durability of Mortars
3.3.1 Sulfuric Acid Effect
Mortars | Weight loss (%) | |||
---|---|---|---|---|
7 days | 14 days | 21 days | 28 days | |
OM | 27.48 | 37.50 | 56.22 | 57.26 |
MSF | 13.27 | 14.03 | 36.83 | 42.76 |
MCS1 | 3.61 | 5.44 | 27.63 | 29.21 |
MCS2 | 7.35 | 10.45 | 23.38 | 38.47 |
MCS3 | 3.59 | 3.59 | 26.28 | 40.83 |
3.3.2 Sulfatic Solution Effect
Mortars | Gain or loss of compressive strength compared to OM in 60 days in (%) |
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OM | 0.0 |
MSF | +7.2 |
MCS1 | +0.2 |
MCS2 | +17.3 |
MCS3 | −10.9 |
3.3.3 Chloride Ions Penetration Effect
4 Conclusion
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Metakaolin obtained from calcined silt (CS), has a pozzolanic reactivity relatively high (75 %) which has the value closest to that of silica fume.
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The study of mechanical performance showed retention of mechanical strength of mortars based on calcined silt and under certain conditions an improvement. The largest gain was obtained for a substitution rate of 10 % of cement with calcined silt; an improvement of about 18 % of the compressive strength at 28 days compared to the ordinary mortar is obtained.
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The substitution of cement by calcined silt has a beneficial effect on the durability, because this addition contributes to the densification of the hardened cement paste, by consumption of portlandite and forming CSH and CAH, also the presence of high lime content in calcined silt (17 %) have contributed to conserve the basic PH of mortars which improve their durability, consequently the weight loss is limited.
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An increase of the strength of all mortars exposed to sulfatic attack appears, due to the reaction of Na2SO4 with Ca(OH)2 to form gypsum, that complete the micro pores leading to a denser structure, which increases the compactness and this positively affects the mechanical strength at 28 and 60 days. It should increase the age of specimens’ immersion in the sulfate to see the effect of this latter.
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The addition of calcined silt reduces the penetration of chloride ions in mortars.
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The composition which substituted 15 % of cement is not appeared the good performance; compressive, flexural strength and durability but it has a similar performance, what is beneficial economic viewpoint, since it was replaced a large quantity of cement having a high manufacturing cost.