Potential Use of Calcined Silt of Dam as a Pozzolan in Blended Portland Cement
© The Author(s) 2014
Received: 18 July 2013
Accepted: 26 February 2014
Published: 12 July 2014
This paper presents results of an experimental study which investigates the effect of industrial pozzolan made from calcined silt of dam at 750 °C for 5 h, on mechanical properties and durability of ordinary mortar, compared to the silica fume. Mortar specimens prepared with 5, 10 and 15 % of calcined silt to substitute cement were evaluated for their compressive and flexural strength, sulfate, acid and penetration of chloride ions resistance. The results were compared with ordinary mortar (without addition) and mortar containing 10 % of silica fume. The results obtained showed that the calcined silt of dam has a high potential to be used as a pozzolanic material, it improves the strength and the durability of mortar and compete the silica fume.
A large number of studies (Mehta 1981; Ramezanianpour 1987; Massazza 1993; Kouloumbi et al. 1995; Rodriguez-Camacho 1998; Tagnit-Hamou et al. 2003) have shown that natural pozzolana have been widely used as a substitute for Portland cement in many applications because of their advantageous properties which include cost reduction, reduction in heat evolution, decreased permeability and increased chemical resistance. However, they are often associated with shortcomings such as the need to moist-curing for longer time and a reduction of strength at early ages and up to 28 days.
The use of calcined clay as a pozzolanic material for mortar and concrete has received considerable attention in recent years (Ambroise et al. 1992; Sabir et al. 2001; Al Rawas et al. 2001; Shvarzman et al. 2002). This interest is part of the widely spread attention directed towards the utilization of wastes and industrial byproducts in order to minimize Portland cement consumption, whose the manufacture whether harmful to the environment (Nkinamubanzi and Aitcin 2000).
It has been reported that the concrete incorporating 10 % of metakaolin (MK) had a higher compressive strength than the control Portland cement concrete at all ages up to 180 days (Sabir et al. 1996; Zhang and Malhotra 1995).
Siltation of dams can have the most dramatic erosion consequences; it takes a sufficient extent in Algeria. The spill is an effective way to fight, nevertheless, it creates another problem which is; “where we put the evacuated silt?”, without causing an ecologic problem!. This problem is always aroused the interest of researchers in the field of water resources. The purpose of this study is focused to this issue in recovery view, which makes it possible to valorize this silt by use in the field of civil engineering.
2 Experimental Investigation
The mortar mixtures investigated in this study were prepared with Portland cement (PC) CEMI.42.5R. Natural sand having a fineness modulus of 1.5 with 2 mm maximum aggregate size was used. The specific gravity and water absorption values were obtained as 2.62 and 0.8 % respectively.
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.
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).
Chemical composition (% in mass) and specific gravity of starting materials.
Portland cement (PC)
Silica fume (SF)
Calcined silt (CS)
Na2O + K2O (%)
[SiO2 + Al2O3 + Fe2O3] (%)
2.1.1 Preparation of Metakaolin
After drying in an oven at 105 °C, the clay (silt of dam) was crushed and sieved. The calcination temperature is about 750 °C for 5 h (Aquino et al. 2001; Guneyisi et al. 2008; Safi et al. 2011, 2012), Generally, an optimum for kaolin clay calcinations recognized in literature is between 600 and 900 °C (Ambroise et al. 1986; Sayanam et al. 1989). The product obtained (calcined silt) was kept away from air and humidity. Particles passing 80 μm are recovered for later use in the preparation of mortars.
The size analysis of the all additions using a Laser grain size indicates that the fraction of grains lower than 80 μm is 80 %.
2.2 Mixture Proportions
Mix proportions of the mortars (kg/m3).
Portland cement (PC)
Calcined silt (CS)
Silica fume (SF)
Mortar with 5 % calcined silt
Mortar with 10 % calcined silt
Mortar with 15 % calcined silt
Mortar with 10 % silica fume
2.3 Casting, Curing and Testing Specimens
3 Results and Discussions
3.1 Pozzolanic Reactivity of Additions Used
Amount of lime fixed by the additions.
Concentration of CaO (g/L)
Fixed amount of CaO
Metakaolin obtained from calcined silt (CS), has a pozzolanic reactivity relatively high (75 %) which has the value closest to that of silica fume.
The calcinations of silt at 750 °C allows the departure of the water content (the dehydroxylation) and the formation of metakaolin (Samet et al. 2007; Michel 1989) with an amorphous structure which makes it more reactive than the starting clay. It is an acid–base reaction, which explains the strong decrease in CaO concentration of the solution. The heat treatment causes the transition of the crystalline phase ordered (kaolinite) to a disordered phase (metakaolinite) by a collapse of the crystal lattice. Metakaolinite is considered a good synthetic pozzolans, through its particular reaction with lime in the presence of water to form compounds of the silicate and aluminate calcium hydrates (Wild and Khatib 1997; Frias et al. 2000; Poon et al. 2001).
3.2 Mechanical Strength
Strength activity index during 28 days.
Compressive strength at 28 days (MPa)
Strength activity index (%)
3.3 Durability of Mortars
3.3.1 Sulfuric Acid Effect
To characterize the chemical resistance in acidic medium, mortars were cast into prismatic molds (4 × 4 × 16) cm3. 24 h later, samples were removed from the molds and kept in cleansed water. After 28 days of treatment, the samples are immersed in a solution of 5 % of a strong acid sulfuric (H2SO4): the change in weight of specimens was examined after 7, 14, 21 and 28 days. The chemical solution is renewed every 7 days. In the examination day, affected parts of mortar are cleanse with water, and the samples were dried for half an hour (ASTM C 267-96 2001), and finally chemical resistant were assessed by measuring the mass loss of the specimen.
Evolution of the weight loss of mortars conserved in H2SO4.
Weight loss (%)
The weight loss of mortars increases with the immersion period. Also note that the weight loss of the mortar with additions is significantly lower than the ordinary mortar, and it reduces especially with 5 and 10 % of calcined silt and silica fume. Silica fume mortar showed 42.76 % weight loss compared to 29.21 and 38.47 % for mortar with 5 and 10 % of calcined silt, respectively. This is can certainly be related to the presence of high lime content in calcined silt (17 %) compared to the low content lime (1 %) in silica fume, the lime contribute to conserve the basic PH of specimens witch improve their durability in acid medium, consequently the weight loss is limited.
Sulfuric acid (H2SO4) is an inorganic acid. These are more harmful to concretes and mortars than the organic acids. Inorganic acids come into reaction with Ca(OH)2 contained in the hardened cement paste formed compounds readily soluble in water.
It is useful to note that after 7 days of conserving in the solution of sulfuric acid, we have recorded a loss in mass.
The gypsum formed is then washed, which gives us a whitish solution, at the age of 14 and 28 days, for all types of mortars the weight loss increases but with different speeds. It is also noted that the mass loss is less important for mortars with additions to the reference mortar; which confirms the good behavior of these mortars in sulfuric attacks (Zivika and Bajz 2002; Ghrici et al. 2007).
3.3.2 Sulfatic Solution Effect
To assess the durability of mortars in the sulfates attacks, we have determined the compressive strength according to the duration of exposure to sulfates.
To characterize the chemical resistance in sulfatic solution, mortars were cast into prismatic molds (4 × 4 × 16) cm3. 24 h later, samples were removed from the molds and kept in clean water. After 28 days of treatment, the samples are immersed in a solution of 5 % Na2SO4. The change in weight of specimens was examined after 7, 14, 21 and 28 days. The chemical solution is renewed every 7 days. In the examination day, affected parts of mortar are cleansed with water, and the samples were dried for half an hour.
Gain or loss of compressive strength.
Gain or loss of compressive strength compared to OM in 60 days in (%)
NaOH: has a high alkalinity which promotes stabilization of CSH, while the gypsum is formed only in the internal spaces of the hydrated cement paste (little or no expansion) which helps to increase the compactness of concrete and mechanical resistance in the Middle Ages therefore the compressive and flexural strength have increased at the age of 28 and 60 days. We should increase the age of specimens’ immersion in the sulfate to see the effect of this latter.
The ettringite is an expansive element which creates a physical and mechanical damage (changes in transport properties and porosity, cracks, loss of strength and cohesion). This leads to the ruin of the cementitious material, more or less long-term function of the attack (nature, content and concentration of sulfates in contact).
3.3.3 Chloride Ions Penetration Effect
The chloride-ions penetration depth of mortars, increase with the immersion period. Also note that the chloride-ions penetration depth of the mortar with additions is significantly lower than the ordinary mortar, and it reduces especially with 10 % of calcined silt and silica fume. This is can certainly be related to the high pozzolanic reactivity of calcined silt and silicafume, can promote pozzolanic reaction of particles of this addition with the calcium hydroxide Ca(OH)2 released from cement hydration leading to pore size and grain size refinement processes which can strengthen the microstructure and reduce microcracking. This is explained also by the great finesse of these additions, these additions fill the pores and capillaries, increasing the compactness of the mortar which hinders the penetration of chloride ions.
The use of metakaolin in cement has gained significant importance because of the requirements of environmental protection and sustainable construction in the future.
Metakaolin obtained from calcined silt (CS), has a pozzolanic reactivity relatively high (75 %) which has the value closest to that of silica fume.
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.
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.
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.
The addition of calcined silt reduces the penetration of chloride ions in mortars.
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.
Finally, we can say that this experimental work opens the way for the use of silt of dam as metakaolin in the cement industry and to improve the physico-mechanical properties and the durability of cementitious materials.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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