Potential Use of Calcined Silt of Dam as a Pozzolan in Blended Portland Cement

2.1.1 Preparation of Metakaolin

We prepared metakaolins in the following steps (Fig. 1):

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.

Figure 2, shows the granulometric distribution of materials established by a Laser-granulometer of Malvern instrument—MASTERSIZER 2000.

The size analysis of the all additions using a Laser grain size indicates that the fraction of grains lower than 80 μm is 80 %.

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.

Figure 8 shows the change in aspect of the various mortars (OM, MSF, MCS1, MCS2 and MCS3) after 28 days of immersion in a solution of 5 % of strong acid (H₂SO₄).

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.

Figures 9 and 10, presents the variation of flexural and compressive strength of the different types of mortar based on the duration of immersion in sulfate solution, after 28 and 60 days. Table 6 shows the gain or loss in compressive strength compared to the reference mortar for 60 days of immersion in solution of Na₂SO₄.

Mortars	Gain or loss of compressive strength compared to OM in 60 days in (%)
OM	0.0
MSF	+7.2
MCS1	+0.2
MCS2	+17.3
MCS3	−10.9

Figure 11, shows the state of various mortars immersed in sulfuric solution Na₂SO₄ after 60 days.

The results obtained showed a gain of strength for all mortars except the mortar with 15 % of calcined silt (Fig. 12). We observe an increase of the strength that appears to be due to the reaction of Na₂SO₄ with Ca(OH)₂ to form gypsum according to the following reaction (Fig. 13):

$$\text{Ca}{\left(\text{OH}\right)}_2+{\text{Na}}_2{\text{SO}}_4+{\text{2H}}_2\text{O}\to {\text{CaSO}}_4·2{\text{H}}_2\text{O}+2\text{NaOH}.$$

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 resistance to chloride ions penetration in the mortar and the concrete is one of the most important questions concerning the durability of the concrete structures. When the chloride concentration exceeds a certain threshold, a depassivation of steel occurs and there is a beginning of corrosion for reinforced steel (Thomas 1996; Alonso et al. 2000). Consequently, the development of protection materials with a resistance to chloride ions penetration is required for the concrete structures. For the tests we used cubic specimens (10 × 10 × 10) cm³. The mortars were made in accordance with the standard NFP 15-403 NF P (1996). Then the specimens are introduced into a concentrated NaCl solution with 5 %. To evaluate the durability of the mortars from the chloride ions penetration, we followed the penetration depth evolution of the ions chlorides to 28 and 60 days. The solutions were changed every 7 days until the age of 28 days and every 28 days until the age of 60 days. Each test specimen is divided into two parts and then solution of silver nitrate AgNO₃ was poured on each section according to the standard UNI 79287 (UNI 79287 1978). A whitish color appeared on the specimen surface, using a caliper measuring the chloride-ions penetration depth; the results are shown in Fig. 14. The test method followed in the present investigation to determine the chloride-ions penetration depth is reliable and accurate, and has also been recommended by other researchers (Erhan and Kasım 2007; Wee et al. 2000; Otsuki et al. 1992; Wee et al. 1999; Meck and Sirivivatnanon 2003).

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.