DURABILITY OF CONCRETE UNDER THE INFLUENCE OF SULPHATES ATTACK IN THE REGION OF OUARGLA – ALGERIA
Mohammed-Amin BOUMEHRAZ
Civil engineering department, Faculty of sciences and technology.
University of Biskra.
amine18gc@yahoo.com
Mekki MELLAS
Faculty of sciences and technology.
University of Biskra
Abstract: Several damages were found on remediation networks (pipes and manholes) in Ouargla -Algeria, due a to product quality of concrete and aggressive external environment. The objective of this work is to study the sustainability of pipes for sanitation networks in this region under the effect of sulfates attack (especially H2S). To do this, Sulphate resisting cement (SRC) specimens have been preserved in the real world and the results were compared with those of control specimens.
According to the study, we concluded that the cement concrete SRC submitted acceptable mechanical properties in wastewater compared to concrete witnesses and penetration of aggressive agents rather slow, where a small decline in mechanical compressive strength from 3% to the duration of 365 days of storage. But the exposure of cement concrete SRC to H2S gas, shows a degradation accelerated concrete under the effect of H2S gas in particular after 90 days of age, when a 40%
regression of approximately compressive strength compared witnesses specimens at the age of 365 days.
Key -words: Durability, sulphate resisting cement (SRC), sanitation, waste water, H2S gas.
1. INTRODUCTION
The basin of Ouargla is characterized by a dry climate and hot, with high temperatures in summer reaching over 47.91 °C [1]. The sewerage system of the city of Ouargla is unitary, we also distinguish sewerage networks according to the unattended mode, but the majority of citizens are connected to the sewer system. The wastewater in the city of Ouargla are essentially domestic type, even though these waters are discharged with industrial wastewater in one collector without any prior treatment, these waters are highly sulfated and cause a high release H2S gas [2].
According to H2S gas measurements by the National Sanitation Office of Ouargla (ONA), the concentration of gas varies depending on the temperature and relative humidity, it reaches the maximum values to the summer months. For example this pressure has reached more than 100 ppm to Rouissat - Ouargla. Photos (01) show some deterioration of sanitation networks in Ouargla region under the effect of wastewater sulphates.
Photo (01): Deterioration of the concrete and reinforcement corrosion sewerage concrete N'Goussa - Ouargla.
The attack of the concrete by the only sulphate is done by the expansive ettringite formation, it is obtained by the formation of gypsum (CaSO4.2H2O) by the reaction between portlandite (Ca(OH)2) and the sulfates [3]. The expansive ettringite formation (C3A.3CaSO4.32H2O) is the result of the reaction between gypsum and anhydrous calcium aluminate, equations (01), (02) and (03) show the case of sodium sulfate (Na2SO4) and magnesium sulphate (MgSO4) [4, 5].
Ca(OH) 2 + Na2SO4 + 2H2O → CaSO4.2H2O + 2NaOH (01)
Ca(OH)2 + MgSO4 + 2 H2O → CaSO4.2H2O + Mg(OH)2 (02)
C3A + 3CaSO4.2H2O + 26H2O → C3A.3CaSO4.32H2O (03)
The hydrogen sulfide gas (H2S) is the result of bacterial decomposition of organic matter in anaerobic conditions, it is produced by human and animal waste, and also it can build up in wastewater systems, usually the concentration of organic sulfurs in the waste water is from about 3 to 6mg / l [6] .The hydrogen sulfide is a gas heavier than air and thus stagnates just above the surface of the organic matter decomposition being. There is currently no standard for limits of exposure to hydrogen sulfide on farms [7]. On the surface, the H2S gas in contact with the air oxide and then decomposes as sulfur by aerobic bacteria, a small amount of 2% -6% oxygen (O2) may cause corrosion of reinforcement by sulphates formed. [8]
The released H2S gas condenses on the walls of sewer networks, it is converted by anaerobic bacteria in closed environments with moisture (a strong, highly corrosive acid to the plates) and will have the formation of sulfuric acid (H2SO4) [9, 10] .The contact between the sulfuric acid and portlandite (Ca(OH)2) form gypsum (CaSO4.2H2O), then the contact between the gypsum and the anhydrous calcium aluminate (C3A ) form ettringite (3CaO. Al2O. 3CaSO4. 32H2O), finally ettringite is a friable material that forms from the incomplete reaction of the sulfuric acid and the pulp of the set cement [11]. from the results of MESSAOUDENE.I et al, we record a strength reduction of compression of the order of 41%, was recorded for mortars retained for a period of one year, at a concentration of 0.25 M, where they concluded that the mortar attack in case the mechanism exposed to sulfuric acid is mainly an external phenomenon, and that the expansible action of gypsum is
responsible for the gradual opening of the material structure by dislocation of its surface and that the attack surface causes a reduction in the section of the test pieces [12]. In the temperature environments was higher or lower than 15
°C, the Thaumasite (CaSiO3. CaCO3. CaSO4.15H2O) was formed sulphatic the attack, the latter is the product of reactions between the hydrated calcium silicate (CSH) and sulfates and carbonates ions according to equation (04). It can also be formed from ettringite and be associated with the formation of gypsum, concrete degradation linked to thaumasite training so comes from the degradation of hydrated calcium silicate (CSH) [4,11].
SO4−2 + 3Ca+2 + 3CO3−2 + 3SiO3−2 + 15H2O → 3CaOSiO2.CO2.SO3.15H2O (04) 2. EXPERIMENTAL PROGRAM
2.1. Characteristics materials used
For making concrete specimens we use the SRC cement (sulphate resistant cement) factory LAFARGE (M'Sila), this cement is made up of 95% clinker containing low levels of calcium aluminate, with a proportion of gypsum smaller than the portland cement and 5% minor components.
The aggregates used are: natural sand granular class (0/5), and crushed gravel granular class (3/8) and (8/15), these aggregates quarries origins of Ben Brahim El Haoud - Hamra. Finally the study of the different characteristics of sand and gravel shows that the materials are acceptable for making the current concrete.
2.2. Formulation of concrete specimens
Table (01) shows the weight percentages of the concrete components, after the calculset the use of the graphical method of Dreux Gourisse. [13]
Table (01): Percentages of mass constituents Cement
in %
Sand in %
Gravel 3/8 in
%
Gravel 8/15 in
%
Amount of water
(L)
Abrams cone slump
(cm)
report L / C
17,34 27,28 7,44 47,94 256 8 0,64
2.3. Making and curing specimens
Our experimental work aims to study the degradation mechanism physicochemical concrete specimens in the real aggressive environment as the building blocks of pipes and manholes used to sanitation in the Ouargla region. In this study, using prismatic mold dimensions (70X70X280) mm3 according to European standards NF EN 12390-1 and NF P 18-427, for the manufacture of concrete specimens. After pouring the concrete, the specimens are kept in the molds to the indoor laboratory (T = 20 ± 2 °C and RH = 70 ± 5%) for a period of 24 hours, for the curing of the concrete and prevent evaporation of water.
After releasing a series of witnesses specimens are kept in the lab of Ouargla in a tray filled with drinking water in the lab (20 ± 2) °C.
Another series is retained in the basin filled with waste water to the treatment plant (WWTP) Ouargla, The final set is raised to a metal substrate and exposed to hydrogen sulfide (H2S) gas in a manhole closed of Ouargla, aiming to ensure a high concentration of H2S (see pictures ((02), (03), (04)). These specimens are preserved in environments cities, up to the date of testing.
3. RESULTS AND INTERPRETATIONS
3.1. Compression Test
This test was conducted in the lab, through the use of concrete cubic specimens stored in different media, dimensions (70X70X70) mm3 according to European Standard EN 12390-3. The results of the compression test are shown in Figure (01).
Figure (01): Compressive strength of concrete samples stored in different environments.
From figure (01), we note that the compressive strength of the specimen test pieces is greater than that of the samples stored in the waste water and in the H2S gas, and the samples stored in the H2S gas exhibit lower values of
compressive strength. For samples stored in the waste water resistance curve is increasing continuously up to 365 days, and the strength reaches its maximum which is 32.65MPa, or regression of resistance is equal to 3.02% with respect to 0
5 10 15 20 25 30 35 40
0 50 100 150 200 250 300 350 400
Compressive strength (MPa)
Age of concrete (days)
witness waste water H2S gas
the tubes witnesses. For the specimens exposed to H2S gas, we find that the tensile strength curve is increasing continuously up to 90 days, where the resistance reaches its maximum 27.04 MPa, but after 90 days the tensile strength decreases up to 20.41 MPa at the age of 365 days, or regression of resistance is equal to 39.38% compared to control specimens. This explains the very negative effect of H2S on concrete durability unlike witnesses’ specimen.
These results show the negative effect of H2S on concrete durability unlike control specimen, or the samples stored in the wastewater, these results are consistent with those of the trial compression. Given that the pressure of H2S measured using a Multigas Detector in the eye where the test pieces were retained was 12 ppm as a maximum value.
4. CONCLUSION
According to the study of the durability of concrete cement SRC for the sanitation network of Ouargla region, we conclude that the concrete of cement SRC is low durability in the remediation of Ouargla region, including the presence of H2S gas. And we record a decline of about 40% of the compressive strength of the specimens exposed to H2S gas compared to control specimens at the age of 365 days.
5. BIBLIOGRAPHY
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