INVESTIGATION ON PROPERTIES OF
CONCRETE CONTAINING NATURAL
POZZOLAN VOLCANIC ORIGIN DUE
TO SULFATE ATTACK
MERIDA AHCENE
UNIVERSITY OF SCIENCES AND TECHNOLOGY HOUARI BOUMEDIENNE (USTHB) BP32 EL ALIA 16111
BAB EZZOUAR, ALGIERS, ALGERIA merida.hacene@yahoo.fr
webmaster@usthb.dz
KHARCHI FATTOUM
UNIVERSITY OF SCIENCES AND TECHNOLOGY HOUARI BOUMEDIENNE (USTHB) BP32 EL ALIA 16111
BAB EZZOUAR, ALGIERS, ALGERIA kharchifcong@yahoo.fr
webmaster@usthb.dz
Abstract:
Sulfate attack and its effects are important from both scientific and industrial viewpoints. It is perceived that concretes containing pozzolan have better performance in sulfate solutions, since the pozzolanic reactions reduce the quantity of calcium hydroxide and increase calcium silicate hydrate. This paper investigates the contribution of natural pozzolan volcanic origin on the physico - mechanical and physico-chemical characteristics of the concretes. When it is coupled to a water reducing superplasticizer by a correct adjustment of the composition, it greatly improves the concrete properties. The analysis of the experimental results on pozzolan concrete at 5% content and sharpness of 9600cm2/g, in a sulphated environment, showed that it contributes positively to the improvement of its mechanical characteristics and its durability with respect to water absorption, and to the permeability to the chlorine ions as well as to the resistance to the sulphates. Keywords: Natural pozzolana, Pozzolanic reactivity, Sulphate
1. Introduction
Studies on sulfates attack have demonstrated the importance of physical factors, viz: porosity, micro cracking and type of cation of sulfate. Sulphate corrosion being one of the most frequent and detrimental processes. The sulfate ions if contened in the ground may diffuse through the capillary pores of concrete due to the concentration gradient,and react with unhydrated components of hardened cement paste. In consequence, these chemical reactions may lead expansive reaction products such as ettringite(C3A.3CaSO4.32H2O) [1]. In turn, the ettringite may cause the overall expansion of a structural element and its extensive damage progressing from the outer surface towards the specimen inner core [2]. This process may result in changes concrete in the physical, mechanical and physico-mechanical order.
Physical changes:
In surface: abrasion, erosion, cavitations, spalling.
Internal (cracks): structural loading, gradients of humidity or temperature, pressure of crystallization, exhibition to the extreme temperatures.
Mechanical changes:
Fall of resistance and rigidity, cracking and distortion of the material. Physico-chemical:
Weakening of the binding properties, modification of the porosity and the transfer properties (porosity, permeability, diffusivity).
concrete is not equally severe. The effect of magnesium sulfate is found to be one of the most severe, concrete is affected by sulfates depends on several factors including its permeability, water to cement (w/c) ratio, type of cement, exposure conditions and the environment[4].
Sulfate attack is due to a series of chemical reactions between sulfate ions and principle components of the cement paste microstructure. In external sulfate attack, the migration of sulfate ions into concrete may be accompanied by a gradual dissolution of Portlandite[5-6] (Ca (OH)
2) and decomposition of the C–S–H phase.
In the latter case, the C/S ratio of this phase eventually declines as increasing amounts of Ca2+ are removed from the structure, thus resulting in a strength loss of the hardened cement paste. Simultaneously, the formation of ettringite crystals and consequent volumetric strains in the hardened material are also considered to be responsible for expansive forces and micro-cracking [7].
For modeling purposes we shall focus here on the sulphate attack associated with ettringite and gypsum formation caused by the sodium sulphate (Na2SO4) When the sodium sulphate is brought into contact with
anhydrous particles of the hardened cement paste gypsum and ettringite may be produced which are responsible for concrete expansion and micro cracking[8-9]. This process is initiated by the reaction between the sodium sulphate and calcium hydroxide (CH):
The newly produced gypsum can react with some alumina-bearing phases like unhydrated tricalcium aluminate 3CaO. Al2O3 or hydrated calcium sulfoaluminate (monosulphate) to form ettringite:
Pozzolanic materials improve the microstructure of concrete due to their particle size, and may alter chemical composition and hydration reactions. Pozzolan as an amorphous or glassy silicate material that reacts with calcium hydroxide formed during the hydration of Portland cement in concrete. The substance that contributes to the strength of the concrete called calcium silicate hydrate (C-S-H)[10]. Calcium hydroxide will reduce the strength of the concrete. Pozzolans contains silica that react with calcium hydroxide in concrete to form extra calcium silicate hydrate compound and diminish calcium hydroxide[11],further strengthening the concrete due to increase of C-S-H compound and making it stronger, denser, and durable during its service life Pozzolans also serve to reduce the permeability of the concrete, which helps to make it more resistant to deterioration and swelling associated with various exposure conditions. Many researches on the performance of concretes containing pozzolan in sulfate solutions have been performed [12-16].
The aim of this study is to experimentally investigate the effect of replacing 5% of cement by natural volcanic pozzolana [17] in the mixture of high performance concrete (HPC) on the compressive strength, permeability to the chlorine ions, sulphate resistance and ultrasonic pulse velocity of specimens exposed to solutions of 5% sodium sulphate( Na2SO4) in comparison with traditional concrete (CC).
The specimens were stored for one year in drinking water (environment 1) and in aggressive solution containing 5% sodium sulphate (environment 2).
Checking the sodium sulphate solution once a week and replacing it if needed with a fresh solution ensured that the difference did not exceeded 5% from the initial concentration.
2. Experimental program
2.1 Materials
The materials investigated in this paper are: Aggregates, Cement, natural volcanic Pozzolana and Superplasticizer,
2.1.1 Aggregates
Natural rolled sand obtained from a local river and crushed limestone with a maximum particle size of 16 mm made fine and coarse aggregates respectively. The size, the finesse modular (FM = 3.2), the sand equivalent Ca (OH) 2 + Na2SO4+2H2O CaSO4. 2H2O + 2NaOH
Gypsium
3CaO.Al2O3 + 3(CaSO4.2H2O) + 26H2O 3CaO.Al2O3 + 3(CaSO4.2H2O) + 26H2O
2.1.3 Natural Pozzolana
Natural volcanic pozzolana, extracted from the deposit Beni-saf (Algeria) was used as supplementary cementing material.
The chemical compositions and physical properties of the cement and the pozzolana are shown in table1 and table 2
Table. 1. Chemical compositions of cement and pozzolana
Note: IR-insoluble residue, LOI-loss on ignition, CaOl-free lime
Table. 2. Physical properties of cement and pozzolana.
The natural pozzolana used was mostly amorphous; however certain crystalline components were identified using X-ray diffraction analysis, Fig 1. The following species were identified: quartz (SIO2), cordierite (Mg2Al3
AlSi5O18), hematite (Fe2O3) analcime Na (AlSi2O6) H2O and axinite Ca2 (Fe,Mn) Al2(SiO4)4.
2.1.3.1 Optimization of natural pozzolana
A series of concrete mixtures with varying percentages of pozzolana was prepared aimed at increasing the compressive strength and optimizing the pozzolana dosage.
The pozzolana content in the mix was fixed at 5 percent by weight of cement, Figure 2 shows the results. Elements CaO SiO2 Al2O3 Fe2O3 MgO SO3 K2O Na2O IR LOI CaOl
CEM-1 63.05 21.28 3.85 4.61 1.19 2.54 0.80 0.18 1.11 1.58 0.75 Pozzolana 14.59 44.95 16.91 9.47 3.70 0.20 1.35 1.34 0.56 4.30 -
CEM-1 Pozzolana
Specific gravity = 3100 Kg/m3 Specific gravity = 2660Kg/m3 Specific surface = 322 m2/Kg Specific surface = 960 m2/Kg
Le chatelier = 1.00mm Activity (ratio of reactive lime/free lime = 11.5 percent
2.1.4 Optimization of superplasticizer
A commercially available sulphonated naphthalene formaldehyde-based superplasticizer was used to give a consistent workability.
The study of concrete composition is always to seek simultaneously two essential qualities: strength and workability, but these two qualities are linked to each other but vary in the opposite direction. The idea was to develop a dense concrete from a compact granular skeleton using cement and water and meeting the strength, durability and workability requirements.
The optimized superplasticizer content was 2% at 0.3 w/c ratio giving a slump of about 21cm[19].
Table.3. Optimization of superplasticizer content in concrete
3. Formulation and concrete mixtures
The study of the concrete composition is to define the optimal dosage of aggregates, cement and water to make a concrete with required qualities: strength and durability. This study used “Dreux Gorisse” method of mix proportioning which is based on the size analysis (sand and gravel different fractions) [20].To investigate the use of natural pozzolana on the performance properties of concrete, two different concrete mixes were employed, details of which are given intable 4.The control mix (CC) contained only Portland cement and mix of HPC (High Performance Concrete) the Portland cement was partially replaced with 5% natural pozzolana (by weight). All concrete mixtures were prepared according to ASTM C 192 standard. The superplasticizer was added at the time of mixing.
Super plasticizer SP.30 (%)
Freshly-mixed concrete Hardened concrete
No w/c Sag cm
Density Kg /m
Compressive strenght MPa
1 1 0.25 5 2524 27
1.5 2 0.25 8 2536 31
2 3 0.25 14 2542 33
1 4 0.3 15 2539 28
1.5 5 0.3 20 2546 29
2 6 0.3 21 2549 30
20
Fig. 2. Compressive strength at 28 days, MPa
5 10
4. Curing of specimens
After mixing, concrete specimens were cast into the moulds as specified in EN 12390-2. Concrete cubes of 280x70x70 mm in size, and cylinders of dimensions 110Ø x220 mm were used in this investigation. The specimens were kept under laboratory conditions for a day, then removed and transferred to the curing basin. In the first curing condition, the specimens were immersed in water until the age of testing, while in the second curing condition, those were immersed in aggressive solution (5% NaSO4)until the age of testing.
5. Test methods
5.1 Compressive strength
This test was carried out in accordance with ASTM C39[21].
Cylindrical specimens 110Ø x220 mm were used. The strength measurements of concrete were performed at 28, 90,180 and 365 days.
5.2 Chloride permeability
This test was performed using the procedures of ASTM C 1202[22]. Soon after the specified curing periods, the periphery of the specimens was dried with paper towels and coated with rapid setting epoxy resin to prevent drying of the specimens during the testing. The epoxy resin was allowed to harden for up to two hours .The resistance of concrete to penetrating chloride ions was measured by the charge passed through two 50 mm disk concrete specimens maintained under an electric tension of 60V during 6 hours by means of electrodes made of rustproof steel between the two cells of the two compartments. One of the faces of the specimen was in contact with 3% NaCl solution, and the other face was in contact with 0,3N NaOH solution. The test was conducted at 28, 90,180 and 365 days.
5.3 Ultrasonic Pulse Velocity (UPV)
This test was carried out in accordance with ASTM C597-02 [23]. The ultrasound device indicates the time put by the wave of distribution to cross the mass of the concrete. The speed of sound is a function of several parameters: strength, density, module of elasticity, age, temperature. The speed is raised all the more as the concrete is thus denser, more strength.
5.4 Sulphate resistance
In the immersion test of ASTM C1012[24] concrete specimens were immersed in a solution containing 5 % sodium sulphate (NaSO4) and their expansions periodically measured for periods of 6 months.
NaCL
3% NaOH 0.3N
Sample
Anode (+) Cathode (-)
Voltage regulator Epoxy
50 mm
6. Results and discussion
Table 5 summarizes the compressive strength, ultrasonic Pulse Velocity, chloride permeability and expansion of the different specimens and Figures 5, 6, 7 and 8, are graphical presentations of these results.
Table. 5.Summary of main results
6.1 Compressive strength
Figure 5 show compressive strength of the specimens kept in water and aggressive solution. It illustrates the results for compressive strength of concrete versus age. An increasing trend of compressive strength is observed for both specimens. For the specimens kept in water, the increase in compressive strength continuous as the duration of immersion increases. The specimen concretes curing in water, the strength of control concrete increase from 34 to 45 whereas the high performance concrete it increase from 56 to 73 MPa. The results indicates that pozzolana addition helps gain compressive strength.
The specimens kept in aggressive solution; the strength of the control concrete is reduced by 17.77% whereas the high performance concrete the reduction was by 5.48% only.
Compressive strength of samples containing mineral admixture (pozzolana) was greater than control sample. It is evident that the pozzolanic admixture creates a more compact concrete. The density increases when admixture is added because it physically occupies pores in the past by virtue of their particle size. Also, there is a partial or Property Specimen 28days 90 d 180 d 365 d Gain at
365d/CC, %
loss at 365,%
Compressive strength, MPa
CC1 34 39 42 45 - -
CC2 32 34 36 37 - 17,77
HPC1 56 64 69 73 62.3 -
HPC2 52 60 63 69 86,5 5,48
Charge passed (Coulomb)
CC1 3000 2830 2765 2614 - -
CC2 3046 2938 2852 2744 - 4,73
HPC1 825 692 605 476 81,8 -
HPC2 886 735 648 519 81 8.3
Velocities m/s
CC1 4200 4240 4255 4260 - -
CC2 4080 4090 4110 4115 - 3,4
HPC1 4295 4335 4385 4410 3,52 -
HPC2 4250 4265 4320 4340 5.46 1,58
Expansion %
CC1 - - -
CC2 0.03 0.045 0.115 - - -
HPC1 - - - -
HPC2 0.015 0.025 0.04 - 65,21 -
Fig. 4. Measurement of expansion rate of specimen concrete
The specimen concretes curing in water, the charge passed of the control concrete decrease from 3000 to 2614 whereas the high performance concrete it decrease from 825 to 476 coulomb.
The specimens kept in aggressive solution, the charge passed of the control concrete is decrease by 4,73% whereas the high performance concrete the decreasing was by 8,3%.
The presence of pozzolana has a very beneficial effect on the chloride permeability of concrete with significant reductions in the charge passed.
This reduction is more at later ages because the pozzolana modifies the microstructure of the concrete in terms of its physical and chemical characteristics. The introduction of this mineral into the cement paste leads to a segmentation of the larger pore and capillaries which reduces the amount of hydrated lime in the cement matrix. 6.3 Ultrasonic Pulse Velocity
Velocities are measured for each set of samples and are presented in figure8. The specimen concretes curing in water, the velocities of the control and pozzolana concretes increase from 4200 to 4260 m/s and 4295 to 4410 m/s respectively in aggressive solution, its decrease by 3, 4% and 1, 58% respectively. It can be seen from these results that the concrete with pozzolanic mineral admixture has a higher pulse velocity than the control; this was a further evidence of densification and low porosity of the concrete due to the natural admixture.
6.4 Sulphate resistance
Figure 8 present the results of sulfate resistance of specimen concrete immersed in aggressive solution was measured by procedures in ASTM C 1012. It was found that the expansion rates are low at the beginning of the control and high performance concrete, and increases substantially after three months of curing for the first specimen concrete. The expansion of the control concrete and high performance concrete curing in aggressive solution decrease by 65, 21.The control concrete presents a matrix porous which facilitates the penetration of the sulfate solution. When the sodium sulfate is brought into contact with anhydrous particles of the hardened cement paste ettringite and gypsum may be produced which are responsible for concrete expansion. On the contrary the high performance concrete presents a dense and low porosity of the matrix preventingpenetration of the sulfatesolution.
Fig. 6. Variation of charge passed
28 90 180 365 28 90 180 365
7. Conclusions
Based on the obtained data in this study, the use of natural volcanic pozzolana replacing 5% by weight of cement in the mixture of high performance concrete influences positively the durability specimens concrete cured in sulphate environment. The pozzolana modifies the microstructure of the concrete in terms of its
physical and chemical characteristics. It was observed that during the early stages, the filler effect results due to reduction in porosity. With aging, the pozzolanic action further evidence of densification and low porosity of the concrete due to the natural admixture by the formation of CSH with binding properties similar to those formed in mineral-based cements. I can be concluded that the mineral admixture improved the physical characteristics of concrete relatively to the control concrete sample.
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