THERMO-MECHANICAL PROPERTIES OF HIGH-PERFORMANCE NATURAL POZZOLAN CONCRETE.

Researches and obtainable results on natural pozzolan particles, to make lightweight concretes, are proven to be very rare. This work has been conducted in the thermo-mechanical properties of the natural pozzolans in lightweight concrete. The use of lightweight concrete has allowed us to expand the field of use of concrete. The goal of this work is to achieve a high-performance lightweight concrete at a competitive cost and minimal environmental impact with natural pozzolans.

Researches and obtainable results on natural pozzolan particles, to make lightweight concretes, are proven to be very rare. This work has been conducted in the thermo-mechanical properties of the natural pozzolans in lightweight concrete. The use of lightweight concrete has allowed us to expand the field of use of concrete. The goal of this work is to achieve a high-performance lightweight concrete at a competitive cost and minimal environmental impact with natural pozzolans.

…………………………………………………………………………………………………….... Introduction:-
The essential novelty that appeared in concrete technology in the 1980s is the increasing use of superplasticizer additives [1]. These products have the ability to turn a firm consistency concrete into a fluid suspension, which spreads and fills a container under its own weight [2].They can also be used to reduce the amount of mixing water in a proportion of the order of 30% while keeping the mixture maneuverability allowing its implementation by the usual methods [3]. Madagascar is one of the developing countries, it adapts the sustainable development program in all areas especially in the field of construction industry, so it is necessary to manufacture sustainable concrete. Recent developments in cement and cement additive technology (silica fume, natural pozzolan) and superplasticizers have led to the production of lightweight, high-strength concrete [4], its high resistance is an inverse function of its total vacuum content.
This work is part of the implementation of natural mineral resources Malagasy. It aims to minimize the cost of construction through the use of proper technique and materials. More specifically, this work studies natural pozzolana aggregates from the Ambohinaorina_Madagascar. The current trend in the individual building, is to favor light products able to fulfill several uses.
As the porosity increases, the mechanical strength of a concrete decreases [5]. This phenomenon has been confirmed by JBu and Z Tian [6] on the compressive strength of light concretes, which is a decreasing function of the volume fraction of aggregates. The second originality of this work concerns the specific characteristics of natural pozzolana concrete. This specificity is due to the properties of each constituent as well as the resulting microstructure of the mixture thereof. In this context, the exploitation of the experimental results facilitates the definition of the reformulation of pozzolan concretes.

Pozzolan:-
The natural pozzolana used to prepare the specimens originates from the site of Ambohinaorina, district of Betafo, Antsirabe. Its main mass, which has not been estimated, is formed by projection products, among which slags largely dominate. Pozzolan samples were subjected to the identification and characterization tests. Chemical analyses of pozzolan powder samples were conducted. The X-ray powder diffraction method was performed on the samples using a monochromatic X-ray beam. A Siemens D500 diffractometer operating with a monochromatic CuKa radiation at a wavelength k = 1.7903 A˚, a voltage of 40 kV and a current of 30 mA was used. The results obtained are shown in fig. 1.  Table 2 summarizes the main characteristics of the scoria samples used for this study while Table 3 shows the results of the chemical analyses for these samples.
1448  Note that we have followed ASTM C618 [9] standard which intend to set pozzolan characteristics. Pozzolan is a natural rock, generally black or red, made of volcanic scorias; the pozzolan silica, alumina, and ferric oxide average value has to be between the following limits: SiO2 from 43 up to 55%, Al2O3 from 12 up to 24%; Fe2O3 from 8 up to 20%. Those limits intend particularly to ensure cohesion between chemical elements. The fines used for some of the mixtures were 100m constituents taken from the pozzolans grinding.

Sand:-
The influence of the sand on the resistance of mortars will be investigated. Their physico-chemical characteristics are set out in Table 4.

Method:-
From the theoretical and experimental point of view, a number of authors have carried out work on the behavior of granular mixtures in order to obtain an optimal formulation of concretes [11]. The latter is nevertheless acknowledged to be unreachable because the various parameters to be considered in the study, according to DREUX [12] are extremely numerous.

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The research process has led to some rules for usefully channeling the practice of concrete composition.
With regard to common concretes, the main objective has always been to produce and obtain concretes with minimal porosity which, by the logical rule, offer the best mechanical resistance on a sustainable time scale.
The goal for lightweight aggregate concrete which is slightly different from conventional concrete is to obtain rules of mixtures compatible with the composition of concretes commonly having acceptable mechanical strengths, a low density and good physical characteristics [13].
Thus, it is important to extend the theory of mixtures of two ordinary aggregates for the case of porous aggregates. Added to this is the experimental study of the existing problem.
We will take into account two different aggregates between them by their grain size. We will call conventionally one of them, the coarse aggregate and the other the fine aggregate, if one refers to the comparison of their respective dimensions.
If it is assumed that the fine aggregate has a size d, an apparent volume Vap and an absolute volume Vab, its expansion f is equal to Vap / Vab.
Weight by weight, if a few grains are replaced by a single large grain of volume v, the apparent volume of the granulate increases by v (f -1).
Taking into account the wall effect of fine grains on coarse grains, moreover, if V is the volume gained per unit area of a coarse grain and at its surface, the total volume gained is AV.
At each substitution, an apparent volume increase is obtained which is expressed by: If we start from the choice of unit volume of matter and let v / the volume of fine grains remaining after n substitutions, their apparent volume is v / f. By hypothesis, in this volume which is reduced, the fine aggregate retains the same structure and the same index of voids out of the immediate vicinity of the coarse grains.
The volume of coarse grains is expressed by (1v / ) and the surface effect leads to an increase in nAV. Thus we obtain a total apparent volume of: However, A = cv/d where c is the coefficient of proportionality between the volume of large grains and that of fine grains. If we put c/d = C, then Vap = v / /(f -1) + CnvV + 1 with: nv coarse volume: nv If we assume that:  = density of the material, the weight of the fine grains in the mixture is given by the relation: it is an expression of the form: Vap = XP + const because C, , f and V prove constant. Since e represents the void index of the mixture, as follows: Vap = Vvide + Vmaterial = e + 1 Hence the relation (3) becomes: It can be seen that the value of f is close to 2. Therefore, the substitution of the large granulate for the fine granulate tends to decrease the apparent volume corresponding to the void ratio of the mixture studied.

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If one starts from a large aggregate and if one supposes that one could replace it in part and by weight by weight for a fine aggregate and, in addition if f / present the expansion of the big aggregates, thus the apparent volume during which the fine grains associate with the fat is expressed by the relation: Vap = f / v r wherev r = volume of the rest of the coarse grains Hypothesis: « it is assumed that the structure of the coarse aggregate remains unchanged » With: 1-vr = volume of fines in substitution, the empty index is expressed by: e = f / v r -1 We obtain as follows: Vap= ( The combination of relations (5) and (7) leads to the existence of a binary mixture where the apparent volume (the void ratio) is maximal. In theory, this volume must be at the intersection of the two lines formed by the relations (6) and (8).
Another type of interaction is experimentally encountered in this study other than the wall effect. This is the granular interference effect [65] in coarse aggregates in the mixture. This threshold disrupts them at the granular level, leading to the increase of the void index with respect to the theoretical index.

Case of two porous aggregates:-
The void ratio expressed in relation (7) can be expressed by: e = (f / -1) v r -(1-v r) As all the aggregates are porous, so: It can be seen that the line described by relation (9) is more or less in the form of an increase in slope by means of the relation (7). The index of grain voids also influences the constant term.

When only the large granulate is porous:-
The addition of coarse aggregate and non-porous fines does not affect the modification of equation (7). It remains e = f / (1+E) v r -1 So, The same slope is relative to the relation (9). In conjunction with the weight: As a conclusion, by interchanging large granules and fine granules, we have a relation: e = αVab + β (10) Where,  and  are the void ratio coefficients and grain forms, and Vab is aggregate absolute volume in concrete 1m 3 .
Cement compatibility / superplasticizer:-Slump loss, setback, stiffening, or disruption of the entrained air content are all synonymous with cement / superplasticizer incompatibility [14]. The study of cement / superplasticizer compatibility (C / SP) is done by the flow test of Marsh cone types. The method used is to prepare a slurry and measure its flow time at 5 and 60 minutes.
The C / SP combination studied is compatible and the saturation point is very high (4.2%) because the solids content of the superplasticizer used is more or less low of about 20%.

Concrete composition:-
We noticed that the grain forms have an influence on the void ratio corroborated with the plate effects and the aggregate interference. So, an experimental study is imperative for a better understanding of the interaction between fines and big aggregates. This behavior can be observed on a scoriaceous form such as natural pozzolans. For ternary mixtures: the sand, fine and gravel interaction has to be analyzed. On the one hand, concerning sands, fines 1451 and gravels, they confirm their behavior as binary mixtures. Ternary mixtures require an experimental study to understand the influence of the void ratio variations.
For binary mixtures, the pozzolans aggregates real volume (Vr) is constant for the compositions categorized by specimen from bc1 to bc6. We reduced progressively the cement dosage to respect void ratio results. The fines quantities used have been calculated to respect the mixture absolute volume total value (in cement substitution).
It is worth noting that for bc7 and bc8 specimens, we reduced the Vr quantity to make its effectiveness significant on the void ratio. Table 5 gives the mixture compositions made in this study. Where: Vc is volume of cement, Vf is volume of fines, Vab is absolute volume of aggregates in 1m3 of concrete, and Vr is real volume of the pozzolan aggregates.
For ternary mixtures, the experimental volumetric method is determined according to the ASTM C33 [15] norm. For this latter, concerning solid constituent dosages, big aggregates are considered on their real volume after presoaking. These aggregates are pozzolan pea gravels including to 5/10 series. The sand used was an ordinary one. The various concrete compositions are listed in table 6. For each series, we added progressively and quantitatively sand.
For series A, fines were not used and the pea gravels quantity was the same for specimens n°1, 2 and 3. During the operations, the sand quantity and cement dosage were increased. For specimen n°4, we reduced the pea gravel amounts in increasing the sand amount compared to the previous ones. The cement dosage is listed in table 8.
For series B, we did not use the fine for specimens n°4 and 5. In these batches, the sand amount used was increased and the pea gravel, fine and cement amounts were progressively reduced. You can obtain these results from a binary concrete belonging to "semi-cavernous" series.
For series C, we made a cement dosage fixed to 450Kg/m3, for specimens from n°1 to n°4, by increasing progressively the sand amount. On the other hand, the cement dosage was reduced to 350Kg/m3 for batch n°5. Its special detail is the fine sequential reductions.

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The aggregate amount calculations used will help us to define the mixtures according to the concrete that we want to make, either a fat or no-fine one. The formulations choice is made according to the elements we want to analyze: the aggregate quality influences, volumetric concentration and on natural lightweight pozzolan concrete mechanical, thermal and acoustical behavior.
Mechanical properties:-Concretes mechanical characteristics are usually more analyzed, and, better known because of the very important structural role played by concretes in civil engineering works.
For a lightweight concrete, the apparent density is considered to be one of the material essential properties. In this study, the apparent density is determined by finding the mass with a precision weighting balance (Perrier, French), 4*4*16 cm parallelepiped sample hardened.
The concretes mechanical properties are given at 28 d. The measurements were performed following ASTM C39 [16], Standards Test Method for Compressive Strength of Cylinder Concrete Specimens.

Thermophysical properties:-
The samples subjected to these tests have a parallelepiped shape with a 27 cm side and a thickness that varies from 2 to 7 cm.
If one considers a steady state, where the flow of heat is supposed to be non-directional and crossing a homogeneous wall of thickness e under uniform temperatures T1 and T2 on both sides. The equation of the heat flux ф that passes through the face A of the sample, is expressed by: ф = (λ/e) (T 2 -T 1 ) A (11) We can deduce the thermal conductivity by the following relation: In practice, the heat exchanges between the box and the outside are not always null. This is because of the difficulty of uniformly obtaining a temperature TB inside the box equal to that outside TA. Thus, a certain loss is recorded ф 0 such that the flow becomes: Ф T = ф + ф 0 et ф 0 = C / / (T A -T B ) (13) Where C / is the coefficient of loss of the box The theoretical value of the coefficient of loss C / is ordinarily determined by the formulas of Carslaw and Jeager on one side, and by that of Langmuir on the other side. This being because of the parallelepipedal shape of the box. The first expressions describe the flow of heat through a dihedron in steady state, the second establishes the value of the form factor for the corners of a cube.
According to the characteristics of the box, we obtain the following relation: with: The value of C / is obtained experimentally by means of two different measurements of the conductivity of the temperature on a given sample. At the heating level, two voltages U 1 and U 2 can be applied while noting the two corresponding temperature differences (T a -T B ) 1

and (T a -T B )
Thermal Conductivity:-To measure the thermophysical characteristics, our samples were dried in an oven at a temperature of 50 ° C. An unidirectional heat flow passed through the samples (E) which were placed between the cold and constant isothermal heat flux sources. The thermal gradient that evolved between these two faces was measured. Once the stationary state is established, the apparent thermal conductivity [17] can be expressed by the formula: where: λa: apparent thermal conductivity; V: voltage applied to the terminals of the heating plate; R: plate resistance; C1: coefficient of heat loss; Ta: ambient temperature of the experiment room; Tb: temperature inside the box; Tc: temperature of the hot face of the sample; Tf: temperature of the cold side of the sample; S: surface of the sample; e: thickness of the sample.
For temperature readings to calculate the thermal conductivity, it is essential that the steady state is reached. In our experiments, this was achieved six hours after the beginning of the test. The device used to measure the thermal diffusivity is the same as that of the box method for the measurement of thermal conductivity. The sample is placed in a box with reflective faces and thermally well insulated. It receives a thermal pulse from a source of constant flux radiation consisting of a 500W incandescent lamp.

Thermal diffusivity :-
The analytical model is based as follows: 1. One of the flat faces of the sample receives radiation from the uniform temperature for a time to. The absorbed energy density q is assumed constant over the duration of the temperature signal; 2. Transfers are assumed to be unidirectional. This hypothesis is verified practically by: ro / e> 5 where ro is the radius of the irradiated surface. T(x,t) = a o + a 1 t + b 1 x + a 2 t² + b 2 x² + c 2 xt + T / (x,t) Boundary conditions of the third homogeneous type are obtained by judicious choice of the coefficients ai, bi and ci. The variable separation makes it possible to obtain the resolution itself.

1454
During the excitation, one can obtain the following expression: For the case of a brief signal, that is to say, tends to 0, the exponential -a U k ²t/l² can be approximated by the quantity 1-a U k ²t/l² and the thermal expression at following the excitement is expressed:    We noticed big results dispersion around the strength average values for the densities from 1408Kg/m3 to 1425 Kg/m3. The higher dispersion, compared with the average value, is 23% for the compressive strength against 1.5% for the tensile strength.
These results were unexpected; the two plots of strength (compressive and tensile) should have the same aspect since samples have nearly equal porosity value. So, we concluded that strengths are modified by a volume scale effect.   On this figure, we notice an increase in compressive strength while density is increasing (Compressive strength max =22MPa for density=1720 Kg/m3) and then it decreases. The concretes tensile strength is 10 times weaker than the compressive strength.
Compressive strength varies according to the cement dosage (concrete 250 to 450 Kg/m3) from 4.39 to 21.92 MPa at 28 d. Fig 5 and 6 show a significant perturbation on strengths. For sure, the cement dosage has an impact on them, but these ones decrease when there is a fine significant amount because of the aggregates specific surface increase. We can also assert that strongest concretes are less porous ones.  The fine fraction increases from 0.53 to 0.68 triggers strength reduction (compressive and tensile). The granular arrangement smallest packing density is due to the partial reduction of the big aggregates. It increases concrete porosities. The grain size is also an element which has an impact on lightweight concretes compressive strength which is straighter because the binder-aggregate bonding quality is better and because the aggregate units and bonding 1458 quantities are similar. In fact, the big aggregate fragmentation takes place through the biggest pores which are thus eliminated. We can notice the positive influence on strength in reducing the aggregate maximum sizes (Ganesh Babu and Saradhi Badu's) [35] This phenomenon is also confirmed by Miled K. and co [36] , who worked also on polystyrene concrete, especially on concrete with low aggregates percentage. This phenomenon is not considerable on significant volumetric concentrations.  Usually, only characteristics such as the aggregate porosities are taken into account. However, other characteristics, mentioned in the bibliography, have an impact on concrete properties. These characteristics are: the granulometric distribution, the fine percentages and the material densification. At the beginning, when we put sand, the compressive and tensile strength started to increase. Then, we noticed a progressive dispersion decrease with a ratio formula: (s+f)/(s+f+g), between the tensile strength and the compressive strength.
The presence of fine among these aggregates can also influence the relation between their densities and mechanical properties.  It shows concretes compressive strength as a function of densities. Comparisons between the different series show that the concretes mechanical properties differences are also due to their aggregate percentages and microstructure. Only the concretes density values C3 and C4 are superior to 1800 Kg/m3. They increase, on one side, with the fine introduction for the binary concretes, and on the other side, with the sand and fine introduction for the ternary concretes. The ratio E/C has a significant impact on the hydrated cement paste because it has an influence on the initial space between cement and grains suspended in the mixing water (Chen and co. [37]). The more the water/ cement ratio is high, the more the porosity is significant (Baaroghel -Bouny and co [38]). This triggered the strength decrease.
This type of production is applied to almost all types of lightweight concretes and can be used in many fields at a mechanical level. So, according to the classification made by the ACI [39], there are: highly resistant concretes (Rc> 20MPa) that can be used as structural concretes, moderate strength concrete (compressive strength included between 10 and 20 MPa) that can be used for load-bearing elements and low strength concretes that can be used to make beam filling for masonry. Thermal variation impacts on the concretes behavior are to discuss.

Conductivity and diffusivity:-
The results of the thermal conductivity measurements are shown in Tables 7 and 8.  We have used stationary methods even if this entails a great difficulty of making large compacts. For an appreciable saving of time and an ease of implementation of the sample, we compared the results obtained by the unsteady methods.
It may be noted that the values of the conductivities of concretes without sand are more or less low and less than 0.403 W / mK.
Humidity significantly affects thermal conductivity, as well as other physical parameters. But in this study, our goal is to follow rather the disruption of the concrete structure on the physical parameters.
From the results obtained, we found that the thermal conductivity increases according to the rate of sand. This result is valid throughout the range of variation of the water content of the samples, which can be explained by the fact that the addition of sand brings to the initial material many fine elements that clog the pores created by the proximity large particles. We also found that the thermal conductivity increases with the water content. Tables 9 and 10 give the values of the thermal diffusivity of concretes.
The values of the diffusivities are between 2,70x10 -7 m ² /sand 5,03x10 -7 m ² /s.  The variations of diffusivities are similar in appearance to those of the conductivities following the introduction of fines and sand. Those heavily dosed in sand are the most diffusive concretes. This situation is normal because of the compactness of these grains.