The present work aims at studying the formulation and characterization of self-compacting concrete equivalent mortars, using calcined mud from the dredged sediments brought from the dam (Western Algeria) and fly ash from the Central Thermal EDF (France). Three SCCEM samples were prepared; a control mortar sample and two mortar samples containing 22% of mineral additions, with a ratio W/B = 0.47. The analysis of the experimental results obtained indicates that mortars comprising calcined mud develop greater compressive strengths than those containing fly ash. Regarding free shrinkage, mortar with calcined mud is characterized by an autogenous shrinkage similar to those of control and fly ash mortars. However, it is more sensitive to total shrinkage and drying as compared to the other mortars. Under the conditions of restrained shrinkage, control and calcined mud mortars are more sensitive to early cracking than mortar based on fly ash.
The siltation phenomenon of dams in Algeria has significantly increased in recent times. This has greatly reduced their storing capacity (significant decrease in the volume of water stored). Today, desilting is an effective technique to get rid of sediments from the bottom of dams. However, the deposition and storage of these sediments is a serious and costly problem (
The valorization of sediments from dams has both an environmental and economic objective, it represents an interesting alternative solution to dumping sediments (mud and silt) in nature on the one hand. On the other hand, it is advantageous to use calcined mud as a cement substitute in the preparation of concrete, because today cement manufacturing is quite costly and considerably polluting, since considerable amounts of carbon dioxide CO2 are released in the atmosphere. The search for a cheaper binder can be beneficial because nowadays cement is expensive and sometimes unavailable (
Concrete shrinkage can be defined as the deformation of a concrete element that is not subjected to any external mechanical stress in a constant thermodynamic environment (
Several studies have been carried out, in recent years, on the effect of mineral additions, such as fly ash, silica fumes, blast furnace slag, calcareous filler, metakaolin, natural pozzolan and artificial pozzolan (calcined mud) on shrinkage in mortars and concretes (
Based on studies conducted on the effect of mineral additions on self-compacting concrete shrinkage, it is possible to classify research works into two groups; a first group involving the majority of authors (
Cracking is a complex phenomenon. Therefore, it would be interesting to emphasize that the risk of cracking depends not only on the extent and rate of shrinkage but also on other material properties, such as the modulus of elasticity, creep, flexural strength and fracture resistance (
The present work is devoted to the study of the influence of Chorfa dam mud compared to that of fly ash on the mechanical properties (compressive and flexural), free shrinkage, crack development due to restrained shrinkage, using ring tests, porosity accessible to water and bound and free water content of self-compacting concrete equivalent mortars. The study was conducted at the Laboratory of Civil and Mechanical Engineering Laboratory (LGCGM) of the University Institute of Technology of Rennes, and at the National Institute of Applied Sciences of Rennes (France).
All tests were carried out under hygrothermic conditions, at a temperature of 20 °C±1°C and 50% ± 2% relative humidity
The cement CEM I 52.5N-SR5 used for the different mortar formulations in accordance with the requirements of the NF EN 197-1 standard was from the Lafarge cement plant (France). It had a Blaine surface of 3800 cm2/ g (
Mineralogical composition of clinker (
Minerals | C3S | C2S | C3A | C4AF |
---|---|---|---|---|
% | 62 | 22 | 8 | 8 |
We used in our study two types of additions: the calcined mud and fly ash. The results obtained by the laser granulometry technique of these additions are shown in
Laser granulometry of calcined mud and Fly ash.
The laser granulometry analysis was carried out using the CILAS 1180 granulometer. The samples were also subjected to wet analysis. The granular distribution of the grains of the calcined mud showed a higher proportion of the fine particles compared to the fly ash. It can clearly be noted that the proportion of 95% of the particles of the calcined mud was less than 50 microns in comparison with 75% for the fly ash, which confirms the great fineness of treated mud.
The mud used in the formulation of self-compacting concrete was brought from Chorfa Dam, which is located in the province of Mascara, in western Algeria. It was first dried in an oven at a temperature of 50 °C; then, it was crushed and passed through an 80 µm, and after that it was subjected to heat treatment by calcination. To avoid thermal shock, the baking rate was set at 5 degrees per minute. The temperature was then kept constant (750 °C) for 5 hours (
Physical characteristics of the calcined mud.
Tests | Standards used | Results obtained |
---|---|---|
Fineness (Blaine) (cm2/g) | EN196-6 | 8310 |
Specific gravity (g/cm3) | NFP18-555 | 2.65 |
Pozzolanic activity index i 28 | NFP 18-506 | 0.93 |
Additional analyses, aimed at identifying the calcined mud, were conducted by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The spectra of the microanalysis indicated the presence of several chemical elements (
Chemical elements in calcined mud.
Element | C | O | Mg | Al | Si | K | Ca | Ti | Fe |
---|---|---|---|---|---|---|---|---|---|
Mass % | 9.69 | 44.74 | 1.19 | 6.66 | 16.77 | 1.91 | 13.09 | 0.67 | 5.27 |
Atomic % | 16.21 | 56.15 | 0.98 | 4.95 | 11.99 | 0.98 | 6.56 | 0.28 | 1.90 |
SEM and EDS analysis of calcined mud.
The fly ash used in the formulation of the self-compacting concrete equivalent mortar came from the Central Thermal EDF to Cordemais (France), it was a silico-aluminous
Physical characteristics of Fly ash (
Tests | Standards used | Results obtained |
---|---|---|
Fineness (Blaine) (cm2/g) | EN196-6 | 4334 |
Specific gravity (g/cm3) | NFP18-555 | 2.05 |
Pozzolanic activity index i28 | NFP 18-506 | 0.79 |
Chemical composition of fly ash (
Oxides | SiO2 | CaCO3 | CaO | Fe2O3 | Al2O3 | MgO | Na2O | SO3 | HCL |
---|---|---|---|---|---|---|---|---|---|
Content (%) | 49.6 | - | 3.0 | 17 | 23.8 | 1.3 | 0.6 | 0.1 | - |
An alluvial siliceous sand was used, of particle size 0/4, from the quarry of La Repennelais (France), with a density of 2.58 g/cm3 (NFP 18-555) and a water absorption coefficient of 1.2 % (
A superplasticizer Sika Viscocrete Tempo 11 was used, of a new generation multi-purpose water reducer based on acrylic copolymer, with a density of 1.06 ± 0.01 and a solid content between 28.5 and 31.5 %.
Considering the different data on the materials used, we derived three compositions of SCCEM from one composition of Self -Compacting Concrete containing fly ash (SCC5) formulated by Mesbah (
Composition of SCCEMs.
Designation SCCEM (g/l) | W/B | Cement | Mud | Fly ash | Water | Admixture | S (0/4) |
---|---|---|---|---|---|---|---|
SCCEM C | 0.47 | 649 | - | - | 304 | 1.5 | 1277 |
SCCEM CM | 0.47 | 500 | 143 | - | 299 | 4.5 | 1266 |
SCCEM FA | 0.47 | 494 | - | 141 | 295 | 1.5 | 1250 |
where:
SCCEMC: Control self-compacting concrete equivalent mortar
SCCEM CM: self-compacting concrete equivalent mortar containing calcined mud
SCCEM FA: self-compacting concrete equivalent mortar containing fly ash.
As an integral part of a SCC, self-compacting mortars (SCEMs) may serve as a basis for the design of concrete since the measurement of the rheological properties of SCCs (
After leaving the cone, the final diameter reached by the slump flow slab is measured.
The initial and final setting times of the self-compacting concrete equivalent mortars prepared were measured using a classical automatic VICAT apparatus, according to standard NF EN 480-2.
The tests were carried out on prismatic specimens of dimensions 4x4x16 cm3, previously preserved in water as soon as they were demolded, until the crushing deadline (28 days) in order to determine the flexural (tensile) strength, using the three-point bending test, and the compressive strength according to the standard EN 196-1.
The main objective of the thermal analysis was to follow the evolution of bound and free water content of the binder pastes. The compositions of these pastes are given in
Formulations of binder pastes.
Identification of Pastes (g/l) | PC (Control paste) | PCM (Paste containing calcined mud) | PFA(Paste containing fly ash) |
---|---|---|---|
Cement | 649 | 500 | 494 |
Calcined mud | - | 143 | - |
Fly ash | - | - | 141 |
Water | 304 | 299 | 295 |
W/B | 0.47 | 0.47 | 0.47 |
After that, the crucible containing the dry ground matter was introduced into the oven at 105 °C, for 2 hours; then, it was allowed to cool for 15 minutes inside a desiccator containing silica gel; next, it was weighed. The operation was repeated, but with the oven temperature set first at 400 °C and then at 600 °C, for 1 hour, for the two temperatures. The relative loss of mass at 600 °C is then given by [2]:
The operation was repeated in the oven where the temperature was previously set at 1000 °C. The relative loss of mass between 600 °C and 1000 °C is therefore equal to [3]:
This test consisted of measuring the percentage of voids, inside the mortar mass, which are connected to the surface, in accordance with standard NF P 18-459. Porosity is measured on prismatic specimens of dimensions 4x4x16 cm3, at 28 days. The porosity accessible to water, ε, which is expressed as a percentage of the volume, is given by the following relation [4]:
Where:
Mwater is the mass in grams of the half specimen immersed in water
Mair is the mass in grams of the soaked half-specimen weighed in air
Mdry: the mass in grams of the dry half-specimen
The present study was carried out on mortar samples of dimensions 4x4x16 cm3, in accordance with the French standard NF P 15-433. The autogenous shrinkage and total shrinkage measurements were carried out from 24 hours after the beginning of malaxation. The specimens used to investigate the autogenous shrinkage were fully coated with adhesive aluminum foil to prevent any water exchange with the outside environment. All test pieces were then stored in a room with a controlled atmosphere. The measurements were carried out using a micrometer comparator, at a rate that is adapted to the kinetics of the observed phenomena. The difference between the total shrinkage and autogenous shrinkage allows obtaining the drying shrinkage.
In order to study the early-age cracking tendency of SCCEMs, it was decided to carry out restrained shrinkage tests using ring tests (
Geometry of the steel and concrete rings used in this study (
The test results of self-compacting concrete equivalent mortars (SCCEMs) in the fresh state are given in
Characterization of SCCEMs in the fresh state.
SCCEM | Slump flow (cm) Ø | Initialsetting time (min) | Finalsetting time (min) | Settingtime (min) |
---|---|---|---|---|
SCCEMC | 19.25 | 245 | 458 | 213 |
SCCEMCM | 20 | 282 | 520 | 238 |
SCCEMFA | 19.25 | 360 | 590 | 230 |
All mixtures of mortar were made to give slump flow diameter of 20±1cm (avoiding segregation) which was acquired by adding the used admixture (
Slump flow variation of SCEMs studied.
The measurement results of the setting times of the different concrete equivalent mortars are presented in
The initial times of setting, the final of setting and the setting time of SCEMs studied.
This may be explained by the fact that the fineness of the mixture, which is attributed rather to calcined mud (8310 cm2/g) and fly ash (4334 cm2/g) than to cement (3800 cm2/g), allowed having a larger area of contact with water. In addition, the setting time increased is attributed to the low hydration heat of mortars containing additions, because of the lower clinker content. Indeed, Jian. S et al. (
The evolution of the compressive strength, at 28 days, as a function of the mineral additions is illustrated in
Evolution of the compressive strength at 28 days of SCEMs studied.
Depending on the nature of the addition, the calcined mud SCEM developed better strengths as compared to the fly ash SCEM. This can be explained by the physical effect of the calcined mud as a result of great finesse (8310 cm2 /g) compared to the fly ash (4334 cm2/g). Indeed, Sabir and al 2001 have shown that finesse, the morphology of the particles is also an influential parameter on the intensity of the Pozzolanic activity (
In addition, this can also due to the latent property of hydration of cements containing fly ash (
For better visualizing the evolution of the compressive strength, we compared the various ratios of the SCEM containing the mineral additions (calcined mud and fly ash) with the SCEMC, as illustrated in
Evolution of the fraction of compressive strengths of SCEMs based on mineral additions compared to control SCEM.
The 28-day flexural strengths were shown graphically in
Evolution of flexural strengths of SCEMs, at 28 days
According to the results obtained, control SCCEM displays higher flexural strength than those of calcined mud SCCEM and fly ash SCCEM.
For better visualizing the evolution of the flexural strength, we compared the various ratios of the SCEM containing the mineral additions (calcined mud and fly ash) with the SCEMC, as shown in
Evolution of the fraction of flexural strengths of SCEMs based on mineral additions compared to control SCEM.
Free and bound water content of the mortar pastes studied.
CO2 content of the mortar pastes studied.
The carbonation depth also depended on the nature of the addition, in particular the CaO content of the fly ash. The higher this content, the lower the carbonation depth (
According to Audenaert et al. (2007), the incorporation of fly ash had no noticeable effect on improving the strength of SCCs to carbonation (
Porosity accessible to water of the different SCEMs studied.
Autogenous shrinkage was a phenomenon that was specifically related to cement hydration, which testifies to its evolution and to the quantity of hydrates formed. This shrinkage increased rapidly as the amount of water decreases, it ended (95%) after 1 to 4 weeks (
From
Evolution of autogenous shrinkage as a function of time.
Based on the definition of SCEMs containing mineral additions and adjuvants, with proportions higher than those in ordinary mortars, it can be said that these two elements have an influence on the hydration of cement and on its consequences (
Hydration and water exchanges with the atmosphere caused shrinkage deformations on mortars, irrespectively of any external mechanical stress. Spontaneous variations due to the total shrinkage were due to changes in moisture content that was engendered by the hydration reactions and evaporation (
It can be seen from
Evolution of total shrinkage as a function of time.
The deformation of the specimens was due to the drying shrinkage when there was an exchange of water between them and the environment. The shrinkage values represented the combination of drying and carbonation which testify to the porosity of the microstructure and to its ability to promote drying (
Evolution of drying shrinkage with time.
Regarding drying, it was noted that shrinkage depended on the amount of evaporable water within the cement paste (
The restrained shrinkage tests were performed during a 49-day period. The formwork removal took place 24 hours after the manufacture of the SCEMs.
Measuring crack widths using a crack sensor.
Flexural stresses developed under the effect of the restrained shrinkage; cracks appear when these stresses exceed the flexural strength of the mortar (
When monitoring the restrained shrinkage using the ring test under desiccation conditions, the SCEM CM and SCEMC were the first to crack, as illustrated in
The SCEM CM and SCEMC cracked earlier than the SCEMFA. However, the crack openings in the control concrete equivalent mortar were smaller than those in the mortars containing mineral additions. The SCEMC crack presented an aperture that was 1,2 and 1,3 times lower than those of SCEM FA and SCEM CM, respectively. This difference was certainly due to the low porosity of SCEMC. The differences between widths of cracks observed in this study should be related to the differences in the fracture energy, flexural strength, and stress developed in mortar rings at early age (
The results relative to bound water content showed that there was an indirect correlation between the amount of bound water in mortar and the age of cracking. In fact, the increase in bound water lead to a decrease in the age of cracking, thus increasing the risk of cracking. In effect a normal consistency test was performed by the Vicat apparatus, on a paste of 100% of calcined mud was in normal consistency with a ratio water / mud of 0.45 whereas a paste of 100% of cement was in normal consistency with an E / C equal to 0.26. (
This study makes a useful contribution to the formulation and characterization of more environmentally friendly concrete equivalent mortars that are based on the use of mineral additions in substitution for Portland cement. Two different mineral additions were considered, namely calcined mud (CM) and fly ash (FA).
Three SCEMs were formulated, a reference (control) mortar and two mortars containing 22% mineral additions, with a ratio W / B = 0.47.
The effects of the type of binder used on the kinetics of development of strengths (compressive, flexural) at 28 days, free shrinkage, and cracking tendency using the ring test were quantified. The bound and free water content and the porosity accessible to water were evaluated as well.
The results of the study of the behavior of fresh concrete equivalent mortar show the possibility of formulating mortars with calcined mud and fly ash, which can be characterized as self-compacting mortars.
The 28-day compressive strength of the calcined mud mortar is greater than that of the fly ash mortar. Indeed, the eventual pozzolanic reaction of calcined mud may well be the cause, with the physical effect of this mud which its fineness is quite high and its activity index which is close to 1. These findings are contrary to those obtained for flexural strengths at 28 days.
After 100 days of drying, the fly ash mortar displayed the best free shrinkage results by decreasing the amplitude of autogenous, total and drying shrinkage. The compactness of the microstructure and the improvement of pore refinement led to a decrease in permeability and prevented the diffusion of water, and therefore decreased the desiccation shrinkage. These results are similar to those found in the literature. Calcined mud-based mortar develops deformations, due to shrinkage under autogenous conditions, similar to those of the control mortar and fly ash mortar; however, it remains the most sensitive mortar with regard to total shrinkage and drying shrinkage. These last two conditions are directly related to the age at which cracking occurs.
In the case of restrained shrinkage, a crack was observed on the SCEMs, at different ages, depending on the mineral additions used. The crack was observed, on the same day and at close times, first on the SCEMCM and on the SCEMC, then on the SCEMFA. The crack width on mortars containing the mineral additions is greater than the one on the reference mortar. The presence of heat-treated mud in self-compacting concrete equivalent mortars seems to increase the risk of earlier cracking as compared to mortars containing fly ash. This is due to its lower flexural strength, also it is due to its higher drying shrinkage and low autogenous shrinkage. In fact, the increase in drying shrinkage leads to a decrease in the age of cracking so the reduction in drying shrinkage reduces the risk of cracking. However, the good correlation between free shrinkage and age of cracking should be analyzed carefully (
However, the opening of the crack in the control concrete equivalent mortar is at least 1,2 and 1,3 times lower than that of SCEMFA and SCEMCM, respectively. Therefore, it can be concluded that the crack width is directly proportional to the porosity of the material and the amount of free water, while the age at which cracking occurs is inversely proportional to the amount of bound water.
The partial replacement of Portland cement by fly ash did not decrease the CO2 content, on the contrary the porosity accessible to water and the total (bound and free) water content diminished.
Overall, the mortar with treated mud exhibited comparable behaviors towards autogenous shrinkage and early cracking to those of the control mortar. This substitution of cement with mud shows a real economic, environmental and technological advantage.
The authors would like to thank Dr. Siham Kamali- Bernard from INSA University for his valuable advice and Mr. Christian Garand for his assistance in the measures of the restrained shrinkage and Mr. Jean-Luc Métayer for his help in the laboratory (LGCGM).