This work describes in detail the experimental investigation of the physico-mechanical properties of nonstructural hemp concrete (usually used as insulating wall material) when the Air-lime based Tradial PF70 binder is partially replaced using Metakaolin. The objective is to reduce the amount of free Ca2+ ions in the binder as these are responsible for the degradation of vegetables particles and can therefore induce a loss of mechanical performances. In order to assess the effectiveness of pozzolanic reaction, amounts of 0%, 10%, and 20% vol. of Air-lime binder were replaced by the Metakaolin material, while testing the mechanical properties of concrete specimens containing 200% and 300% of hemp particles. Through SEM and EDX analysis, a tight relationship has been found to exist between the Metakaolin content and physical-mechanical properties of specimen. The pozzolanic reaction consumes calcium hydroxide from binder to produce Hydrated Calcium Silicates (C-S-H) and in turn, this leads to a decrease in the pH-value of the pore solution which is the main factor responsible for hemp particle degradation.
Building materials based on plant particles are currently considered among the very promising materials, due to renewable resources, their low carbon impact and their hygrothermal performance. However, their problem generally lies in low mechanical strength compared to conventional concretes. Problems related to environmental issues have prompted extensive research into new environmentally friendly building materials. The construction sector is indeed responsible for a high consumption of primary energy and CO2 emissions [
In this context, particular interest has been given to the use of vegetable particles obtained from renewable sources in composite materials [
The use of renewable raw material derived from vegetable products has been the subject of extensive research. Various types of vegetable by-products (flax, hemp, coir, jute, bamboo, palm…), after being processed, have been used as aggregates and/or fibers replacement of sand and gravels in concrete and mortars [
Nevertheless, these new materials are often used as a filling material in construction, and not as load-bearing elements, due to their limited mechanical strengths. In addition, the hydrophilic nature of plant particles leads to a notable decrease in physical, mechanical and thermal properties and eventually rotting due to fungi attack. However, for several decades, several authors have studied the feasibility of improving the thermal, mechanical and physical insulation properties of these composite materials through the use of various types of vegetable materials in aggregates and/or fiber forms including pineapple leaf, date palm, kenaf, Alfa, Diss…,etc, as sand and/or aggregates replacement in concrete and mortars to enhance several properties [
In the same way, composites based on other vegetable materials such as straw bales, rapeseed straw, corn stalk, jute straw, Diss, and rice husks mixed with different types of binder have found to be energy efficient materials and provide concrete with high performances of thermal insulation [
The degradation problem may be due to a weakening of the fibers by alkaline attack, mineralization of the aggregates by the migration of hydration products [
The novelty approach in this study consists in a separate investigation of the degradation mechanism of hemp particle under a combination effect of alkali-attack and mineralisation, by analysing the change in the surface morphology and microstructure modifications. This work also allows the description of the metakaolin addition effect to restrain the particles degradation degree through the reduction in the pH pore solution of the binder medium, while the work reported in the literature investigate only the physic-mechanical properties of the specimen-based hemp particles without considering the environmental medium effect as regards binder-hemp particles interactions.
The aim of this work is to investigate the effect of alternative binder based-Metakaolin on the properties of hemp concrete, while a separate investigation of the physico-chemical interactions between hemp particles and hydraulic binder has been assessed. The objective is to reduce the migration of Ca2+ ions from the basic binder towards the hemp particles through the mineralization process, which causes their degradation and thereby a loss of mechanical performances of the specimen. It consists to partial replacement of lime binder amount by Metakaolin material in order to consume the free Ca2+ ions derived from Calcium Hydroxide contained in lime binder. The reaction occurred between the calcium ions from dissolved Calcium Hydroxide hydrates and the amorphous silica, contained in Metakaolin, produces stable components of C-S-H, according to the reaction mechanism of:
However, investigation of the effect of different amounts of Metakaolin on the neat lime binder has been carried out, while specimens of hemp concrete containing hemp particles for 200% and 300% by volume of the binder were tested.
Hemp is one of the world’s earliest cultivated crops and has a variety of applications including construction. It is one of the most widely used and studied plant particles for the manufacture of building materials. It is cultivated mainly in temperate climate areas. France is the world leader in its culture, although production has fluctuated a lot over the past centuries. The shape of hemp particles used is shown in
Bulk density [kg/m3] | Absolute density [kg/m3] | Porosity [%] | Water absorption [%] |
---|---|---|---|
104 ± 45 | 1438 ± 50 | 93 ± 5 | 290 ± 15 |
The preformulated Air-lime binder (Tradical® PF70) is a mixture of 75% air lime, 10% hydraulic lime and 15% pozzolans, produced by Lhoist industry which is located in the Northern region of France [
Bulk density [kg/m3] | Soluble silica [%] | CO2 content [%] | Adjuvants [%] | Water retention [%] | Maximum grading size [mm] |
---|---|---|---|---|---|
720 ± 20 | 10 ± 2 | 8 ± 1 | 0.5 ± 0.02 | 75 ± 6 | 0.09 |
Metakaolin is a complex amorphous structure obtained after dihydroxylation of clay mineral kaolinite at temperatures between 650°C and 800°C. It is a known admixture for cement-based applications as considered to be more reactive than most other pozzolans. Called ARGICAL-M 1000 and has a chemical formula of Al2O3.2SiO2, it is an amorphous non-crystallised material, with a maximum particle size of 0.063 mm It is mainly composed of amorphous silica (SiO2) and alumina (Al2O3) with an average SiO2/Al2O3 mass ratio = 1.2, and can have impurities, mainly quartz. The pozzolanic character of Metakaolin allows it to react with calcium hydroxide (Ca(OH)2) to form Hydrated Calcium Silicate and Hydrated Calcium Silico-Aluminate that achieve higher mechanical strength than Calcium Carbonate [
Chemical composition/physical properties | Value |
---|---|
SiO2 | 59.00% |
Al2O3 | 36.30% |
Fe2O3 | 1.40% |
CaO | 0.10% |
MgO3 | 0.10% |
Pozzolanic Activity index (28 days) | 1.03 |
Specific area (BET) | 20 m2/g |
Bulk density | 400 kg/m3 |
Specific gravity | 2400 kg/m3 |
Two types of samples were prepared. The former consist of neat binder without aggregates, in which lime binder was substituted by 10% or 20% by weight of Metakaolin. A second series of samples was then prepared for the hemp concretes where the binder contained 20% of Metakaolin and hemp particles for 200% and 300% by volume of the binder, respectively.
For all samples, Metakaolin and Air-lime based binder were initially mixed in a planetary mixer. The total mixing water added for all specimens corresponds to that used for fresh control mortar in order to achieve the normal workability (as measured by flow test). For the hemp mortars, the plant particles were then slowly and evenly dispersed throughout the binder, and the fresh material was allowed to mix for an additional three minutes. All the specimens were then compacted on a vibrating table and moist-cured for 28 days at 20°C ± 2°C and 98% relative humidity.
The different specimens studied are designated by: Tr: Pure lime binder Tradical without Metakaolin; TrMK10: Tradical and 10% Metakaolin; TrMK20: Tradical and 20% Metakaolin; BCT2: Hemp concrete based on pure Tradical by using 200% by volume of hemp particles; BCT2MK: BCT2 with 20% Metakaolin; BCT3: Hemp concrete based on pure Tradical by using 300% by volume of hemp particle; BCT3MK: BCT3 with 20% Metakaolin.
The designation of each formulation as well as its composition is given in
Specimen-ID | Tradical PF70 [%] | Metakaolin MK [%] | Water/binder ratio | Hemp particles [%] |
---|---|---|---|---|
Tr | 100 | 0 | 0.5 | 0 |
TrMK10 | 90 | 10 | 0.5 | 0 |
TrMK20 | 80 | 20 | 0.5 | 0 |
Specimen-ID | Tradical PF70 [%] | Metakaolin MK [%] | Water/binder ratio | Hemp particles [%] |
---|---|---|---|---|
BCT2 | 100 | 0 | 1 | 200 |
BCTMK2 | 80 | 20 | 1 | 200 |
BCT3 | 100 | 0 | 1.1 | 300 |
BCTMK3 | 80 | 20 | 1.1 | 300 |
For measurements of hardened properties, prism and cylindrical samples of 70 mm × 70 mm × 280 mm and 110 mm × 220 mm in sizes were prepared, for flexural and compressive tests, respectively. The shapes of different specimens after demolding are shown in
The tested properties on the hardened specimens include dry unit weight, as determined by means geometrical measurement and weighting. The compressive tests were carried out on 110 mm × 220 mm cylindrical samples in accordance with Standard EN 196-1 [
The three-point bending flexural tests were carried out on prismatic specimens of dimensions 70 mm × 70 mm × 280 mm. These tests were carried out in accordance with the Standard EN 196-1 [
where,
As the first part of the stress-strain curves shows à linear quasi-elastic behavior, the initial elastic stiffness is determined from the slope of the curves. The elastic modulus from flexural-test was determined using
where
A first approach to evaluate the effect of fiber addition on specimen toughness is to measure the amount of energy to be applied to the material to realize total failure in the compressive and flexural strength tests, respectively. The value of the tenacity was obtained by calculating the total area under the stress-strain diagram for the compressive test and the load deflection curve for flexural-test.
The influence of the alkaline medium on the hemp aggregates was analyzed by scanning electron microscopy (SEM). A scanning electron microscopy (SEM) Environmental Quanta 200 FEG type was used to study the structure of the binder, the vegetable particles and their bonding to the matrix. Before the SEM analysis, a carbon metallization step is necessary to increase the conductivity and allow quality images to be acquired. The technical characteristics are as follows: resolution from 1 nm to 15 kV; low energy allowing to have an almost monochromatic beam; samples up to 50 mm in diameter.
For the observations on the hemp particles, they were first immersed in a binder-based solution with or without Metakaolin. In order to homogenize the mixtures, the aggregates were stirred for 48 h using a magnetic stirrer then the aggregates were rinsed with demineralized water and then dried. The observations were made first on the outer surface of the hemp particles, then, after cutting them on the cross section. In a second phase, we recovered hemp particles after bending tests on concrete specimens, and observed the particles directly using SEM micrograph analyses.
In order to complete the observations made by scanning electron microscopy, X-ray analyzes (EDX) were carried out on the binder and the hemp aggregates. This EDS analysis makes it possible to determine the concentration of Ca2+ ions present in the binder itself and in the aggregates after immersion in the binder-based solution.
The purpose of this measurement is to determine the degree of alkalinity of Air-lime based binder with an without Metakaolin addition. The pH-variation was assessed throught the measurement of the pore solutions by a pH-electrode, using the suspension method [
The physico-mechanical properties of hardened mortars specimens were tested at 28-days. Three types of samples were tested: the Tradical lime binder alone (Tr), lime binder with 10% and 20% Metakaolin (TrMK10 and TrMK20); their composition has been given in
The objective of this part is to investigate the effect of Metakaolin addition on the strengthening of mechanical properties of neat Air-lime Tradical binder over time. Metakaolin amount was varied from 0% to 20% vol. in order to assess the pozzolanic activity which refers to the degree of reaction over time occurred between a pozzolan and Ca2+ ions from dissolved calcium hydroxide(Ca(OH)2). Based on the works conducted by Wei et al. [
Specimen-ID | Dry unit density [kg/m3] | Compressive |
Ultimate strain |
Elastic modulus [MPa] |
---|---|---|---|---|
Tr | 840 ± 20 | 5.1 ± 0.5 | 4.43 ± 0.6 | 1343 ± 15 |
TrMK10 | 854 ± 25 | 6.01 ± 0.7 | 5.85 ± 0.3 | 1382 ± 25 |
TrMK20 | 872 ± 40 | 7.2 ± 0.4 | 4.90 ± 0.5 | 1453 ± 20 |
The flexural tests were carried out on prismatic samples of 70 mm × 70 mm × 280 mm in size. The obtained results after 28 days are reported in
Specimen-ID | Load (N) | Flexural strength (MPa) | Ultimate deflection (mm) |
---|---|---|---|
Tr | 1412 ± 20 | 1.48 ± 0.10 | 0.14 ± 0.02 |
TrMK10 | 1518 ± 10 | 1.59 ± 0.25 | 0.18 ± 0.02 |
TrMK20 | 1607 ± 8 | 1.69 ± 0.20 | 0.16 ± 0.03 |
The previous results show that the partial replacement of lime binder by Metakaolin enhances the mechanical performances of the binder matrix. The reaction occurred between metakaolin and Ca(OH)2 generates new products of C-S-H and Alumino-Silicates hydrates which are considered as the main components for the binder strengthening.
To understand this effect, we observed a section of the material with a scanning electron microscope. From these observations, one notes a greater formation of C-S-H crystal hydrates in the presence of 20% of Metakaolin (
After having crushed the test pieces, they are diluted in water, we found that the pH of the solution decreases
pH (Tr) | pH (Tr+10% MK) | pH (Tr+20% MK) |
---|---|---|
12.2 ± 0.2 | 12 ± 0.1 | 11.7 ± 0.1 |
The hemp concrete studied is based on Tradical without or with 20% of Metakaolin and includes 200% and 300% volumes of hemp particles, respectively BTC2 and BTC2MK or BTC3 and BTC3MK. The materials are formulated from the composition of a conventional mortar composed of a binder in which vegetables particles have been incorporated at volume contents of 200% and 300% for hemp, with a Water/Binder ratio close to 1. For each formulation the results are compared with the ones of the binder alone, Tr and TrMK. Compressive and flexural strength measurements are performed with the same press as in the previous section.
Apparent density-value of dry hemp concrete at different hemp volumes is shown in
The Tr, TrMK (20%), BCT2, BCT2MK, BCT3, BCT3MK specimens were tested along 28-days and the results, including ultimate strain and elastic moduli, are listed in
Specimen-ID | Dry unit weight [kg/m3] | Compressive strength [MPa] | Ultimate strain [mm/m] | Elastic modulus [MPa] |
---|---|---|---|---|
Tr | 840 ± 10 | 5.10 ± 0.5 | 4.43 ± 0.5 | 1343 ± 20 |
TrMK (20) | 872 ± 15 | 7.20 ± 0.6 | 4.9 ± 0.7 | 1453 ± 5 |
BCT 2 | 525 ± 18 | 1.52 ± 0.2 | 51 ± 2 | 138 ± 8 |
BCT2MK | 559 ± 10 | 1.71 ± 0.1 | 60 ± 1.5 | 144 ± 5 |
BCT3 | 420 ± 20 | 0.29 ± 0.08 | 132 ± 6 | 62 ± 2 |
BCT3MK | 445 ± 20 | 0,42 ± 0.05 | 188 ± 5 | 78 ± 2 |
As already seen with the neat binder, replacing part of Tradical with Metakaolin increases the compressive strength of the hemp concrete, whether for 200% and 300% volumes of plant particles. At the same time, it can be seen that increasing the hemp particle content in the mortar increases the ultimate strain value, which induces higher ductility of the specimen. However, the loss in mechanical strength is significant when the rate of plant particles is increased, i.e., on average nearly 80% between BCT2-BCT2MK and BCT3-BCT3MK. It is obvious that the mechanical compressive strength is correlated to the concrete density. In addition, this decrease in compressive strength can be related to porous structure of sample. The more the air-voids ratio, the lighter the specimen and the lower its mechanical strengths.
The typical variations and the corresponding mean values of the peak stress, strain, and elastic modulus for all specimens are shown in
The BCT2 and BCT2MK specimens exhibit a ductile failure; the ultimate strain-value varied from 51 mm/m for BCT2 to 60 mm/m for BCT2MK. Despite a low level in compressive strength, samples BCT3 and BCT3MK show the same mechanical behavior, with a more extended plastic phase and a larger strain before failure, respectively 132 mm/m and 188 mm/m for BCT3 and BCT3MK.
It is interesting to examine the in failure mode change of specimens after reaching a maximum load.
The Load-deflection diagrams obtained under three binding-tests of all specimens are reported in
Material mix | Load [N] | Flexural strength [MPa] | Ultimate deflection [mm] | Elastic modulus [MPa] |
---|---|---|---|---|
Tr | 1412 ± 20 | 1.48 ± 0.1 | 0.12 ± 0.01 | 1323 ± 15 |
TrMK20 | 1607 ± 25 | 1.69 ± 0.2 | 0.16 ± 0.02 | 1438 ± 10 |
BCT 2 | 553.9 ± 12 | 0.58 ± 0.05 | 0,97 ± 0.03 | 123 ± 10 |
BCT2 MK | 689.6 ± 15 | 0.72 ± 0.08 | 0.68 ± 0.03 | 169 ± 12 |
BCT3 | 135.2 ± 10 | 0.14 ± 0.05 | 0.47 ± 0.01 | 54 ± 5 |
BCT3 MK | 185.3 ± 24 | 0.19 ± 0.02 | 0,44 ± 0.02 | 73 ± 3 |
As reported in
During the tests, a bridging phenomenon was observed for materials based on hemp particles, making it possible to delay the propagation of cracks.
It is noted that the behavior of the reference material (Tr) is characterized by a short elastic phase, followed by a sudden rupture. For this control specimen, the breaking is sudden and rapid, without plastic zone. On the other hand, the presence of vegetable particles gives the material a behavior in a linear behavior until the load recovery, then by the appearance of a very marked non-linearity, followed by a ductile and flexible behavior, with the absence of post-peak. However, despite an unfavorable impact on compressive strengths, the presence of vegetables particles improves the elastic behavior of specimen.
The brittle or ductile behavior of specimens has been also characterized by evaluating the Brittleness Index (BI), using hysteresis loops, obtained during the loading/unloading cycle.
The stress-strain hysteresis loops obtained for the hemp concretes (BCT2, BCT2MK, BCT3 and BCT3MK), corresponding to 200% and 300% volumes of hemp particles are shown in
Specimen-ID | Brittleness Index-value |
---|---|
Tr | 7.9 ± 1 |
TrMK | 9.1 ± 0.8 |
BCT 2 | 2.1 ± 0.2 |
BCTMK2 | 2.4 ± 0.3 |
BCT3 | 1,43 ± 0.08 |
BCTMK3 | 1,9 ± 0.05 |
As described in
In the same way, we can examine the cross-section of plant particles. It is noted that the raw fiber has an imperfect polygonal section with a central hole: the lumen (
The same observation can be made from plant aggregates recovered after grinding a hemp concrete based on (Tr) and (TrMk) binder. As shown in
These results are confirmed by EDX (X-ray dispersive energy). We notice that there is a decrease in calcium concentration of about 41% when Metakaolin is added. X-ray analysis shows important peak of Ca2+ ions in the case of neat Tradical binder (
This work deals with the investigation of effective means to enhance the mechanical properties of hemp concrete based on Air-lime binder. The objective consists to restrain the hemp particles degradation occurred along time due to the combination effect of alkaline attack and mineralization process, which is considered as the main factor leading to decrease the mechanical strengths of specimen. The investigation was carried out by testing the mechanical properties of hemp concrete containing 20% of Metakaolin as partial replacement of Air-lime binder, while the results have been compared to the usually specimen produced without Metakaolin.
The effectiveness of used Metakaolin on the strengthening of Air-lime binder was separately investigated by evaluating the effect of several amounts of Metakaolin (0%, 10%, and 20% vol.) as partial binder replacement on the mechanical properties. The test-results have shown that the increase of Metakaolin amount enhances the mechanical performances of binder, due to the pozzolanic reaction which produces additional amounts of C-S-H hydrates. For 20% of Metakaolin replacement, the specimen exhibited 41% and 14% greater compressive and flexural strengths, respectively, than the sample without Metakaolin. The SEM micrograph analyses of hydrated components of binder have shown that the Air-lime containing Metakaolin is largely hydrated to produce a high amount of C-S-H hydrates. In contrast, abundant free Ca(OH)2 hydrates needle shaped have been observed for binder without Metakaolin. The X-ray analyses have shown that the low content-value of free Ca2+ ions means that a large part of these ions has been reacted with amorphous silica from Metakaolin to produce C-S-H crystals, which are the primarily responsible for the strength development of air lime binder. These results highlighted the capability of Metakaolin to consume free Ca2+ ions, and to make the embedded hemp particles less vulnerable.
In the second part, the efficiency of Metakaolin addition on the performances of hemp concrete containing 200% and 300% by volume of vegetable particles has investigated. The results have shown that the addition of 20% vol. of Metakaolin increases the mechanical strength, compared to the hemp concrete with neat Air-lime binder. However, despite the decrease of 20% in dry density of hemp concrete for hemp particles ranged from 200% to 300%, the compressive strength level increased by approximately 80%. It should be noted, that the presence of hemp particles results in ductile behavior of specimen, characterized by the low Brittleness Index-value. As observed in post-cracking shape of specimen, this trends leads to maintain the structure of the material, thus delaying the failure phase and subsequently increasing its safety.
It is concluded that the combination of Metakaolin and Air-lime binder appears to enhance the mechanical strengths of hemp concrete, due to the production of additional C-S-H hydrates and also to the protective effect of hemp particles occurred as regards the mineralization mechanism and alkali-attack.