Compared with the control,Ni110 significantly reduced the hemp emergence rates for the three hemp cultivars,and Ni220 even completely inhibited germination.Unlike Ni stress,Pb1500 had no marked effects on the emergence rates for any of the hemp cultivars compared with the control,however,Pb3000 significantly increased the rates of Guangxibama and Yunma No.1.Both Sb50 and Sb100 significantly increased the germination rate of Guangxibama compared with the control,however,no marked differences between treatments for rates were found for the other two cultivars.Husain et al.also reported a reduction in seed germination of six hemp varieties sown in mine soils and most of the germinated plantlets died one to two weeks after their emergence.Heavy metal and Sb stresses significantly negatively affected the plant height,stem diameter,and the number of leaves of flax.For Cd,Ni,and Pb,most measurements of the plant height,stem diameter,and the number of leaves of the three flax cultivars were significantly reduced by both treatment levels,and the negative effects increased with increasing stress level.In general,compared with the control,Y2I329 was affected only slightly by the different levels of Cd because the three morphological indices of Y2I329 changed the least under Cd stress.Similarly,under the stress of Pb,Y2I329 still showed the best endurance.The plants under high Ni treatment died,and similar negative effects of Ni in flax were also reported by Amna and Syed.The effects of Sb on flax are complex and there is no uniform rule.Plant height was the least affected.The plant height,stem diameter,cannabis grow equipment and the number of leaves of Guangxibama and Yunma No.1 were significantly or slightly reduced by Cd stress compared with that of the control.Similar results were also observed by Luyckx et al..Treatment Cd40 increased the plant height of Zhongdama No.1 but Cd80 significantly reduced it; no significant difference was found for stem diameter and number of leaves.Treatment Ni220 completely inhibited the growth of the three hemp varieties; Ni110 significantly reduced the three morphological parameters for all varieties of hemp.
Pb addition at both levels significantly or slightly reduced plant height,stem diameter,and the number of leaves of all varieties.Sb did not significantly affect the plant height of Guangxibama and Yunma No.1 but significantly increased this characteristic for Zhongdama No.1 compared with that of the control; Sb had no marked effects on the stem diameter of hemp except for the significant reduce in Yunma No.1 by the Sb50.Treatment with Sb50 showed no significant effects on the number of leaves for Guangxibama and Zhongdama No.1 compared with the control but significantly reduced that for Yunma No.1; Sb100 did not significantly affect the number of leaves for Guangxibama and Yunma No.1 compared with Sb50,but notably reduced them compared with the control; Sb100 markedly increased the number of leaves compared with that for Sb50.Cd stress significantly reduced the fresh biomass of HP,but no significant reduction was found for Hongyou No.2 and GGS compared with the control.No significant difference in fresh biomass of the three kenaf varieties was found between Cd40 and Cd80.The addition of Ni and Pb notably or slightly reduced the fresh biomass of all three kenaf varieties compared with the control,and the reduction of biomass increased with increasing metal concentration.Similarly to its effects on flax,Sb50 significantly improved the biomass of all kenaf varieties compared with the control; Sb100 notably or slightly reduced the biomass compared with Sb50 but still increased the biomass compared with the control.Nickel was not lethal for kenaf varieties,however,it significantly inhibited plant growth.Lead was the second most toxic element,especially Pb3000,followed by Cd and lastly by Sb.Concrete is the single most widely used material in the world.Due to the worldwide substantial expansion of our built environment the demand for cement,as a par excellence material used for concrete production,will significantly increase in the near future.For cement production,huge exploitation of Earth’s minerals precedes.Its production is also an energy-intensive process,which contributes to high pollution,especially carbon dioxide emission.Since the increase in global carbon dioxide emissions is unacceptable,a search for other binder alternatives is necessary.
The solution for cement substitution or even its total replacement could be found in industrial by-product pozzolanic materials.Until now,the mostly used pozzolanic material for cement substitution is fly ash that comes from thermal power plants after coal combustion.When fly ashes are mixed with an alkali activator,they form alkali-activated material.This group of materials is comparable in physical and mechanical properties with cementitious materials,but has better durability and can come even at a cheaper price,leaving a much lower environmental footprint.However,these materials still do not have a broad usage in the building construction.The standardization of their solid based materials and activators should be firstly defined.Besides,there are also some disadvantages of the materials that should be solved before they find a broad usage on construction sites.AAMs are brittle or quasi-brittle materials and to move toward more ductile behaviour they need to be reinforced,similarly to cementitious materials.In the field of fibre reinforced materials,natural fibres have attracted a lot of attention in the last decade due to their low production cost and low consumption of energy during their production.Natural fibres are fully biodegradable,renewable resources and globally available.Their tensile strength and E-modulus are comparable to those of synthetic fibres.However,the mechanical properties of natural fibre reinforced composites are somewhat weaker compared to their counterpart reinforced with synthetics fibres.Natural fibres are generally nonhomogeneous,have high hydrophilicity and reduced long term durability in the matrix.One of the most distinctive parameters influencing the properties of the composite is the characteristic of the fibre/matrix interface itself.The quality of the bond is related to the fibres’ surface morphology and topography.To optimise the bond,so far in cementitious composites various chemical treatments on the surface of natural fibres have been applied.In terms of its simplicity and effectiveness,sodium hydroxide treatment could be a viable treatment also in AAMs.There are no results of the hemp fibres’ surface roughness changes and their influence on the pre-and post-peak of the flexural stress–strain curve,after the usage of the sodium hydroxide treatment.However,the enhancement of the fibres’ physical and mechanical properties when the fibres are treated with sodium hydroxide is confirmed.The researches on the alkali-treated fibres show that 5% sodium hydroxide solution treatment of hemp fibres resulted in the fibres’ density increase ,since the treatment cleans the impurities from the fibres’ surfaces.
The treatment also increased the tensile strength and Young’s modulus of the hemp fibres for ca.4% and 28% respectively.Mwaikambo and Ansell tested mechanical characteristics of the hemp fibres after treatment with 0.03%,0.08%,0.16%,0.24% and 0.32% of sodium hydroxide solution.They stated that there was an increasing trend in the tensile strength of the fibres after increasing the concentration of sodium hydroxide.The fibres’ highest tensile strength is reached after the usage of 0.24% of sodium hydroxide.The usage of the concentration of 0.16% of sodium hydroxide solution resulted in the fibres’ highest Young’s modulus.The highest concentration of sodium hydroxide provided the highest fibres’ strain at failure.Alawar et al.showed that data palm tree fibres treated with 1% of sodium hydroxide increased their tensile strength by 300% compared to non-treated fibres.Up to 11%,higher tensile strength of the kenaf fibres after the treatment with 6% sodium hydroxide is achieved in the research conducted by Edeerozey et al..After treating the jute fibres with 0.5% sodium hydroxide solution,the tensile strength of fibres increased by 82%.The sodium hydroxide treatment of natural fibres proved to be very efficient in enhancing fibres’ mechanical characteristics.However,there is no published work on the influence of the sodium hydroxide treated fibres on any group of alkali-activated mortars.According to the authors’ knowledge,there are only the works conducted by Amalia et al. and by Sean et al.where previously treated short natural fibres are used in alkali-activated matrices.In both works,natural fibres were used,but none of them used hemp fibres.Additionally,both studies focused on the alkali-activated pastes,having no aggregates in the matrix.Due to the pure mechanical characteristics and high cost of the pastes they should be upgraded to composites containing aggregate to be considered as building materials in civil engineering applications.Very importantly,the work conducted in this study is the first research dealing with the durability of the natural fibre reinforced alkaliactivated materials in terms of wet/dry cycles.Composites reinforced with natural fibres show comparable mechanical properties tested at an early age as their counterparts reinforced with synthetic fibres.However,after some months,natural fibres degrade in the alkaline environment and their long-term physical and mechanical properties are questioned.Therefore,it is extremely important to estimate the durability of the natural fibre reinforced high alkaline materials.Only with a proof of their durability,they can find a broader usage in the replacement of some traditional fibres and the usage where high energy absorption capacity,mobile grow system resistance to impact- and dynamic loading or prevention of cracking is required.The objective of this research was to study the physical and mechanical properties of hemp fibre reinforced alkali-activated materials before and after wet/dry cycles.The constant change of the relative humidity can seriously attack and destroy the fibre–matrix interface,which reduces the composite’s mechanical properties.With the wet/dry cycles we wanted to simulate a real weather change for the specimens that could be exposed to outdoor environmental conditions and to examine the composites’ properties change.Additionally,the influence of sodium hydroxide surface treatment of hemp fibres on the physical and mechanical properties of fly ash-based alkali-activated mortars was examined.Three different concentrations,i.e.,3%,6% and 9% of sodium hydroxide treatment were applied on fibres and the mortars’ porosity,water absorption,bulk density,compression- and flexural strength,as well as energy absorption capacity under flexure,prior and after the wet/dry cycles,were measured.Besides,the X-ray Powder Diffraction and Thermogravimetry-Differential thermal analysis were conducted on the mortars.
To estimate the fibres surface changes after the treatment,Scanning Electron Microscope and Atomic Force Microscope are used.In addition,the water absorption of the fibres was measured.For fibre reinforcement,primary bast hemp fibres were used Hemp fibres were selected since they are widely used as fibre reinforcement in cementitious materials and have a long cultivation tradition in Europe.Their properties are listed in Table 1.The fibres length and dosage were chosen based on the literature review,as optimal values for a compromise between mechanical properties in hardened state and fresh state properties such as work ability and fibres good dispersion within the matrix.To cut fibres to the length of 10 mm,a small bundle of fibres were taken,measured with a ruler and cut with scissors manually.Due to their hydrophilic nature,the fibres were added to the mixture in water-saturated dry surface condition.They were completely soaked in water for 24 h,taken out,softly squeezed by hand and their surface was wiped with a cloth.The water absorption was measured on 200 g of fibres.From a fibre bundle,fibres were separated manually,and their weight is measured firstly in a dry condition.Afterwards they were put in water and on defined time intervals their water absorption was measured.Even though it could be seen that the fibres’ highest water absorption happens within the first 20 min,they were left in water for 24 h in order to be fully sure that they absorbed maximum water they could.It is measured that the weight of the water-saturated non-treated fibres is 2.3 times the weight of the dry fibres.Due to the ease of application,effectiveness of the fibres’ structure and surface change,availability of the material for fibres treatment,the sodium hydroxide was used for fibres treatment.The sodium hydroxide was also used within the composites’ activator and it was not necessary to additionally provide a new material for fibres treatment.It is true that sodium hydroxide has a negative environmental footprint.However,there is a very minor amount of the sodium hydroxide that could be used for the fibre treatments.In this context,it should be noted that NaOH is the main activating agent for other types of alkali-activated materials bound in a hardened matrix,which has been documented in the publications or even applied in practice.Besides,natural fibres have enormous advantages over traditional fibres.Not only regarding the environmental footprint,but also that they are renewable materials.Therefore,even though they are not used as raw materials,their usage within the composites in the research is still justified.For fibre treatment three different concentrations of sodium hydroxide solution were used: i) 3%-; ii) 6%- and iii) 9% sodium hydroxide solution.To obtain the required sodium hydroxide concentrations,185.57 g,382.98 g and 593.41 g of sodium hydroxide were dissolved in 6000 g of distilled water respectively and left for 24 h to homogenize.Three plastic containers were half-filled with respective sodium hydroxide solutions.Afterwards,in each container 100 g of hemp fibre bundles were added,the containers were filled completely with sodium hydroxide solutions and tightly closed.The containers were placed in a drying oven at 80 ◦C temperature for one hour.