The fibre bundles were then taken out from the containers and washed several times with distilled water until all the sodium hydroxide from the surface of the fibres was eliminated.This was verified by the neutral response of pH measurement of water in which fibres were washed.Afterwards,the fibre bundles were cut to the required length.Additional treated fibres were prepared for evaluation of their water absorption percentage share measured at different time intervals,i.e.the same intervals as for the non-treated fibres.The treated fibres resulted with the same percentages of their total water absorption as for the non-treated fibres.For all mortar mixtures,the fly ash to sand mass ratio was set to 1:3.The activator to fly ash dosage was defined by equaling the mass of Na2O from the activator with 10% of the fly ash mass.All mortar specimens were prepared according to the EN 1015-11.From each mixture,six prism specimens with dimensions of 40 × 40 × 160 mm3 were cast.The exact composition of all the mixtures is listed in Table 2.After the mixture is poured into the moulds and left for one hour in laboratory conditions ,the specimens were covered with a plastic foil,placed in an oven and cured at 80 ◦C for six hours.The oven was turned off and the specimens were left to rest inside the oven for the next 12 h.The specimens were demoulded and kept in a climate chamber until their testing at the age of 28 days.Non-treated hemp fibres have many small fibrous materials that are connected to the main fibres’ structure.These filaments result from the fibres’ processing and are not to be confused with the small structural fibres.Besides,mobile vertical rack other impurities and wax materials could be found on the surface of non-treated fibres.Generally,one single hemp fibre bundle consists of many fibrils that are glued together with waxy and oily materials.

During the fibres’ surface examination,it was noticed that 3% concentration of sodium hydroxide solution can significantly clean the fibres’ surfaces and remove small filaments.With the sodium hydroxide treatment also waxy and oily materials can be rinsed off and removed from a fibre,which leads to the separation of larger fibre bundles into smaller fibrils with cleaner surfaces.Using higher concentrations of sodium hydroxide treatment leads to further significant improvements in the cleaning of the fibres’ surface.The positive impact on the morphology of natural fibres after the sodium hydroxide treatment has also been proven in the study of others.Mwaikambo and Ansell showed that after the treatment with 0.24% sodium hydroxide solution,the surface of hemp fibres became cleaner and fibre bundles more separated,with a highly serrated surface.Sedan et al.proved that the hemp fibre treatment with 6% sodium hydroxide solution removed hemicelluloses,waxes and impurities from the fibres’ surface and thus increased its homogeneity.Edeerozey et al.showed that the treatment of kenaf fibres with 3%,6% and 9% of sodium concentrated solutions significantly enhances the purity of the fibres’ surface.They concluded that the optimum concentration of sodium solution for fibres’ treating in regards to fibres’ surface cleanness and tensile strength is 6%.After the chemical treatment of the fibres even by touching them and with visual inspection could be noticed that the fibres are rougher and show jagged edges.The level of roughness before and after the treatment can provide important information for further interfacial adhesion between the fibres and matrix.Measuring the roughness on the surface of non-treated hemp fibres was very difficult.Due to the inhomogeneous surface of the fibres ,the cantilever from the AFM could not move easily on the surface of the fibres.After the chemical treatment of the fibres it is indicated that sodium hydroxide not only cleans the fibres surface ,but also attacks more sensitive surface parts,disrupting them and providing the roughness and jagged edges of the fibres surfaces.

Chemically treated fibres had more uniform surface structure in the longitudinal direction.The fibres after alkali treatment also resulted in higher root mean square roughness,which indicates that the treatment increases the roughness of the fibre surface.After the alkali treatment,fibres also decrease their water absorption capacity ,due to the elimination of the fibres’ filaments and washing out the hemicellulose and waxes.The pore diameters in the range of 2.5–10 nm,10–50 nm and 50 nm − 10 µm are considered to be the small,medium and large capillary pores,respectively and were adopted according to Mindess et al..All pores with diameters larger than 10 µm are considered as entrained air pores.Plain mortar is characterized by 31% lower total pore volume and 24% lower porosity in comparison to the non-treated hemp reinforced mortar.The main pore structure parameters and the pore sizes distribution give evidence of the finer pore structure formed in the plain mortar than in the non-treated fibre reinforced mortar.In a plain mortar,the entrained air pores occupy only around 10% of its total porosity ,whereas in nontreated hemp fibre reinforced mortars even up to 25%.The fibre reinforcement contributes to aeration of the matrix by the entrained air,as in cement-based matrices,and consequently to pore structure coarsening.The medium capillary pores occupy in both mortars around 70% of their total porosity,however,the ratio of small capillary pores significantly differs,i.e.,in the plain mortar,it is around 20%,whereas in hemp fibre mortar – only around 7%.The pore size distribution curve of the plain mortar has a lower slope in the area of the coarser pores than in hemp fibre-reinforced mortar ,which also indicates its finer pore structure.However,the mortar’s total porosity and the effect of the pore structure coarsening by fibres addition are lowered when the fibres are previously treated with sodium hydroxide solutions.After the fibre treatment,reinforced mortars resulted in a lower volume of total pores and lower porosity.When fibres are treated with 3%-,6%- and 9% sodium hydroxide solution,the total pore volume of the fibre reinforced mortars’ decreases by 28%,3% and 20% and their total porosity – by 22%,3% and 19% respectively.The fibre treatment leads to a pores refinement of the reinforced mortar.Furthermore,the fibre treatment results in a decrease of the entrained air pores by 46%,19% and 22% when the fibres are treated with 3%-,6%- and 9% sodium hydroxide solution,respectively.Additionally,the alkali treatment of fibres results in a reinforced mortar’s lower slope of the pore size distribution curve in the area of coarser pores followed by a more pronounced rise in the small range pore area around 1 µm.

The alkali treatment leads to a cleaner fibres’ surface and better separation of the fibre bundles and consequently in a better mixability with more optimal distribution within a matrix.All this could be the reason for lower porosity and pore refinement in reinforced mortars containing treated fibres.In addition to this,treated fibres show higher surface roughness.Consequently,it is expected that treated fibres have a better bond with the matrix than non-treated fibres,resulting in fewer pores on the fibre–matrix interface itself.As a result of the entrained air,no fibre-reinforced mortar achieves the pore structure parameters of the plain mortar.Nevertheless,no significant deterioration is observed in the case of alkali-treated fibre reinforced mortars.In cementitious mortars,a similar but not so pronounced trend of total porosity change was obtained by Jo et al..It was shown that when using 5 mm-long 0.5% sodium hydroxide treated jute fibres in a dosage of 1 wt%,cement-based mortars reduced their porosity by 3.5% when compared to non-treated jute fibre reinforced mortars.After wet/dry cycles,the porosity of plain mortar measured with MIP is 41% lower than in the case of non-treated fibre-reinforced mortar,which shows a significant decrease.The reason could be that nontreated fibres absorb water and swell,causing micro-cracks in the matrix.The alkali treatment of fibres seems to lower the reinforced mortar’s porosity also after the wet/dry cycles.Reinforced mortars show a decrease in the total pores volume by 33%,35% and 26% and in porosity by 27%,28% and 23%,after the fibres treatment with 3%,6% and 9% sodium hydroxide,respectively.In addition to this,the alkali-treated fibre reinforced mortars show a more distinctive drop of the pore size distribution curve in the area of the main curve’s peak ,which is evidence of a pore refinement.Lower porosity in alkali-treated fibre reinforced mortars could arise from the reduced water absorption capacity of treated fibres ,which consequently reduces the fibres’ swelling under wet/dry cycles.Additionally,cleaner fibres’ surfaces and removal of surface filaments are expected to reduce the fibre agglomeration and improve the distribution of fibres within the matrix.In comparison to non-aged mortars,after wet/dry cycles,the porosity of the plain mortar increases by 15%,whereas the porosity of the non-treated fibre reinforced mortar increases significantly,i.e.by 47%.The alkali treatment of fibres decreases this value.Decreased porosity by 36%,10% and 40% is observed for 3%-,6%- and 9% sodium hydroxide treated fibres,respectively.Most importantly,it is noticed that mortars reinforced with 6% sodium hydroxide treated fibres experience a smaller change in porosity after wet/dry cycles in comparison to the plain mortars.It could be concluded that 6% sodium hydroxide treatment helps to clean the fibres’ surface sufficiently and at the same time,it is not a too strong concentration for a possible deterioration of the structure of the fibres.

Water ingresses in open pores that are larger than approximately 1 μm.These pores belong to the large capillary and entrained air pores.The plain mortar results in 19% lower water absorption in comparison to non-treated fibre reinforced mortar.This occurs due to an increase of the coarser pores after the addition of non-treated hemp fibres.Since the fibres alkali treatment decreases the water absorption capacity of the fibres itself and refines the pore structure of mortars ,it also leads to an overall decrease in the fibre reinforced mortar’s water absorption capacity.The same trend was reported in cement-based mortars reinforced with 5 mm-long jute fibres with a dosage of 1% in the matrix.The addition of non-treated jute fibres to the matrix increased the water absorption of the plain mortar by 5.55%.Due to a better interface between 0.5% sodium hydroxide-treated jute fibres and the matrix,the water absorption of alkali-treated fibre reinforced mortars was 3.16% lower than by non-treated fibre reinforced mortars.After the wet/dry cycles,vertical grow rack the plain mortar shows 12% lower water absorption capacity when compared to the non-treated fibre reinforced mortar.The alkali treatment of the fibres,after the wet/ dry cycles,helps to reduce the water absorption capacity of the fibre reinforced mortars to almost the same level as the plain mortar’s absorption.On average 11% lower water absorption capacity is noticed for alkali-treated fibre reinforced mortars than for the non-treated fibre reinforced mortar.Comparison between the water absorption capacities of the mortars after the wet/dry cycles and their non-aged counterparts ,shows a decrease in the mortars’ water absorption capacity.It can be seen that fibre reinforced mortars experience a higher water absorption capacity drop in comparison to plain mortars.The alkali treatment results in a reduced water absorption capacity of the reinforced mortar after wet/dry cycles.The sodium hydroxide concentration itself has no influence on the fibre treatment in terms of the mortars’ water absorption capacity since all mortars showed on average the same capacity reduction,i.e.23%.The addition of non-treated fibres to the mortar has almost no influence on the mortar’s density,i.e.,a negligible decrease was observed.The decrease results from the entrained air and the increase of the total pore volume by the addition of fibres to the mortar.Additionally,hemp fibres have a lower density than plain mortar ,which in turn slightly decreases the composites’ overall density.In the case of sodium hydroxide treated fibres,the density of the reinforced mortar remains almost the same,.The different sodium hydroxide concentrations used for fibre treatment had no influence on the bulk density of the mortar.In fibre-reinforced cement-based composites,the same trend was reported.After the addition of 1% of 50 mm-long coir fibres to the matrix,the density of the composite decreased by 1.7%.In the case of 5% sodium hydroxide solution-treated fibres,the density of the composite increased by 0.4% in comparison to the nontreated fibre reinforced composite.The same trend with no significant change in density was noticed by Jo et al.in the case of jute fibre reinforced cementitious mortars.The addition of the 5 mm-long,non-treated jute fibres,in the dosage of 1% ,decreased the density of the mortars by 1%.However,after the addition of the 0.5% sodium hydroxide-treated fibres,the density of the nontreated fibre reinforced mortars was increased by 0.7 %.Similar to the results of our study,there was no significant change in density after the fibres were chemically treated.Compared to the non-treated fibre-reinforced mortar,after the wet/ dry cycles,the plain mortar’s bulk density was only 4% higher.