Influence of compaction direction on selected thermal and moisture properties of a lightweight composite based on magnesium binder and organic filler

An alternative binder in thermal insulation composites based on hemp shives is magnesium cement. It provides higher mechanical strength of the composite in comparison to the lime binder. Thanks to this, it is possible to reduce the amount of binder in relation to the shives, and as a result, obtain a lower density and better thermal insulation parameters. The process of compacting the composite mixture determines many of its properties. The longitudinal shape of the shives causes that during compaction they are arranged mainly perpendicular to the direction of compaction. The unidirectional course of the fibers in the shives determines the anisotropic nature of both the shives and the composite. The article presents the results of tests of thermal conductivity and capillary rise of the compacted composite in the direction perpendicular and parallel to the heat flux and moisture transport. A composite with a low binder content, characterized by a density of about 250 kg/m3, was tested. The direction of the mix compaction had an impact on the differences in the obtained results. This phenomenon can be used, for example, in the production of blocks or other prefabricated elements, using the appropriate direction of compaction depending on the expected properties and applications of the composite.


Introduction
Composites based on hemp shives are mainly used as a thermal insulation material for external walls in a wooden frame structure.The most commonly used binder is hydrated lime modified with additives that accelerate setting or increase water resistance and strength.Examples are pozzolanic materials such as metakaolin [1,2], zeolite [3], but also biopolymers such as gum arabic [4].The problem with the use of lime binder is its low strength, brittleness and slow setting process.In order for the composite to have strength suitable for use in walls, the weight ratio of the binder to the shives is 1.5:1 and higher.As a result, the composite has a minimum density of about 350 kg/m 3 and it is difficult to reduce the density in order to maintain proper strength.The key parameter of this composite, i.e. the thermal conductivity coefficient, also depends on the density.Composites based on lime and shives are characterized by thermal conductivity in the exemplary range of 0.074 -0.138 W/(m•K) [2,3,5].An alternative binder used in the shive-based composite is magnesium cement [6][7][8][9][10].It is an air binder obtained by combining 2 magnesium oxide and an aqueous solution of magnesium chloride.The advantage is higher mechanical strength compared to lime binder.It is possible to reduce the ratio of binder to shives even below 1:1 [6,7].As a result, the resulting composite is characterized by low density, low thermal conductivity, while maintaining strength appropriate for use in walls.It is a suitable binder for use in the prefabrication of composite elements based on shives.Hemp shives, due to their elongated shape, determine the anisotropic properties of the composite [1].The properties of the composite are determined, among others, by the direction of application of the external factor (e.g.heat flux) in relation to the direction of the mixture compaction.The shives tend to be perpendicular to the direction of compaction.
The paper presents the results of research on a composite based on magnesium binder and hemp shives.
A low ratio of binder to shives was used to obtain a lightweight material with good thermal insulation parameters.The tests of the thermal conductivity coefficient and the capillary rise of the composite were carried out in two directions -perpendicular and parallel to the direction of the composite compaction in the mold.

Materials
Magnesium cement was used as a binder.It consists of magnesium oxide with a MgO content of 78% and magnesium chloride hexahydrate in the form of flakes with a minimum MgCl2 content of 47%.
After dissolving magnesium chloride in water, the solution was mixed with magnesium oxide.Citric acid was used as a setting retarder.Hemp shives of the Polish variety "Białobrzeskie" were used as a filler.The shives used are suitable for construction purposes.In the production process, they were cleaned of excess fibers and dust.The shives were sieved through a sieve with a mesh diameter of 8 mm to eliminate the unfavorable impact of random, long fractions.Figure 1 shows the percentage of the length of each shive.The weight fraction of shives in magnesium oxide was 1:0.8.An aqueous solution of magnesium chloride (1:1) was used in the amount of 63% of the weight of magnesium oxide.The amount of citric acid was 0.5% by weight of magnesium oxide.The total proportion of water in the mixture (excluding the water contained in the solution) is 113% by weight of the magnesium oxide.

Samples preparation
The procedure for making the mixture was as follows: Hemp shives were mixed with part of the water, then magnesium oxide was added.An aqueous solution of magnesium chloride was then prepared together with citric acid and added to the mixture.Finally, the remaining water was added and mixed until the desired consistency was obtained.The mixture was placed in molds in several layers, compacting each one manually with a wooden rammer.They were compacted in two directions: parallel and perpendicular to the direction of influence of the two tested factors (heat flow and water transport).The samples were maturating in laboratory conditions (air temperature 20°C ± 2°C, air relative humidity 55 % ± 5 %) for 30 days after preparation.Before testing, the samples were dried to constant weight at 70°C for at least 24 hours.

Apparent density, total porosity
To determine the apparent density, samples with dimensions of 250×250×50 mm 3 , intended for thermal conductivity tests, were used to determine the dependence of this parameter on density.The apparent density was calculated based on the dimensions of the samples and their weight in a dry state.The total porosity was calculated on the basis of the results of measurements of the apparent density and specific density tested by the pycnometric method.

Thermal conductivity
The study of the thermal conductivity coefficient was carried out on composite samples with dimensions of 250×250×50 mm 3 .The measurement was made perpendicularly and parallel to the direction of compaction of the mixture in the mold.The measurement was carried out using a plate apparatus FOX314 (TA Instruments) in accordance with the ISO 8302 standard [11].The temperature set on the heating plate was 25°C and on the cooling plate 0°C.Three composite samples were tested for each direction of heat flow.

Capillary rise
Three samples compacted in parallel and three samples compacted perpendicularly to the direction of water capillary action were used for the tests.Samples measuring 80×80×240 mm 3 were placed in water to a depth of about 10 mm.The four side surfaces were covered with bituminous mass to ensure water absorption on only one surface (from the bottom).The weight gain of the samples was measured at the following intervals: 5 min, 15 min, 30 min, 1 h, 3 h, 6 h, 12 h, 24 h, 2 d, 3 d, 5 d, 6 d and 7 days.

Apparent density, total porosity
Table 1 shows the basic physical parameters of the hemp-magnesium composite.These are the average values of the three samples.The composite compacted parallel to the direction of heat flow showed an apparent density of 252.4 kg/m 3 , and when compacted perpendicular to the direction of heat flow, 244.5 kg/m 3 .Differences may be due to technical reasons.In the first case, an area of 250 mm ×250 mm was compacted, and the height of the sample was 50 mm.In the second, an area of 50 mm ×250 mm was compacted, and the height of the sample was 250 mm.Smaller compaction area and greater mold depth could result in locally less compacted areas as a result of more difficult access with a wooden rammer.The obtained densities of the tested composites are definitely lower than the average densities of composites based on shives and lime -for example 328-627 kg/m 3 [1,2,4,5].This confirms the fact that it is possible to reduce the density of the composite by using smaller amounts of magnesium binder instead of lime binder, while maintaining its functional properties.The tested composites, due to their low bulk density, are characterized by high total porosity in the range of 87.1-87.5%.The high porosity of the composite results primarily from the structure of the hemp shives, whose total porosity is 78.7% [12].

Thermal conductivity
Figure 2 shows the results of the composite thermal conductivity depending on its apparent density.The value of the thermal conductivity coefficient of the composite compacted parallel to the direction of heat flow was 0.069 W/(m•K) to 0.071 W/(m•K).However, in the case of compaction in the direction perpendicular to the heat flow, the value of the coefficient was between 0.083 W/(m•K) and 0.087 W/(m•K).In both cases, the dependence of the increase in the value of the coefficient with the increase in the density of composite samples can be observed.This is a correct relationship confirmed in many studies on building materials, including composites based on hemp shives [2,5,13,14].Based on the differences in the results, the anisotropic nature of the composite is clearly visible.Hemp shives are made of fibers that are arranged in one direction (along the stem, in the direction of plant growth).Under the influence of compaction, the hemp shives (and thus the fibers) are arranged mainly in the direction perpendicular to the direction of compaction.The capillary pores also run in the same direction.Thus, the heat stream flowing through the composite sample in the direction of compaction (parallel) hits numerous barriers in the form of capillary pore walls.Such arrangement of the shives results in a better thermal insulating ability of the composite.This observation was also confirmed in the case of other fibrous materials, such as straw bales [15], as well as lime-hemp composite [1].Wood, as a fibrous material, also has a lower thermal conductivity across than along the fibers [16]

Capillary rise
The results of the capillary rise are shown in Figure 3.The dynamics of water absorption by each sample and the averaged values are presented.The focus was on the period of 7 days of water rising through the composite.The results of the capillary rise tests also confirm the anisotropic characteristics of the composite.Samples compacted perpendicularly to the direction of water flow absorbed a much larger amount of water than samples compacted parallel to the direction of rising water.The growth rate was also faster.
After 24 hours of testing, the difference in the amount of water absorbed per 1m 2 of the bottom surface was 40.2%, while after 7 days it was 39.3%.Compaction parallel to the direction of water rising causes that the shives are arranged parallel to the water table, and thus the direction of the capillary pores is analogous.The stream of water encounters barriers in the form of pore walls.Otherwise, when the samples are compacted perpendicular to the rising water direction, the water transport path is more open because the capillary pores are perpendicular to the water table.A similar effect was observed when examining the capillary rise of hemp-lime composites in the work [1].In paper [2], the water absorption coefficient by capillary rise of the hemp-lime composite after 24 hours was determined.It was in the range of 2.65 and 3.37 kg/(m 2 •h 1/2 ).Comparing our own results with the cited literature, the coefficient of capillary rise of the composite tested parallel to the direction of compaction was 2.45 kg/(m 2 •h 1/2 ) after 24 hours, while in the case of the composite tested in the direction perpendicular to the direction of compaction it was 3.24 kg/(m 2 •h 1/2 ).

Conclusions
Based on the research on the composite based on hemp shives and low content of magnesium binder, the following conclusions can be drawn: -By using a magnesium binder, it is possible to reduce the amount of binder in the shive-based composite, below 1:1 in relation to the shive weight, while maintaining the functional properties of the composite.This is difficult to achieve with typical lime-based binders.
-The use of a small amount of binder contributed to obtaining a composite with low bulk density (244.5 kg/m 3 and 252.4 kg/m 3 ) and low thermal conductivity (0.07 W/(m•K) and 0.085 W/(m•K)).-Thermal conductivity of the composite compacted parallel to the direction of heat flow was lower by about 18% in relation to the composite compacted perpendicularly to the direction of heat flow.
-The composite compacted parallel to the direction of capillary water absorption absorbed about 40% less water after 7 days of testing than the composite compacted perpendicularly to the direction of the capillary water flow.

Figure 1 .
Figure 1.Sizes of the shives used in the study.

Figure 2 .
Figure 2. Relationship between thermal conductivity of the composite and its apparent density.

5TH 6 -
CENTRAL EUROPEAN SYMPOSIUM ON THERMOPHYSICS (CEST 2023) Journal of Physics: Conference Series 2628 (2023) 012002IOP Publishing doi:10.1088/1742-6596/2628/1/012002The composite has anisotropic properties.The direction of compaction affects the arrangement of the shives in the composite.They tend to lay their fibers perpendicular to the direction of compaction.

Table 1 .
Basic physical properties of the composite.