Energy saving in buildings may be achieved by using materials with high thermal performance (1). The easiest way to improve the thermal performance of solid fired-clay bricks is to increase their porosity. It is well known that thermal conductivity and mechanical properties are strongly influenced by void fraction with a direct effect: an increase of the porosity increases thermal performance but decreases mechanical strength. This limits the maximum porosity to levels that ensures a reasonable mechanical strength, for specific application requirements in constructions (2-6). Principal factors which affect the materials values of thermal conductivity and mechanical strength are their microscopic and macroscopic structure (pore volume, size and shape), mineralogical composition and anisotropy (7-12).

For solid clay units the reduction of the thermal conductivity parameter may be obtained by incorporating different waste materials which play the role of pore-forming agents (6, 13-18). The well-known pore forming agents’ materials incorporated up to now into clay matrix by scientific researchers are residual biomass (15, 18), paper waste (6, 16), vermiculite (19), etc.

An efficient pore forming agent is a building material with high content of organic matter, which has the role to improve the thermal properties of ceramic materials by increasing their porosity. One of the waste materials which can be used as an organic pore forming agent is sawdust. In the clay matrix it plays multiple roles: act as a pore forming agent which burns during the firing, increasing the porosity and reducing the thermal conductivity (12, 15) of the final product and save energy in the manufacturing process, due to its high caloric power (15-20,5MJ/kg) (20).

Based on studies performed by other authors, sawdust content into the clay matrix is up to 30% (11, 15, 21-24), depending by the raw material characteristics.

Depending on the particle size, the sawdust in the firing process will develop in the clay matrix a network of different types of pores (capillary, subcapillary or microcapillary pores). In ceramic specimens these pores are supplementary to those resulted due to the clay’s particle arrangement and those resulted during the chemical and mineralogical transformations which appear at different firing temperatures (12).

The study investigates the influence of sawdust particles size on mechanical and thermal properties of solid fired clay bricks, and the anisotropy of porosity and thermal conductivity measured in two directions (parallel and perpendicular to the pressing direction). The aim of the experimental study was to determine the influence of the pore size in improving the thermal properties of fired clay bricks while the mechanical properties are kept in acceptable limits.

The XRD pattern of the starting mixture (Figure 2) consists of quartz, kaolinite, calcite, dolomite and some iron oxides. Part of these phases suffer transformations during the firing process.

Part of the phase transformations that occur during the firing process are associated with heat effects, so these transformations are also monitored by thermal analysis. The DTA-TGA curve is presented in Figure 3. The water evaporation, the carbonates decomposition and the burning of the added biomass are the main cause of weight loss in brick manufacturing.

The first effect visible in the TGA curve is a wide endothermic peak corresponding to the water evaporation. This water evaporation starts at temperatures over 50 °C and is still ongoing up to temperatures around 200 °C. The second effect present is a strong exothermic peak composed of several effects. The major effect is the burning of the sawdust particles in the 270–620 °C temperature range. Over this range a smaller endothermic effect is overlapping, the kaolinite- metakaolinite transformation with the release of the OH^- functional group. Further increasing the temperature, the carbonates’ decomposition begins. During firing, carbonates act as pore-forming agents and generate crystalline phases, which can enhance mechanical strength (28). The calcium carbonate that begins to decompose as the CO₂ partial pressure indicates, around 700°C and continues up to 860°C. These observations are similar with the findings of other authors on clays (29). No clear evidence of liquid forming was observed in the DTA-TGA analysis.

Samples contains no pore-forming agent except for the naturally occurring carbonates having densities of 1.69g/cm³ (control sample-CS). By adding sawdust, the other samples had their density reduced by approximately 12-17% and are presented in Table 1. Some scattering of the results was observed but they remained within acceptable limits and were due to the uneven nature of the naturally occurring materials.

An increase of the total porosity by 11-13% was observed, some of the porosity was lost during the firing shrinkage.

The water absorption properties of bricks are a key factor that affects their durability and describes the open porosity through which water can pass. An important increase (~30%) in the water absorption was obtained by the addition of the sawdust and it is also visible in the total porosity increase. Most of the pores responsible for the water absorption are capillary pores which represent about 23% of the total volume. These pores are generated by two mechanisms: gas releases during the firing representing about 16% (5 % from water and 11 % from calcium carbonate) and 7% from the clay particle arrangement. The difference (up to total porosity) is given by the sawdust particles after burning.

The anisotropic nature of the sawdust particle gives another unusual effect in the samples; the orientation of the pore former in the die’s filing direction. This effect was evidenced by measuring the porosity in the transversal and longitudinal direction by image analysis. The results are presented in Figure 4. Since the porosity is the main factor that affects the compressive strength and the thermal conductivity, it is important to take these variations into account.

In the fired bricks, the carbonates from the raw materials and the interparticle voids are responsible for generating micrometer sized pores. The large pores generated by the burning of the organic particles create a pore network that can compromise the brick’s properties by acting as a weak link. These large pores (millimetre sized), are easily visible by optical microscopy. Using the ImageJ software, the pore size distribution of the different samples was measured on transversal and longitudinal direction for five samples for each sample type. The results are presented in Figure 5.

The measured pore size fraction differs from the used sawdust particle at a first glance but is important to remember that the sawdust particles were separated by sieving, and during this process the particle passes through the mesh on its smallest section, and the image analysis in the used configuration measures all the pore’s three dimensions. A gradual decrease of the pore size distribution is observed with the decrease of the used sawdust particles; in each case most of the pores have dimensions less than 0.5 mm. This can be explained by the elongated shape of the pores that varies from 0.3 up to 1 mm. Due to this variation the true dimensions of the pores vary from the calculated equivalent dimension.

Compressive strength on clay mixtures is between 7.9 MPa (M5) and 11.3 MPa (M1) (see Figure 6). One can observe a wide variation between samples for the same quantity of sawdust. Higher strengths occur in the case of samples M1, M2, which can be explained by the fact that the contact points between the clay particles are stronger and provide a better resistance to samples. As one can observe in Figure 4 a pore network is formed around the clay agglomerates, which tend to act as weak links between them even after firing. This fact explains the reduction of the mechanical properties. The effect of the coarser particles is due to the less bonding bridges between the clay agglomerates, the number of the bonding areas increases as the sawdust particle size decreases.

The thermal diffusivity of the pyroelectric sensor is independent on the thermal behaviour of the investigated sample. In this case, the sensor’s thermal diffusivity is α = 1.1 x10^-6 m²/s, in good agreement with the literature. This result confirms the fact that the theoretical and the experimental conditions are fulfilled. The obtained results for the investigated samples (M1 - M5) are displayed in Figure 7. Figure 8 shows the optimization of the fits performed in order to get the thermal effusivity. The correct value of the thermal effusivity minimizes the root mean square (RMS) of the experimental data performed with Equation [1].

IR investigations: The DC amplitude and phase images, together with the corresponding profiles are shown in Figures 9-11. The DC and the amplitude images show the dissipation of the heat at the surface of the sample, due to the absorption of the laser. The profiles are quite smooth and thus the emissivity of the investigated surface can be assumed to be constant. The average temperature of the surface has increased from 26 ^°C up to 41 ^°C (see Figure 10). The profile of the phase image shows that the thermal wave diffuses to the surface of the sample symmetrically with respect to the excitation source (the laser is targeted close to the pixel 170 on the IR images). The thermal wave spreads over a distance of about 1.5 mm around the excitation source. The disturbance of the thermal wave (see the phase profile, around pixel 210) reveals the presence of a small pore located on the surface of the sample M2. The thermal diffusivity of the samples was calculated from phase profiles, according to Equation [4].

The obtained results with the associated standard deviations are listed in Table 2. The thermal effusivity was measured for all four transversal sections of a cubic specimen and the average value was calculated. The thermal diffusivity was taken on the same surface, at four different points, and the mean value was calculated.

The thermal conductivity values lie between 0.69W/mK (the control sample) and 0.49W/mK. The thermal conductivity of the samples containing pore-forming agent decreases by 30% compared to the reference sample. At the same time, the samples (M4 and M5) containing the sawdust particles with bigger size (0.6mm÷1.2mm) have a slightly lower conductivity (k=0.49 W/mK) compared to M2 and M3 samples (k=0.58W/mK) which contain the same percentage of pore-forming agent with smaller size (0.3mm÷0.6mm). May be observed that thermal conductivity coefficient is lower for samples with large particle size of sawdust which are in accordance with other authors observations (12, 23).

In the sections perpendicular to the pressing plane (transversal sections) the pore distribution is slightly larger than in the longitudinal direction (parallel with respect to the pressing plane), and the pores are quite elongated. Nevertheless, only a slight anisotropy in the thermal conductivity is revealed, as shown in Table 2. The heat flow is privileged in the parallel direction due to a slightly higher thermal conductivity. Compared to extruded bricks the ratio k_///k⊥ ≅ 1.1 for conventional bricks is in the range 1.08-1.9 obtained by authors (7,11). Anisotropy of the sample appear due to the pore forming agent and clay layers orientation which affect the thermal and mechanical properties of pressed (10) and extruded clay bricks (7, 8, 11).

Room temperature X-ray diffraction shows that the starting crystalline phases are quartz, kaolinite, calcite, dolomite and some iron oxides. High temperature X-ray diffraction combined with the DTA-TGA-MS analysis shows that these phases are subjected to different transformations as follows: quartz suffers a transformation phase around 580°C, followed by the beginning of the dehydration of kaolinite. Around 720°C a sharp rise in the CO₂ partial pressure is observable in the MS curve correlated with a mass loss which corresponds to the calcite decomposition. Further increasing the temperature another peak in the CO₂ partial pressure is observable corresponding to the dolomite’s decomposition. In the fired samples the different crystalline phases are present. The silicon dioxide reacts with the aluminum oxide (and forms mullite and illite), incorporating different other cations, some quartz reacts with the iron and forms iron silicon dioxide. A significant quantity of iron oxide remains for the samples to have the specific reddish color.

The optical microscopy shows the formation of a high porosity pore network, significantly reducing the mechanical properties. A difference in the porosity determined by optical microscopy was observed along the length of the scaled down brick due to the anisotropic shape of the sawdust particles.

The pores size distribution measured by image analysis shows a decreasing trend with the decrease of the used sawdust particle size range. Water absorption and total porosity variation with particle size range is in the range of the experimental uncertainties, no clear trend is observable. Since the determining parameter is the porosity not the pore size distribution.

The effect of anisotropy in thermal transport is insignifiant for conventional bricks, compared to ceramic block obtained by extrusion. The thermal conductivity is about 10% smaller in the perpendicular direction than in the longitudinal one. Thus, the laying directions of conventional bricks to the wall geometry is not critical such as the bricks obtained by extrusion.

On the other hand, the compressive strength follows a linear decrease due to the formation of the weak links between the clay particles. By increasing the sawdust particles size, the bigger pores act as more intense crack initiation sites, drastically reducing the samples compressive strength. Compared to the mean particle size of the starting mixture (d₅₀=2.1 mm) one can observe that the compressive strength increases with the increase of the clay particle/sawdust particle ratio.