On the hygrothermal properties of sandcrete blocks produced with sawdust as partial replacement of sand

1 Introduction

Throughout the world, the term ‘shelter’ has a strong connection to the ultimate purpose of housing. Also, for every human family, housing that is both safe and affordable is a basic necessity along with water, food and companionship. As far as provision of protection from the local environment is concerned, shelters are architectural buildings constructed to serve various societal needs. Such buildings may be used for residential, mercantile, business, educational, institutional, religious or industrial purposes, and so on. For many decades now, sandcrete blocks have remained vital and the most popular walling materials used in construction of buildings. Sojobi et al. [1] remarked that sandcrete blocks constitute more than 90% of residential building construction in developing countries especially in West Africa.

Sandcrete blocks are produced using a mixture of cement, sand and water in prescribed quantities. Among the mixture constituents, cement is the primary binder used in the cementitious composite [2] and good quality sand is a key material utilised even for the production of other masonry units. As a result of the suitability of dredged sand for such undertaking [3], dredging has been done significantly on large scale. Eventually, this situation causes flooding, landslides, and losses of agricultural land, thereby raising serious environmental concerns. These impacts of sand dredging activity are evident in Thomson Reuters Foundation News of May 7, 2019 and Premium Times Newspaper publication dated December 19, 2020 as well as research reports [4, 5]. Based on the findings from various researches carried out to find solutions to the emerging problems, it has been firmly established that sand can be partially replaced with waste materials to produce sandcrete blocks that are suitable for building construction purpose. For instance, partial replacements of sand with certain proportions of polythene fibres [6], plastic wastes [3], PET plastic bottle flakes [7], wood residue [8, 9], and coconut husk [10] have been reported to yield sandcrete blocks that can perform satisfactorily.

Though the performance ability declines with increasing proportions of the wastes in all the reported cases, it is of great interest to understand that the level of replacement for optimum performance depends a great deal on cement quality, water-cement ratio, and method of block production adopted. Interestingly, the practice of utilising wastes as a partial substitute for sand in the production of sandcrete blocks has additional advantages like provision of routes for effective waste management, minimisation of adverse effects posed by accumulation of such wastes on environment, safeguarding of land for agricultural activities, and enhancement of affordable housing construction. In the real sense, wastes that are recyclable have potential uses and as such, are promising raw materials for beneficial purposes. Unfortunately, very few of such wastes are generated continuously in large volumes every year. A typical example under purview is sawdust. This wood residue is cheap and sustainable. In Nigeria, sawmills account for over 93% of the entire wood processing industries and they are the largest sector of wood processing. Onochie et al. [11] asserted that sawmills in Nigeria produce well over 1.7 million cubic metres of wood waste per year. Not only that, in [12], the authors noted that people always rely on wood, thereby making it the most multipurpose raw material the world has ever known. Indeed, this is a fact when considering the submission of Peter et al. [13] which pointed out that generation of wood waste is constantly on the increase due to increased demand for wood and its products.

However, sawdust is majorly under-utilised, a situation which warrants its disposal by open burning as a prevalent technique in Nigeria and other developing countries. This practice is obviously detrimental to the environment and public health. Aside that, it has been observed that studies carried out and reported so far on wastes utilisation in sandcrete blocks manufacturing focussed on investigation of mechanical and micro-structural properties, porosity, and water permeability of the developed blocks. Determination of density of the blocks is considered in rare cases, though it is an important requirement as stipulated in Ref. [14]. Consequently, there is a dearth (and in most cases complete lack) of information on other important properties that can be used to accurately predict the performances of sandcrete blocks produced by partially replacing sand with waste materials. Since building walling materials may be exposed to attack by water and heat, the knowledge of hygrothermal properties of such materials is generally very vital as it helps in understanding moisture effect and heat flow processes in relation to the materials. This, in turn, enables the selection of appropriate walling materials that will suit the functional requirements of building structure and interior space. Keeping the aforementioned situations in mind, this work is designed to address the emerging concerns based on utilisation of sawdust in the production of solid core sandcrete blocks.

2 Experimental framework

2.2 Processing of the aggregates

The sawdust was submerged in water at room temperature for 20 seconds in order to remove impurities like sand and any accompanying dirts from it. After that, it was removed from the water and dried in air until it became moisture-free. The dried sawdust was comminuted and then divided into two equal portions. One portion of it was soaked in a fresh alkaline solution prepared by dissolving 80g of sodium hydroxide pellets in a litre of the solution. This alkaline treatment was allowed for 6 hours after which the treated sawdust was removed and thoroughly washed with distilled water. The untreated sawdust, treated sawdust, and river sand were sun-dried completely. Figure 1 shows the dry forms of the untreated and treated sawdust utilised for the production of sandcrete block samples in this work.

Figure 1

Dry forms of (a) untreated sawdust (b) treated sawdust

2.5 Testing of the sandcrete block samples

2.5.1 Water absorption

According to [18], this is one major test that is required for verification of the quality of sandcrete block produced to be used as a walling material in building construction. It indicates the ability of a material to absorb and retain water. In this work, the mass of each block sample was determined with a scale balance before all of them were submerged in water at 26°C. After 24 hours, the blocks were removed from the water and the mass of each of them was determined again. The mass of water absorbed and retained by each block sample was calculated as the difference between the mass before and after immersion in the water. Water absorption was then calculated as a percentage, thus

(3) WA=(MwM)×100%

where WA = percentage water absorption, M_w = mass of water absorbed and retained by the block sample, and M = mass of the block sample before immersion in water.

2.5.2 Sorptivity

Since sandcrete block is porous, water ingress into it is inevitable, especially, in flood environments. An appropriate way to evaluate water ingress so as to characterise sandcrete block based on water transport management is by sorptivity measurement. Each block sample produced in this work was first weighed. Five digital hanging scales (A 08) were suspended separately from a fixed height. From the free end of each scale, a block sample was hung by means of a very strong wire. The block was hung in a manner to allow water ingress occur by capillary action upwardly along its length. In each case, a transparent vessel (of diameter 300mm and height 100mm) was kept directly under the suspended block and the position of the block was adjusted until the lower end of the block was about 40mm from the bottom of the vessel. Then cold water was poured into the vessel and immediately the water level was 30mm above the lower edge of the block (that is, 70mm from the bottom of the vessel), timing of water infiltration was commenced. Figure 2 shows the diagram of the set-up used.

Figure 2

Schematic illustration of the set-up for sorptivity test

With the aid of a digital thermometer (Model No. 305) calibrated and equipped with type-K probe, the temperature of the water was monitored. At every 2 minutes interval, the mass of water that ingressed into each block was determined as the difference between the current reading on the scale and the initial reading on it before the commencement of water infiltration into the block used in the set-up. This was done until 50 minutes elapsed. The density of water at that temperature, as provided in [19], was noted. To know the volume of water in the block, its mass was divided by density at that temperature. Then the water infiltration depth was determined as the ratio of the water volume to the block sample contact surface area. The principle put forward by Philip [20] and adopted by Sabir et al. [21] on determination of sorptivity from horizontal infiltration, where water flow is mostly controlled by capillary absorption, was applied. A graph of cumulative infiltration depth of the water against the square root of time taken was plotted. The sorptivity was then deduced from the slope of the linear portion of the graph in line with the relation [22]

(4) Sp=dt

where S_p = sorptivity, d = cumulative infiltration distance of water in the block sample, and t = water infiltration time.

2.5.3 Bulk density

This property expresses the extent to which matter is compacted in a given volume of a material. As such, it has influence on the pore geometry of the material and then becomes vital for assessment of not only the mechanical performance, but thermal behaviour of a material as well. For each block sample under investigation, the mass was determined by weighing and the bulk volume was measured by geometry method (in which case, the length, width and height were multiplied). By applying the concept of mass to volume ratio scrupulously used as the definition of density [23, 24, 25], the bulk density was computed thus

(5) D=MV

where D = bulk density, M = mass of the block sample, and V = bulk volume of the block sample

2.5.4 Specific heat capacity

One consequence of heat transmission through a material is change in temperature and the quantity of heat needed to effect the change is governed by the specific heat capacity of the material. Using mixture method of calorimetry, the specific heat capacity was determined for each block sample in this work. In doing so, temperature-cooling correction was employed [26, 27]. A copper calorimeter of height 100mm and diameter 75mm was used. The block samples subjected to this test were cut into reasonable sizes capable of penetrating the calorimeter unit. For the purpose of heating, an electric oven (Model N30C, Genlab) was made use of. After well lagging of the calorimeter, transfer of the heated sample into water contained in it was done by means of tongs. Also, heat losses were assumed to be negligible and the specific heat capacity was obtained by calculation using the equation

(6) c=(qc−qwMΔθ)

where c = specific heat capacity, q_c = total amount of heat gained by the calorimeter and its stirrer, q_w = total quantity of heat gained by the water, Δθ = change in temperature of the heated block sample on cooling completely.

2.5.5 Thermal conductivity, thermal diffusivity, and thermal effusivity

Heat Flow Meter (HFM 446 Lambda series, NETZSCH) was employed for thermal conductivity test according to the procedure outlined in [28]. This instrument is a standalone unit with two plates between which a block sample was inserted. It has Peltier system for plates temperature control. The data obtained were applied to determine the required thermal conductivity based on Fourier’s law for one-dimensional heat conduction [29, 30], expressed mathematically as

(7) k=QLAΔT

where k = thermal conductivity, Q = rate of heat flow, L = thickness of the block sample, A = area of the block sample surface through which the heat flows, ΔT = temperature difference between the block surfaces in contact with the plates.

In order to obtain the corresponding thermal diffusivity, volumetric heat capacity was calculated as the product of bulk density and specific heat capacity values, which was then applied in the widely used relation [31, 32, 33] to compute thermal diffusivity, λ as

(8) λ=kCv

where C_v = volumetric heat capacity of the block sample.

Also, thermal effusivity, e was calculated using

(9) e=kCv

2.5.6 Time lag, and solar radiation absorptivity

For the calculation of time lag due to thermal disturbance that may be distributed within the block sample during periodic heating, the following relation was used on a 24-hour period

(10) TL=(Tp4πλ)L

where T_L = thermal time lag of the block sample, and T_p = period of the heating cycle.

In the event of photothermal heating of the block while being used as a walling unit, there is a likelihood that it will interact with the surroundings throughout a day due to change in environmental temperature and solar radiation. By simplifying the formula used in some studies including those by [22, 34], the following convenient form of the relationship was derived and applied in this study to calculate the solar radiation absorptivity, α for each block sample on a 24-hour basis:

(11) α=(πλTp)

All the tests were performed at room temperature with ± 1.0°C variations. In each case, the mean and corresponding standard error values were computed and recorded.

3 Results and discussion

Table 1 shows some important parameters that characterise the sand, untreated (raw) sawdust, and treated sawdust utilised in this work to produce the sandcrete block samples. From the results presented, it is observed that, among the aggregates, untreated sawdust is the lightest whereas sand is the heaviest. The coefficient of uniformity values obtained are not less than 6, thus indicating that the aggregates are fine and also well graded [35] for sandcrete blocks manufacturing. Also, as illustrated in Figure 3, the particle size distributions for the sand and sawdust fall within the grading limits for zone one stipulated in [36] for well graded fine aggregates. Since angle of repose values ranging from 31° to 35° are interpreted in the scale of flowability to mean good flow property for acceptation of materials for manufacturing purpose [37], it is evident from the obtained values that the aggregates utilised in this work are suitable for application in sandcrete blocks making.

Figure 3

Gradation curves for the aggregates

Chemically, the treated sawdust is found to be richer in cellulose content but poorer in the hemicelluloses and lignin contents compared to the untreated sawdust (Table 2). It can be deduced from the results that for the untreated sawdust, the cellulose content is 15.22% less whereas the hemicelluloses and lignin are respectively 17.91% and 13.56% greater than in the case of the treated sawdust. The observed increase in cellulose component and remarkable reductions in hemicelluloses and lignin fractions of the sawdust may be attributed to the effect of alkalisation. Stating in another way, the raw sawdust becomes treated sawdust after undergoing surface modification through treatment with sodium hydroxide solution. This mercerisation brings about the removal of a significant amount of hemicelluloses and lignin but causes changes in arrangement of units in cellulose macromolecule thereby resulting in increase in cellulose component. Such variations in lignocelluloses constituents are indicative of the fact that, though both are fibres, the untreated sawdust and treated sawdust can differ in the extent to which they respond to stresses caused by water attack, heat flow, and so on.

In Table 3, the results of tests performed to evaluate the hygrothermal properties of the block samples are recorded. Among the various sawdust loading levels considered in this study, the differences in the water absorption values are minimum (1.87%) and maximum (8.35%) at 10% and 40% contents of the fibres respectively. Since the block samples were produced under the same conditions, it is obvious that water absorption tendency is material-dependent. In this case, the dissimilarity in water absorption capabilities of the studied samples is mainly due to the effect caused by the alkaline treatment on the chemical nature of the USD and TSD. Explaining in another way, the alkaline process directly brings about dewaxing of the raw fibre, influences the cellulose fibril as well as degree of polymerisation, thus making the TSD to be richer in cellulose fraction but poorer in lignin content than the USD. Also, cellulose is very hydrophilic whereas lignin is highly hydrophobic. Enhancement of the cellulose content is accompanied by release of free hydroxyl groups and these groups absorb water through formation of hydrogen bonding. As such, for equal volumes, the affinity of the TSD for water is greater than that of the USD. Consequently, the blocks containing the TSD absorb more water than their counterparts produced with the USD. This means that a greater resistance to attack by water can be achieved by utilising the USD as a partial substitute for sand compared to the use of the TSD in making the blocks. The observation in this case resonates with the report of Mittal and Chaudhary [38] that alkaline treatment of Pineapple Leaf Fibre and Coconut Husk Fibre with sodium hydroxide solution further increases water absorption properties of the fibres. On the basis of the results obtained in this work, it can be adjudged that the blocks produced by utilising more than 20% of the USD or 10% of the TSD fail to meet the maximum water absorption requirement specified as 12% for sandcrete blocks according to [18].

Sorptivity coefficient is very essential for prediction of service life of a structural material and improvement of its performance. From the sorptivity test results, it can be seen that block samples with the USD content have greater sorptivity values than those made with the TSD at similar loading levels of the fibres. Since sorptivity depends on capillary pressure and effective porosity, this is possible because the alkaline treatment applied to obtain the TSD causes change in morphology of the raw sawdust, thereby making the TSD to have rougher surfaces than the USD. In effect, a greater interfacial adhesion is ensured at the boundaries between the TSD and cement paste matrix, thus reducing the pore size unlike in the case of incorporating the USD into a similar matrix. It is a known fact that hydration of cement leads to a product consisting of solid and pore network. In this case, the pore structure mainly involves size and volume of the interconnected capillary pores. It therefore means that, with utilisation of USD in producing the studied blocks, the existing pores are larger than when TSD is made use of. With 0% content of sawdust (USD or TSD), the block sample produced has the lowest sorptivity value. As the sawdust loading increases in steps of 10%, the mean sorptivity values obtained for the block samples made with the USD exceed those obtained for their counterparts produced with the TSD by 7.0%, 7.0%, 7.2%, and 5.6%. This portrays that utilisation of 10% to 20% of the fibres could result in a constant difference in the water infiltration capabilities of the blocks made using the USD and TSD. Now, by comparing the percentage increase in the sorptivity values due to incorporation of the fibres, the least percentage is 6.9% and is obtained at 10% level of the TSD content. Howbeit, the chemical treatment is responsible for the improvement in the lightweightness of the blocks over the pristine samples. In other words, one vital modification resulting from the alkaline treatment is the disruption of hydrogen bonding in the network structure, thereby increasing surface roughness by removing a certain amount of wax, lignin, and oil covering the external surface of the raw sawdust. Jain et al. [39] observed in their study that alkaline treatment using sodium hydroxide solution modified the surface of basalt fibre. Thus, the USD has smooth surfaces and also contains many dusty particles whereas fewer impurities are present in TSD, a situation that improves its packing behaviour. This, in turn, affects the bulk density of the block samples in such a way that those produced with USD fractions have lower values than the ones containing similar fractions but with TSD as a component. The said effect is most pronounced at 40% loading level, leading to a maximum difference as 300.44kgm⁻³ between the block developed with the USD and the one similarly produced with the TSD. By having approximately the same bulk density (as 1700kgm⁻³), it could be argued that the block containing 40% of the TSD might contribute the same weight to a building load as the one produced with 20% content of the USD. It can be deduced that increasing the USD content to 40% leads to a reduction in bulk density by 30.3% whereas in the case involving the use of the TSD, such reduction is 15.5%. Comparatively, this shows about 50.0% difference in the bulk density reductions. Based on the value (1500 kgm⁻³) stated as the minimum bulk density required for sandcrete blocks made with light-weight aggregates [14], it can be remarked that only the block samples produced with 40% of USD fail to satisfy the condition. Figure 4 shows the trends in sorptivity and bulk density of the block samples with respect to the sawdust loadings. It can be observed that the sorptivity of the blocks exhibits increasing tendency with added proportions of the USD or TSD. This is simply because the sawdust is more porous than the cement paste and as such, the pore space increases with increasing content of the sawdust. A sharp bend is noticed at 10% level indicating it to be the minimum proportion of sawdust incorporation for a reasonable influence to manifest in sorptivity of the blocks. As should be expected for the use of either of the fibres (USD and TSD), higher water absorption corresponds to higher water ingress into the blocks and the driving force for water penetration depends on capillary suction within the pore spaces. It is pertinent to opine that, in order to minimise penetration of water into the interior of a building through blocks purposed for use as a walling material, selection has to be done such that blocks of low sorptivity value are preferable. For the fact that sorptivity is an important index of concrete durability [40], the implication of the results is that sandcrete blocks produced with TSD fractions can be more durable than those manufactured with USD at similar levels for application in tropical humid climate. In the case of bulk density, a negative relationship is revealed with respect to the sawdust loadings because sand is the densest component compared to the USD and TSD used in making the block samples. On the basis of pore geometry influence, it can be posited that the more porous the block is, the less bulk density but greater sorptivity it has provided the production conditions remain the same.

Figure 4

Variations of sorptivity and bulk density of the block samples with sawdust proportions

Specific heat capacity quantifies the ability of each block sample to store internal energy. It is observed in this work that whether USD or TSD is used to partially replace sand in the block samples, specific heat capacity values increase with increasing proportions of each fibre. Based on the obtained values, the differences in specific heat capacity (in Jkg⁻¹K⁻¹) at 10%, 20%, 30%, and 40% between blocks with USD content and those similarly produced but with TSD content are respectively 24.00, 57.94, 64.59, and 71.95. Also, at similar loading levels, blocks containing USD have greater specific heat capacity values, thus showing an improvement in heat-storing capability over the blocks made with TSD. That is to say, blocks produced with a particular proportion of USD can maintain temperature for a longer time than those containing same level of sand replaced with TSD. Though the block containing 30% of TSD has the specific heat capacity value (1293.33 Jkg⁻¹K⁻¹) that is comparable to that of the block made with 10% of USD, the volumetric heat capacity of the USD is greater than that of the TSD. In that case, it shows that utilisation of USD yields blocks that exhibit more effectiveness for thermal storage than in the case of applying TSD at same level of sand replacement. This is attributable to the USD having more freeness and increased charge which promote a greater lowering effect on the heat capacity compared to the TSD.

Regarding heat transmission, the results of thermal conductivity test reveal that, at similar content level of sand, blocks that contain USD can exhibit a better thermal insulation performance than those made with TSD. For example, while the thermal resistivity (reciprocal of thermal conductivity) of the blocks, in W⁻¹mK, increases from 2.90 to 3.18, 3.40, 3.75, and 4.39 as a result of utilising the USD at 10%, 20%, 30%, and 40% levels respectively, the corresponding increase due to incorporation of the TSD is 3.03, 3.18, 3.34, and 3.72. This indicates that though improvement in the ability of the developed blocks to resist heat transfer is greater with the use of the USD than TSD as a partial sand replacement material, the block made with 10% of the USD and the one containing 20% of the TSD have the same heat-insulating efficiency. The observed variation is partially attributable to the amount of air present within the block samples. In other words, conduction of heat through the blocks takes place via the solid phases and existing voids. Since the blocks are porous and also completely dry, the voids/pores are filled with air and thus, serve as scattering centres for phonons. In this case, the volume of air present has a positive relationship with the pore sizes. Because air is an excellent thermal insulator, its thermal conductivity value is lower than that of a whole block sample. As earlier noted, more voids exist in blocks with USD content than in the case of those containing TSD of similar proportions. Consequently, blocks produced with USD have greater ability for thermal insulation compared to their counterparts with TSD content. However, the value obtained for each block studied in this work falls within the recommended range given as 0.023 Wm⁻¹K⁻¹ to 2.900 Wm⁻¹K⁻¹ [41] for heat-insulating and construction materials.

Thermal conductivity plays a crucial role to determine thermal diffusion in the blocks. From the obtained values of thermal diffusivity, it can be inferred that with increase in sawdust loading from 10% to 40%, the thermal diffusivity decreases by about 12.0% due to partial sand replacement using the USD but by 10.2% in the case of utilising the TSD. Just like in the case of thermal conductivity, block samples with the USD content have lower thermal diffusivity values than those similarly produced with the TSD. This observation agrees well with the outcome of thermal diffusivity test carried out on composite board produced with untreated and treated coconut husks [42]. In this work, it can be deciphered from the results of thermal diffusivity determination that if each of the block samples is used as a building envelope, those that contain the TSD fractions have greater heat transfer capability than similar ones containing the USD. This means that incorporation of the TSD enhances a faster propagation of temperature variation than making use of the USD. As a matter of fact, being more thermally conductive than the USD is a clear indication that the TSD allows thermal disturbance to be distributed by diffusion at a faster rate. Since thermal energy generated at the surface of a solid is dissipated into its bulk by diffusion, this further shows that blocks containing the USD can maintain thermal comfort and improve overall thermal efficiency better than those produced with similar proportions of the TSD. This insinuates that among the blocks with sawdust component, the one produced with 10% loading level of the TSD is the least efficient as far as thermal insulation is a priority. However, by comparing its mean thermal diffusivity value (1.337 × 10⁻⁷ m²s⁻¹) with the thermal diffusivity values (2.11 to 3.30) × 10⁻⁷ m²s⁻¹ obtained for compressed earth blocks stabilised with sawdust [43], it can be posited that its heat spread rate is 36.6% slower than the slowest rate reported for the compressed earth blocks used for building purposes. Thus, it can ensure thermal convenience better than the earth blocks if both of them are subjected to thermal disturbance under same conditions. Graphically, it is revealed that the more the loading of sawdust in the block is, the less is the speed at which heat conducted by the block is distributed to cause a change in temperature within it (Figure 5). A similar behaviour is displayed in the case of thermal conductivity, thus indicating that with constancy of thermal capacity, thermal diffusivity correlates positively with thermal conductivity.

Figure 5

Variations of thermal conductivity and thermal diffusivity of the block samples with sawdust proportions

As a consequence of periodic heating, thermal waves are generated in a solid. When such waves propagate between two media, thermal effusivity determines the value of the reflection and transmission coefficients at the interface. Thermal effusivity is very relevant for surface heating or cooling processes and it represents the capacity of a material to absorb and release heat. At similar loading percentages, the smallest difference (39.45 Jm⁻²K⁻¹s^−1/2) is found when 10% sawdust loading is applied and the largest difference (109.53 Jm⁻²K⁻¹s^−1/2) is realised when utilising 40% of the sawdust fibres. The higher values of thermal effusivity obtained for blocks containing TSD indicate that, as soon as surrounding temperature drops, those blocks are unable to hold heat long enough. This is because heat will quickly dissipate from their surfaces, unlike in the cases involving the blocks that contain USD. Observably, blocks containing USD tend to respond slowly to such temperature changes. Thermal effusivity of the blocks decreases as the loading of sawdust increases. The block made with 10% of USD appears to possess the same ability to release the absorbed heat like the one containing 20% of TSD, meaning that they fit into same class as long as thermal energy exchange with the environments is concerned.

For acting as a better thermal diffuser compared to USD, the TSD causes the block samples containing it to have lower thermal lag. This means that under periodic conditions of heat flow, the time difference between the temperature maximum at the outside and inside is less when using blocks containing TSD than in the case involving blocks similarly produced but with USD as a partial substitute for sand. The observed trend in decrement decay is in agreement with lower values of thermal conductivity and thermal diffusivity obtained due to incorporation of USD into the blocks. Frankly speaking, thermal conductivity is not infinite and there is some resistance offered by a material to heat transfer. Keeping that in mind, the studied blocks are no exception. With the results obtained in this work, thermal resistance (ratio of thickness to thermal conductivity) of the blocks is greater by 4.6%, 6.9%, 12.6%, and 17.7% for applying USD than in the case of using TSD at 10%, 20%, 30%, and 40% respectively. Thus, time is needed by the blocks to increase their temperature with the limited amount of heat which, in essence, takes time to propagate in each of the blocks. A greater thermal insulation efficiency exhibited by the sawdust over the sand component of each of the blocks shows a pronounced increase not only in thermal time lag, but solar radiation absorptivity as well. It is worth noting that in most cases, buildings are erected to protect the occupants from extreme temperatures, wind, solar radiations, etc, thus providing good indoor environment for comfortable living. As building envelopes, blocks used for walling purpose in buildings are unavoidably subjected to severe influence from external environments on 24-hour cycles. Applying those facts to the case at hand, it is obvious that solar radiation absorptivity expresses the ability of each of the developed block samples to absorb the radiation equal to the internal absorptance of its homogeneous layer under conditions in which the path of the radiation has unit length and the boundaries of the layer have no influence. The tendency to absorb solar radiation varies positively with the proportions of USD or TSD used, though the cases involving USD have greater values than those associated with utilisation of the TSD. This is because thermal diffusivity correlates negatively with solar radiation absorptivity (as expressed in Eq. (11)) and with the use of USD, thermal equilibrium is reached slower than when similar fractions of TSD are made use of.

4 Conclusion

In this work, the tests performed to determine hygrothermal properties of solid core sandcrete block samples revealed that water absorption, sorptivity, specific heat capacity, thermal time lag, and solar radiation absorptivity increase with increasing loadings of either untreated sawdust (USD) or treated sawdust (TSD). Contrariety was observed in the cases of bulk density, thermal conductivity, thermal diffusivity, and thermal effusivity of the blocks. Though incorporation of sawdust yielded blocks with improved thermal insulation performance over the control block sample, those containing USD were found to exhibit a greater ability to enhance thermal comfort in buildings compared to their counterparts produced with TSD. In addition, it was observed that 20% of USD or 10% of TSD is the optimum level of sawdust to be used as partial replacement of sand so as to meet the bulk density and water absorption requirements stipulated in standard protocols for production of sandcrete blocks with light-weight aggregate content. Utilisation of sawdust in such undertakings can help to minimise the problems associated with its disposal and also serves as a promising way of ensuring safe, sustainable, and affordable housing development. Further studies could be carried out to evaluate the effects of curing age and aggregates batching methods on hygrothermal properties of composite cementitious mortar containing sawdust of various wood types.