Engineering Characteristics and Potential Increased Utilisation of Sawdust Composites in Construction—A Review

Many timber producing countries generate more than 2 million m 3 of sawdust annually. In developing countries, sawdust is often disposed of by open dumping, open burning, or dumping in landfills. This poses huge environmental challenges related to air pollution, greenhouse gas emissions, and de-struction of plant and aquatic life. Findings from this review article reveal that sawdust can be used to make sawdust construction composites with good modulus of elasticity, water absorption and strength characteristics that satisfy international specifications. These composites include particleboards, sawdust concrete blocks or bricks and sawdust concrete. The article concludes that partially replacing 5% to 17% of sand with sawdust, or replacing cement with sawdust ash in proportions of 5% to 15% in concrete mixes can produce structural concrete with compressive strengths greater than 20 MPa. Partially replacing 10% to 30% of sand used in the manufacture blocks and bricks with sawdust can also produce sawdust bricks and blocks with compressive strengths greater than 3 MPa. Sawdust composites are also attractive for their low thermal conductivity, high sound absorption and good sound insulation characteristics. These findings indicate that increased utilisation of sawdust composites in construction will mitigate against potential sawdust environmental pollution, conserve energy and reduce disposal costs.


Introduction
Sawdust is a waste or by-product from a range of timber manufacturing processes Table 1 shows the amount of wood waste and sawdust generated from sawmills as well as some annual sawdust production quantities in selected regions around the world. It is noted from Table 1 that in many timber producing countries, more that 2 million m 3 of saw dust is generated annually from sawmilling operations. In Zambia's Copperbelt province, like in many developing countries, large piles of sawdust, slabs, off cuts and bark are characteristic of operation areas of the province's 13 registered saw millers. This indicates a huge environmental challenge if this material is simply left as waste.
2) Common non-construction usage and disposal of sawdust Common non-construction usage of the sawdust include bedding for poultry and livestock, soil composting and mulching [21]. Before the advent of refrigeration, it was used to keep ice frozen in icehouses during summer. When mixed with water and then frozen, it forms a slow-melting and stronger form of ice.
Sometimes it is used to soak up spilled liquids, thus enabling the easy collection or sweeping away of the spill [1]. Sawdust is also considered a very good raw material for the production of wood pellets and biomass briquettes used as solid fuels [20] [22] [23]. 3 Sweden *Data based 9 out of 10 studied sawmills; **Data from sawmills in 1 out 10 provinces of Zambia; -No data available; † Amount evaluated from volumes using an approximate sawdust density of 210 kg/m 3 ; † † Average values from four years sawdust generation figures.

Current Use of Sawdust Composites in Construction
Sawdust composites have been applied in construction for a long time. For example it has been used to produce sawdust concrete for more than 40 years [1].
Apart from its use in concrete, literature indicates that other sawdust composites used in the construction industry include particleboards, floor panels, partitioning, cladding, ceiling, formwork and concrete blocks and bricks.

Particleboards and Related Products
A considerable amount of sawdust and wood shavings in the United States of America is utilised in the manufacturing of particleboards [24]. The global production of wood based panels that include particleboards, plywood, oriented strand boards (OSBs) and fiberboards increased by 125% between 2000 and 2017 [25]. Between 2012 and 2016 the largest proportion (62%) of these products were manufactured in the Asia-Pacific region, followed by Europe (21%), North America (11%), Latin America and the Caribbean (5%) and Africa (1%) [26]. The low manufacturing figure for Africa and other developing continents compared with the high sawdust produced (Table 1) implies that there is great potential In Zambia, the demand for particleboards and related products like plywood and sawn timber is continuously rising. A 39% increase in demand for these products, from 501,100 m 3 in 2010 to 698,700 m 3 in 2025, has been projected [27]. It is envisaged that incorporation of sawdust in the manufacturing of these particleboards will mitigate against the environmental pollution that this waste poses in Zambia.
Particleboards and related wood products such as low-density fibreboard (LDF), and chipboard are manufactured by mixing various proportions of wood chips, sawmill shavings, or sawdust with a synthetic resin or any suitable binder [9] [28]. For example, Abdulkareem et al. [28] established that particleboards made from sawdust and plastic based resin (PBR) synthesised from waste styrofoam as binder exhibited properties that were in tandem with the requirements of the American National Standard Institute (ANSI) A208.1 requirements. This standard specifies the required dimensions as well as the physical and mechanical properties for different grades of particleboards. The study observed that the sawdust-PBR particleboards exhibited better water penetration resistance, dimensional stability, mechanical properties and resistance to deformation when compared with urea formaldehyde (UF) particleboards. They were thus more durable, tougher, and better suited for application in most environments than the UF particleboards.
A study by Dotun, A.O. et al. [29] observed that sawdust particleboards produced from a combination of sawdust and polyethylene terephthalate plastic waste was favourable for indoor applications. However, the study also showed that these products had limited structural and load bearing applications. Similarly Akinyemi et al. [30] recommended that panels produced as corncob and sawdust composites using urea formaldehyde as binder were suitable for indoor uses in buildings but not for load bearing purposes.
Erakhrumen et al. [31] proved that for mixtures of pine (Pinus caribaea M.) sawdust and coconut husk or coir (Cocos nucifera L.) using cement as binder, parameters such as water resistance, strength properties and density of the particleboards were enhanced with high cement content. However, these properties were lowered with increased inclusion of coir in the mixture.
Sawdust composites made from gluing of sawdust or wood chips together with expanded polystyrene are known to exhibit good thermal conductivity characteristics. These products are deemed appropriate for use in room partitioning and suspended ceilings [32].

Floor Panels
A study by Chanhoun et al. [33] investigated a combination of wood waste, polystyrene waste and plastic waste composites. The study indicated that these composites could be used, not only for interior and exterior flooring but also as self-adhesive sandwich panels or boards in door cores, false ceilings and form-Journal of Building Construction and Planning Research work sandwich boards.
An innovative concrete sandwich panel investigated in Iraq was made using a layer of light weight concrete (LWC), sandwiched between two outer layers of reinforced concrete. These elements were connected together by truss reinforcement as shear connectors. The strength of the sandwich slab panel with sawdust, which was used as aggregate in the inner wythe, was greater than the strength of sandwich slab panel with polystyrene (styropor) or porcilenite [34].
Chung et al. [35] demonstrated the vibration damping potential of a sand-sawdust layer in lightweight timber-framed floor/ceiling systems (LTFSs). The studied LTFS comprised an upper floor made of a sand-sawdust mixture, a cavity space filled with fibre infill for sound damping and a ceiling. The theoretical model and the experimental measurements showed that the sand-sawdust layer dampened the vibration in the frequency range of between 10 and 200 Hz.

Partitioning and Cladding
Sawdust-cement composites could be utilised for cladding and walling. An important consideration for this application, however, is the need to carefully select wood with suitable constituents for cement compatibility [36].

Sawdust Concrete Blocks or Bricks and Mortar
Various studies have been done in the quest to come up with green and less costly construction blocks that incorporate sawdust in raw form or in the form of sawdust ash. Mangi et al. [37] provides a good overview of 17 studies carried out on concrete masonry blocks between 2012 and 2016 in 11 different countries. This review underscores the potential for increased utilisation of sawdust concrete blocks as lightweight masonry units in buildings.
Gil et al. [38] observed that wood sawdust waste has a positive effect on the post-cracking of building mortar. This in turn improves the ductility of mortar. Claudiu [8] studied the use of sawdust in plaster mortars. The study highlighted important characteristics of the investigated plaster mortars that included their good sound and thermal insulation capacity and non-susceptibility to ignition from open flame. These mortars were thus recommended for application in interior walls of buildings.

Lightweight Sawdust Concrete
Lightweight concrete is concrete with densities of between 300 and 1850 kg/m 3 . Structural lightweight concrete has densities of between 1120 and 1920 kg/m 3 and has a minimum compressive strength of 17 MPa [39] [40]. The low density and high thermal insulation value of waste wood aggregate such as sawdust [24] makes it a good alternative ingredient for the production of lightweight concrete and thermal insulation construction composites. Ahmed et al. [41] observed that a mix design of coarse aggregate, sand and cement, with different dosages of sawdust as partial replacement for sand, produced eco-friendly and thermal efficient normal and lightweight concretes.
Okhuen wood sawdust and recycled polyethylene (RLDPE) were blended and then hot-pressed to produce sawdust/recycled polyethylene composite board by Atuanya and Obele [48]. The investigated average tensile strength of the optimised composite board was 13.991 MPa, a value that met the specifications for general applications.
Abu-Zarifa et al. [49] examined particleboards that were manufactured from sawdust and agricultural waste (banana stems, wheat bran and orange peels). Each agricultural waste was mixed with sawdust in two proportions of 25% and 75% while the amount of polypropylene plastic was kept constant at 40%. The mixes were pressed under a 24 ton load, at a temperature of 170˚C for 2.5 hours. The test results showed a maximum modulus of elasticity (MOE) value of 2160.78 MPa for the mix with 75% wheat composition, a maximum modulus of rapture (MOR) value of 11.07 MPa for the mix with 100% sawdust composition, and a maximum-stress value of 7.8 MPa for the mix with 25% banana composition. The range of water absorption values were between 8.19% and 19.3%. These results were better than the commercial type particleboards (Medium Dense Fiber, Fiber and Press wood boards). The particleboard mix with 75% banana composition exhibited the least water absorption and swelling capacity. The Journal of Building Construction and Planning Research one with 75% orange composition showed the highest water absorption and swelling percent.

Sawdust in Concrete Blocks or Bricks and Mortar
Kupolati et al. [50] investigated the utilization of sawdust as partial replacement of crusher sand for the production of bricks as a way of enhancing the greening of the environment. Sawdust was used as a partial replacement for crusher sand at 1%, 3% and 5% by volume. The investigated compressive strength values of the sawdust-sand bricks produced on site was less than the minimum values of 4.0 MPa specified for solid masonry units in masonry walls [51]. The average com- To investigate the potential use of sawdust in blocks, Ravindrarajah et al. [52] evaluated blocks made using cement, lime, fly ash, calcium chloride, Radiata Pine sawdust, sand and water. A sawdust concrete block mix with 12% sawdust content by volume produced a density of 1540 kg/m 3 and a 28-day compressive strength of 14 MPa. The use of 2% calcium chloride led to the achievement of optimum strength at all ages but also caused significant increase in shrinkage.
The study observed that sawdust is a good filler material for the production of lightweight concrete blocks.
Replacing sand with sawdust in a sand-cement block mix, sawdust replacement proportions of 10%, 20%, 30%, and 40%, with water cement ratio of 0.5 was investigated by Dadzie et al. [53]. Compressive strengths of the investigated sawdust composite blocks exceeded the minimum BS 6073 requirement of 2.8 MPa for sawdust replacement of not more 10%.It was further noted that the sawdust replacement content should not exceed 10% if sawdust blocks were to meet standard specifications.
Boob [54] established that sandcrete blocks prepared by partially replacing sand with sawdust gave optimum and desired results from a 1:6 (cement: sand + sawdust) (85% sand + 15% sawdust) mix ratio. The compressive strength obtained from 100 mm × 100 mm × 100 mm blocks for this mix proportion was 4.5 MPa. This is a good result for blocks made with sawdust replacement of not more than 10%, when evaluated in relation to the minimum BS 6073 requirement of 2.8 MPa [55].
Ettu et al. [56] investigated the use of ordinary Portland cement (OPC), saw- Moreira et al. [58] studied the performance of building blocks made with the partial replacement of fine aggregates with sawdust from the Dinizia Excelsa Ducke wood species. The blocks were made by replacing fine aggregates with sawdust at 5% by weight. Two sawdust treatment processes, one comprising the washing of the sawdust in an alkaline solution (lime) and another comprising the immersion of the sawdust in aluminum sulfate were used. The compressive strength results on the 28 th day were 1.39 and 3.98 MPa for the two treatment methods respectively. The water absorption results were 13.13% and 10.40% respectively. The results showed good performance of the blocks made with aluminum sulphate treated sawdust than those made with alkaline solution treated sawdust. The 28 days compressive strength results of 3.98 MPa for blocks with aluminum sulphate treated sawdust satisfied the Brazilian NBR7173 standard that specifies a minimum compressive average strength of 2.5 MPa for construction blocks. The study showed the potential of producing masonry blocks with 5% fine aggregates replaced with aluminum sulphate treated Dinizia Excelsa Ducke sawdust.
Adebakin et al. [59] investigated the use of sawdust as a partial replacement Journal of Building Construction and Planning Research for sand in the production of hollow sandcrete blocks. The study aimed at reducing the cost of construction materials and lowering the dead loads imposed on particularly high rise buildings and those built on low bearing capacity soils.
The investigation showed that replacement of sand by 10% sawdust resulted in blocks with compressive strength values that almost met the required Nigerian standard specification of 3.5 -10 MPa for sandcrete blocks. This 10% sawdust replacement content also yielded blocks with 10% weight reduction and 3% production cost reduction.
Lightweight bricks made from mixes of sawdust to cement ratios of 3:2 and 2:1 were investigated by Zziwa et al. [60]. Bricks measuring 100 × 100 × 100 mm were tested as air-dried samples and as soaked samples after soaking in water at room temperature for 24 hours. The highest compressive strength result of 2.21 MPa was obtained from the dry specimens with sawdust to cement ratio of 3:2.
The corresponding compressive strength result for the soaked specimens averaged 1.38 MPa. The low dry compressive strength and the even lower soaked compressive strength indicated that these bricks did not meet the requirement for use in load bearing walls and walls exposed to wet environments. They could, however, be used for internal wall panelling where there was minimum wetting conditions and little or no loading.
A summary of compressive strength results of selected sawdust bricks and blocks are presented in Table 2. These results indicate good performance of sawdust brick/block composites which should give confidence for their increased use in construction.

Partial Replacement of Sand with Sawdust in the Concrete Mix
Osei and Jackson [61] studied the use of sawdust, crushed granite and rapid hardening cement for the production of sawdust concrete. Using a concrete mix  Nathan [65] showed that sawdust is a potential material for preparation of light weight concrete. By using cement, fine aggregate, coarse aggregate, water and sawdust, a conventional control mix was prepared with mix proportions of The possibility of using reinforcement in sawdust concrete was studied by Olutoge [71]. This study demonstrated that replacing less than 25% of sand by sawdust in reinforced concrete yielded results that satisfied the characteristic strength requirements for structural use of concrete as specified in the BS 8110, 1997.    where: c f is the 28-day compressive strength in MPa.
λ is the percentage sand replacement by sawdust.
It is noted from Equation (1)   content. This is particularly evident from studies by Sawant et al. [67] and [74].

Sawdust Concrete with Sawdust as One of the Main Constituents
Apart from partial replacement of sand with sawdust, other studies have also been made where sawdust is one of the main constituents of the concrete mix.
Comparisons of compressive, split tensile and flexural strength results of sawdust concrete from selected literature are shown in Table 3. The tabulated results indicate reduction in compressive, flexural and split tensile strengths with increase in the amount of sawdust in the concrete mix. It is also noted from Table 3 that the 1:1:2 and the 1:1:1 mixes produce lightweight concrete with good compressive strength results. [78] and Marthong [79] investigated sawdust ash (SDA) concrete by replacing ordinary Portland cement (OPC) with SDA. The studies established that with a 10% SDA replacement it was possible to attain a design strength of 20 MPa at 28 days, which is comparable with the strength attained by conventional concrete at longer curing periods. Marthong [79] however, noted that inclusion of SDA as partial replacement for cement tended to reduce the durability of concrete when exposed to sulphate environment. Later Obilade [80]    showed that SDA led to the attainment of 28 days compressive strengths of between 21.02 and 19.05 MPa at 5% to 15% sawdust ash replacement respectively.

Partial Replacement of Cement with Sawdust Ash (SDA) in the Concrete Mix Udoeyo and Dashibil
The 5% to 15% SDA content was thus considered as the optimum SDA replacement for cement as SDA content of above 15% significantly reduced the concrete compressive strength. This investigation recommended the evaluation of the durability of concrete made with SDA as partial replacement for cement.
Dhull [81] partially replaced the mass of cement by amounts of 5%, 10%, 15% and 20% in a 1:1:2 concrete mix ratio.  i.e., water: cement: sawdust ash: sand: granites, a study by Onwuka et al. [82] produced SDA concrete with an optimum compressive strength result at 28 days of 20.44 MPa. The study concluded that sawdust concrete can suitably be used as a building material in construction industry.
Fapohunda et al. [83] showed that, wood waste, either in the form of SDA, or wood aggregate, or sawdust; can be incorporated into an appropriate concrete mix design which can yield structural concrete that satisfies building requirements. The SDA content must however, not exceed 20%. Concrete incorporating SDA is known to exhibit good durability properties against most of the process that degrade concrete in its service life. However its durability is compromised when it is exposed to carbonation and sulphate attack. Mangi et al. [84] also noted the need to investigate the durability of high-strength concrete developed with SDA and its performance in aggressive alkaline and acidic environments.
An investigation by Raheem et al. [85] further notes that SDA concrete becomes less workable as the SDA content increases. This indicates that SDA has a higher water demand compared with ordinary Portland cement. The study observed that 5% SDA was the optimum substitution content that produced SDA concrete strength gain comparable to the control mix which had no SDA content.
The compressive strength values of SDA concrete in Figure 5 show similar trend to those in Figure 2 in terms of decreasing strength with increase in SDA.

Effect of Sawdust Composites on Thermal Properties of Construction Units
Thermal insulation materials and systems are used to reduce heat flow transmission. The thermal conductivity and thermal transmittance indicate the thermal  insulation performance of such materials. Construction materials with a thermal conductivity of less than 0.07 W/mK are considered as thermal insulators [86].
Thermal conductivities for timber are favourable compared with other materials used in buildings. They vary slightly with different densities, moisture contents and species, the lower densities having lower conductivities. Meyer [24] argues that one primary advantage of waste wood aggregate, such as sawdust and shavings, is the low weight and high thermal insulation value of the material. A study by Sindanne et al. [89] involving earth blocks stabilized by cement, sawdust and lime showed increase in thermal conductivity with increase in cement and lime as stabilising agents. However stabilisation with sawdust decreased the thermal conductivity of the blocks. Sawdust stabilised blocks were thus observed to exhibit increased thermal resistance when compared with the cement or lime-stabilised blocks. The results from this study are summarised in Table 4.
Conventional concrete with a density of between 2100 and 2400 kg/m 3 has a thermal conductivity of between 1.40 and 1.75 W/mK [93] [94]. Thus addition of sawdust in the concrete mix significantly reduces the thermal conductivity of the resulting lightweight concrete.
The thermal conductivity values shown in Figure 6 also satisfy the requirements of ASTM C332-09 [95] which stipulates that the maximum average thermal conductivity for concrete made from lightweight aggregates should be 0.43 W/mK for oven dry concrete that has a density of 1440 kg/m 3 at 28 days.

Sound Absorption
Noise pollution is considered one of the four major environmental hazards that include air, water and solid waste pollution. Sound absorbing materials therefore play an important role in mitigating noise pollution effects on human health such as hearing loss and stress [96]. Low frequency noise, particularly that with a frequency range of 10 Hz to 100 Hz poses a special environmental noise that can cause heightened distress in people that are sensitive to its effects [97]. Sound absorbing materials reduce acoustic energy of a sound wave as the wave passes through it. One way of evaluating the performance of sound absorbing materials is by measuring the sound absorption coefficient, defined as the measure of the acoustic energy absorbed by the material upon incidence of the energy wave [98] [99].
An absorption coefficient of 0.00 entails that no sound has been absorbed whereas a sound absorption coefficient close to 1.00 for a sound frequency range of 125 to 4000 Hz entails good sound absorption [98] [100].
Wood is the most frequently used material for sound absorption in auditoria. Hz. This study also observed that composite porous materials exhibited complex sound absorption characteristics.

Sound Insulation
Sound absorption products absorb echoes inside a room, thereby preventing sound from bouncing around the room. Sound-insulating materials, on the other hand block or stop sound waves from traveling to adjacent spaces.
Timber studded partitions for offices can be designed to obtain any degree of sound insulation required from the barest minimum. Knowledgeable design and attention to detail can result in a very high sound insulation within minimum overall thickness [106].
Chung et al. [107] established that lightweight timber based floor/ceiling systems (LTFSs) can have superior impact sound insulation to that of concrete slab based systems. Examples of such systems include vibration isolation/damping features, such as rubber ceiling batten clips, glass fibre wool, and a sand-sawdust mixture layer. The inclusion of the sand-sawdust layer was found to provide effective vibration damping and thus sound insulation of the whole composite structure over a wide frequency range. Later Chung et al. [35]  Chathurangani et al. [109] studied a combination of sawdust and coconut coir fiber for use as noise reducing wall surface materials. The study verified the potential use of these materials for effective noise reduction. From this study, the noise reduction co-efficient, a ratio between the reduction levels of noise to the intensity of incident sound, values obtained for sawdust and coir fiber tiles ranged from 0.1 to 0.5. Later a study done in Indonesia proved that using panels made from similar materials had good acoustical performances and could be used for wall layering in noisy urban housings [110].

Future Trends
Sawdust is a recyclable waste and a raw material that is readily available and easily accessible in many timber producing countries. It can be collected and transported at minimal cost and energy when compared with the cost and energy required in exploiting natural resources. Value addition to this waste by incorporating it in the production of construction composites will address the quest for eco-friendly and energy efficient materials in building and construction, contribute to a pollution-free environment and create employment. Journal of Building Construction and Planning Research Research and development of sawdust construction composites is, therefore, likely to increase in the nearby future. Possible future research focus and development includes producing versatile sawdust construction composite materials that are more robust, durable, lightweight, energy efficient, cost effective and safe for civil engineering infrastructure, than is obtaining presently. Novel eco-friendly and energy efficient construction composites that are expected to attract research and construction interests include those made from cement-sawdust admixtures, bitumen-sawdust admixtures and polymer-sawdust admixtures. Development of these novel sawdust composites will make a huge contribution to the science of alternative construction materials and greatly influence the reformulation of construction material specifications and standards.
Other potential future uses of sawdust composites in construction include their use as construction formwork and as lightweight roofing tiles. These composites also have the potential of replacing conventional air-conditioning in handling urban heat and thermal discomfort, with the added benefits of energy conservation and climate change mitigation.

Conclusions
Literature shows that many timber producing countries generate more than 2 million m 3 of sawdust annually. In developing countries, this material is often Particleboards incorporating sawdust can exhibit modulus of elasticity values of more than 2100 MPa, thickness swelling of not more than 15% and acceptable water absorption characteristics that satisfy international specifications. Sawdust and sawdust ash can be incorporated as part of the raw materials to produce bricks and blocks that satisfy building specifications for masonry wall units and paving bricks. Lightweight concrete for both structural and non-structural works can be produced with sawdust or sawdust ash forming part of or as one of the main concrete ingredients. Sawdust construction composites are also attractive for their low thermal conductivity, high sound absorption and good sound insulation characteristics. Replacing cement with sawdust ash (SDA) proportions of 5% to 15% also produces concrete with compressive strengths greater than 20 MPa. Higher sawdust and SDA proportions than these significantly reduce the sawdust concrete strength. Replacing 10% to 30% of the sand used in the manufacture blocks and bricks with sawdust can also yield sawdust bricks and blocks with compressive and water absorption characteristics that satisfy international specifications.
Increased use of sawdust in construction will greatly contribute to construction sustainability related to the development and use of environmental friendly and green building materials. Additionally, using sawdust composites in construction would contribute to conservation of non-renewable construction resources, reduction of energy as well as CO 2 emissions from the exploitation of natural construction materials. All this will ultimately greatly contribute to climate change mitigation. Thus sawdust composites have a market as well as environmental mitigation value. Developing countries should not, therefore, regard sawdust as a waste, but as a valuable by-product with increased potential use in the construction industry.