Sawdust is a waste or by-product from a range of timber manufacturing processes that include sawing, planning, routing, drilling, sanding operations, furniture manufacturing and joinery. This waste stream comprises small discontinuous chips or simply fine particles of wood  .
The disposal of sawdust is often by open dumping, open burning, or dumping in landfills  . Sawdust dumped in landfills increases the burden on landfill sites and burning it contributes to greenhouse gas emissions . Despite the air pollution and public health challenges associated with open burning, it is commonly practiced by saw millers as the easiest way to get rid of sawdust  . When dumped on stream and river banks sawdust is transported by rainwater or wind into the surface water and can severely affect aquatic life. Moreover, sawdust indiscriminately dumped on land kills plant life and causes wood dust when blown into the atmosphere .
Creating value from this waste stream would reduce disposal costs and create jobs . Additionally, utilising wood-based products such as sawdust composites in construction contributes to climate change mitigation  . Substituting steel, concrete, and other high energy manufactured products with sawdust composites has the potential to reduce the consumption of large amounts of fossil fuels. Considering that wood based products store carbon for the duration of their life cycle, the use of sawdust composites, accordingly, leads to reduced CO2 emissions   and therefore mitigates against global warming.
The motivation for this review article is that sawdust, an environmental hazard, has great potential for use as a raw material to produce construction composites that meet international standards. This potential use is yet to be fully explored, particularly in developing countries where indiscriminate dumping of sawdust is rife. The article briefly highlights some environmental challenges that sawdust poses and reviews engineering characteristics of sawdust construction composites, namely, particleboards, sawdust concrete blocks, bricks and lightweight sawdust concrete. It is envisaged that the reviewed literature will serve as a catalyst for more research on sawdust composites and for promoting increased utilisation of these composites in construction. This would further contribute to the development of green building materials and mitigate against the sawdust environmental pollution threat. The data presented and discussed in this article is also beneficial to researchers studying alternative construction materials aimed at conserving non-renewable natural resources and energy.
Production, Non-Construction Common Usage and Disposal of Sawdust
1) Amount of sawdust produced from sawmills
The sawmilling operation is one of the major sources of sawdust. The amount of sawdust produced from sawmilling depends on the sawmill efficiency which can be measured by the quality and quantity of sawn plank recovered compared with the resulting wood waste. This wood waste is a combination of bark, sawdust, trimming, split wood, planer shavings and sander dust . The type of machinery used also has a bearing on the quantity of sawdust generated. Kambugu et al.  observed that lack of proper machinery for the timber sawing leads to high generation of saw dust in the timber sawing process.
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 m3 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 . 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 . Sawdust is also considered a very good raw material for the production of wood pellets and biomass briquettes used as solid fuels   .
Table 1. Approximate quantities of sawdust generated annually from sawmills.
*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/m3; ††Average values from four years sawdust generation figures.
Common disposal of most of this waste include open dumping, open burning and sometimes dumping in landfills. Figure 1 shows indiscriminate dumping and burning of sawdust, typical in developing countries.
2. 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 . 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.
2.1. 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 . The global production of wood based panels that include particleboards, plywood, oriented strand boards (OSBs) and fiberboards increased by 125% between 2000 and 2017 . 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%) . The low manufacturing figure for Africa and other developing continents compared with the high sawdust produced (Table 1) implies that there is great potential
(a) (b) (c) (d)
Figure 1. Open dumping of sawdust: (a) Burning of sawdust near residential area; (b) and (c) Burning of sawdust at a sawmill; (d) Dumping of sawdust on the banks of a stream.
for increased manufacture of sawdust construction composites from this waste in developing countries.
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 m3 in 2010 to 698,700 m3 in 2025, has been projected . 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  . For example, Abdulkareem et al.  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.  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.  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.  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 .
2.2. Floor Panels
A study by Chanhoun et al.  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 formwork 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 .
Chung et al.  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.
2.3. 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 .
2.4. 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.  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.  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  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.
2.5. Lightweight Sawdust Concrete
Lightweight concrete is concrete with densities of between 300 and 1850 kg/m3. Structural lightweight concrete has densities of between 1120 and 1920 kg/m3 and has a minimum compressive strength of 17 MPa  . The low density and high thermal insulation value of waste wood aggregate such as sawdust  makes it a good alternative ingredient for the production of lightweight concrete and thermal insulation construction composites. Ahmed et al.  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.
3. Engineering Characteristics and Performance of Sawdust Composites Used in Construction
Badejo  observed that 12 mm thick cement-bonded particleboards made of sawdust from four tropical hardwood species (Mitragyna ciliata, Triplochiton scleroxylon, Terminalia superba and Ceiba pentandra) had strong effect on the board properties tested. The evaluated modulus of rapture (MOR) ranged from 4.72 to 8.20 MPa, 5.00 to 8.00 MPa, 4.35 to 6.05 MPa and 3.75 to 6.20 MPa respectively for the four wood species. The modulus of Elasticity (MOE) ranged from 2750 to 4000 MPa, 2500 to 3500 MPa, 2500 to 3400 MPa and 2100 to 3350 MPa respectively for the four wood species. After soaking in cold water for 72 hours, the percentage thickness swelling ranged from 2.80% to 4.5%, 2.9% to 5.5%, 2.2% to 3.55% and 4.50% to 5.70% for the four woods. The respective approximate densities of these wood species are between 450 and 560, 320 and 400, 450 and 580 and 230 and 260 kg/m3  . The MOE properties, of the experimental boards relate with the densities of the woods used. The Mitragyna ciliata and Terminalia superba species have higher densities and yielded higher MOE values than the other two species. It is also noted that the MOE results from this study satisfy the ANSI 208.1 specifications  for high and medium grade particleboards. However, the MOR results did not meet the ANSI 208.1 requirements. The studied particleboards exhibited acceptable swelling behaviour, considering that BS EN 312:2010  and BS EN 317:1993  stipulate that particleboards should have a maximum thickness swelling (TS) value of 8% if subjected to 2 hours water immersion, or a maximum TS of 15% if the 24 hours water immersion procedure is used.
Okhuen wood sawdust and recycled polyethylene (RLDPE) were blended and then hot-pressed to produce sawdust/recycled polyethylene composite board by Atuanya and Obele . 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.  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 one with 75% orange composition showed the highest water absorption and swelling percent.
3.2. Sawdust in Concrete Blocks or Bricks and Mortar
Kupolati et al.  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 . The average compressive strength of the bricks (290 mm × 150 mm 90 mm) on site at 28 days was 0.67 MPa, 0.23 MPa and 0.21 MPa for the respective sawdust replacement percentages. However, the bricks cubes measuring 100 mm × 100 mm × 100 mm produced in the laboratory yielded average compressive strengths of 6.10 MPa, 5.73 MPa and 3.7 MPa for the aforementioned respective sawdust replacement percentages. This was attributed to better quality control practices in the laboratory. This study stressed the importance of quality control in the bulk production of sawdust bricks. The research further showed the potential use of sawdust as a partial replacement material for crusher sand in the production of bricks.
To investigate the potential use of sawdust in blocks, Ravindrarajah et al.  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/m3 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. . 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  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 .
Ettu et al.  investigated the use of ordinary Portland cement (OPC), sawdust ash (SDA) and pawpaw leaf ash (PPLA) for possible production of sandcrete blocks, (where sand was the major constituent), and soilcrete blocks, with laterite being the major constituent. Binary cementing blends of OPC-SDA and OPC-PPLA and ternary cementing blends of OPC-SDA-PPLA for the production of the blocks were evaluated. The study observed that the produced blocks from these blended cementing materials yielded sufficient strength for their use particularly in civil engineering works where the need for high early strength was not a critical factor. The 150-day strength values for OPC-SDA-PPLA ternary blended cement, for sandcrete and soilcrete blocks were, respectively, 6.00 MPa and 5.20 MPa for 5% replacement, 5.90 MPa and 5.10 MPa for 10% replacement, 5.75 MPa and 5.00 MPa for 15% replacement of OPC, and 5.70 MPa and 4.90 MPa for 20% replacement of OPC. These results were slightly better than the respective control values of 5.20 MPa and 4.80 MPa.
Investigations by Turgut and Algin  used limestone powder wastes (LPW) from quarrying operations and wood sawdust wastes (WSW) produced from the sawing process of raw wood to obtain WSW-LPW bricks. These composite bricks with varying WSW-LPW combinations yielded compressive strength, flexural strength, unit weight, ultrasonic pulse velocity (UPV) and water absorption values that satisfied international standards, namely ASTM C67-03a, BS 6073 and BS 1881. A 30% replacement of WSW in the brick composite mix produced bricks with a compressive strength of 7.2 MPa and a flexural strength of 3.1 MPa. These results comply with the BS6073 requirements for building materials to be used in structural applications. This sawdust composite was evaluated as a potential wall construction unit, a wooden board substitute, as well as an economical alternative to concrete blocks, ceiling panels and sound barrier panels.
Moreira et al.  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 28th 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.  investigated the use of sawdust as a partial replacement 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. . 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.
3.3. Sawdust in Lightweight Concrete
3.3.1. Partial Replacement of Sand with Sawdust in the Concrete Mix
Osei and Jackson  studied the use of sawdust, crushed granite and rapid hardening cement for the production of sawdust concrete. Using a concrete mix of 1:2:4, sawdust was used to replace 25%, 50%, 75% and 100% of sand by volume. The 28 days strength for the respective sawdust replacement proportions were 12.13 MPa, 9.15 MPa, 4.66 MPa and 3.37 MPa. The study established that sawdust can potentially be used as aggregate in the production of non-structural lightweight concrete for use in situations where the compressive strength was not a major requirement. Further analysis of the compressive strength indicated that a sawdust replacement of less than 14% can produce concrete with a 28 days compressive strength of 20 MPa. This is the minimum concrete strength for structural use. Earlier, Bdeir  observed that 10% replacements of sand with sawdust showed an increase of compressive strength from 23.24 to 27.31 MPa between 7 and 28 days indicating that partial replacement of sand with sawdust in concrete can attain the same order of strength as conventional concrete at longer curing periods.
Suliman et al.  used sawdust, sand, crushed stones and cement to produce sawdust concrete. A replacement of sand with sawdust in measures of 5%,
Table 2. Compressive strength of sawdust blocks or bricks at 28 days.
10% and 15% of the total sand volume was investigated. The resulting compressive strength values at 28 days were 50.06 MPa, 41.48 MPa and 34.7 MPa respectively. The optimum design for producing sawdust concrete was established at 10% sawdust replacement content. The study further showed that the sawdust concrete was free from any harmful health contaminants.
A study by Oyedepo et al.  showed that the compressive strength values obtained with sawdust content equal to or greater than 25% did not conform to the Nigerian minimum requirement of 17 MPa for light weight concrete. The concrete mix ratio of 1:2:4 was prepared using water/cement of 0.65, with 0%, 25%, 50%, 75% and 100% sawdust as partial replacement for fine sand. Compressive strength values for the 25%, 75% and 100% sawdust replacement percentages were 14.15 MPa, 12.96 MPa and 11.93 MPa respectively. Consequently, this study observed that using sawdust in a proportion greater than 25% was detrimental to strength and density properties of concrete. A further suggestion was that use of 0% to 25% sawdust content as partial replacement in concrete would not adversely affect concrete strength.
Nathan  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 1:1.5:3. Replacement of fine aggregate with sawdust was done at 0%, 5%, 10%, 15% and 20%. The average compressive strength values recorded at 28 days were 29.33 MPa, 27.7 MPa, 26.37 MPa, 24.15 MPa and 22.67 MPa respectively. The respective tensile strengths were 2.08 MPa, 1.82 MPa, 1.69 MPa, 1.49 MPa and 1.41 MPa. Using a similar mix design an investigation by Tilak et al.  showed lower compressive strengths of 24.13 MPa, 15.55 MPa, 11.11 MPa and 8.13 MPa, when fine aggregate was replaced with sawdust at proportions of 10%, 20%, 50% and 100% respectively. These two studies indicate potential use of sawdust in structural concrete when the proportion of sawdust replacement of sand does not exceed 10%.
Chitra and Hemapriya  used a mix proportion of 1:1.60:2.78 to reiterate the possibility of using sawdust as an alternative to sand with optimum strengths obtained at 15% replacement of sand with sawdust. The compressive strength values obtained at 28 days were 25.1 MPa, 24.2 MPa, 23.75 MPa and 17.54 MPa when fine aggregate was replaced with sawdust at 0%, 5%, 10%, 15% respectively.
Sawant et al.  investigated sawdust concrete made from a 1:1.62:2.83 mix proportion that included cementitious metakaolin as an admixture, meant to provide good bonding between sawdust and other concrete ingredients. Partial replacement of sand with sawdust in measures of 0%, 5%, 10%, 15%, 20% and 25% was done in the investigation. The resulting compressive strength values were 24.4 MPa, 21.11 MPa, 12.45 MPa, 10.07 MPa, 7.25 MPa and 5.12 MPa respectively, indicating good strength for sawdust content of less than 10%.
A study by Awal et al.  investigated sawdust concrete specimens made with cement to sawdust ratios of 1:1, 1:2 and 1:3 by volume. The respective compressive strength results at the age of 28 days for the aforementioned cement to sawdust mix proportions were 18.65 MPa, 17.20 MPa and 12.80 MPa. The strength of the sawdust concrete increased with increase in curing age. However, the strength and the recorded modulus of elasticity decreased with the increasing amount of sawdust in the mix.
Sawdust concrete from 1:1:2 and 1:1.5:3 mixes, with sawdust replacing the coarse aggregate, was investigated by Ogundipe and Jimoh . The 28 days compressive strength results were 18.33 and 8.78 MPa respectively, while their 28 days flexural strengths were 1.71 and 1.33 MPa respectively. The 28 days water absorption of the mixes were 5.69%, 8.97%, 8.29%, 7.83% and 11.11% with respective 28 days linear shrinkages of 0.67%, 0.50%, 1.83%, 1.83% and 1.95%.
Sojobi  observed that sawdust wastes and laterite as alternative fine aggregate and cementitious material respectively could be used to produce eco-friendly lightweight interlocking concrete paving units (ICPUs). Consequently, Sojobi et al.  produced ultra-lightweight green interlocking paving units using the same materials. With an optimum 10% sawdust content and after 90 days water curing, the paving units attained a compressive strength of 16.6 MPa and exhibited a skid resistance of 64.5 pendulum test value (PVT). The strength results exceeded the minimum requirements of 3.45 to 15 MPa for pedestrian traffic and non-load bearing concrete applications.
The possibility of using reinforcement in sawdust concrete was studied by Olutoge . 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.
Figure 2 shows an overview of the 28 days sawdust concrete compressive strength results in relation to partial replacement of sand by sawdust in various concrete mixes. Data from Figure 2 indicates that concrete mixes with sawdust content of 5% to 15% as replacement for sand can generally produce concrete with compressive strength values exceeding 15 MPa, which is suitable for lightweight structural applications as recommended by Neville .
Figure 2 further shows that mixes with sawdust content of between 5% and 10% as replacement for sand can produce concrete with compressive strength values higher than 20 MPa. These mixes could thus be used in structural applications in line with the ASTM C330/C330M-09  recommendations. Further it is noted that compressive strength significantly decreases with increased sawdust beyond 15% sand replacement content.
A scatter plot showing the effect of sand replacement by sawdust on the compressive strength of sawdust concrete is presented in Figure 3. The average compressive strength results give an exponential relationship with a good correlation value, i.e. R2 = 0.8017. This relationship can be expressed as
Figure 2. Compressive strength of sawdust concrete in relation to the sawdust replacement component.
Figure 3. Scatter plot of sand replacement by sawdust versus compressive strength of sawdust concrete.
is the 28-day compressive strength in MPa.
λ is the percentage sand replacement by sawdust.
It is noted from Equation (1) that the optimum content of sand replacement by sawdust required to produce structural concrete with a compressive strength of 20 MPa is 17%. Sawdust content above this proportion results in sawdust concrete with compressive strengths lower than 20 MPa.
Figure 4 illustrates a decrease in flexural strength with increase in the sawdust content. This is particularly evident from studies by Sawant et al.  and .
3.3.2. 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.
3.3.3. Partial Replacement of Cement with Sawdust Ash (SDA) in the Concrete Mix
Udoeyo and Dashibil  and Marthong  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  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 
Figure 4. Flexural strength test of sawdust concrete in relation to sawdust content.
Table 3. Compressive, flexural and split tensile strengths resulting from various sawdust mix composites.
*Cement to sawdust mix ratio; -No data available.
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. 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  partially replaced the mass of cement by amounts of 5%, 10%, 15% and 20% in a 1:1:2 concrete mix ratio. The 28 days strength of the 5% and 10% replacement contents resulted in compressive strength results of 32.44 and 30.24 MPa respectively. Replacement of cement with higher contents of SDA beyond 10% produced concrete with compressive strengths lower than the strength of the control mix.
Using the Scheffe’s Simpexfive component design ratio of 0.5:0.95:0.05:2.25:4 i.e., water: cement: sawdust ash: sand: granites, a study by Onwuka et al.  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.  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.  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.  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. Figure 5 further indicates that concrete with 5% to 15% SDA as replacement for cement could be used to produce concrete with compressive strength values greater than 20 MPa. This concrete can be used for structural applications.
3.4. 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
Figure 5. Compressive strength of SDA concrete.
insulation performance of such materials. Construction materials with a thermal conductivity of less than 0.07 W/mK are considered as thermal insulators .
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  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.
Sawdust concrete made from cement, sawdust and sand mixed in the ratios 1:1:1, 1:2:1 and 1:3:1 respectively have shown that the 1:3:1 mix ratio exhibited lower thermal conductivity when compared with the other two mixes. This reduction in the heat transfer through the 1:3:1 mix was attributed to the increased sawdust content in this mix compared with the other two  .
Salih and Kzar  used a combination of pre-treated reed and sawdust as partial replacement for natural sand in a 1:2.5 (cement:sand) mix. The reed and sawdust were pre-treated by soaking them in boiling water to which lime amounting to 20% of the weight of reed or sawdust was added. The soaking treatment was done to reduce harmful soluble carbohydrates, tannins, waxes and raisins. The replacement contents were an equal combinations of sawdust and reed in 10%, 20%, 30%, and 40% proportions. For example, the 10% replacement comprised 5% sawdust and 5% reed. A water cement ratio was kept at 0.4 for all the mixes. The values of 28 days oven dry densities ranged between 2060 and 1693 kg/m3 the high values relating to the density of the control mix. The lower density values were obtained for the 40% (i.e. 20% sawdust and 20% reed) sand replacement content. The thermal conductivity significantly decreased from 0.745 to 0.222 W/mK for the control and the 40% sand replacement mix respectively.
A study by Sindanne et al.  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.
Ogundipe and Jimoh  replaced coarse aggregate with sawdust in four mixes, namely 1:1:2, 1:1.5:3, 1:2:4, 1:3:6 and 1:4:8. Respective conductivity results measured after a 28 days curing period were 0.229, 0.232, 0.229, 0.223 and 0.176 W/mK. The results indicate progressive reduction in thermal conductivity with increase in the sawdust content. This trend was also noticed in studies done by AbdulAmeer  , Salih and Kzar  and Cheng et al.  , presented in Figure 6.
Figure 6 further shows that sawdust concrete exhibits lower thermal conductivity compared with the conventional concrete (0% sawdust content in this case). The reduction in thermal conductivity with increase in sawdust, a lightweight
Table 4. Thermal conductivity of stabilised earth blocks (W/mK)—after Sindanne et al. .
Figure 6. Thermal conductivity of sawdust concrete in relation to the amount of sawdust.
aggregate, is in agreement with findings of Asadi et al. . Lightweight aggregates, not only reduces the density, but also the thermal conductivity of concrete. Conventional concrete with a density of between 2100 and 2400 kg/m3 has a thermal conductivity of between 1.40 and 1.75 W/mK  . 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  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/m3 at 28 days.
3.5. Effect of Sawdust Composites on Acoustic Properties in Construction Units
3.5.1. 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 . 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 . 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  .
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  .
Wood is the most frequently used material for sound absorption in auditoria. When used in various forms in combination with complimentary sound absorptive materials it can provide optimum sound absorption properties. In this regard, wood in the form of sawdust incorporated in concrete or mortar and other related construction elements has been found to be effective in sound adsorption.
Kang et al.  studied rice hull-sawdust composite boards for sound adsorption in construction. The target densities for the boards were 400, 500, 600, and 700 kg/m3. The percentage by weight combinations of rice-hull/sawdust/phenol-resin mixes were 10/80/10, 20/70/10, 30/60/10 and 40/50/10 respectively. The sound absorption characteristics of these boards were compared with those of commercial gypsum board and fiberboard. The sound adsorption coefficients of the composite board were about 0.20 at 500 Hz, 0.40 at 1000 Hz, and 0.40 - 0.55 at over 1000 Hz. The sound absorption coefficient of the composite board was found to be two times higher than that of the gypsum board with an 11 mm thickness, especially at a frequency of 1000 Hz. The composite boards further showed higher sound adsorption coefficients than the commercial gypsum boards over the frequency range of 500 to 4000 Hz. The overall results showed that the rice hull-sawdust composite boards could be used as a replacement material for sound absorbing purposes in non-structural construction applications, such as ceilings, wall sheathing and interior wall surfaces.
Tiuc et al.  studied sound absorption of two products made from two waste materials namely, recycled rubber and sawdust. One product comprised of recycled rubber particles and 15% polyurethane binder. The other one comprised sawdust and 30% polyurethane material. Both products were 15 mm thick. For the frequency range of 100 to 1000 Hz both products showed similar sound absorption coefficient characteristics. However, for the higher frequency range of 1000 to 3150 Hz the sample with rubber particles had better sound absorption properties.
Materials made from sawdust and recycled rubber granules were tested for acoustic performance and compared with the existing acoustic products on the market, namely glass wool and flexible polyurethane foam. The sound absorption coefficient was experimentally evaluated over a frequency range of 100 to 3200 Hz. The results showed that composite materials made from sawdust and rubber granules had better acoustic properties than the existing products, particularly at frequencies below 1600 Hz. Sound absorption coefficient measured for material made with sawdust and 30% polyurethane binder had a minimum value of 0.65 in the frequency range of 300 to 3150 Hz. A maximum sound absorption coefficient of 0.979 was recorded at the frequency of 2000 Hz .
Tiuc et al.  further compared the sound absorption of products made from 100% Flexible Polyurethane Foam (100-FPF) and those made with 50% fir sawdust and 50% Flexible Polyurethane Foam (50-FPF). The 100-FPF product exhibited effective sound absorption characteristics for the 100 to 1700 Hz frequency range. This product recorded a maximum sound absorption coefficient value of 0.86 at a frequency of 1700 Hz. The 50-FPF product exhibited effective sound absorption characteristics for the 100 to 700 Hz frequency range, recording a maximum sound absorption coefficient value of 0.89 at a frequency of 700 Hz. This study also observed that composite porous materials exhibited complex sound absorption characteristics.
Table 5 presents sound absorption characteristics of various types of materials. It is clearly noted from this table that sawdust composites have better sound absorption capacity over materials such ordinary wood, conventional concrete and brick work.
Table 5. Sound absorption properties of some common building materials and those incorporating sawdust.
3.5.2. 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 .
Chung et al.  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.  used a mathematical model to predict the vibration of lightweight timber-framed floor/ceiling systems (LTFSs) caused by mechanical excitation. This study consolidated the earlier findings of the good sound insulating properties of the sand-sawdust layer in LTFSs. The theoretical model and the experimental measurements showed that the sand-sawdust layer effectively dampens the vibration in the frequency range of between 10 and 200 Hz.
Emms et al.  examined several problems surrounding lightweight floors, one being the inadequate impact insulation capacity in the low-frequency region of 16 to 250 Hz. The use of a sand/sawdust mix as an infill in battened cavities of these lightweight floors provides good impact insulation results, attributed to a combination of added mass, more damping capacity and floor stiffness.
Chathurangani et al.  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 .
4. 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.
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.
Literature shows that many timber producing countries generate more than 2 million m3 of sawdust annually. In developing countries, this material is often indiscriminately disposed of by open dumping and open burning, posing a huge environmental challenge. Various studies on the use of sawdust in construction, aimed at mitigating this sawdust environmental challenge, have been reviewed in this paper. The reviewed studies include the use and potential use of sawdust and sawdust ash in sawdust construction composites like particleboards, bricks, blocks and lightweight concrete.
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.
It is however noted from literature that increased sawdust proportions in the sawdust construction composites adversely affect the mechanical and physical characteristics of the produced composites. Replacing part of conventional sand in a concrete mix with sawdust proportions ranging between 5% and 15 % can produce good lightweight structural concrete with compressive strength values greater than 20 MPa. Analysis of the collected data gives a relationship between the compressive strength of sawdust concrete ( ) and sand replacement by sawdust content (λ) as . This relationship gives an optimum λ value of 17% for producing structural concrete with of 20 MPa.
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 CO2 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.
The authors gratefully acknowledge the support from Copperbelt University, Kitwe, Zambia.
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