Utilization of Rice Husk Ash for Sustainable Construction: A Review

Applications of Supplementary Cementitious Materials (SCM) in construction industry are getting priority due to environmental friendly behavior as well as enhanced mechanical and durability properties. Considering the characteristics of SCM, properly burnt and ground Rice Husk Ash (RHA) has significant potential on account of rich reactive silica (SiO2) compound. When RHA is applied in cementitious system it improves the systems properties by two fold of effects; chemical or pozzlanic effect and physical or filler effect. The reactive silica compound present in RHA reacts with cement hydration products calcium hydroxide (Ca (OH)2) and formed secondary C-S-H gel, which is counted as chemical or pozzolanic activity of RHA. The physical or filler effect of RHA is denoted as the proper distribution of finer RHA particles into the cementitious system. This study reviews the advantageous use of RHA for sustainable construction. The aim of this study is to promote the idea of using RHA by elaborating upon their various production processes, different properties, pozzolanic activity and its contribution to the cementitious system.


INTRODUCTION
Utilization of cement in the production of concrete is popular all over the world.However, production of cement is not only high energy intensive but also emits greenhouse gases (Mehta, 1999).As a result, construction industry is reaching its limiting value in terms of sustainability.Researchers are exploring alternative sources of cementitious materials for future construction considering sustainability.Some industrial by products (fly ash, slag, silica fume) and agricultural wastes (rice husk ash, palm oil fuel ash, sugarcane baggase ash) are produced every day.These materials are associated with cementitious properties.These cementitious materials are named as pozzolans.Rice Husk Ash (RHA) is one of the popular pozzolan.Generally, rice milling industry generates huge amount of rice husk during milling of paddy every year.However, popular use of this husk either used as fuel in the rice mills to produce steam during parboiling process or land filling which causes pollution (Mehta, 1977).On the other hand, RHA contains high amount of silica compound and it is almost 80-85% (Siddique, 2008).Usually, silica presents in RHA in two particular forms: amorphous silica which is chemically active, crystalline silica which is chemically inactive (Della et al., 2002).The amorphous form of the silica is the key of pozzolanic activity of RHA.The formation of amorphous silica depends on temperature and duration of burning (Chindaprasirt et al., 2007;Zain et al., 2011).Only properly burnt and ground RHA is favorable for use as a pozzolanic material because it contains high amount of amorphous silica (Chindaprasirt and Rukzon, 2008).Its fineness and reactivity is significantly improved after proper grinding (Zain et al., 2011).Higher fineness and specific surface area important property of RHA to increase pozzolanic and filler activity (Karim et al., 2014;Shukla et al., 2011;Raman et al., 2011;Muthadhi and Kothandaraman, 2010;El-Dakroury and Gasser, 2008;Giaccio et al., 2007).Thus, utilization of RHA (an agricultural waste material) in concrete production is very much helpful to reach the goal of the sustainable construction (Serniabat et al., 2014).Extensive research has been carried out by the past researchers using RHA as partial replacement of cement.This study illustrates a brief description on various production processes of RHA, favorable conditions for producing reactive RHA, properties of RHA, pozzolanic activity of RHA and its contribution in concrete and mortar.

PRODUCTION OF RHA
Various incineration processes were followed by the past researchers for the incineration of raw rice husk to produce RHA and reported in various literatures Incineration processes of RHA are briefly discussed in the following sections.
Earlier, Loo et al. (1984) designed a ferrocement furnace in order to produce RHA.The authors reported that, at first raw rice husk was kept in the incineration furnace for burning and after incineration then it was allowed to cool for 24 h.Finally, they improved the fineness of cool incinerated ash using Los Angeles grinding machine.Because, grinding performance of Los Angeles machine incorporated with 40 mild steel rods of 10 mm diameter and 500 mm in length placed in the rotating drum is better than the conventional ball mills (Zain et al., 2011;Raman et al., 2011).Nehdi et al. (2003) introduced another technology for the combustion of rice husk using fluidized beds which based on a different technique Torbed rea presented in Fig. 1.The Torbed reactor can operate gas velocities of 3-12 m/sec.The advantages of this RHA production technology are able to produce smaller RHA particles, more economic and easy to maintain temperature.Torbed reactor initially produced coarser RHA particles, ranges 44-46 µm.After that, this particle size can be reduced to 7 µm using a vibratory ring pulverizer.The authors reported that pozzolanic RHA has very low carbon content and can be obtained at combustion temperatures between 750 using this technology.Habeeb and Mahmud (2010) designed a rice husk incineration furnace having a capacity of 60 rice husks as presented in Fig. 2. It has three small openings for ventilation.The design concept of furnace is almost similar to Loo et al authors reported that to produce reactive RHA using this furnace, the burning temperature of rice husk should not exceed 690°C and particle sizes of RHA should less than 11.5 µm for a grinding time 3 Ramezanianpour et al. (2009) commenced another type of furnace in order to produce RHA furnace reported by the author is shown in Fig. 3. furnace is capable to produce good quality of rice husk ashes with various un-burnt carbon contents.Because, temperature and rate of burning easily measurable in this furnace.Burning temperature can be measured in this furnace using thermocouples at the fire zone of air inlet and outlet.The authors produced RHA at various temperatures rate such as 550, 600, 650, 700 and 750°C with duration of burning time 30, 60 and 90 min respectively.Finally, they suggested that RHA produced at 650°C for 60 min can be considered to be non-crystalline RHA and also save the RHA production time.Zain et al. (2011) developed a simple ferrocement furnace located at Structural and Concrete

PRODUCTION OF RHA
Various incineration processes were followed by the past researchers for the incineration of raw rice husk to produce RHA and reported in various literatures.Incineration processes of RHA are briefly discussed in .( 1984) designed a ferrocement furnace in order to produce RHA.The authors reported that, at first raw rice husk was kept in the incineration furnace for burning and after incineration then it was allowed to cool for 24 h.Finally, they improved the ness of cool incinerated ash using Los Angeles .Because, grinding performance of Los Angeles machine incorporated with 40 mild steel rods of 10 mm diameter and 500 mm in length placed in the rotating drum is better than the conventional ball ., 2011). .( 2003) introduced another technology for the combustion of rice husk using fluidized beds which based on a different technique Torbed reactor as The Torbed reactor can operatewith .The advantages of this RHA production technology are able to produce smaller RHA particles, more economic and easy to maintain initially produced coarser 46 µm.After that, this particle size can be reduced to 7 µm using a vibratory ring pulverizer.The authors reported that pozzolanic RHA has very low carbon content and can be obtained etween 750 and 850°C Habeeb and Mahmud (2010) designed a rice husk incineration furnace having a capacity of 60 kg of raw It has three small openings for ventilation.The design concept of this et al. (1984).The authors reported that to produce reactive RHA using this furnace, the burning temperature of rice husk should not exceed 690°C and particle sizes of RHA should less than 11.5 µm for a grinding time 360 min.
. ( 2009) commenced another type of furnace in order to produce RHA.The detail of furnace reported by the author is shown in Fig. 3.This furnace is capable to produce good quality of rice husk contents.Because, temperature and rate of burning easily measurable in this furnace.Burning temperature can be measured in this furnace using thermocouples at the fire zone of air inlet and outlet.The authors produced RHA at various uch as 550, 600, 650, 700 and 750°C with duration of burning time 30, 60 and 90 min, .Finally, they suggested that RHA produced at 650°C for 60 min can be considered to be crystalline RHA and also save the RHA production (2011) developed a simple ferrocement Concrete Lab in The produced heat during combustion is trapped by the ferrocement cylinder as well as preventing it from spreading into the air.The steel cylinder has dimensions of 760 mm diameter, 1090 mm height and thickness is 5 mm which acts as a container for burning of rice husk.In order to supply sufficient air during combustion air ducts also designed, whose dimensions are 100 mm diameter, 200 mm height and 100 mm thickness.The combustion temperature can be controlled by providing air through air ducts using electric fans.The surfaces of the steel cylinder and air ducts are porous with small holes of 5 mm diameter.
The authors suggested that furnace of UKM is able to produce amorphous silica with the constant furnace combustion temperature at 500 Although grinding for 30 min produced good result, grinding for 60 min or more is recommended by the authors for achieving standard fineness of RHA.Van et al. (2013) furnished a modified incinerator for the incineration of rice husk incinerator as presented in Fig. 5.This incinerator can produce about 20-25 kg of RHA within a burning period of two days in a single batch.This incineration process also able to produce RHA consists of high Illustration of the cyclonic motion of rice husk reactor (Nehdi et al., 2003) Fig. 2: The ferrocement furnace (Habeeb and Mahmud, 2010) National University of Malaysia (UKM), Malaysia as s two parts: one is ferrocement steel cylinder.The dimensions of the ferrocement cylinder are 1030 mm and wall thickness is 60 mm.The produced heat during combustion is trapped by the ferrocement cylinder as well as preventing it from spreading into the air.The steel cylinder has dimensions of 760 mm diameter, 1090 mm height and cts as a container for burning of rice husk.In order to supply sufficient air during combustion air ducts also designed, whose dimensions are 100 mm diameter, 200 mm height and 100 mm thickness.The combustion temperature can be r through air ducts using electric fans.The surfaces of the steel cylinder and air ducts are porous with small holes of 5 mm diameter.The authors suggested that furnace of UKM is able to produce amorphous silica with the constant furnace rature at 500-700°C for 2 h.Although grinding for 30 min produced good result, grinding for 60 min or more is recommended by the authors for achieving standard fineness of RHA.
. ( 2013) furnished a modified incinerator incineration of rice husk.The details of incinerator as presented in Fig. 5.This incinerator can 25 kg of RHA within a burning period of two days in a single batch.This incineration process also able to produce RHA consists of high Fig.3: Schematic shape of rice husk ash furnace (Ramezanianpour et al., 2009) Fig. 4: Furnace at UKM for burning rice husk (Zain et al., 2011) amount amorphous silica content about 97.4%.In order to produce active RHA, the authors ground RHA in a batch ball mill with diameter of 270 mm, length of 400 mm and rotation speed of 67 rpm for different grinding  periods.In each batch, the internal volume was partially filled with 21 kg steel balls (7 kg of 12.5 mm, 8.4 kg of 22 mm and 5.6 kg of 28 mm diameter) and 1 kg RHA.Form discussions related to production process of RHA, it is noticed that temperature control and grinding after burning process are the most significant to produce highly reactive RHA.Only properly burnt and ground RHA is suitable for the reactive amorphous silica content.Higher burning temperature may produce some crystalline formation of silica (Ismail and Waliuddin, 1996).If RHA is produced with some crystalline silica due to higher burning temperature, then reactive RHA could be obtained by fine grinding (Rodrigues et al., 2006).Most of the researchers suggested to maintain temperature below 700°C during incineration of rice husk.Madandoust et al. (2011) also reported that silica content of the ash transforms into non-crystalline or amorphous silica while rice husk is burning between 550 and 700°C temperature for 1 h.Therefore, it is clear that temperature ranges form 550-650°C and using proper grinding media after incineration of raw rice husk can produce higher fineness and reactive silica content RHA.

PROPERTIES OF RHA
Typical physical and chemical properties of RHA obtained from various literatures are presented in Table 1 and 2. Generally, mean particle size of properly ground RHA is less than cement particles.Particles size of RHA normally ranges 5 to 10 µm.The ground RHA particles indicate higher fineness, more than 95% passing through 45 µ sieve.When RHA is applied in cementitious system, it contributes to the hardened properties of concrete or mortar by filling or densification of microstructure due to higher fineness (Khan et al., 2014a;Shukla et al., 2011;Raman et al., 2011;El-Dakroury and Gasser, 2008).Higher specific surface is another important property of RHA which contributes in both pozzolanic and filler action (El-Dakroury and Gasser, 2008;Giaccio et al., 2007).Pozzolanic effect is governed by the pozzolanic reaction of RHA.Pozzolanic reaction of RHA depends on reactive amorphous silica compound that present in properly burnt and ground RHA.When this amorphous silica gets contact with hydration product of cement then it produces secondary binder (Yu al., 1999;Feng et al., 2004;Jamil et al., 2013;Khan et al., 2014b).Table 2 indicates that RHA contains high silica compound and it varies 75-97%.The amount of active silica present in RHA depends on proper burning and grinding.Total percentage of major oxide (SiO 2 + AlO 2 + Fe 2 O 3 ) is over 70% which satisfies ASTM C618 standard for class F pozzolan.The lower loss on ignition values of RHA indicates less amount of carbon content, not more than 10%.Sometimes uncontrolled burning produced crystalline silica which unable to produce pozzolanic reaction, however this inactive silica can take part in filler action.Therefore, mainly reactive silica compound, large specific surface area and higher fineness of RHA makes it suitable to use as partial replacement of cement.

POZZOLANIC ACTIVITY OF RHA
RHA is a highly reactive pozzolanic material; it contains non-crystalline silica and high specific surface area that are accountable for its high pozzolanic reactivity.The reactive non-crystalline or amorphous silica present in RHA react with hydration product of cement and produces an additional C-S-H gel (Yu et al., 1999;Feng et al., 2004).As a result the quality of RHA mixed concrete or mortar is improved significantly.Usually, pozzolanic activity of RHA depends on: • Presence of amorphous silica • Fineness of RHA particles • Its specific surface area (Zain et al., 2011;Shukla et al., 2011;Raman et al., 2011) The pozzolanic reaction can be satisfactorily described by the Jander diffusion equation that is based on Fick's parabolic law of diffusion (Cabrera and Rojas, 2001).The Jander equation for three dimensional diffusion in a sphere is (1 -(1 -x) 1/3 ) 2 = (D/r 2 ) kt where, x is the fraction of the sphere that has reacted, r is initial radius of the starting sphere and k is the diffusion constant.
Presently, Jamil et al. (2013) reported a theoretical estimation for the maximum percentages replacement of RHA based on pozzolanic reaction between RHA and cement hydration product calcium hydroxide (Ca (OH) 2 ).The author mentioned that, anhydrous calcium silicates (C 3 S and C 2 S) present in cement only involves to produce Ca (OH) 2 .
The hydration of cement (C 3 S and C 2 S) follows the equations (Metha and Monteiro, 2006): It is observed that hydration of cement (C 3 S and C 2 S) produces calcium hydroxide (Ca (OH) 2 ).
Based on chemical equilibrium, pozzolanic reaction between silica and Ca (OH) 2 in the presence of water is as follows (Jamil et al., 2013;Yu et al., 1999):

CONTRIBUTION OF RHA IN CEMENTITIOUS SYSTEM
Concrete or mortar incorporating with RHA shows higher strength and durability than concrete without RHA.Strength of RHA blended concrete depends on several factors such as curing period, water to cement ratio, cement replacement level and quality of ash.Early ages strength development of RHA blended concrete is not significant while compared to the longer ages curing.This may due to the formation of secondary C-S-H gel at later ages that provides a homogeneous and compacted microstructure.Water to cement ratio also plays important role in the strength development of RHA blended concrete.Gastaldini et al. (2010) found the highest compressive strength for 20% RHA mixed concrete at water to cement ratio 0.35 while compared strengths for other water to cement ratios.De Sensale (2006) reported that performance of controlled incinerated RHA blended concrete much better than residual RHA mixed concrete.According to Habeeb and Mahmud (2010) study, strength of RHA blended concrete increases with the increment of RHA percentages.The author reported that available silica from the addition of RHA reacted with that portion of C-H released from the hydration process and thus, the C-S-H released from the pozzolanic reaction.As a result, greater portion of RHA may produce greater amount of C-S-H.Saraswathy and Song (2007) reported that up to 30% replacement level of RHA, there is no decrease in compressive strength observed when compared to control concrete after 7 to 28 days of curing.According to Isaia et al. (2003) study, 20% RHA mixed concrete showed higher strength than control concrete at both 28 and 90 days curing period.
Figure 6 shows the compressive strength development concrete containing various percentages of RHA which is a finding of Cordeiro et al. (2009).It is see RHA concrete shows significant strength at all ages.
From above discussions, it is clear that strength of RHA associated concrete increases up to 30% replacement of cement.Mortar with RHA also shows similar manner.Chindaprasirt et al. (2008) reported that strength 40% RHA mixed concrete is higher than control mortar after 90 days of curing.Ganesan (2008) concluded that, cement can be replaced up to 30% by RHA without any adverse effect on strength and durability properties of mortar.Tensile strength of RHA mixed concrete and mortar is also better than control concrete as shown in Fig. 7 which is obtained from Madandoust et al. (2011).However, early tensile strength of RHA mixed concrete is not good enough when it is compared to control concrete.Therefore, cement can be replaced up to 30% by RHA without effecting strength of concrete and mortar.The difference of strength between RHA mixed mortar Fig. 7: Development of tensile strength with age ( Evolution of compressive strength of concretes against RHA mixed concrete showed higher strength than control concrete at both 28 and 90 days curing period.Figure 6 shows the compressive strength development concrete containing various percentages of RHA which . (2009).It is seen that 20% RHA concrete shows significant strength at all ages.From above discussions, it is clear that strength of RHA associated concrete increases up to 30% replacement of cement.Mortar incorporating with RHA also shows .( 2008) reported that strength 40% RHA mixed concrete is higher than control mortar after 90 days of curing.Ganesan et al. (2008) concluded that, cement can be replaced up to 30% by RHA without any adverse effect on strength s of mortar.Tensile strength of RHA mixed concrete and mortar is also better than control concrete as shown in Fig. 7 which is obtained .( 2011).However, early tensile strength of RHA mixed concrete is not good enough ared to control concrete.Therefore, cement can be replaced up to 30% by RHA without effecting strength of concrete and mortar.
rength between RHA mixed mortar and control mortar is much significant at long term ages.

CONCLUSION AND RECOMMENDATIONS
This literature review deals with the production procedure of RHA as well as its effect on concrete and mortar properties.It is noticed that in order to get desired properties RHA, controlled burning and well grinding are essential.It is difficult to get all ash in reactive form of amorphous silica since some of ash converted to crystalline form.Incorporating incinerated rice husk ash has a significant effect on the properties of concrete or mortar.Compressive strength of mortar and concrete increased when cement is partially replaced by RHA.This is due to reactive amorphous silica content that contains in RHA.This active silica reacts with hydration product of cement and produced secondary C-S-H gel, which brings a significant change in microstructure.Finer RHA fills pore or voids into the microstructure of concrete and mortar.Finally, this literature has been reported some recommendations for future research in this area.These include the following: • Need to develop higher capacity of RHA burning furnace for commercial or industrial purpose.• Need to elaborate cost analysis between concrete with or without RHA.• Need to test strength of concrete that contains higher percentages of RHA.

Fig. 1 :
Fig. 1: Illustration of the cyclonic motion of rice husk particles in a torbed reactor (Nehdi