Effect of elevated temperature on rice husk ash based self-compacting alkali activated slag concrete mixes under different cooling regimes

Here an attempt has been made to develop self-compacting alkali activated slag concrete mix using Rice Husk Ash as Binder. Investigation on the mechanical properties and residual strength when subjected to temperature of 200,400,600,800°C retention period1hour. The Rice husk ash Self Compacting Alkali Activated Slag Concrete mixes was made with GGBS, Rice husk ash and Lime as the binder, with its content varying from 550 to 650 kg/m3of fresh concrete. The W/B ratio of mixes varied in 0.7-0.88 range. The alkaline solutions had Na2O percentages in the range 5-7 with constant activator modulus maintained at 1. Final mix considered had lower water content, higher percentage of RHA and higher compressive strength. Trails showed Processed Rice Husk Ash30%, lime20% and GGBS50% for binder content 600kg/m3 and 6% Na2O dosage showed greater compressive strength and lower water binder ratio compared to other trail mixes. Results showed the slump flow, L–Box, U-box and V-Funnel as greater than 650mm, 0.85, 20mm and 10s resp. Fire damage of RSAASC was measured by subjecting to elevated temperature between 200-800°C for a retention period of 1hour. Cooled to room using furnace cooling, air cooling and sudden cooling. Results indicated reduced compressive strength with increased temperature.


Introduction
Cement is the most important constituent of a concrete mix.Since 1800 Ordinary Portland Cement (OPC) is the most used binder material in concrete.The most widely used construction material in the world is concrete.Research estimates that each ton of Cement produced, about 0.84 ton of Carbon Dioxide is released into the environment.The total contribution of Cement to global Carbon Dioxide emissions is estimated to be between 5 and 8 percent Today around 4.2 billion tons of Cement are produced annually with an expected growth rate of 2.5% per annum.[1][2][3].
Geo-polymers are a new class of concrete mixes, with no Portland cement, and are produced through the reaction of alumino silicate-rich materials with an alkaline solution .Fly ash, Rice husk In the earlier ranges, studies become extra focused at the behavior of alkali activated slag pastes and mortar mixes.In recent times, studies on AASC mixes have additionally gained greater attention Rice husk ash (RHA) will be found in rice-growing regions around the world, such as China, India, and the Middle East.RHA is a by-product of rice husk incineration.During the burning of rice husk, the majority of the evaporable components are lost slowly, and the silicates are the main left overs .Rice husks having a calorific value of about half that of coal, and assuming that the husks have about 8%-10% moisture content and no bran, the calorific value is calculated to be 15 MJ/kg.RHA is found to have pozzolanic properties as it i s a silica-rich material.The major constituents were SiO2 -90%, K2O -4.60%, and P2O5 -2.43% [14][15][16][17][18][19][20] Based on available literatures, there are several researches carried out on Alkali -Activated concrete mixes with different binders namely, GGBS, Metakaolin, Fly ash, RHA and many more.However, there are only few research carried out on Rice Husk Ash based alkali activated concrete mixes and there is currently no research on RHA based self-compacting alkali activated slag concrete mixes.Only few research has been carried out on elevated temperatures under different cooling regimes which could give an idea of a structure subjected to fire.Hence, the attempt has been made on effect of elevated temperature on RHA based self -compacting alkali activated slag concrete mixes under different cooling regimes.[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35].
Rice husk ash based self-compacting alkali activated slag concrete mixes were developed with minimum to attain M30 grade concrete and compared with conventional OPC mix of similar grade.The main aim of the present study is to maximum utilization of Rice h usk ash in self-compacting alkali activated slag concrete mixes.RSAASC mixes were prepared with GGBS, Rice husk ash and lime as a binder and alkali solution combination of sodium silicate and sodium hydroxide (NaOH) as an alkali activator.The fresh and hardened properties such as workability, compressive strength, split tensile strength, flexural strength, and modulus of elasticity of concrete mix were evaluated as per standard test procedures.RSAASC mixes were casted and exposed to different temperatures (200℃ -800℃) for a retention period of 1 hou Then they were cooled to room temperature with different cooling regimes like furnace cooling, air cooling and sudden cooling.After exposure to elevated temperature and cooling regimes, the residual compressive strength characteristics were studied.
The present study focused on development of Rice husk ash Self Compacting Alkali Activated Slag Concrete mixes (RSAASC) and to evaluate the mechanical properties of RSAASC mixes was main objective of this investigation.Further evaluation of residual strength of RSAASC mixes when subjected to elevated temperature under different cooling regimes and studies on microstructural characterization of RSAASC mixes after subjected to elevated temperature.

Experimental Program 2.1 Materials 2.1.1
Ground Granulated Blast Furnace Slag (GGBS) GGBS is the most commonly used material in AAS mixes, due to its higher hydraulic activity when compared to other types [16].Ground Granulated Blast-Furnace Slag (GGBS) conforming to IS 12089-1987 was procured from JSW Iron and Steel Plant, Bellary, India.GGBS as tested for Blaine's fineness, resulted about 390m 2 /kg and specific gravity of 2.81.The Chemical composition (given by manufacturer) of the GGBS used is given in Table 1.The material procured from Sri Shreesha Rice Industries, Tumkur.Rice Husk is the material burned under the temperature range between 600℃-800℃ forms Rice Husk Ash [4,14].The chemical composition of RHA consists of 90 percent amorphous silica, 5% carbon, and 2% K2O make up the average composition of well-burned RHA [20,21].The material burned without constant temperature and may contain unburned rice husk called as Unprocessed Rice Husk Ash (URHA).The URHA is then taken to Grinding process called as Processed Rice Husk Ash (PRHA) as shown in Fig 1 .Table 2 shows properties of PRHA and URHA.

Aggregates
In the present study, crushed stone sand also called manufactured sand is used as fine aggregate with maximum nominal size 4.75mm down size and natural aggregates of maximum nominal size of 12.5mm down size is used as coarse aggregates.The specific gravity, fineness modulus and water absorption of the fine aggregate were 2.65, 3.25 and 10%, respectively.The specific gravity and water absorption of the coarse aggregate were 2.67 and 0.5%, respectively.All the aggregates used herein were tested as per relevant Indian standard codes.

Alkaline Solution
Alkaline solution for alkali activation is a combination of sodium silicate solution (Na2SiO3) and sodium hydroxide (NaOH).The alkaline solution procured from Kamath's Rice Mill & Industries, Mangalore.Sodium hydroxide (NaOH) flakes having a purity of 97% and sodium silicate solution containing 14.7% Na2O +32.8 SiO2 + 52.5% H2O, by mass and with a density 1570 kg/m 3 (given by manufacturer).In the present study, activator modulus i.e., ratio of SiO2/ Na2O is kept constant as 1 but three different dosages of Na2O such as 5%, 6% and 7% are used as alkaline solution.

Mixture proportioning, preparation and casting of mixtures
Alkali activated concrete mix design is based on trial-and-error basis.EFNRC guidelines were used to achieve self-compacting concrete.As we designed on trial-and-error basis, the water to binder ratio started with 0.4 and continued till the flow of fresh concrete achieves flow ability around 650-800mm.For reaction of alkalis present in the concrete, the Alkaline solution combined with Sodium silicate and Sodium hydroxide is used.The mix design starts with Na2O dosage and Modulus of sodium silicate.The binder content as per EFNARC guidelines is 500-700kg per meter cube.The binder used in this experiment are GGBS, Rice Husk Ash (URHA and PRHA) and Lime, the total binder content varying from 550-650 kg per meter cube.For all the mixes 1% of total binder content of super plasticizer is been used.The super plasticizer used in this study is Glenium ACE JP (30).

Arriving at Final mix
Initially started with maximum utilization of URHA by 75%, 50% and 25% replacement of binder content and in next mix we increased Na2O dosage by 5%, 6% and 7% of binder content.In addition to reduce setting time we introduced Lime in 10%,15% and 20% of binder content.In further trials, we reduced URHA by 10%, 15% and 20% of total binder.And finally, PRHA by 20%, 30% and 40% of total binder was used in the mix.As different combinations of trial mixes were used, lower water to binder ratio, flow ability and higher compressive strength were all the factors considered to finalize the mix.The final mix design consists of processed rice husk ash (PRHA) 30% of total binder, Lime 20% of total binder and GGBS 50% of total binder.The water to binder ratio is 0.7, flow ability is greater than 650mm and compressive strength for a period of 1,3 and 7day is 5.5MPa,14.15MPaand 20.5MPa respectively.The mix design consisting of unprocessed rice husk ash 15% and lime 20% comparably shown equal results of the mix consisting of PRHA 30% and Lime 20%.As we are looking for maximum utilization of Rice husk ash, the PRHA 30% and lime 20% and 50% GGBS of total binder was considered for further evaluation of mechanical properties and residential strength when subjected to elevated temperature.Finalized mix is shown in Table 3 for 1m 3 of concrete.

Fresh Properties of RSAASC Mix
In the present investigation the levels of workability of the mixes were measured using the standard Abraham's slump cone and flow table.The slump cone with bottom diameter of 20cm, top diameter of 10cm and height of cone is 30cm.For all the trial mixes flow ability is checked to satisfy the EFNARC guidelines of flow 650-800mm.To achieve flow of the mixes, the water binder ratio is increased from 0.4 to 0.9 till the flow is greater than 650mm.The final fixed mix showed the slump flow values greater than 650mm showed in figure 2, with their L-Box ratios, U-box and V-Funnel values ranging between 0.85, 20mm and 10s respectively, satisfying the EFNARC guidelines.

Compressive strength of RSAASC and Control mix
Based on the trails, all self-compacting alkali activated slag concrete mixes with GGBS, URHA and PRHA replacing GGBS and introduction of lime in process of faster setting action herein tested as per IS 516:1999.From the trails, the mix with lower water binder ratio and higher compressive strength was finalized for further mechanical properties and residual strength when subjected to elevated temperature.From the trials conducted, it was observed that maximum utilization of Processed Rice husk ash (PRHA) shown compressive strength of about 30MPa, for a binder content of 600kg/m3 as shown in table 4. which consists of 30% of processed rice hush ash, 20% lime and 50% GGBS.Even though 15% Unprocessed rice hush showed 30MPa compressive strength, as we targeted maximum utilization of rice husk ash, hence 30% processed rice husk ash was fixed for further mechanical and residual strength.Control mix of concrete consisting of cement, fine and coarse aggregate is casted for M30 grade and the compressive strength at different age were shown in Table 4.

Residual Compressive strength of RSAASC and Control mix
Figure 3 shows an electric furnace.Temperature is set using the knob and temperature increased till it reached the target temperature.The specimen was subjected to exposure tests in an electrical furnace for temperatures of 200°C, 400°C, 600°C and 800°C with a retention period 1 hour.Three types of cooling regimes studied here are, furnace cooling, air cooling and sudden cooling.By furnace cooling, it is meant that, after completion of 1hour retention period at designated temperature, the furnace power supply is cut off and the door of the furnace shall not be opened till the specimen inside attain room temperature.The interior of the furnace temperature can be monitored digitally from outside display panel.It is a function of the temperature exposed to, it takes nearly one day for specimen subjected to 800°C, to reach room temperature.By air cooling, it is meant that, after retention period, that furnace door is opened and specimen are removed and allowed to cool in ambient/ room temperature.By Sudden cooling it is meant that, after completion of retention period at designated temperature, the door of the furnace shall be opened and the specimen shall be immediately quenched in water at room temperature.

Figure 3. Electric Furnace
The specimens for a period of 28 days were casted and subjected to different elevated temperatures and followed by three different cooling regimes (Sudden cooling, air cooling and furnace cooling).Table 5 and Table 6 shows cube compressive strength at 28 days of RSAASC and Control mix which were subjected to different elevated temperature.Figure 4 shows variation of compressive strength when subjected to elevated temperature and three cooling regimes.From Table 5 and Table 6 it is observed that as temperature increases from 200℃ to 800℃, the compressive strength decreases.As compared to control mix, RSAASC mix subjected elevated temperature relatively showed lower compressive strength.From both RSAASC and control mix, air cooling method is best cooling method which shows higher compressive strength compared to other cooling methods.Furnace cooling showed least compressive strength and sudden cooling showed medium compressive strength.For a temperature 800℃, the cube is being exposed to red hot and formation of wide range of cracks shown in figure 5.   From the Table 5 and Table 6, it is observed that maximum compressive strength 20.48 MPa and 23.05 MPa of RSAASC and control mix respectively exposed to air cooling method for temperature 200℃.Hence there is a reduction of 33% and 23% of compressive strength of RSAASC and control mix respectively.In a furnace cooling method, it is observed that the least compressive strength of 4.17MPa and 7.89MPa for a temperature of 800℃.About 86% and 73.5% reduction in compressive strength of RSAASC and control mix respectively.With different cooling regimes, air cooling showed higher compressive strength when compared with other two cooling regimes.Furnace cooling gave the least compressive strength while sudden cooling gave medium compressive strength of the cubes.

Split Tensile strength, Flexural strength and MOE of RSAASC and Control mix
Tensile strength test was conducted in laboratory by casting the cylinder specimen of size 100 mm dia.200 mm height and tested using compression testing machine at 28 days for both ordinary concrete and RSAASC mixes.The results for split tensile strengths of RSAASC mix and control mix at 28 days is 1.56 MPa and 1.76MPa.Figure 6 shows split tensile test.Flexural experiment is carried in laboratory by casting beam specimen size 500mm x 100mm x 100mm using twopoint loads in UTM at different duration for both conventional concrete and RSAASC mixes.The RSAASC mix consist of 30% PRHA, 20% lime and 50% GGBS with 6% Na2O dosage and modulus of sodium silicate as 1.The flexural strength of RSAASC mix and Control mix of 28 days is 2.33 MPa and 2.46MPa.

Water absorption
Rice husk ash based self-compacting alkali activated concrete slag mix were subjected to water absorption test to know the pores inside the hardened concrete.It is observed that the water absorption of RSAASC mix showed an average water absorption of 3.25%.When comparing with control mix the water absorption of RSAASC mix showed higher water absorption value indicating that RSAASC mix sample is porous.The control mix showed water absorption of 2.5%.

Studies on microstructures of RSAASC mix
The RSAASC mix were subjected to elevated temperatures and treated with different cooling methods, and were tested for compressive strength.After testing for compression test, the samples were crushed to pieces.The crushed piece of compression tested samples were taken for microstructural studies.The smaller size pieces of respective temperature and cooling methods were subjected to SEM (Scanning electron microscopy) and EDX (Energy Dispersive X-Ray Analysis) analysis.Before subjecting to SEM and EDX, the samples were in oven dried condition.Figure 9 shows SEM analysis with different magnification RSAASC mix subjected to 400˚C and aircooled sample.The sample has been crushed to pieces and taken for SEM analysis in oven dried condition.SEM analysis shows that the slag grains and ash has been activated effectively.Densified structure of the sample can be seen in lower magnification.Smaller portion of the images are magnified further to check for activation.Magnified images show activation of slag grains and rice husk ash with negligible space occupied by unreacted ash and slag grains.From lower magnification significant number of pores and minor cracks can be seen which in turn lead to lower strength.At higher magnification the bonding of slag grains and ash material leading to strength development in the concrete.Formation of densified layer due to the activation of ash and slag grains, has probably led to disappearance of ITZ in and around the aggregates.As increase in magnification of the images, morphology of white patches shows the activation of slag grains and ash powder which are completely reacted by alkaline solution.At 400˚C and exposed to air cooled method, the images show lower number of pores and minor cracks in the concrete.Similarly, 400˚C and Furnace cooled (FC) shows number of pores and minor cracks in SEM analysis figure 11.As cooled in furnace, the sample cooled undergoes gradual decrease in temperature as it is kept inside muffle furnace and cooled to room temperature.SEM analysis shows white patches which indicates the activation of slag grains and ash material with alkaline solution complete reaction.EDX analysis shows elemental composition in Figure 12 which clearly shows EDX analysis of RSAASC mix subjected to 400˚C and furnace cooled sample.From the image it clearly indicates the peaks for Si, Ca, Al, Mg, Na, K and O.As per this investigation, EDX analysis indicates the clear higher peak of Si element than Ca and O. Figure 13 shows SEM analysis with different magnification RSAASC mix subjected to 600˚C and sudden (water) cooled sample.The sample has been crushed to pieces and taken for SEM analysis in oven dried condition.SEM analysis shows that the slag grains and ash has been activated effectively with alkaline solution complete reaction.White patches in the images explains the reaction of alkaline solution.Densified structure of the sample can be seen in lower magnification.Smaller portion of the images are magnified further to check for activation.Magnified images shows activation of slag grains and rice husk ash with negligible space occupied by unreacted ash and slag grains.Magnification of 3500x shows significant amount of pores and 2000x, 5000x shows micro cracks.As increase in magnification of the images, morphology of white patches shows the activation of slag grains and ash powder which are completely reacted by alkaline solution.Figure 17 shows SEM analysis with different magnification RSAASC mix subjected to 800˚C and Air cooled sample.The sample has been crushed to pieces and taken for SEM analysis in oven dried condition.White patches in the images explains the complete reaction of alkaline solution.Magnified images shows activation of slag grains and rice husk ash with negligible space occupied by unreacted ash and slag grains.As temperature increased to 800˚C, wide range of cracks can be seen in 1000x and 2000x and even pores in the structure.As increase in magnification of the images, morphology of white patches shows the activation of slag grains and ash powder which are completely reacted by alkaline solution.At higher magnification of 8000x shows pores in the image and concludes the weak internal structure of the concrete.Figure 19 shows SEM analysis with different magnification RSAASC mix subjected to 800˚C and furnace cooled sample.The sample has been crushed to pieces and taken for SEM analysis in oven dried condition.White patches in the images explains the complete reaction of alkaline solution.Magnified images shows activation of slag grains and rice husk ash with negligible space occupied by unreacted ash and slag grains.As temperature increased to 800˚C, wide range of cracks can be seen in 1000x and 2000x and even pores in the structure.As increase in magnification of the images, morphology of white patches shows the activation of slag grains and ash powder which are completely reacted by alkaline solution.At higher magnification of 8000x, image clearly shows the crack developed which leads to lower strength of the concrete.Comparing to air cooling method and furnace cooling method, air cooling is best cooling method with lower cracks and pores in the concrete.As rice husk ash is composed of higher silica content is the reason for concrete sample to show clear peak of silica element in EDX analysis.

Conclusions
1.A new form of self-compacting alkali-activated slag concrete mixes, there produced effectively using Rice husk ask as binder, flow greater than 650mm with L-Box ratios, U-box and V-Funnel values ranging between 0.85, 20mm and 10s respectively, satisfying the EFNARC guidelines.2. Comparing to both unprocessed rice husk ash and Processed rice husk ash, processed rice husk improves reactivity and shows better result.3. GGBS can be replaced by Processed rice husk ash (PRHA) by 30% which relatively gave equal compressive strength compared with cement based concrete.4. Residual compressive strength of RSAASC mix showed lower compressive strength as temperature increased from 200℃ to 800℃. 5.In a furnace cooling method, it is observed that the least compressive strength of 4.17MPa and 7.89MPa for a temperature of 800℃.About 86% and 73.5% reduction in compressive strength of RSAASC and control mix respectively.6.With different cooling regimes, air cooling showed higher compressive strength when compared with other two cooling regimes.Furnace cooling gave the least compressive strength while sudden cooling gave medium compressive strength of the cubes.7. Satisfactory performance in the split tensile strength, flexural strength and modulus od elasticity of 1.56 MPa, 2.21 MPa and 21GPa respectively.8. From the microstructural studies, damage of internal structure can be seen due to rise in temperature from 400℃ to 800℃ of RSAASC mix leading to lower strength.Microstructural study concludes that as the temperature increases the number of pores and number of cracks increases.9. Microstructure study also concludes that from three cooling regimes, air cooling regime showed micro cracks and pores while furnace cooling regime showed wide cracks and large number of pores in the sample at 400℃, 600℃ and 800℃ while sudden cooling at 600℃ showed minor cracks and pores.10.Microstructure study shows the activation of slag grains and ash material with complete reaction of alkaline solution and empty space occupied by unreacted rice husk ash and slag.It is clear evident that the formation of ITZ is very less.

Figure 1
Figure 1 Rice Husk Ash

Figure 2 .
Figure 2. Slump flow and U-box test of RSAASC mix 3.2 Hardened Properties of RSAASC mix

Figure 4 .
Figure 4. Variation of 28day compressive strength subjected to elevated temperature (a) RSAASC mix (b) Control mix

Figure 5 .
Figure 5. Specimens at 800℃ and sudden cooling at 28 days

Figure 7
flexural strength test.Modulus of elasticity (MOE) experiment carried out by casting cylinder of 150mm dia.and 300mm height and tested in universal testing machine.Figure8shows the experimental setup of MOE and testing.As per experimental results the RSAASC mix gave the result of 21GPa and Control mix showed result of 24GPa.

Figure 9 .
Figure 9. SEM analysis of RSAASC mix subjected to 400˚C and Air cooled (AC) sample with Different Magnification

Figure 10 . 11 Figure 10 Figure 11 .
Figure 10.EDX analysis of RSAASC mix subjected to 400˚C and Air cooled (AC) sample

Figure 14 .Figure 15 .
Figure 14.EDX analysis of RSAASC mix subjected to 600˚C and Sudden cooled (SC) sample Figure 14 clearly shows EDX analysis of RSAASC mx subjected to 600˚C and sudden cooled sample.From the graph it is clearly indicating elements of Si, Ca, O, Na, Al, Mg, K and Fe.From the image it clearly indicating more prominent signatures of peaks for Si, Ca and O.As rice husk ash is composed of higher silica content which is why clear peak of silica element is high in EDX analysis.

Figure 15 shows 15 Figure 16 .Figure 17 .
Figure15shows SEM analysis with different magnification RSAASC mix subjected to 600˚C and furnace cooled sample.The sample has been crushed to pieces and taken for SEM analysis in oven dried condition.White patches in the images explains the complete reaction of alkaline solution.Magnified images shows activation of slag grains and rice husk ash with negligible space occupied by unreacted ash and slag grains.Magnification of 3500x shows significant amount of pores and 1000x, 2000x shows wider cracks in the images.As compared to sudden cooling, furnace cooling shows wider cracks.As increase in magnification of the images, morphology of white patches shows the activation of slag grains and ash powder which are completely reacted by alkaline solution.

Figure 18 .Figure 19 .
Figure 18.EDX analysis of RSAASC mix subjected to 800˚C and Air cooled (AC) sample

Figure 20 .
Figure 20.EDX analysis of RSAASC mix subjected to 800˚C and Furnace cooled (FC) sampleFigure 20 clearly shows EDX analysis of RSAASC mx subjected to 800˚C and furnace cooled sample.From the image it is clearly indicating elements of Si, Ca, O, Na, Al, Mg, K and Fe.From the image it clearly indicating more prominent signatures of peaks for Si, Ca and O.As rice husk ash is composed of higher silica content is the reason for concrete sample to show clear peak of silica element in EDX analysis.

Table 3 .
Finalized Mix of RSAASC mix

Table 5 .
RSAASC mix cubes subjected to Elevated temperature at 28 days

Table 6 .
Control mix Cubes subjected to Elevated temperature at 28 days