Experimental Studies on Bagasse Ash-Based Self-Compacting Alkali Activated Slag Concrete

In the present study, an attempt was made to develop Self Compacting Alkali Activated Slag Concrete mixes incorporating Bagasse Ash as binder content. This study investigates the physical and mechanical properties of Bagasse Ash based Self Compacting Alkali Activated Slag Concrete (BSAASC) with varying proportions of Bagasse Ash. Lime and GGBS (Ground Granulated Blast Furnace Slag) are used along with Bagasse Ash as binder materials in the BSAASC. The total binder content was restricted to 600kg/m3 containing Bagasse Ash at 10%, 20% and 30% of total binder content. The solution-to-binder ratio was 0.6. the dosage of Na2O was varied by 5%, 6% and 7%. The molarity of the alkaline solution is 1M. By using Minitab Statistical Software nine mixes were produced with 3 factors and 3 levels. The analysis was done by Desirability Function Approach (DFA) to check the effectiveness of the considered factors. The workability tests for properties of concrete samples are carried out according to EFNARC guidelines. Microstructural studies such as SEM, EDX and XRD analysis were also carried out, showing denser morphologies indicating effective activation of slag by the alkaline solution.


Introduction
Since the 1800s, Ordinary Portland cement is widely used in the construction industry.According to the research, for every ton of cement produced 0.84 tons of carbon dioxide is released to the environment directly during decomposition and indirectly during production as energy consumed.Approximately 5 to 8 percent of Global Carbon Dioxide emission is contributed by the production of cement].With an expected growth rate of 2.5% per annum, around 4.2 billion tons of Cement are produced annually.At the same time, latently hydraulic cementitious matrices such as Fly Ash, Silica Fumes, and Slag which are by-products from various industries have been used as admixtures to replace some of the cement content.However, the presence of cement is required for cementitious matrices reaction to occur and produce binding material in cement.The construction industry has taken considerable strides forward over the last two or three decades with regard to trials in the use of one or another cementitious material generally identified as pozzolans, for the compounding of various cement-based products.This has not only resulted in improving the compressive strength value attained thereby but also in qualities like the ability to set and harden under water.Among this coal fly ash, blast furnace slag, rice hush ash, bagasse ash, silica fume, or metakaolin are the most common ones.Due to economic and environmental concerns, different methods of making cement products are being considered.One method to achieve the goal of reducing carbon dioxide emissions and greenhouse gases is to formulate cement using a lower portion of calcinated material, thereby reducing carbon dioxide emissions per unit of product.Another approach is to use a lower percentage of cement and or gypsum compared to standard cement or gypsum to ensure an increased compressive strength and or flexural strength is yet attained thereby.This one is durable and suitable for all types of applications, and also benefits the environment.Additionally, a need exists for improved cement and gypsum products that permit the use of less expensive aggregates to reduce the cost of the cement product.Alkali-Activation is a technique for starting the reactivity of cementitious matrices without using cement.The reaction of concrete including both cement and a cementitious matrix, such as Fly Ash, is the finest demonstration of the Alkali-Activation process.Two chemical processes are going on at the same time.The first is the classic cement reaction, in which the active components of cement, Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S), react with water to generate Calcium Silicate (CSH) and Calcium Hydroxide (CH).In the reaction, Calcium Hydroxide from the first reaction reacts with the Silicon Dioxide (S) present in Fly Ash to produce more Calcium Silicate (CSH) [11][12][13][14][15][16][17][18][19][20][21][22].

Materials
Bagasse ash is the prime material used in the study.Sugarcane bagasse ash was procured from NSL Sugars Lmt., Chikkonahalli, Koppa, Maddur, Karnataka.Generally, every ton of sugarcane bagasse ash contains approximately 48% cellulose, 26% of hemicellulose, 26% lignin, and 26% of bagasse [4][5].From the review of the literature, it can be found that up to 30% of replacement can be done [8] [10].The specific gravity and fineness of the Bagasse Ash used for the study are 1.84 and 220 m2/kg respectively and the chemical properties of Bagasse Ash [5] [9] from the review of literature are in Table 1.GGBS, a by-product of iron production mainly consists of melted calcium silicate and alumino silicate removed from the blast furnace on regular basis.The chemical composition of GGBS is shown in Table 2 (as given by the manufacturer) [6] [7].The specific gravity, fineness, and bulk density obtained from the tests carried out in the laboratory are 2.81, 390 m2/kg, and 1120 kg/m3.Lime is the inorganic substance calcium hydroxide (also known as slaked lime) and has the chemical formula Ca(OH)2.When quicklime (calcium oxide) is combined or slaked with water, it produces a colourless crystal or white powder.Hydrated lime is also known as caustic lime, builders' lime, slack lime, and pickling lime.In this study, lime is used as a replacement for binder content along with GGBS varying in the range of 10, 15, and 20% of total binder content in the concrete mix.It is used in the mixes to have a rapid setting property in the presence of an alkaline solution in the BSAASC mix.The aggregates used in this study were tested according to Indian Standard codes.As a coarse aggregate, self-compacting concrete.The coarse aggregate had a specific gravity of 2.67 and a water absorption of 0.5 percent, respectively.For this study, manufactured sand that is readily available is employed, and the sand is evaluated in the laboratory to determine its physical qualities.IS stands for "fineness."IS 383 -1970 fine aggregate modulus.The sand utilized in the experiment belongs to zone II.M sand is free of silt and clay particles, resulting in improved abrasion resistance, increased unit weight, and decreased permeability.Because it decreases sand mining from riverbeds, it is less detrimental to the environment.M Sand's precise grading and cubical shape provide strong strength and long-term durability to concrete.The specific gravity, water absorption, and fineness modulus of the used fine aggregate are 2.65, 10%, and 3.25 respectively.
Alkaline solution for Geopolymer is a combination of sodium silicate and sodium hydroxide.The alkaline solution was procured from Kamath's Rice Mill & Industries, Mangalore, in which sodium silicates contain 32.8% SiO2, 14.7% Na2O, and 52.5% of water by weight (as per the information provided by the distributor).

Optimum mix design of BSAASC mixes
In the present study, the Desirability Function Approach (DFA) method was adopted for the development of BSAASC mixes.DFA is generally used for optimizing the number of experiments to be carried out.Here 3 factors were considered on 3 levels.Percentage of Bagasse ash content, lime content percentage, and dosage of Na2O.an initial set of nine trial mixes were formulated using the Box Behnken method as shown in Table 3 and the performance of these mixes was tested in the laboratory.

Mixture proportioning, preparation, and casting of mixtures
The details of the proportions of concrete mixtures are shown in Table 4. Test specimens were cast and tested, using the initial nine trial mixes, designated as BSAASC-1 to BSAASC-9.The mix design for BSAASC concrete is calculated following the volume batching of EFNARC guidelines.In this method, the total binder content is restricted to 600 kg/m3.The ratio of proportions of fine to coarse aggregates in all the mixes was maintained constant at 60: 40.The molarity of the alkaline solution was kept at 1M throughout.The water-to-binder ratio was 0.60 and was kept constant for all the mixes.

Fresh properties of BSAASC mixes
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 a bottom diameter of 20cm, a top diameter of 10cm, and a height of the cone is 30cm is used.For all the trial mixes flowability is checked to satisfy the EFNARC guidelines of flow 600-800mm.To achieve the flow of the mixes, the water binder ratio is increased from 0.4 to 0.9 till the flow is greater than 650mm.

Compressive strengths of SCAASC mixes
All the BSAASC mixes tested here have shown the targeted strength values on testing as per IS 516:1999 as shown in Table 5 and Fig. 2 shows the variation in compressive strength.It was observed that compressive strength of about 40-50 MPa was obtained as shown in Fig. 2 for 600 Kg/m3 binder content.
It is observed as the percentage of bagasse ash increases, there is a slight decrease in the compressive strength.The highest compressive strength value was observed for BSAASC-1 mix with 10% Bagasse Ash, 10% Lime content, and 6% dosage of Na2O.And BSAASC-7 mix shows the least strength consisting of 10% Bagasse Ash, 15% lime, and 7% dosage of Na2O.
The results of 7-day and 28-day compressive strength obtained for various BSAASC mixes were statistically analyzed using Minitab Software.The main effect plot for compressive strength was generated as shown in Fig. 3.

Split tensile strengths of BSAASC mixes
The split tensile strength tests were performed according to IS 5816 -1999 and the results are shown in Table 7.The values of split-tensile strengths of the various mixes, at different ages, are ranging from 1.7 MPa to 2.6 MPa, as shown in Fig. 4.
The result of the 28-day split tensile strength obtained for various BSAASC mixes was statistically analyzed using Minitab Software.The main effect plot for split tensile strength was generated as shown in Fig. 5.

Flexural strength of BSAASC mixes
The Flexural strength tests were performed according to IS 516 -1959 and the results of both normal strength and analysed strength are shown in Table 9.The values of normal flexural strengths of the various mixes over 28 days are ranging from 2 MPa to 2.7 MPa, as shown in Fig. 6.

Performance of regression equationstests on mixes in the verification Phase
The initially identified nine trial mixes (shown in Table 4) showed satisfactory properties for both fresh and hardened states.The predictive capabilities of these strengths were further tested by casting test specimens using six additional mixes (Verification phase) (denoted BSAASC-10 to BSAASC-15) and testing their strengths.All the details of the mixes and test results obtained are shown in Table 10 and 11 respectively.The predicted strengths, as computed from regression equations (1), ( 2), (3) and ( 4) are also tabulated in Table 11 for all verification BSAASC mixes

Studies on microstructures of BSAASC mixes
Here five BSAASC were chosen for analysing the microstructural details.Among these BSAASC-1, BSAASC-5, BSAASC-8 and BSAASC-9 were selected for high compressive strength and BSAASC-12 for low compressive strength when compared with other BSAASC mixes.
Crushed pieces of failed compression test samples (28 days) were taken for microstructural analysis.
The smaller particles were subjected to Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Analysis (EDX) and finely powdered fractions passing through a 90µm sieve of the representative sample were taken for X-Ray Diffraction (XRD).

Conclusions
• Bagasse Ash-based Self Compacting Alkali Activated Slag Concrete (BSAASC) mixes were effectively developed using Bagasse Ash at varying proportions.The mixes showed satisfying EFNARC guidelines, showing enhanced behaviour in terms of flowability and engineering properties.• The compressive strength of 10% replacement of Bagasse Ash showed good strength when compared with 20% and 30% replacement of Bagasse Ash.• BSAASC mixes developed here show compressive strength values ranging between 40 MPa to 50 MPa without affecting the flowability properties of BSAASC mixes.The split tensile strength and flexural strength also show satisfactory performance with values ranging from 1.7 MPa to 2.6 MPa and 2.0 MPa to 2.7 MPa respectively.• The enhanced compressive strength may be due to the presence of lime and GGBS as binder content leading to the formation of a large amount of C-S-H and C-A-S-H gel.It is evident from the microstructural analysis.

Figure 2 .
Figure 2. Variation of compressive strength for different BSAASC mixes.

Figure 3 .
Figure 3. Variation of compressive strength for different BSAASC mixes after DFA analysis.

Figure 4 .
Figure 4. Variation of Split Tensile Strength for different BSAASC mixes.

Figure 6 .
Figure 6.Variation of Flexural Strength for different BSAASC mixes on 28 days and DFA analysed Flexural Strength.

Fig. 7 to
Fig.11shows the SEM, EDX and XRD analysis of the considered samples.They distinctly show the activation of the slag particles depending on the composition of the mixes.From the EDX images, it is clear that Si and Ca are the peak elements when compared to other elements.

Table 1 .
Chemical properties of bagasse ash.

Table 3 .
Levels and factors of concrete mixes.

Table 4 .
Details of mix designations and mix proportions of concrete mixes.

Table 6
shows the compressive strength of 7 days and 28 days as analyzed by the DFA method.

Table 5 .
Compressive strength results

Table 6 .
Compressive Strength of 7 days and 28 days as analysed by the DFA method

Table 8
shows the compressive strength of 7 days and 28 days as analyzed by the DFA method.

Table 7 .
Split Tensile strength results

Table 8 .
Split Tensile Strength as analysed by the DFA method.
8 Figure 5. Variation of Split Tensile strength for different BSAASC mixes after DFA analysis.

Table 9 .
Flexural Strength for different BSAASC mixes on 28 days and DFA analysed Flexural Strength.