Effect of Sodium Oxide on Physical and Mechanical properties of Fly-Ash based geopolymer composites

Objective: To analyze the effect of variation of Na2O content on the compressive strength and water absorption of geopolymer paste and mortar specimens. Method: Alkali Activator solution was prepared by mixing sodium silicate solution and sodium hydroxide pellets in appropriate proportions 1 day before its use. The prepared solution was mixed with fly ash (FA) for 5 minutes in Hobart Mixer for geopolymer paste preparation and further sand wasmixed to obtain geopolymermortar. These specimens were placed in amold and kept in an oven at 48oC for 48 hours. Compressive strength and water absorption tests were conducted on the specimens afterward. In this study, Na2O with 5%, 6.5% & 8.0% was analyzed for its effect on geopolymer paste and mortar. Findings: This research helped in determining the appropriate quantity of NaOH pellets required for geopolymer composite preparation and reducing the cost of construction, as NaOH pellets are more expensive than sodium silicate solution. Maximum compressive strength was achieved by using 8% Na2O content and water absorption was observed maximum at 6% of Na2O content.Novelty: This study is the first of its kind, which has analyzed the effect of Na2O content variation on water absorption and compressive strength of geopolymer paste and mortar specimens.


Introduction
Fly ash (FA) obtained from biomass and coal combustion in thermal power plants has been extensively used for the preparation of alkali-activated materials and geopolymer (1) . To obtain other binding properties similar to that of the Portland cement (PC), FA is generally mixed with basic alkaline activators such as Na 2 SiO 3 , CaO, and NaOH (2) . These are a class of mineral binders synthesized by activation of the source materials that could be natural or by-products of the https://www.indjst.org/ industries, which has a high content of alumina and silica with an alkali metal hydroxide/ silicate solution at moderate temperatures (3) . These binders have been reported to possess better properties than ordinary Portland cement (OPC) based binders (4) .
Unlike Portland cement hydration, "interstitial or structural water" has been incorporated into the geopolymer gel production, due to which it has caused a reduction in demand of the mixing water (5) . Also, a certain percentage of free water is required for the product to be homogeneous and workable (6) and this water evaporates at 150 • C (7) . Geopolymer concrete production using FA is an alternative to conventional OPC concrete due to high early strength gain, durability, less carbon emission, and less cost involved (8) . With fixed composition (75%wt of wood biomass FA and 25%wt of metakaolin), sodium hydroxide and sodium silicate (1.0 and 1.5 weight ratios), two concentrations of alkali activators, one with and another without moisture and temperature control, five curing methods were applied (9) . Also, micro-pore formation began on the replacement of 100% of POFA in FA based geopolymer when exposed to high temperatures from 300 • C to 500 • C (10) .
It was analyzed that mechanical properties were enhanced with alccofine with a simultaneous reduction in GPC transport properties. Without elevated heat curing, compressive strength of 42 MPa was achieved in maximum. (11) . It was told that to gain good mechanical strength, water content is an important parameter in the synthesis of FA based geopolymer. Reduction in water content during geopolymer synthesis generally results in an increment in the compressive strength. Also, compressive strength is affected due to sodium hydroxide concentration in geopolymer, with increasing molarity of the NaOH, the strength generally increases (12) , however after a certain point, it affects the strength negatively. The molarity corresponding to NaOH concentration of 6.6 M resulted in geopolymer materials with a 38.5 MPa compressive strength. The authors also stated that for their particular mix composition, the optimum concentration of the NaOH is 6.6M and specimens developed greater or lower NaOH concentration than 6.6M resulted in lower compressive strength (13) .
It was further observed that with an increase in NaOH molarity, there is a significant effect on the setting time of geopolymer paste. The decrease in setting times from 66-95 minutes to 37-68 minutes with an increase in FA content from 45% to 75% along with an increase in NaOH molarity from 8M to 12M was found. It was also found that there was a rapid increase in the compressive strength of specimens in the first 24 hours, afterwards, there was a sharp decline in the rate of increase as concluded by the author (14) .
Relevant studies have also found that initial SiO 2 and Al 2 O 3 content have effects on the stability of the amorphous geopolymer phase in the long term concerning crystallization. A gradual transformation from the amorphous phase(s) to crystalline phase(s) in some mixtures was observed when subjected to prolonged curing. In particular, molar ratios of Al 2 O 3 / SiO 2 if not equals to 3.8 then an increase in Na 2 O content supports the transformation (15) .

The objective of the study
1. Study of physical and mechanical properties of manufactured geopolymer composites, which are FA based with varying composition of sodium oxide in a hardened state.
To accomplish the objective, the experimental work consists of testing a total of six geopolymer mortar and paste specimens prepared by varying alkali content (%Na 2 O) in unexposed condition. Compressive strength was determined to study the behavior of FA-based geopolymers. Also, simple yet important tests like bulk density, water absorption were conducted.

Materials used
Fly Ash: Class F FA (16) of low calcium concentration was procured from the Thermal Power Plant of Kolaghat, West Bengal, India, and used as the solid aluminosilicate source material for manufacturing FA-based geopolymer concrete. Before the commencement of the research work, a required quantity of FA was brought in bulk packed in bags from the thermal power plant. The FA was then remixed thoroughly to ensure homogeneity and replaced in sealed plastic containers until its use for the manufacture of a geopolymer. The specific gravity of FA was found to be 2.73. The particle size distribution in percentage by volume passing size is shown in Figure 1. Nearly 75% of the particles of the FA were smaller than 45 micrometers. The specific surface area determined by Blaine's method was 380m2/kg.

Fig 1. Particle size distribution
Fine Aggregate: Fine aggregate such as well graded river sand which passed through 4.75mm IS sieve was used. The sand was washed for removal of any dirt associated with it, air-dried, sieved, and used in saturated surface dry conditions for making geopolymer mortar specimens. For mortar, locally available sand was used from the nearby river. The fineness modulus and a specific gravity of the sand were 2.50 and 2.65 respectively. Sieve analysis results are shown in Figure 3. Selected sand falls in Zone-II due to its particle size distribution, as per IS: 383-2016 classifications (17) . Figure 2 depicts the percentage of fine aggregate passing through IS sieves of different sizes. Alkali activator: A mixture of Na 2 SiO 3 solution, NaOH solids, and water was used as an alkaline activating solution. NaOH solids used were of laboratory grade in the forms of pellets with 98% purity and a specific gravity of 2.15. Sodium silicate (Na 2 SiO 3 ) solution had a chemical composition of Na 2 O=8%; water=65.5%; SiO 2 =26.5% and bulk density of 1410 kg/m 3 . Chemicals were procured from Loba Chemie Ltd, India. To make sodium hydroxide solution of desired Na 2 O content, required quantities of water and sodium hydroxide pellets were mixed in the initial stage. Afterward, it was mixed with an appropriate quantity of Na 2 SiO 3 solution for the desired content of SiO 2 . The mixture of activator solution finally obtained was left for 1 day at ambient temperature before its use in geopolymer mix preparation.
Ground Granulated Blast Furnace Slag (GGBFS): It is a by-product obtained from the cooling of molten iron found in a blast furnace within steam or water, which is afterward dried and grounded into a fine powder and thus finally used in practical applications. It provides excess heat to mix the alkali activator and FA (18) . It also increases the workability and gives a good surface finish. The main constituents present in the blast furnace slag are SiO 2 , CaO, MgO, Al 2 O 3 , and their concentrations are mentioned in Table 1 . It has been found that these compounds are common in most of the cementation substances. Water: For geopolymer preparation, drinkable water is found to be suitable. Hence, potable water has been used in this study to cure and mix.

Preparation of geopolymer composites
Specimens prepared by mixing only FA with alkali activator solution are called geopolymerpaste. The mixture is usually a homogeneous slurry and dark grey. When fine aggregate, i.e. sand is added to the mixture, the resulting product is termed as geopolymer mortar. Both geopolymer paste and mortar show a cohesive nature in the fresh state. Table 2 shows the materials required and their composition. The above constituents were used in the manufacture of geopolymer composites, the mixing, and the curing procedure followed after Thakur and Ghosh (19) .
The following steps describe the manufacturing processes of geopolymer composites.
1. Preparation of alkaline activator solution by mixing sodium hydroxide pellets, sodium silicate solution, and water in the required quantities to obtain desired alkali and silica content along with predetermined water to FA ratio. It should be prepared at least one day before its use for the manufacturing of geopolymer paste/mortars. 2. In a Hobart mixer, an activating solution and FA is mixed for 5 minutes to obtain a homogeneous slurry.
3. Introduction of fine aggregate (sand) gradually into the slurry and further followed by mixing in the Hobart mixer for another 5 minutes to get geopolymer mortar. 4. Transferring the geopolymer mix (paste or mortar) into steel molds of 50×50×50 mm cubes and vibrating it for 2-3 minutes to remove any entrapped air from the vibrating table. 5. Rest period of 60 minutes before heat curing. 6. Replacing the filled molds into an oven for heat curing at a temperature of 85 • C for 48 hours. 7. Storing the geopolymer specimens at a dry place at ambient temperature until testing is to be done. 8. Various tests such as compressive testing, water absorption, etc.
The ratio of 0.32 of water to FA was maintained for the preparation of the paste and 0.33 for the mortar specimens. It was found to give sufficient workability after several trial mixes. Mortar and paste having Na 2 O content varying from 5 -8% were prepared as shown in Table 3 .

Study on Physical and Mechanical properties
The first part of the experimental investigation was to manufacture FA-based geopolymer composites of different compositions varying in alkali content (%Na 2 O)of the activator solution. Specimens were prepared by activating dry FA with activator solutions, which were a mixture of NaOH solids, Na 2 SiO 3 solutions, and water. For studying the effect of alkali content on the physical and mechanical properties of resulting geopolymer composites, Na 2 Ocontent in the activator solution varied from 5% to 8.5% of the weight of FA. Variations in Na 2 Ocontent were controlled by mixing required quantities of NaOH solids, water, and Na 2 SiO 3 solutions to make activator solutions. After 28 days from the manufacture, physical and mechanical properties such as water absorption and compressive tests were studied and inter-related.

Compressive strength
Relevant details related to the determination of the compressive strength of geopolymer paste and mortar are shown in Table 4.  (15) It was measured first by completely drying the specimen to a constant mass, after that submerging the specimen in water, and then measuring the increase in weight as a percentage of dry mass. The procedure followed for the determination of water absorption of geopolymer specimens was following ASTM C-642 (20) . 28 days old specimens were dried at 105 o C for 24 hours and immersed in water after measuring its weights. The specimens were taken out from submergence and surface water was wiped out after this it was weighed immediately in saturated surface dry (SSD) condition to find out the increment in the weight of the composite.   Figure 3. However, as the Na 2 O content is further increased up to 8%, an increase in compressive strength is found to be very little. P3 and M3 having 8% Na 2 O have a compressive strength of 35.03 MPa and 40.63 MPa, which are only 12.52 % and 7.37% higher than the strength measured for paste and mortar specimens of 6.5% Na 2 O. There is an increment in the compressive strength with the increment in sodium oxidecontent which is due to the higher reactivity of FA in a stronger alkaline solution. It can be noted here that a highly alkaline solution is needed for complete polymerization. Hence, with a higher content of alkali in the activator solution, better dissolution of the FA takes place thereby forming more geopolymer gel accompanied by lesser pores, which result in higher compressive strength. But, the rate of increase in compressive strength is significantly higher in the lower ranges of Na 2 O content also. Therefore, when Na 2 O is increased from 5% to 6.5%, a remarkable rise in compressive strength has been observed. However, the change in strength increase is not significant when specimens having 6.5% Na 2 O content is compared to specimens of 8% Na 2 O content. This indicates that most of the dissolution of FA is possible even at 6.5% Na 2 O which causes a highly improved microstructure than the specimen having 5% Na 2 O. While improvements in the microstructure are noticed after increasing Na 2 Ocontent from 6.5% to 8% and it was not significant. This is the reason for little improvement in compressive strength.

Effect of Na 2 O content on Water absorption
Test Conducted on 28 days, the effect of Na 2 O content on water absorption for geopolymer paste and mortar specimens is shown in Figure 4 . It can be noticed that water absorption goes on decreasing with increasing Na 2 O content from 5% to 8%. For specimens P1 and M1, both having 5% Na 2 O, water absorption values are found to be 22.85% and 10.97% respectively, which shows a remarkably high water absorption in paste specimens. P2 and M2 with higher Na 2 O content of 6.5% yielded water absorption of 20.28% and 9.57%, respectively. Further, an increase in the Na 2 O content up to 8% (P3 and M3) resulted in a further decrease in water absorption to 15.13% and 6.24%. In the case of paste specimens, water absorption was found relatively higher than the mortar counterparts for the same Na 2 O content. It may be noted here that the porosity has been observed higher with lesser Na 2 O content and this has caused greater water absorption in these specimens. Details of water absorption variation of geopolymer composite due to sodium oxide variation are shown in Figure 4.