Effect of Curing Conditions on Properties of Fly Ash-Based Geopolymer Bricks
Submitted By masiweyy
The Effect of Curing Time on The Properties of Fly Ash-based Geopolymer Bricks
W. I. Wan Mastura1,a, H. Kamarudin1,b, I. Khairul Nizar2,c, A.M. Mustafa Al Bakri 1,d, H. Mohammed3,e
1 Center of Excellence Geopolymer and Green Technology, School of Material Engineering, Universiti Malaysia Perlis (UniMAP), P. O. Box 77, d/a Pejabat, Pos Besar, 01007 Kangar, Perlis, Malaysia.
2School of Environmental Engineering, Universiti Malaysia Perlis (UniMAP), P. O. Box 77, d/a Pejabat, Pos Besar, 01007 Kangar, Perlis, Malaysia.
3 King Abdul Aziz City Science & Technology (KACST), P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
Keywords: Geopolymer, Fly ash, Bricks, Geopolymerization, Compressive strength
Abstract. This paper reports the results of an experimental work conducted to investigate the effect of curing conditions on the properties of fly ash-based geopolymer bricks prepared by using fly ash as base material and combination of sodium hydroxide and sodium silicate as alkaline activator. The experiments were conducted by varying the curing time in the range of 1-24 hours respectively. The specimens cured for a period of 24 hours have presented the highest compressive strength for all ratio of fly ash to sand. For increasing curing time improve compressive strength and decreasing water absorption.
A geopolymer, first defined by Davidovits, should more appropriately be referred to as a type of ‘‘inorganic polymer’’. More specifically, a geopolymer is a three dimensional aluminosilicate mineral polymer that contains a variety of amorphous to semicrystalline phases . Geopolymer products have the potential to provide many environmental advantages such as significantly reduce greenhouse gas emission, use large volumes of industrial (waste) by-products, increase resource efficiency by producing products with longer services lives . The use of wastes in various segments of the brick through geopolymerization process is increasing continuously [3, 4, 5]. Fly ash is ash separated from the flue gas of coal-fired power plant which contains sufficient amounts of reactive alumina and silica that can be used as source materials for geopolymer . The use of fly ash in the geopolymerization will contribute to the reduction of CO2 gas emission, and at the same time complies with the more complicated demands of the construction industry in terms of characteristics and the quality of the construction materials . Curing of freshly prepared fly ash geopolymer bricks is the most crucial aspect and plays an important role in the geopolymerisation process. The process of curing and hardening is different for geopolymer products than for Portland cement products. Geopolymers tend to give higher compressive strength when cured at elevated temperatures, ranging from room temperatures to nearly 100 °C depending on the source materials . Previous researches have shown that both curing time and curing temperature significantly influence the compressive strength of fly ash-based geopolymer products [9, 10, 11]. The objective of this research is to investigate the effect of different curing time with different ratio of fly ash to sand on properties of fly ash geopolymer bricks.
In the present study, class C  Fly ash was used as a source material for the synthesis of fly ash geopolymer bricks. Fly ash was obtained from Manjung Power Station, Lumut, Perak, Malaysia. The chemical composition of Fly ash as determined by X-Ray Fluorescence (XRF) analysis is shown in Table I. Natural Malaysian sand was used as fine aggregates. Fine aggregates was sieved for the size less than 4.75mm and used in air dry condition. For the alkaline activator, a combination of sodium hydroxide and sodium silicate solution was used. Sodium hydroxide in a form of pellets with 97% purity and Sodium silicate solution was supplied by South Pacific Chemicals Industries Sdn. Bhd. (SPCI), Malaysia with the chemical compositions SiO2 30.1%, Na2O 9.4%, and H2O 60.5% has been used in this study.
Table 1: Chemical composition of fly ash
|Chemical composition |Percentage (%) |
|SiO2 |26.4 |
|Al2O3 |9.25 |
|Fe2O3 |30.13 |
|CaO |21.6 |
|P2O5 |0.67 |
|SO3 |1.3 |
|K2O |2.58 |
|TiO2 |3.07 |
First, the sodium hydroxide (NaOH) solution was prepared by adding sodium hydroxide pellet to distilled water in volumetric flask. Due to the generated heat, enough time was allowed for the solution to cool down to room temperature before it was used. In this research, the concentration of NaOH solution used is 12M due to the study done by Mustafa et al.  where they have found that this concentration give the highest in compressive strength. Then, the alkaline activator was prepared by mixing the NaOH solution 12M concentration with sodium silicate solution (Na2SiO3).
Design of Mix Proportions
In this experimental work, several mix proportions of fly ash-based geopolymer bricks were designed to study the effect of curing time on the properties of fly ash-based geopolymer bricks. Four levels of fly ash: sand ratio i.e. 1:2, 1:3, 1:4, 1:5 was calculated by mass and three ranges of curing time i.e. 1, 6 and 24 hours were used. The ratio of sodium silicate (Na2SiO3) to sodium hydroxide (NaOH) was kept 2.5  whereas the ratio of fly ash to alkaline activator used was 2:1. This composition has been designed on the basis of literature data and consideration of better workability which expected to give hardened products of good properties.
The materials (fly ash, sand, sodium hydroxide solution and sodium silicate) were weighed accordingly. Before mixing it, the sand should be sieved to remove foreign constituents. This procedure is important to make sure that the sand has the same size which is less than 4.75mm and to avoid any constituents disturbing the geopolymerization process. Then the materials were put into the mixer followed the sequence and mix together for about 25 minutes. The sequence is important as different materials have different properties. The source of material which is fly ash and sand need to be mixed first for 10 minutes before the alkaline activator is added to the mixes. After the alkaline activator was added, the mixture was mixed for 15 minutes or until integrated all mixture. Afterword, the mixes were weighed approximately 2.5kg for each sample of brick, then poured into the mould and compressed to get the compact sample bricks. Table 2 shows the size and dimension of bricks according to the British Standard BS 3921:1985.
Table 2: Size of bricks
|Coordinating size (mm) | | |Work size (mm) | | |
|Length |Width |Height |Length |Width |Height |
|225 |112.5 |75 |215 |102.5 |65 |
Curing and Testing
After assessing the necessary size of bricks, the molded samples were cured in the oven for 1, 6 and 24 hours. The temperature used during curing period is 60°C . At the end of the curing period, the specimens were taken out from the oven and left undisturbed in room temperature for about 20 minutes before testing to cool down the samples. Compressive strength is one of the most common measures used to evaluate the quality of hardened bricks. Compressive strength test for brick was carried out according to ASTM C67-11 by using Hydraulic Compression Testing Machine VU-2000. The reported compressive strength values were an average of the results obtained for the five samples produced for each ratio and different curing time. Water absorption is very important to determine the amount of water absorbed by the bricks and the rate of absorption to investigate their resistance to the penetration of rain. Water absorption test was conducted according to ASTM C140-07a. To determine the water absorption of brick, three specimens for each ratio were oven dried at a temperature of 60°C for 1, 6 and 24 hours. The samples were then immersed in water for 24 hours and its saturated surface dry weight was recorded as the saturated weight (Ws). Next, the samples were dried in oven at 100°C and the oven-dry weight was recorded as the dry weight (Wd). Water absorption of specimens is reported as the percentage. Calculation of water absorbed for each samples were expressed by using the following equation:
Water absorption (%) = [(Ws - Wd)/Wd] x 100%.
Results and Discussion
Fig. 1 shows the influence of curing time on the compressive strength of fly ash-based geopolymer bricks. The test specimens were cured in the oven at a temperature of 60°C. The curing time varied from 1 hour to 24 hours. By referring to the Fig. 1, it shows that the compressive strength increase with increasing curing time for each ratio of fly ash to sand. It is believed that longer curing time improved the geopolymerisation process resulting in higher compressive strength. Compressive strength results shown in Fig. 1 indicate that increase in the ratio of fly ash to sand from 1:2 to 1:5 decreased the compressive strength of the fly ash geopolymer bricks. This is due to the decreased amount of fly ash and less content of alkaline activator in the bricks for the complete geopolymerization reaction; consequently decreased the compressive strength of the fly ash geopolymer bricks. The result obtained shows that 1:2 ratio of fly ash to sand gives the higher compressive strength for each curing time. However, the structure of the 1:2 ratio is not in good shape and do not have good workability compared to 1:3 ratio. It is because, the 1:2 ratio of fly ash to sand have more alkaline activator content compared to others.
Fig. 1: Effect of Curing Time on Compressive Strength
The results of the water absorption for each ratio with different curing time are presented in Table 3. From the results obtained, it clearly shows that ratio 1:2 fly ash: sand has the lowest water absorption while ratio 1:5 fly ash: sand has the highest percentage of water absorption. A decrease in water absorption is accompanied by a corresponding increase in strength, means the specimen durable. This may be attributed to the fact that higher alkali content which is contained in the alkaline activator in the mix gives better reactivity with the fly ash resulting in denser microstructure, hence, makes the water more difficult to penetrate the bricks. For increasing curing time improve compressive strength and decreasing water absorption due to complete geopolymerization process.
Table 3: Water Absorption of fly ash geopolymer bricks for different curing time
|Ratio of fly ash/sand |Curing time (hours) |Water absorption (%) |
| |1 |9.5 |
|1:2 |6 |6.27 |
| |24 |5.36 |
| |1 |10.9 |
|1:3 |6 |7.32 |
| |24 |6.5 |
| |1 |10.7 |
|1:4 |6 |8.6 |
| |24 |7.9 |
| |1 |11.4 |
|1:5 |6 |10.1 |
| |24 |9.6 |
In this study, the effect of curing time on the compressive strength of fly ash geopolymer bricks was investigated. Test results indicate that curing time significantly affects the compressive strength of hardened bricks. Based on the data obtained, it can be concluded that longer curing time improves the geopolymerisation process resulting in higher compressive strength. The compressive strength was highest when the specimens were cured for a period of 24 hours for all ratio of fly ash to sand. In addition, from investigation the effect of curing time on th properties of fly ash-based geopolymer bricks, it was found the increasing curing time and ratio fly ash: sand increasing the water absorption of brick. Better bricks using for construction should be less water absorption. The fly ash-based geopolymer bricks produced in this study seem to be suitable and environment friendly for use as construction material by choosing appropriate ratio fly ash: sand.
This study was supported by Center of Excellence Geopolymer and Green Technology UniMAP and School of Material Engineering, UniMAP. The authors of this research are very grateful to King Abdul Aziz City Science and Technology (KACST) for the financial support of this study.
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