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Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures
Construction and Building Materials xxx (2016) xxx–xxx
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Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures Khoa Tan Nguyen a, Tuan Anh Le b, Jaehong Lee a, Dongkyu Lee a, Kihak Lee a,⇑ a b
Sejong University, Department of Architectural Engineering, Seoul 143-747, South Korea Vietnam National University of Ho Chi Minh City, Faculty of Civil Engineering, Viet Nam
h i g h l i g h t s Proposing two new alternative curing method are using preheated materials and thermogentic admixtures. For using preheated materials, a larger amount of preheated materials results in a higher compressive strength. For using thermogentic admixtures, quicklime is more efficient than hot pack material. The suggested amount of quicklime is about 3–5% of fly ash by mass.
a r t i c l e
i n f o
Article history: Received 2 May 2016 Received in revised form 27 October 2016 Accepted 27 October 2016 Available online xxxx Keywords: Geopolymer mortar Self-cured geopolymer Preheated Quicklime Hot pack Fly ash
a b s t r a c t Heat curing in an oven is the traditional method to obtain mechanical properties of geopolymers. This characteristic has significantly affected operations in terms of construction and energy consumption. The target of this paper is proposing alternative curing methods namely self-cured technologies for fly ash based geopolymer materials that are cured in ambient conditions without use of a heat resource. Two alternative methods are to use three different mixing processes with preheated materials and to use two thermogenetic admixtures (hot pack material and quicklime). The results show that for the mixing processes, a larger amount of provided heat energy results in a higher compressive strength of the geopolymer mortar (GM). For the use of a thermogenetic admixture, the results from compressive strength testing, Scanning Electron Microscope (SEM), energy dispersive X-ray analysis (EDX) micrographs and X-ray diffraction (XRD) analysis confirm that quicklime is more efficient than hot pack material. In the case of using quicklime, the suggested amount is about 3–5% of fly ash by mass. Ó 2016 Published by Elsevier Ltd.
1. Introduction The impact of Portland cement production on the environment will issue a significant challenge to concrete industry in the future. The Portland cement manufacturing process depletes natural resources and emits a huge amount of greenhouse gases [1–3]. New materials have been researched to totally or partially replace traditional Portland cement [4–6]. Among these new materials, the most potential material is geopolymeric material. The production
Abbreviations: GM, geopolymer mortar; SEM, Scanning Electron Microscope; EDX, energy dispersive X-ray analysis; XRD, X-ray diffraction; CO2, carbon dioxide; Na2SiO3, sodium silicate; NaOH, sodium hydroxide; KOH, potassium hydroxide; FA, fine aggregate; CaO, calcium oxide; CaCO3, calcium carbonate; NaCl, sodium chloride. ⇑ Corresponding author. E-mail address: [email protected] (K. Lee).
of alumina-silicate based geopolymer cement is usually a combination of an alkaline solution, like water glass or sodium silicate (Na2SiO3), sodium or potassium hydroxide (NaOH or KOH) and source material such as fly ash, rice husk ash or granulated blast furnace slag to form homogenous slurry [7–9]. The heat used for curing process is above the ambient temperature and is in a range from approximately 60–100 °C for 24–48 h. Subsequently, the geopolymer will be fully cured or left in room temperature for further handling [10]. Previous studies have determined that the properties of geopolymer binders, tested against the standard of Portland cement, are identical to or even better than those of materials made from traditional cement. Furthermore, geopolymers utilize solid industrial waste material and their production can be accomplished without an energy intensive process like that necessary for the production of Portland cement. Replacing traditional cement with a geopolymer binder can yield environmental benefits such
http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110 0950-0618/Ó 2016 Published by Elsevier Ltd.
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as reduction in consumption of natural resources and decrease in the net production of carbon dioxide (CO2) [11]. Nevertheless, the requirement of heat curing for geopolymer materials has significantly affected the operations of construction and energy consumption. The common fly ash-based geopolymer paste cannot set within 24 h at ambient temperature [12], however, the presence of calcium source could significantly shorten the setting time of those common geopolymer paste. An increase of calcium content in geopolymer cement can be achieved by adding granulated blast furnace slag, high calcium fly ash, calcium hydroxide or even Portland cement [13–16]. Based on these facts, this study concentrates on the curing processes and properties of geopolymer mortar (GP) using thermogenetic admixtures (quicklime and hot pack material) and different mixing processes. Also, the optimum percentages for the materials in the thermogenetic admixture is determined. Afterwards, instructions or recommendations for fabrication of geopolymer mortar without curing in the oven or self-cured geopolymer mortar are proposed. 2. Materials The materials used for making fly ash based geopolymer mortar were low-calcium dry fly ash as source material, fine aggregate (FA), alkaline liquids and water. The fly ash used in this study, known as a ‘Class F’ material based on ASTM 618 [22], with specific gravity 2500 kg/m3, came from a power station. The chemical compositions of the fly ash are given in Table 1. Sand was used as fine aggregate. The specific gravity and fineness modulus of sand were 2.6 and 2.1 respectively. The fine aggregate to fly ash ratio used in the test was 7:3 by mass. The alkaline liquid was a combination of sodium silicates (Na2SiO3) and sodium hydroxide (NaOH). The components of the sodium silicates solution were Na2O and SiO2 (approximately 36–38% by mass). The ratio of SiO2-to-Na2O by mass was approximately 2. The concentration of sodium hydroxide solution was 14 M (M). Water glass and sodium hydroxide were mixed at a ratio of 1:1 by mass. The ratio of alkali solutions (including water glass and sodium hydroxide) to fly ash was 0.35. Details of the mix proportions used in this study are provided in Table 2. In this table, the name of mixture is started with ‘‘G” which stands for geopolymer. The following characters represent for the properties of mixture. 3. Experimental procedures To investigate the properties of geopolymer mortar, experimental tests were conducted with different curing methods: natural curing (at room temperature 28 ± 2 °C), curing using an oven, natural curing using three mixing processes and natural curing using thermogenetic admixtures. Geopolymer mortar cylinders of 60 mm diameter and 120 mm height were used for the compressive strength test. Details of the mixing processes and of the thermogenetic admixtures are explained in the following section. 3.1. Mixing processes with preheated materials In this study, three kinds of mixing processes were used to manufacturing GP, namely, processes 1, 2 and 3, as follows: process 1: fine aggregate was firstly heated at 80 °C, 100 °C and 120 °C for 4 h and then mixed with fly ash for about 30 s. After that the combina-
tion of sodium silicate and sodium hydroxide were added to the solid and the resulting materials were mixed for 60 s. Process 2: Fly ash was heated in the same conditions as were used for the FA in process 1; then, alkali solution (sodium silicate and sodium hydroxide) was poured into the solid and resulting materials were mixed for 60 s. Process 3: the procedure was the same processes 1 and 2 except that the mixtures that included FA and fly ash were heated at 80, 100 and 120 °C for 4 h. A flowchart of the testing procedures is provided in Fig. 1. The mixtures GPHS80, GPHS100 and GPHS120 are designed for Process 1; GPHF80, GPHF100 and GPHF120 are for Process 2; GPHM80, GPHM100 and GPHM120 are for Process 3. 3.2. Thermogenetic admixtures Based on the properties of geopolymerization process, which requires heat, this research uses two admixtures to provide heat. Two thermogenetic admixtures that we considered in this study are quicklime and hot pack material. Details of the two admixtures are summarized below. 3.2.1. Quicklime Quicklime is the common name of calcium oxide (CaO); it is a white powder as can be seen in Fig. 2. Calcium oxide is usually produced in a lime kiln by thermal decomposition of materials such as limestone, or seashells that contain calcium carbonate (CaCO3). This is accomplished by heating the material to above 825 °C, a process called calcination, to liberate a molecule of carbon dioxide (CO2); leaving quicklime as shown in Eq. (1) [17].
CaCO3 ! CaO þ CO2
ð1Þ
Quicklime releases heat energy by the formation of the hydrate, calcium hydroxide, according to the following equation (Eq. (2)):
CaO þ H2 O CaðOHÞ2 þ DHr
ð2Þ
where DHr = 63.7 kJ/mol of CaO. When hydration occurs, the heat energy is released; this energy will be useful for the geopolymerization process. In this study, quicklime is used at percentages in the range of 2– 10% and of 15% of fly ash by mass. The percentages 5%, 10% and 15% were used in comparison experiment with identical amounts of hot pack material. The remaining percentages were used to determine the optimum amount of quicklime that can be used to impart the best compressive strength. The properties of the quicklime used for this research are given in Table 3. 3.2.2. Hot pack material (hand warmer) Hot packs or hand warmers, as shown in Fig. 3 are small (mostly disposable) packets that are held in the hand and produce heat on demand to warm cold hands. They are commonly used in outdoor activities such as hiking and skiing to keep extremities warm and to supplement the person’s insulated clothing. Depending on the type and source of heat, hot packs can last from 30 min up to 24 h [18]. The hot packs contain iron (Fe) powder, cellulose, sodium chloride (NaCl), vermiculite, activated charcoal and moisture. Most hot packs work base on a simple chemical reaction similar to that of rusting; the reaction occurs when the packs are exposed to air. When the iron in the pack is exposed to oxygen in the air, it
Table 1 Chemical composition of fly ash. Oxide
SiO2
Al2O3
Fe2O3
CaO
K2O & Na2O
MgO
SO3
Loss On Ignition (LOI)
(%)
51.7
31.9
3.48
1.21
1.02
0.81
0.25
9.63
Please cite this article in press as: K.T. Nguyen et al., Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110
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K.T. Nguyen et al. / Construction and Building Materials xxx (2016) xxx–xxx Table 2 Mixture Proportions of Experimental Geopolymer Paste (GP). Name
FA (g)
Fly ash (g)
Sodium silicate solution (g)
Sodium hydroxide solution 14 M (g)
Hot pack (g)
Quicklime (g)
Description of mixtures
GP0 GP40 GP60 GP80 GPHS80 GPHS100 GPHS120 GPHF80 GPHF100
445.9
191.1
33.4
33.4
– – – – – – – – –
– – – – – – – – –
GPHF120
–
–
GPHM80
–
–
GPHM100
–
–
GPHM120
–
–
GAHP5 GAHP10 GAHP15 GAC2 GAC3 GAC4 GAC5 GAC6 GAC7 GAC8 GAC9 GAC10
9.55 19.11 28.65 – – – – – – – – –
– – – 3.82 5.73 7.64 9.55 11.47 13.38 15.29 17.2 19.11
GAC15
–
28.65
Natural curing Cured at 40 °C, oven Cured at 60 °C, oven Cured at 80 °C, oven 80 °C sand, natural curing 100 °C sand, natural curing 120 °C sand, natural curing 80 °C fly ash, natural curing 100 °C fly ash, natural curing 120 °C fly ash, natural curing 80 °C sand + fly ash, natural curing 100 °C sand + fly ash, natural curing 120 °C sand + fly ash, natural curing 5% hot pack, natural curing 10% hot pack, natural curing 15% hot pack, natural curing 2% quicklime, natural curing 3% quicklime, natural curing 4% quicklime, natural curing 5% quicklime, natural curing 6% quicklime, natural curing 7% quicklime, natural curing 8% quicklime, natural curing 9% quicklime, natural curing 10% quicklime, natural curing 15% quicklime, natural curing
Water-to-solid ratio (w/s)
0.17
Fig. 1. Flowchart of testing procedure.
oxidizes as in the reaction given in Eq. (3). During that process, heat is created. The salt (NaCl) acts as a catalyst and the charcoal help spread the heat throughout the pack. The vermiculite acts as an insulator, keeping the heat from dissipating too rapidly [18,19]. The chemical reaction occurs slowly and releases heat. This heat energy can be used in the geopolymerization process.
2Fe þ 3O2 ! Fe2 O3 þ DH
ð3Þ
where DH: the heat energy released. The amounts of hot pack material used for the test were 5%, 10% and 15% of fly ash by mass. In the mixing step, the hot packs were shaken for 5 min before mixing with the base materials fly ash and sand. This means that the hot packs were activated and ready to provide heat energy for the geopolymerization process.
3.3. Testing methods To evaluate the properties of geopolymer mortar, a total of four kinds of analyses such: a compressive strength test, Scanning Electron Microscope (SEM) test, energy dispersive X-ray analysis (EDX) technique and X-ray diffraction (XRD) analysis were used. The compressive strength test applies a compressive axial load to the molded cylinder at rates ranging from 0.15 to 0.35 MPa/s, until failure occurs. The compressive strength of the specimens is determined by dividing the maximum load attained during the test, by the cross-sectional area of the specimen [20]. The SEM photo was used to define the microstructure, particles and formation of specimens. The EDX technique was used to observe the chemical compositions of the materials. On the other hand, the XRD analysis was used to analyze and explain the results of the experimental tests. The temperature of specimens were recorded by using pocket digital thermometer. The range of this thermometer is from
Please cite this article in press as: K.T. Nguyen et al., Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110
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Fig. 2. Quicklime or calcium oxide powder.
Fig. 4. Compressive strength of geopolymer paste according to different curing conditions.
Table 3 Properties of quicklime.
50 to 260 °C, and the division is 0.1 °C. The top of specimen was chosen as a position for measuring the temperature. The data was recorded every 5 m until the temperature of specimens was equal to room temperature. Then, the result is reported in degree Celsius (°C).
the effects of curing temperature on the compressive strength for GP0, GP40, GP60 and GP80 after curing of the specimens. For mixture GP0 cured under ambient condition, the compressive strength increases according to time. At 9 d after the start of the test, the compressive strength of the mortar reached 13.42 MPa, which is about 4.8 times the value obtained at 3 d after the start of the test. The compressive strength keeps increasing and achieves a value of 22.59 MPa at 28 d and 29.15 MPa at 60 d. For GP40, GP60 and GP80 cured in the oven, higher curing temperature resulted in larger compressive strength of the geopolymer mortar. For example, the highest compressive strength is 28.81 MPa for GP80; the lowest value is 25.43 MPa for GP40. However, the specimens of the three mixtures GP40, GP60, and GP80 do not increase the strength in the same way as GP0. In general, the compressive strength of a geopolymer mortar cured under ambient conditions increases according to time while the compressive strength of geopolymer mortar cured in an oven achieves its highest value after curing. In the geopolymerization process, the heat energy helps molecules to diffuse, transport, orient or condense into monomers. This energy also impulses the reaction kinetics. If the curing temperature is high, this means that the heat energy is high and the chemical reaction happens quickly. As a result, the polymer will be produced in great quantities. These polymers connect together to become a long chain. This chain imparts compressive strength to the GP [21,23]. Thus, the specimens can reach a high value of compressive strength early by heating to the high curing temperature. For curing under ambient conditions, geopolymerization happens slowly; it takes about 60 d to achieve the same compressive strength as that achieved for the specimen that was cured in an oven. In order to obtain a high compressive strength geopolymer mortar, high curing temperature using an oven must be employed as prior condition. However, using an oven for curing of a specimen is not always the most advantageous way. Thus, other curing methods must be found out and applied to geopolymer materials.
4. Results and discussion
4.2. Curing process of GP with different pre-heated materials
4.1. Influence of curing temperature
Fig. 5 shows the effect of three mixing processes on the compressive strength of the geopolymer mortar. Fig. 5a shows the test data for Process 1 – which FA was preheated, Fig. 5b is for Process 2 – which fly ash was preheated and Fig. 5c is for Process 3 – which both FA and fly ash were preheated. For each process, the materials
Items
Property
Free CaO Total CaO Slaking time Slaking temperature
>75% >90% 8–15 min 85–95 °C
Fig. 3. Hot pack materials or hand warmers.
In this section, mixture GP0 was cured under ambient conditions (28 ± 2 °C) and mixture GP40, GP60, and GP80 were cured in the oven at 40, 60 and 80 °C, respectively for 8 h. Fig. 4 shows
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K.T. Nguyen et al. / Construction and Building Materials xxx (2016) xxx–xxx
(a) Process 1 – Preheated FA + Fly Ash
(b) Process 2 – FA + Preheated Fly Ash
(c) Process 3 – Preheated (FA + Fly Ash)
5
Process 3 is performed by mixing a combination of sand and fly ash with the alkali solution. After a close comparison, it was found that with the same temperature provided for materials, Process 3 produced much more heat energy for the geopolymerization process than did either Process 1 or Process 2 because the amount of heated materials in Process 3 was larger than those in Processes 1 and 2. Based on the properties of the geopolymerization process, a lot of polymer can be produced when the heat energy is sufficiently large. In the studied case, Process 3 had the highest heat energy because of the quantity of heated materials inside the mixture (according to Table 2: 637 g preheated materials for Process 3, 445.9 g for Process 2 and 191.9 g for Process 2). Following this rule, the heat energy of Process 1 is in the middle and Process 2 produces the lowest level of heat. As a result, the compressive strength values of the specimens in Process 3 are the highest among the three mixing processes. The temperature curve shown in Fig. 6, which indicates the heat evolution during these processes, can be used to confirm the results for compressive strength. Time 0 is the point of finishing the mixing step and starting the curing step. At point 0, Process 3 reached the highest temperature among the three processes. The temperature then steadily decreased to room temperature in 3 h. The heat from the preheated materials spread out to the other materials inside the geopolymer mortar. This heat also provided energy for the geopolymerization process. Because it had the highest amount of heat, Process 3 unavoidably had the largest compressive strength. The higher preheat temperature of this material resulted in a larger compressive strength. After a detailed comparison, it was found that the values of compressive strength of the geopolymer mortars in the three processes are identical to or higher than that value of mixture GP0. However, from 7 d until 28 d, there are large differences among the three processes and GP0. For example, at 28 d, all specimens of the three processes had higher values of compressive strength than did the specimens of GP0. The reason for this can be easily explained: during the first several hours of curing time, the heat from preheated materials (fly ash and sand) provided energy for geopolymerization process. This energy make the chemical reaction happen quickly. During this time, the specimens of the three processes also had temperatures higher than room temperature. Thus, the polymers are created in great quantities. These polymers creates strength for the geopolymer mortar. Obviously, the compressive strength values during the three processes are higher than the value of the specimens cured at room temperature. In conclusion, in order to use preheated materials to produce self-cured geopolymer mortar, a higher temperature of the preheated materials resulted in a higher compressive strength of the geopolymer mortar. 4.3. Properties of geopolymer mortar with different thermogenetic admixtures
Fig. 5. Compressive strength of geopolymer paste after different mixing processes.
a. Hot pack material were mixed together as mentioned in Section 3.1; then the geopolymer mortar specimens were cured in the ambient condition. The compressive strength values of specimens during all three processes show a trend of increasing quickly in the first 28 d. After that, this strength keeps increasing until 60 d. This behavior is identical to that of the specimens of mixture GP0. However, all the compressive strength values of the specimens subjected to the three processes are higher than the values of the mixture GP0. As can be seen in Fig. 1, Process 1 is performed by mixing preheated sand with fly ash and alkali solution, Process 2 is performed by mixing the preheated fly ash with sand and alkali solution;
For the study of the properties of geopolymer mortar using hot packs, mixtures GAHP5, GAHP10, and GAHP15 were employed. The composition of the hot packs of GAHP5, GAHP10 and GAHP15 were 5, 10 and 15% of fly ash by mass, respectively. The specimens were cured in ambient conditions after materials were cast and compacted into the molds. Fig. 7 shows the compressive strength values of the geopolymer mortars used as admixtures in the hot pack material. As can be seen in Fig. 7, the compressive strength of GAHP5 was larger than those of GAHP10 and GAHP15. Within 28 d, the compressive strength increases quickly and then increase more slowly after 28 d: at 9 d, GAHP5 reaches 17.70 MPa, and GAHP10 and
Please cite this article in press as: K.T. Nguyen et al., Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110
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(a) Process 1
Fig. 7. Compressive strength of geopolymer paste with admixture HP.
(b) Process 2
Fig. 8. Temperature of geopolymer paste using hot pack admixture.
(c) Process 3 Fig. 6. Temperature of geopolymer pastes with different mixing process.
GAHP15 are 14.98 MPa and 5.18 MPa. At 28 d, the compressive strengths of GAHP5, GAHP10 and GAHP15 are, in turn, 25.40 MPa, 22.1 MPa and 7.96 MPa. After 28 d, this trend goes up only slightly. For example, at 60 d, the values of compressive strength are 28.12 MPa, 22.42 MPa and 9.55 MPa for GAHP5, GAHP10 and GAHP15. By comparison, all three groups of specimens using thermogenetic admixtures were subjected to the compressive strength test at 5 d, and it was possible to subject GP0 to the test at 3 d. Among the three mixtures of GAHP, only GAHP5 has a higher compressive strength than did GP0 at 28 d. However GP0 has a value higher than those of all three GAHPs at 60 d, all three mixtures of GAHP have temperature higher than that of GP0 as shown in Fig. 8. After finishing mixing, the temperatures of the three GAHPs are higher than that of GP0. GAHP 15 has the highest temperature
because it used the largest quantity of hot pack material. After this, it takes about 3 h for the GAHPs to drop to room temperature; the same drop took 2 h for GP0. Logically, the compressive strength of GAHP15 must be the highest of all the specimens because GAHP15 has more heat energy than do the remaining specimens. However, the test data show the opposite result. That is, GAHP5 has the highest compressive strength at 28 d and GP0 stands at the top position at 60 d. The reason for this result can be found in the mechanism of the hot packs. That is, hot packs work based on a simple chemical reaction similar to rusting; the reaction occurs when the packs are exposed to air. When this chemical reaction happens, heat is released simultaneously with the creation of ferric oxide (Fe2O3), which is the product of rusting. This material is soft and creates pores inside the geopolymer mortar. These things cause the geopolymer mortar to have many empty spaces; the final result is low compressive strength. For GAHP5, the quantity of hot packs materials is only 5% of fly ash by mass. Thus, the amount of iron powder for this specimen is lower than the amounts of GAHP10 and GAHP15. As a result, the chemical reaction happens slowly and ferric oxide is produced in smaller quantity. The structure of the geopolymer mortar at that time is strong enough to reach high compressive strength at 28 d; however, after 28 d, the chemical reaction of rusting continues and more ferric oxide appears. This prevents the geopolymer mortar from reaching high strength. For
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example, the compressive strength values of all the mixtures GAHPs are lower than that of GP0 at 60 d. b. Quicklime In this section, mixtures GAC5, GAC10 and GAC15 were used to evaluate the properties of geopolymer mortar using a quicklime admixture. The usage percentages of quicklime for GAC5, GAC10 and GAC15 are 5%, 10% and 15% of fly ash by mass, respectively. The specimens were cured in ambient condition after casting. The test data is shown in Fig. 9. From the results shown in Fig. 9, the compressive strength of mixture GAC5 can be seen to be much higher than those of GAC10 and GAC15. At 9 d, the compressive strength of GAC5 reaches 19.09 MPa; at 28 d it is 32.85 MPa and at 60 d it is 33.27 MPa, while GAC10 and GAC15 have values of compressive strength lower than 5 MPa, even though GAC10 and GAC15 have higher temperature as shown in Fig. 10. The temperatures of mixture GACs are higher than that of GP0; GAC15 has the highest temperature (50 °C). The time for the temperature to drop to room temperature is about 10 h for GAC10 and GAC15; it is 8 h for GAC5. Despite the great amount of heat that they generate over a long period of time, GAC10 and GAC15 show very low compressive strength compared with that of GAC5. At 28 d, the compressive strength of GAC5 is higher than that of GP0 by about 45%; this value is 15% at 60 d. For the geopolymer mortar, the percentage of quicklime must be lower than 10%; the ideal amount is around 5%. Theoretically, calcium oxide can react with water to form calcium hydroxide and generate heat, as shown in Eq. (2). This heat provides energy to make chemical reactions happen quickly as discussed above. Besides this, the formed calcium hydroxide is inside the GP, which means that hydroxide (OH-) appears. As a results, the pH of GP increases. Fortunately, with high pH, the linking Si-O-Si is easy to break and the dissolving of Si and Al atoms happens easily. Thus, GP has more raw materials with which to create the geopolymer structure. In order to find the optimum percentage of quicklime for use in geopolymer mortar, mixtures GAC2, GAC3, GAC4, GAC6, GAC7, GAC8 and GAC9 were designed. The percentages of quicklime are 2%, 3%, 4%, 6%, 7%, 8% and 9%. The test results are given in Fig. 11. According to the test data shown in Fig. 11, the mixtures GAC3 and GAC4 show higher compressive strength values than those of the other mixtures. The compressive strength of GAC3 is slightly higher than that of GAC5. The difference between the compressive
Fig. 10. Temperature of geopolymer paste using quicklime admixture.
Fig. 11. Compressive strength of geopolymer paste with quicklime admixture.
strength values of these two mixtures is trivial. Also from these results, it can be seen that the maximum percentage of quicklime needed to make geopolymer mortar must be 5%; the minimum percentage is 3%. Beyond this range, the compressive strength cannot reach a high value at 28 d or at 60 d. c. Microstructure of geopolymer mortar using thermogenetic admixture
Fig. 9. Compressive strength of geopolymer paste using quicklime admixture.
A Scanning Electron Microscope (SEM) was used to analyze the microstructure of the geopolymer mortar employing the thermogenetic admixture (hot pack and quicklime) at under 28 d of age. The results of the microstructural observations of GAHP5, GAC5 and GP0 are provided in Fig. 12a–c respectively. It is apparent that GAHP5’s structure is not as dense as those of GAC5 and GP0. For the geopolymer mortar using hot pack and quicklime, there are fewer unreacted spherical ash particles than there are for the geopolymer mortar GP0. It was also possible to observe the presence of silica (Si) and alumina (Al) in the EDX spectrum. Compared with GAHP5 and GAC5, GP0 has many more unreacted ash particles. As a result, the geopolymer structure of GP0 is not fully developed and the compressive strength is lower than those of GAHP5 and GAC5 at 28 d. However, these unreacted ash particles keep reacting slowly. Then, when the geopolymer structure is
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(a) SEM-EDX micrograph of GAHP5
(b) SEM-EDX micrograph of GAC5
(c) SEM-EDX micrograph of GP0 Fig. 12. SEM-EDX micrograph of geopolymer paste specimens.
completely developed, the compressive strength of GP0 increases significantly. This result can be explained by considering that the geopolymer mortar using thermogenetic admixtures is provided with heat energy at the beginning of the geopolymerization process and that energy makes the process happen quickly. Therefore, the geopolymer mortar can reach high compressive strength in a shorter period of time compared with geopolymer mortar GP0. Within 28 d, the compressive strengths of GAHP5 and GAC5 are almost fully developed; from 28 d to 60 d, the change of compressive strength is insignificant. Besides this, the structures of GP0 and
GAC5 are similar structure expect for the quantity of unreacted ash particles. d. XRD analysis of geopolymer mortar using thermogenetic admixture The XRD data for the geopolymer mortar using the thermogenetic admixture are shown in Fig. 13. Crystalline phases of quartz were detected as sharp peaks in GAHP5 and GAC5. This means that almost the entire product of the hardening process of the
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(a) GAHP5
9
(b) GAC5
Fig. 13. XRD patterns of geopolymer paste using thermogenetic admixtures.
geopolymer mortar using the thermogenetic admixture is geopolymer gel. Besides this, these data reveal that there is no formation of gypsum or ettringite even though calcium oxide was added to the geopolymer mortar. For normal concrete, calcium oxide is not a good component because of its properties. Calcium oxide is the main factor in the chemical reaction that creates expansive gypsum and ettringite which cause expansion, cracking, and spalling in concrete. Thus, considering the results of the XRD analysis, calcium oxide can be used for self-cured geopolymer mortar with a maximum 5% of fly ash by mass. In conclusion, according to the compressive strength, SEM-EDX, and XRD pattern testing, the best for making self-cured geopolymer mortar is to use quicklime as a thermogenetic admixture. The amount of quicklime should be in the range 3–5% of the fly ash by mass. 5. Conclusions These are different mixing processes, but all three mixing processes are related to preheated materials. Process 1, Process 2, and Process 3 lead to higher compressive strength values than does the normal mixing process under the same natural curing conditions. Among the three processes, Process 3 obtains the highest strength. It can be concluded that Process 3 has the largest heat energy due to the quantity of preheat materials (sand and fly ash). Thus, the reaction kinetics is high and the speed of chemical reaction increases. As a result, the polymer is created in great quantities. For the preheated materials, a larger amount of provided heat energy results in the higher compressive strength of geopolymer mortar. Among the two thermogenetic admixtures, quicklime shows better results than does the hot pack material. For these types of geopolymer mortar, the usage amount is limited at 5% of fly ash by mass. Especially, when using quicklime, the recommended amount is from 3% to 5%. The compressive strength of the geoplymer mortar using 5% thermogenetic admixtures is larger than that of the mixture GP0 at 28 d. However, at 60 d, only the geopolymer mortar using quicklime still has a higher compressive strength value than that of GP0. The heat energy generated from quicklime is higher and lasts longer than that from the hot pack material. Then, the geopolymer structure is created in a denser or more fully developed way. Thus, the compressive strength values of the specimens using quicklime are higher than those obtained using the hot pack material. These results are confirmed by SEM-EDX micrographs. Besides this, XRD patterns prove the geopolymer produc-
tion of the mortar using the thermogenetic admixtures. These patterns also reveal that there is no formation of expansive gypsum or ettringite in the case of using quicklime as admixture.
Acknowledgement This research was supported by a grant (16AUDP-B066083-04) from Architecture & Urban Development Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures Khoa Tan Nguyen a, Tuan Anh Le b, Jaehong Lee a, Dongkyu Lee a, Kihak Lee a,⇑ a b
Sejong University, Department of Architectural Engineering, Seoul 143-747, South Korea Vietnam National University of Ho Chi Minh City, Faculty of Civil Engineering, Viet Nam
h i g h l i g h t s Proposing two new alternative curing method are using preheated materials and thermogentic admixtures. For using preheated materials, a larger amount of preheated materials results in a higher compressive strength. For using thermogentic admixtures, quicklime is more efficient than hot pack material. The suggested amount of quicklime is about 3–5% of fly ash by mass.
a r t i c l e
i n f o
Article history: Received 2 May 2016 Received in revised form 27 October 2016 Accepted 27 October 2016 Available online xxxx Keywords: Geopolymer mortar Self-cured geopolymer Preheated Quicklime Hot pack Fly ash
a b s t r a c t Heat curing in an oven is the traditional method to obtain mechanical properties of geopolymers. This characteristic has significantly affected operations in terms of construction and energy consumption. The target of this paper is proposing alternative curing methods namely self-cured technologies for fly ash based geopolymer materials that are cured in ambient conditions without use of a heat resource. Two alternative methods are to use three different mixing processes with preheated materials and to use two thermogenetic admixtures (hot pack material and quicklime). The results show that for the mixing processes, a larger amount of provided heat energy results in a higher compressive strength of the geopolymer mortar (GM). For the use of a thermogenetic admixture, the results from compressive strength testing, Scanning Electron Microscope (SEM), energy dispersive X-ray analysis (EDX) micrographs and X-ray diffraction (XRD) analysis confirm that quicklime is more efficient than hot pack material. In the case of using quicklime, the suggested amount is about 3–5% of fly ash by mass. Ó 2016 Published by Elsevier Ltd.
1. Introduction The impact of Portland cement production on the environment will issue a significant challenge to concrete industry in the future. The Portland cement manufacturing process depletes natural resources and emits a huge amount of greenhouse gases [1–3]. New materials have been researched to totally or partially replace traditional Portland cement [4–6]. Among these new materials, the most potential material is geopolymeric material. The production
Abbreviations: GM, geopolymer mortar; SEM, Scanning Electron Microscope; EDX, energy dispersive X-ray analysis; XRD, X-ray diffraction; CO2, carbon dioxide; Na2SiO3, sodium silicate; NaOH, sodium hydroxide; KOH, potassium hydroxide; FA, fine aggregate; CaO, calcium oxide; CaCO3, calcium carbonate; NaCl, sodium chloride. ⇑ Corresponding author. E-mail address: [email protected] (K. Lee).
of alumina-silicate based geopolymer cement is usually a combination of an alkaline solution, like water glass or sodium silicate (Na2SiO3), sodium or potassium hydroxide (NaOH or KOH) and source material such as fly ash, rice husk ash or granulated blast furnace slag to form homogenous slurry [7–9]. The heat used for curing process is above the ambient temperature and is in a range from approximately 60–100 °C for 24–48 h. Subsequently, the geopolymer will be fully cured or left in room temperature for further handling [10]. Previous studies have determined that the properties of geopolymer binders, tested against the standard of Portland cement, are identical to or even better than those of materials made from traditional cement. Furthermore, geopolymers utilize solid industrial waste material and their production can be accomplished without an energy intensive process like that necessary for the production of Portland cement. Replacing traditional cement with a geopolymer binder can yield environmental benefits such
http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110 0950-0618/Ó 2016 Published by Elsevier Ltd.
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as reduction in consumption of natural resources and decrease in the net production of carbon dioxide (CO2) [11]. Nevertheless, the requirement of heat curing for geopolymer materials has significantly affected the operations of construction and energy consumption. The common fly ash-based geopolymer paste cannot set within 24 h at ambient temperature [12], however, the presence of calcium source could significantly shorten the setting time of those common geopolymer paste. An increase of calcium content in geopolymer cement can be achieved by adding granulated blast furnace slag, high calcium fly ash, calcium hydroxide or even Portland cement [13–16]. Based on these facts, this study concentrates on the curing processes and properties of geopolymer mortar (GP) using thermogenetic admixtures (quicklime and hot pack material) and different mixing processes. Also, the optimum percentages for the materials in the thermogenetic admixture is determined. Afterwards, instructions or recommendations for fabrication of geopolymer mortar without curing in the oven or self-cured geopolymer mortar are proposed. 2. Materials The materials used for making fly ash based geopolymer mortar were low-calcium dry fly ash as source material, fine aggregate (FA), alkaline liquids and water. The fly ash used in this study, known as a ‘Class F’ material based on ASTM 618 [22], with specific gravity 2500 kg/m3, came from a power station. The chemical compositions of the fly ash are given in Table 1. Sand was used as fine aggregate. The specific gravity and fineness modulus of sand were 2.6 and 2.1 respectively. The fine aggregate to fly ash ratio used in the test was 7:3 by mass. The alkaline liquid was a combination of sodium silicates (Na2SiO3) and sodium hydroxide (NaOH). The components of the sodium silicates solution were Na2O and SiO2 (approximately 36–38% by mass). The ratio of SiO2-to-Na2O by mass was approximately 2. The concentration of sodium hydroxide solution was 14 M (M). Water glass and sodium hydroxide were mixed at a ratio of 1:1 by mass. The ratio of alkali solutions (including water glass and sodium hydroxide) to fly ash was 0.35. Details of the mix proportions used in this study are provided in Table 2. In this table, the name of mixture is started with ‘‘G” which stands for geopolymer. The following characters represent for the properties of mixture. 3. Experimental procedures To investigate the properties of geopolymer mortar, experimental tests were conducted with different curing methods: natural curing (at room temperature 28 ± 2 °C), curing using an oven, natural curing using three mixing processes and natural curing using thermogenetic admixtures. Geopolymer mortar cylinders of 60 mm diameter and 120 mm height were used for the compressive strength test. Details of the mixing processes and of the thermogenetic admixtures are explained in the following section. 3.1. Mixing processes with preheated materials In this study, three kinds of mixing processes were used to manufacturing GP, namely, processes 1, 2 and 3, as follows: process 1: fine aggregate was firstly heated at 80 °C, 100 °C and 120 °C for 4 h and then mixed with fly ash for about 30 s. After that the combina-
tion of sodium silicate and sodium hydroxide were added to the solid and the resulting materials were mixed for 60 s. Process 2: Fly ash was heated in the same conditions as were used for the FA in process 1; then, alkali solution (sodium silicate and sodium hydroxide) was poured into the solid and resulting materials were mixed for 60 s. Process 3: the procedure was the same processes 1 and 2 except that the mixtures that included FA and fly ash were heated at 80, 100 and 120 °C for 4 h. A flowchart of the testing procedures is provided in Fig. 1. The mixtures GPHS80, GPHS100 and GPHS120 are designed for Process 1; GPHF80, GPHF100 and GPHF120 are for Process 2; GPHM80, GPHM100 and GPHM120 are for Process 3. 3.2. Thermogenetic admixtures Based on the properties of geopolymerization process, which requires heat, this research uses two admixtures to provide heat. Two thermogenetic admixtures that we considered in this study are quicklime and hot pack material. Details of the two admixtures are summarized below. 3.2.1. Quicklime Quicklime is the common name of calcium oxide (CaO); it is a white powder as can be seen in Fig. 2. Calcium oxide is usually produced in a lime kiln by thermal decomposition of materials such as limestone, or seashells that contain calcium carbonate (CaCO3). This is accomplished by heating the material to above 825 °C, a process called calcination, to liberate a molecule of carbon dioxide (CO2); leaving quicklime as shown in Eq. (1) [17].
CaCO3 ! CaO þ CO2
ð1Þ
Quicklime releases heat energy by the formation of the hydrate, calcium hydroxide, according to the following equation (Eq. (2)):
CaO þ H2 O CaðOHÞ2 þ DHr
ð2Þ
where DHr = 63.7 kJ/mol of CaO. When hydration occurs, the heat energy is released; this energy will be useful for the geopolymerization process. In this study, quicklime is used at percentages in the range of 2– 10% and of 15% of fly ash by mass. The percentages 5%, 10% and 15% were used in comparison experiment with identical amounts of hot pack material. The remaining percentages were used to determine the optimum amount of quicklime that can be used to impart the best compressive strength. The properties of the quicklime used for this research are given in Table 3. 3.2.2. Hot pack material (hand warmer) Hot packs or hand warmers, as shown in Fig. 3 are small (mostly disposable) packets that are held in the hand and produce heat on demand to warm cold hands. They are commonly used in outdoor activities such as hiking and skiing to keep extremities warm and to supplement the person’s insulated clothing. Depending on the type and source of heat, hot packs can last from 30 min up to 24 h [18]. The hot packs contain iron (Fe) powder, cellulose, sodium chloride (NaCl), vermiculite, activated charcoal and moisture. Most hot packs work base on a simple chemical reaction similar to that of rusting; the reaction occurs when the packs are exposed to air. When the iron in the pack is exposed to oxygen in the air, it
Table 1 Chemical composition of fly ash. Oxide
SiO2
Al2O3
Fe2O3
CaO
K2O & Na2O
MgO
SO3
Loss On Ignition (LOI)
(%)
51.7
31.9
3.48
1.21
1.02
0.81
0.25
9.63
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K.T. Nguyen et al. / Construction and Building Materials xxx (2016) xxx–xxx Table 2 Mixture Proportions of Experimental Geopolymer Paste (GP). Name
FA (g)
Fly ash (g)
Sodium silicate solution (g)
Sodium hydroxide solution 14 M (g)
Hot pack (g)
Quicklime (g)
Description of mixtures
GP0 GP40 GP60 GP80 GPHS80 GPHS100 GPHS120 GPHF80 GPHF100
445.9
191.1
33.4
33.4
– – – – – – – – –
– – – – – – – – –
GPHF120
–
–
GPHM80
–
–
GPHM100
–
–
GPHM120
–
–
GAHP5 GAHP10 GAHP15 GAC2 GAC3 GAC4 GAC5 GAC6 GAC7 GAC8 GAC9 GAC10
9.55 19.11 28.65 – – – – – – – – –
– – – 3.82 5.73 7.64 9.55 11.47 13.38 15.29 17.2 19.11
GAC15
–
28.65
Natural curing Cured at 40 °C, oven Cured at 60 °C, oven Cured at 80 °C, oven 80 °C sand, natural curing 100 °C sand, natural curing 120 °C sand, natural curing 80 °C fly ash, natural curing 100 °C fly ash, natural curing 120 °C fly ash, natural curing 80 °C sand + fly ash, natural curing 100 °C sand + fly ash, natural curing 120 °C sand + fly ash, natural curing 5% hot pack, natural curing 10% hot pack, natural curing 15% hot pack, natural curing 2% quicklime, natural curing 3% quicklime, natural curing 4% quicklime, natural curing 5% quicklime, natural curing 6% quicklime, natural curing 7% quicklime, natural curing 8% quicklime, natural curing 9% quicklime, natural curing 10% quicklime, natural curing 15% quicklime, natural curing
Water-to-solid ratio (w/s)
0.17
Fig. 1. Flowchart of testing procedure.
oxidizes as in the reaction given in Eq. (3). During that process, heat is created. The salt (NaCl) acts as a catalyst and the charcoal help spread the heat throughout the pack. The vermiculite acts as an insulator, keeping the heat from dissipating too rapidly [18,19]. The chemical reaction occurs slowly and releases heat. This heat energy can be used in the geopolymerization process.
2Fe þ 3O2 ! Fe2 O3 þ DH
ð3Þ
where DH: the heat energy released. The amounts of hot pack material used for the test were 5%, 10% and 15% of fly ash by mass. In the mixing step, the hot packs were shaken for 5 min before mixing with the base materials fly ash and sand. This means that the hot packs were activated and ready to provide heat energy for the geopolymerization process.
3.3. Testing methods To evaluate the properties of geopolymer mortar, a total of four kinds of analyses such: a compressive strength test, Scanning Electron Microscope (SEM) test, energy dispersive X-ray analysis (EDX) technique and X-ray diffraction (XRD) analysis were used. The compressive strength test applies a compressive axial load to the molded cylinder at rates ranging from 0.15 to 0.35 MPa/s, until failure occurs. The compressive strength of the specimens is determined by dividing the maximum load attained during the test, by the cross-sectional area of the specimen [20]. The SEM photo was used to define the microstructure, particles and formation of specimens. The EDX technique was used to observe the chemical compositions of the materials. On the other hand, the XRD analysis was used to analyze and explain the results of the experimental tests. The temperature of specimens were recorded by using pocket digital thermometer. The range of this thermometer is from
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Fig. 2. Quicklime or calcium oxide powder.
Fig. 4. Compressive strength of geopolymer paste according to different curing conditions.
Table 3 Properties of quicklime.
50 to 260 °C, and the division is 0.1 °C. The top of specimen was chosen as a position for measuring the temperature. The data was recorded every 5 m until the temperature of specimens was equal to room temperature. Then, the result is reported in degree Celsius (°C).
the effects of curing temperature on the compressive strength for GP0, GP40, GP60 and GP80 after curing of the specimens. For mixture GP0 cured under ambient condition, the compressive strength increases according to time. At 9 d after the start of the test, the compressive strength of the mortar reached 13.42 MPa, which is about 4.8 times the value obtained at 3 d after the start of the test. The compressive strength keeps increasing and achieves a value of 22.59 MPa at 28 d and 29.15 MPa at 60 d. For GP40, GP60 and GP80 cured in the oven, higher curing temperature resulted in larger compressive strength of the geopolymer mortar. For example, the highest compressive strength is 28.81 MPa for GP80; the lowest value is 25.43 MPa for GP40. However, the specimens of the three mixtures GP40, GP60, and GP80 do not increase the strength in the same way as GP0. In general, the compressive strength of a geopolymer mortar cured under ambient conditions increases according to time while the compressive strength of geopolymer mortar cured in an oven achieves its highest value after curing. In the geopolymerization process, the heat energy helps molecules to diffuse, transport, orient or condense into monomers. This energy also impulses the reaction kinetics. If the curing temperature is high, this means that the heat energy is high and the chemical reaction happens quickly. As a result, the polymer will be produced in great quantities. These polymers connect together to become a long chain. This chain imparts compressive strength to the GP [21,23]. Thus, the specimens can reach a high value of compressive strength early by heating to the high curing temperature. For curing under ambient conditions, geopolymerization happens slowly; it takes about 60 d to achieve the same compressive strength as that achieved for the specimen that was cured in an oven. In order to obtain a high compressive strength geopolymer mortar, high curing temperature using an oven must be employed as prior condition. However, using an oven for curing of a specimen is not always the most advantageous way. Thus, other curing methods must be found out and applied to geopolymer materials.
4. Results and discussion
4.2. Curing process of GP with different pre-heated materials
4.1. Influence of curing temperature
Fig. 5 shows the effect of three mixing processes on the compressive strength of the geopolymer mortar. Fig. 5a shows the test data for Process 1 – which FA was preheated, Fig. 5b is for Process 2 – which fly ash was preheated and Fig. 5c is for Process 3 – which both FA and fly ash were preheated. For each process, the materials
Items
Property
Free CaO Total CaO Slaking time Slaking temperature
>75% >90% 8–15 min 85–95 °C
Fig. 3. Hot pack materials or hand warmers.
In this section, mixture GP0 was cured under ambient conditions (28 ± 2 °C) and mixture GP40, GP60, and GP80 were cured in the oven at 40, 60 and 80 °C, respectively for 8 h. Fig. 4 shows
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K.T. Nguyen et al. / Construction and Building Materials xxx (2016) xxx–xxx
(a) Process 1 – Preheated FA + Fly Ash
(b) Process 2 – FA + Preheated Fly Ash
(c) Process 3 – Preheated (FA + Fly Ash)
5
Process 3 is performed by mixing a combination of sand and fly ash with the alkali solution. After a close comparison, it was found that with the same temperature provided for materials, Process 3 produced much more heat energy for the geopolymerization process than did either Process 1 or Process 2 because the amount of heated materials in Process 3 was larger than those in Processes 1 and 2. Based on the properties of the geopolymerization process, a lot of polymer can be produced when the heat energy is sufficiently large. In the studied case, Process 3 had the highest heat energy because of the quantity of heated materials inside the mixture (according to Table 2: 637 g preheated materials for Process 3, 445.9 g for Process 2 and 191.9 g for Process 2). Following this rule, the heat energy of Process 1 is in the middle and Process 2 produces the lowest level of heat. As a result, the compressive strength values of the specimens in Process 3 are the highest among the three mixing processes. The temperature curve shown in Fig. 6, which indicates the heat evolution during these processes, can be used to confirm the results for compressive strength. Time 0 is the point of finishing the mixing step and starting the curing step. At point 0, Process 3 reached the highest temperature among the three processes. The temperature then steadily decreased to room temperature in 3 h. The heat from the preheated materials spread out to the other materials inside the geopolymer mortar. This heat also provided energy for the geopolymerization process. Because it had the highest amount of heat, Process 3 unavoidably had the largest compressive strength. The higher preheat temperature of this material resulted in a larger compressive strength. After a detailed comparison, it was found that the values of compressive strength of the geopolymer mortars in the three processes are identical to or higher than that value of mixture GP0. However, from 7 d until 28 d, there are large differences among the three processes and GP0. For example, at 28 d, all specimens of the three processes had higher values of compressive strength than did the specimens of GP0. The reason for this can be easily explained: during the first several hours of curing time, the heat from preheated materials (fly ash and sand) provided energy for geopolymerization process. This energy make the chemical reaction happen quickly. During this time, the specimens of the three processes also had temperatures higher than room temperature. Thus, the polymers are created in great quantities. These polymers creates strength for the geopolymer mortar. Obviously, the compressive strength values during the three processes are higher than the value of the specimens cured at room temperature. In conclusion, in order to use preheated materials to produce self-cured geopolymer mortar, a higher temperature of the preheated materials resulted in a higher compressive strength of the geopolymer mortar. 4.3. Properties of geopolymer mortar with different thermogenetic admixtures
Fig. 5. Compressive strength of geopolymer paste after different mixing processes.
a. Hot pack material were mixed together as mentioned in Section 3.1; then the geopolymer mortar specimens were cured in the ambient condition. The compressive strength values of specimens during all three processes show a trend of increasing quickly in the first 28 d. After that, this strength keeps increasing until 60 d. This behavior is identical to that of the specimens of mixture GP0. However, all the compressive strength values of the specimens subjected to the three processes are higher than the values of the mixture GP0. As can be seen in Fig. 1, Process 1 is performed by mixing preheated sand with fly ash and alkali solution, Process 2 is performed by mixing the preheated fly ash with sand and alkali solution;
For the study of the properties of geopolymer mortar using hot packs, mixtures GAHP5, GAHP10, and GAHP15 were employed. The composition of the hot packs of GAHP5, GAHP10 and GAHP15 were 5, 10 and 15% of fly ash by mass, respectively. The specimens were cured in ambient conditions after materials were cast and compacted into the molds. Fig. 7 shows the compressive strength values of the geopolymer mortars used as admixtures in the hot pack material. As can be seen in Fig. 7, the compressive strength of GAHP5 was larger than those of GAHP10 and GAHP15. Within 28 d, the compressive strength increases quickly and then increase more slowly after 28 d: at 9 d, GAHP5 reaches 17.70 MPa, and GAHP10 and
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(a) Process 1
Fig. 7. Compressive strength of geopolymer paste with admixture HP.
(b) Process 2
Fig. 8. Temperature of geopolymer paste using hot pack admixture.
(c) Process 3 Fig. 6. Temperature of geopolymer pastes with different mixing process.
GAHP15 are 14.98 MPa and 5.18 MPa. At 28 d, the compressive strengths of GAHP5, GAHP10 and GAHP15 are, in turn, 25.40 MPa, 22.1 MPa and 7.96 MPa. After 28 d, this trend goes up only slightly. For example, at 60 d, the values of compressive strength are 28.12 MPa, 22.42 MPa and 9.55 MPa for GAHP5, GAHP10 and GAHP15. By comparison, all three groups of specimens using thermogenetic admixtures were subjected to the compressive strength test at 5 d, and it was possible to subject GP0 to the test at 3 d. Among the three mixtures of GAHP, only GAHP5 has a higher compressive strength than did GP0 at 28 d. However GP0 has a value higher than those of all three GAHPs at 60 d, all three mixtures of GAHP have temperature higher than that of GP0 as shown in Fig. 8. After finishing mixing, the temperatures of the three GAHPs are higher than that of GP0. GAHP 15 has the highest temperature
because it used the largest quantity of hot pack material. After this, it takes about 3 h for the GAHPs to drop to room temperature; the same drop took 2 h for GP0. Logically, the compressive strength of GAHP15 must be the highest of all the specimens because GAHP15 has more heat energy than do the remaining specimens. However, the test data show the opposite result. That is, GAHP5 has the highest compressive strength at 28 d and GP0 stands at the top position at 60 d. The reason for this result can be found in the mechanism of the hot packs. That is, hot packs work based on a simple chemical reaction similar to rusting; the reaction occurs when the packs are exposed to air. When this chemical reaction happens, heat is released simultaneously with the creation of ferric oxide (Fe2O3), which is the product of rusting. This material is soft and creates pores inside the geopolymer mortar. These things cause the geopolymer mortar to have many empty spaces; the final result is low compressive strength. For GAHP5, the quantity of hot packs materials is only 5% of fly ash by mass. Thus, the amount of iron powder for this specimen is lower than the amounts of GAHP10 and GAHP15. As a result, the chemical reaction happens slowly and ferric oxide is produced in smaller quantity. The structure of the geopolymer mortar at that time is strong enough to reach high compressive strength at 28 d; however, after 28 d, the chemical reaction of rusting continues and more ferric oxide appears. This prevents the geopolymer mortar from reaching high strength. For
Please cite this article in press as: K.T. Nguyen et al., Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110
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example, the compressive strength values of all the mixtures GAHPs are lower than that of GP0 at 60 d. b. Quicklime In this section, mixtures GAC5, GAC10 and GAC15 were used to evaluate the properties of geopolymer mortar using a quicklime admixture. The usage percentages of quicklime for GAC5, GAC10 and GAC15 are 5%, 10% and 15% of fly ash by mass, respectively. The specimens were cured in ambient condition after casting. The test data is shown in Fig. 9. From the results shown in Fig. 9, the compressive strength of mixture GAC5 can be seen to be much higher than those of GAC10 and GAC15. At 9 d, the compressive strength of GAC5 reaches 19.09 MPa; at 28 d it is 32.85 MPa and at 60 d it is 33.27 MPa, while GAC10 and GAC15 have values of compressive strength lower than 5 MPa, even though GAC10 and GAC15 have higher temperature as shown in Fig. 10. The temperatures of mixture GACs are higher than that of GP0; GAC15 has the highest temperature (50 °C). The time for the temperature to drop to room temperature is about 10 h for GAC10 and GAC15; it is 8 h for GAC5. Despite the great amount of heat that they generate over a long period of time, GAC10 and GAC15 show very low compressive strength compared with that of GAC5. At 28 d, the compressive strength of GAC5 is higher than that of GP0 by about 45%; this value is 15% at 60 d. For the geopolymer mortar, the percentage of quicklime must be lower than 10%; the ideal amount is around 5%. Theoretically, calcium oxide can react with water to form calcium hydroxide and generate heat, as shown in Eq. (2). This heat provides energy to make chemical reactions happen quickly as discussed above. Besides this, the formed calcium hydroxide is inside the GP, which means that hydroxide (OH-) appears. As a results, the pH of GP increases. Fortunately, with high pH, the linking Si-O-Si is easy to break and the dissolving of Si and Al atoms happens easily. Thus, GP has more raw materials with which to create the geopolymer structure. In order to find the optimum percentage of quicklime for use in geopolymer mortar, mixtures GAC2, GAC3, GAC4, GAC6, GAC7, GAC8 and GAC9 were designed. The percentages of quicklime are 2%, 3%, 4%, 6%, 7%, 8% and 9%. The test results are given in Fig. 11. According to the test data shown in Fig. 11, the mixtures GAC3 and GAC4 show higher compressive strength values than those of the other mixtures. The compressive strength of GAC3 is slightly higher than that of GAC5. The difference between the compressive
Fig. 10. Temperature of geopolymer paste using quicklime admixture.
Fig. 11. Compressive strength of geopolymer paste with quicklime admixture.
strength values of these two mixtures is trivial. Also from these results, it can be seen that the maximum percentage of quicklime needed to make geopolymer mortar must be 5%; the minimum percentage is 3%. Beyond this range, the compressive strength cannot reach a high value at 28 d or at 60 d. c. Microstructure of geopolymer mortar using thermogenetic admixture
Fig. 9. Compressive strength of geopolymer paste using quicklime admixture.
A Scanning Electron Microscope (SEM) was used to analyze the microstructure of the geopolymer mortar employing the thermogenetic admixture (hot pack and quicklime) at under 28 d of age. The results of the microstructural observations of GAHP5, GAC5 and GP0 are provided in Fig. 12a–c respectively. It is apparent that GAHP5’s structure is not as dense as those of GAC5 and GP0. For the geopolymer mortar using hot pack and quicklime, there are fewer unreacted spherical ash particles than there are for the geopolymer mortar GP0. It was also possible to observe the presence of silica (Si) and alumina (Al) in the EDX spectrum. Compared with GAHP5 and GAC5, GP0 has many more unreacted ash particles. As a result, the geopolymer structure of GP0 is not fully developed and the compressive strength is lower than those of GAHP5 and GAC5 at 28 d. However, these unreacted ash particles keep reacting slowly. Then, when the geopolymer structure is
Please cite this article in press as: K.T. Nguyen et al., Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110
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(a) SEM-EDX micrograph of GAHP5
(b) SEM-EDX micrograph of GAC5
(c) SEM-EDX micrograph of GP0 Fig. 12. SEM-EDX micrograph of geopolymer paste specimens.
completely developed, the compressive strength of GP0 increases significantly. This result can be explained by considering that the geopolymer mortar using thermogenetic admixtures is provided with heat energy at the beginning of the geopolymerization process and that energy makes the process happen quickly. Therefore, the geopolymer mortar can reach high compressive strength in a shorter period of time compared with geopolymer mortar GP0. Within 28 d, the compressive strengths of GAHP5 and GAC5 are almost fully developed; from 28 d to 60 d, the change of compressive strength is insignificant. Besides this, the structures of GP0 and
GAC5 are similar structure expect for the quantity of unreacted ash particles. d. XRD analysis of geopolymer mortar using thermogenetic admixture The XRD data for the geopolymer mortar using the thermogenetic admixture are shown in Fig. 13. Crystalline phases of quartz were detected as sharp peaks in GAHP5 and GAC5. This means that almost the entire product of the hardening process of the
Please cite this article in press as: K.T. Nguyen et al., Investigation on properties of geopolymer mortar using preheated materials and thermogenetic admixtures, Constr. Build. Mater. (2016), http://dx.doi.org/10.1016/j.conbuildmat.2016.10.110
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(a) GAHP5
9
(b) GAC5
Fig. 13. XRD patterns of geopolymer paste using thermogenetic admixtures.
geopolymer mortar using the thermogenetic admixture is geopolymer gel. Besides this, these data reveal that there is no formation of gypsum or ettringite even though calcium oxide was added to the geopolymer mortar. For normal concrete, calcium oxide is not a good component because of its properties. Calcium oxide is the main factor in the chemical reaction that creates expansive gypsum and ettringite which cause expansion, cracking, and spalling in concrete. Thus, considering the results of the XRD analysis, calcium oxide can be used for self-cured geopolymer mortar with a maximum 5% of fly ash by mass. In conclusion, according to the compressive strength, SEM-EDX, and XRD pattern testing, the best for making self-cured geopolymer mortar is to use quicklime as a thermogenetic admixture. The amount of quicklime should be in the range 3–5% of the fly ash by mass. 5. Conclusions These are different mixing processes, but all three mixing processes are related to preheated materials. Process 1, Process 2, and Process 3 lead to higher compressive strength values than does the normal mixing process under the same natural curing conditions. Among the three processes, Process 3 obtains the highest strength. It can be concluded that Process 3 has the largest heat energy due to the quantity of preheat materials (sand and fly ash). Thus, the reaction kinetics is high and the speed of chemical reaction increases. As a result, the polymer is created in great quantities. For the preheated materials, a larger amount of provided heat energy results in the higher compressive strength of geopolymer mortar. Among the two thermogenetic admixtures, quicklime shows better results than does the hot pack material. For these types of geopolymer mortar, the usage amount is limited at 5% of fly ash by mass. Especially, when using quicklime, the recommended amount is from 3% to 5%. The compressive strength of the geoplymer mortar using 5% thermogenetic admixtures is larger than that of the mixture GP0 at 28 d. However, at 60 d, only the geopolymer mortar using quicklime still has a higher compressive strength value than that of GP0. The heat energy generated from quicklime is higher and lasts longer than that from the hot pack material. Then, the geopolymer structure is created in a denser or more fully developed way. Thus, the compressive strength values of the specimens using quicklime are higher than those obtained using the hot pack material. These results are confirmed by SEM-EDX micrographs. Besides this, XRD patterns prove the geopolymer produc-
tion of the mortar using the thermogenetic admixtures. These patterns also reveal that there is no formation of expansive gypsum or ettringite in the case of using quicklime as admixture.
Acknowledgement This research was supported by a grant (16AUDP-B066083-04) from Architecture & Urban Development Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
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