Effect of cement and lime on strength and high-temperature resistance of class F and C fly ash-based geopolymer mortars

2.1. Materials

Two types of fly ashes (F and C) were used as aluminosilicate sources in the study. The Class F fly ash was supplied from Izdemir Enerji/Izmir and the Class C fly ash was supplied from Cayirhan Thermal Power Plant/Ankara. An ordinary CEM I 42.5 R type cement and slaked lime were used as replacement materials. The chemical composition and some physical properties of fly ashes and cement are shown in the Table 1. SEM images of fly ashes are shown in Figure 1. These images were obtained using by “Thermo Scientific Apreo S” device with the range of 5 and 7.5 kV. SEM images revealed that the Class F fly ash particles were more rounded than those of the Class C fly ash. Moreover, the particles of Class F fly ash were found to be more uniform in size than that of Class C fly ash particles.

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Figure 1. SEM images of fly ashes (a: Class F, b: Class C) Sodium silicate solution, containing 11.32% Na2O, 28.22% SiO2 and 60.45% H2O by weight, and sodium hydroxide in the form of pellets with 98% purity were used as activators. Sodium hydroxide pellets were dissolved in a given amount of sodium silicate solution to obtain the desired Ms ratio (total SiO2 to Na2O ratio by weight in activator). The obtained solution was used after 24 hours of rest. Tap water and crushed limestone sand was used in the preparation of mortar mixes. The gradation of the sand is shown in the Figure 2.

2.2. Method

The experimental part of the study consisted of 2 stages. At the first stage, the proportions of mixtures to obtain the highest compressive strength for each aluminosilicate were determined. For this purpose, 9 mortar mixtures were prepared with each aluminosilicate by using activators designed with 3 different Ms ratios and 3 different Na2O percentages (total Na2O ratio of the activator by weight of aluminosilicate). The compressive strengths as well as flow diameters of the mortar mixtures were determined. The flow diameters of the mortars were determined according to the ASTM 1437-20 [15] standard. In order for the mortars to have similar flow diameters, trail mixtures were prepared using different amounts of water. The target flow diameter for the mixtures was chosen as 18.5±1 cm. For the target flow value, Class C fly ash-based mortars water demand was higher than that of the Class F fly ash-bearing mixtures. The fact seems to be arisen from the morphological characteristics (angular shape) of the Class C fly ash particles which cause greater internal friction in the mixture reducing its consistency. The ingredients were placed in the Hobart mixer bowl and mixed for 45 seconds at low speed. After scraping the materials adhering to the container, the mixer was operated for another 45 seconds at the same speed and the flow diameters were determined. The mortars were placed in the 50 mm cube molds in 2 layers and each layer was compacted by 25 drops in a jolting table. Immediately after preparing, the specimens were placed in an oven and cured at

Figure 3. Designating of geopolymer mortar mixtures. 60°C for 1, 3 and 5 days. At this stage, the mixture giving the highest compressive strength for each fly ash was chosen as the reference mixture. The proportions of the mixtures produced at this stage are shown in the Table 2. Mixtures were designated as shown in Figure 3. The compressive strength tests were carried out in a 2000 kN capacity concrete press. The loading rate was set as 0.9 kN/s. The reported strength values are the average of 3 samples. At the second stage, definite amounts of fly ashes were replaced with cement or slaked lime. The effects of substituted materials on the compressive strength and high-tem-

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Table 3. Proportions and flow diameters of mixtures containing cement and lime

perature resistance (300, 600, 900°C) of geopolymer mortars were determined. At this stage, it was aimed to keep the flow diameters of the mixtures as close as possible to that of the reference mixture. For this purpose, differences between the flow diameters of cement- and lime-bearing mixtures and that of the control mixtures were kept within the range of ±0.3 cm. There was a rapid loss of consistency in some of the test mixtures. Thus, a retarder admixture was added to these mixtures. Trial and error were applied

to determine the retarder admixture requirement of the mixtures. The proportions of the mixtures containing cement and lime are shown in Table 3. Payakaniti et al. [13] investigated the high temperature resistance of Class C fly ash based geopolimer pastes and determined that the compressive strength starts to decrease after temperatures of 200°C, the rate of decrease was more considerable in the range of 400–600°C, and the rate of strength loss decreased after 600°C. Researchers also

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Figure 4. Compressive strength of class F fly ash-based geopolymer mortars. reported that new crystalline phases appeared in the matrix 800°C and beyond that these crystalline phases had an positive or negative effect on the strength depending on the numbers of these new crystals. Lahoti et al. [16] stated that the first major shrinkage in geopolymers under the influence of high temperature starts at 150–300°C, and the second major shrinkage starts around 600°C in geopolymers produced with activators containing sodium ions. Regarding these findings and probability of presence of components such as C-S-H and CH in the matrix, it was decided to conduct high temperature resistance tests at 300, 600 and 900°C in the present study. The experiments were carried out in a muffle furnace. The temperature was increased at a rate of 20°C/min and the exposure time at the target temperature was set as 3 hours. At the end of this time, the samples were left to cool in the closed furnace. The compressive strength tests were applied immediately when the specimens cooled to the room temperature.

3.1. Determination of Reference Mixtures

The compressive strength of Class F and C fly ash-based geopolymer mortars are shown in Figure 4 and Figure 5, respectively. With the increase in the curing time, the compressive strength of both Class C and Class F fly ash-based mortars increased. At the same Ms ratio and Na2O percentage Class F fly ash mortars were found to be superior to the Class C fly ash mixtures. It is thought that the high water requirement of Class C fly ash-based mortars for a given flow is the cause of their low strength. The highest compressive strengths at all Ms ratios and Na2O percentages were obtained with 5 days of curing. However, the increase in compressive strength was limited after 3 days. Chithambaram et al. [17] investigated the compressive strength of fly ash-based geopolymer mortars containing sodium silicate-sodium hydroxide activator in different concentrations at different curing times (3, 7, 28 days) and reported that the compressive strength increased with the increase in curing time, but the rate of gaining strength decreased in time. Bellum et al. [18] examined the 7, 14, 28, 60 and

Figure 5. Compressive strength of class C fly ash-based geopolymer mortars.

Figure 6. Compressive strength of Class F fly ash-based geopolymer mortars including cement and lime. 91- day compressive strength of slag-bearing fly ash-based geopolymers and stated that the increase in curing time had a positive effect on the compressive strength. When the strengths of the mortars prepared with the same Ms ratio and cured for the same time are examined, it is seen that the compressive strengths of the geopolymers produced with both fly ashes increased significantly with the increase in the Na2O percentage. The highest compressive strength achievable in mortars produced with activator having 4% Na2O content was approximately 10 and 6 MPa for Class F and C fly ash mixtures, respectively. However, with the increase of Na2O ratio, compressive strengths beyond 55 MPa in Class F fly ash geopolymers, and around

1.3. Ms ratio and 10% Na2O content subjected to 5 days

curing showed the highest compressive strength. However, since the compressive strengths of the mixtures cured for 3 and 5 days were very close to each other, thus, 3 days was decided to be sufficient for the curing of the mixtures.

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Figure 7. Compressive strength of Class C fly ash-based geopolymer mortars including cement, lime and retarder.

Figure 8. Compressive strength of geopolymer mortars exposed to high temperatures.

Figure 9. Relative compressive strength of geopolymer mortars exposed to high temperatures.

3.2. Determination of Effect of Cement and Slaked

Lime Replacement The effect of cement and lime on compressive strength of Class F and C fly ash-based geopolymer mortars are shown in Figure 6 and Figure 7, respectively. Within the scope of the study, 10, 20 and 30% lime and cement were replaced with Class F fly ash and the effect of the replacement of these materials on the compressive strength was investigated. The replacement levels of Class C fly ash with cement were same as those of Class F fly ash. However, the substitutions levels of lime with Class C fly ash was somewhat different, i.e., 5, 10, 20 and 30%. Since it was determined that the lime substituted Class C fly ash geopolymers set very quickly (with in 3–4 minutes), mixtures with set retarder were also produced with two different admixture dosages (2.5 and 5.0% by weight of fly

ash). In spite of presence of set retarder admixture, the Class C fly ash-based geopolymer containing 30% lime showed flash set during mixing. Therefore, the mixture containing 30% lime could not be produced. Similarly, workability deficiencies were observed in Class F fly ash mixtures containing 30% lime. Vafaei and Allahverdi [20] studied the effect of partial substitution of natural pozzolan with calcium aluminate cement on the compressive strength of geopolymer mortars. It was reported that the compressive strengths increased with calcium aluminate cement replacement. The strengths increased gradually further with increasing the cement replacement level. The researcher stated that the increased amount of alumina in the geopolymer mixture is important for the formation of N-A-S-H gel, and extra CaO in cement may react with SiO2 and Al2O3 to form C-A-S-H. Yip and van Deventer [21] investigated the formation of C-S-H in geopolymers produced using metakaolin and blast furnace slag. Researchers reported that when calcium-containing aluminosilicates are used in the production of geopolymers, C-S-H could form in the matrix in addition to the geopolymer gel. However, the Ca/Si ratio of this C-S-H was found to be lower than that formed upon portland cement hydration. The increase in strength of the geopolymer mixtures observed in this study was attributed to the C-S-H formation. In the present study, it was observed that the compressive strength of Class F fly ash mortars decreased by 8.9 to 24.4% with cement and lime replacement. However, calcium inclusion was expected to improve strength by formation of additional phases (like C-S-H or C-A-S-H) in the matrix. The fact seems to be due to the effect of the extra water requirement of the mixture for the same workability. A similar situation was observed in Class C fly ash upon substitution of small amount of lime. With lime replacement, a strength loss of up to 10.9% occurred in the compressive strength of these mixtures. The addition of retarder resulted in a further reduction in strength. The water present in the retarder is thought to be the reason for the further strength reduction. However, in spite of its high water content, the compressive strength of cement-bearing Class C fly ash mixtures improved at all inclusion ratios reaching 34.5% and 46.6% upon 20% and 30% cement addition. The improvement in compressive strength of these mixtures, in spite of their high water content, was probably due to the presence of new phases in the matrix. Chindaprasirt et al. [22] also reported that calcium hydroxide and cement substitution improved the compressive strength of alkali-activated fly ash pastes. According to the researchers, this was related to the formation of additional C-S-H and C-A-S-H phases in the matrix and the shortening of the both initial and final setting times. Temuujin et al. [11] stated that the compressive strength of fly ash-based geopolymer pastes cured at ambient temperature increased with the inclusion of CaO and Ca(OH)2, but on the contrary, the compressive strengths decreased upon curing at 70°C. The researchers attributed the fact to the increased rate of evaporation of water at high temperature increasing the porosity.

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3.3. Determination of the Resistance of Selected

Mixtures to High-Temperature High- temperature resistance is also one of the important durability issues for geopolymers. There are many studies on the high- temperature resistance of geopolymers in the literature. F-C30, C-C30, F-L20 and C-L20 mixtures were selected for high-temperature resistance tests. Except for F-L20 mixture, the others showed the highest strength in their own group. Owing to its very short setting time, F-L30 mixture having the highest strength in its group was not selected. The compressive strengths of the selected geopolymer mortars before and after exposure to 300, 600, 900°C are shown in Figure 8, and the relative compressive strengths are shown in Figure 9. The compressive strength of both Class C and Class F fly ash-based mortars decreased gradually with the increase in temperature. The reduction in strength of Class F fly ash mixtures after exposing to 300°C was in the range of 19.6 to 28.8%. The corresponding range was 51.2 to 60.7% in Class C fly ash mortars. The higher amount of calcium in Class C fly ash-based mixtures seems to be the reason for this situation. With the increase in temperature, the strength losses also increased so that upon exposure to 900°C the residual strengths in the Class F and C fly ash-based mortars were merely in the range of 29.1 to 34.0% and 6.8 to 19.7%, respectively. The presence of 16–17 MPa residual strength in Class F fly ash mixtures even after exposure to 900°C is a good measure of the high-temperature resistance of these mixtures. Klima et al. [23] stated that the evaporation of the water present in the geopolymers at high temperatures caused both thermal shrinkage and vapour pressure damaging the structure. Payakaniti et al. [13] reported that the compressive strength losses at temperatures higher than 400°C in geopolymers may be caused by thermal stresses, and the changes in the crystal structure at 1200°C was found to be responsible for further reduction of the mechanical properties. A very important part of the strength loss experienced within the scope of the present study was observed at temperatures up to 600°C. It is thought that the evaporation of water coupled with thermal shrinkage/stress are the causes of the strength loss. As it can be seen from Figure 9, the loss in strength of Class F fly ash mixtures either with or without inclusions upon exposure to extreme temperatures are very close to each other, irrespective of the temperature level. This indicates that either the presence or type of the substitute has no significant effect on residual strength of Class F fly ash geopolymer exposed to high temperature. However, the opposite is true for Class C fly ash-based mortars. The cement and lime substituted Class C mixtures showed considerably higher loss in strength than their control mixture particularly when exposed to 600 and 900°C. The fact probably is arisen from the increase in the amount of C-S-H in the matrix due to the increased calcium content, which also negatively affects the compressive strength after exposing to high temperatures.