3.1. Compressive strength
The compressive strengths at 7-, 28-and 90-day cement mortar composites incorporating slag, fly ash, and bottom ash with similar Blaine fineness at 5% and 20% replacement ratio were explored and are given in Table 3.
Table 3
The compressive and flexural strengths at 7-, 28- and 90-days old cement mortar composites incorporating GGBS, FA, and BA with similar Blaine fineness.
Samples | Compressive strength (MPa) |
7-day | 28-day | 90-day |
Control | 35.65 | 46.60 | 54.43 |
FA5 | 34.22 | 45.77 | 54.15 |
FA20 | 30.51 | 41.88 | 53.23 |
BA5 | 33.78 | 42.60 | 53.44 |
BA20 | 30.05 | 41.17 | 50.74 |
GGBS5 | 34.36 | 45.98 | 54.21 |
GGBS20 | 32.23 | 43.17 | 53.87 |
When the percentage change of compressive strengths at 7 days cement mortar composites incorporating SCMs were examined, the percentage change of compressive strength results of the GGBFS5, FA5, BA5 samples with 5% replacement ratio were found 3.6%, 4.0%, and 5.3% lower than the control sample (35.65 MPa), respectively. At 7 days old samples with a 5% replacement ratio, the highest compressive strength result was observed in samples with slag and the lowest compressive strength result was observed in samples with bottom ash replacement. The percentage change of compressive strength results of GGBFS20, FA20, BA20 samples with a 20% replacement ratio were found 9.6%, 14.4%, and 15.7% lower than the control sample, respectively. At 7 days old samples with a 20% replacement ratio, the highest compressive strength result was observed in samples with slag replacement, and the lowest compressive strength was observed in samples with bottom ash. When the percentage change of compressive strengths at 28 days cement mortar composites incorporating SCMs were examined, the percentage change of compressive strength results of the GGBFS5, FA5, BA5 samples with 5% replacement ratio were found 1.3%, 1.8%, and 8.6% lower than the control sample (46.60 MPa), respectively. At 28 days old samples with a 5% replacement ratio, the highest compressive strength result was observed in samples with slag and the lowest compressive strength result was observed in samples with bottom ash replacement. The percentage change of compressive strength results of GGBFS20, FA20, BA20 samples with a 20% replacement ratio were found 7.4%, 10.1%, and 3.4% lower than the control sample, respectively. At 28 days old samples with a 20% replacement ratio, the highest compressive strength result was observed in samples with slag replacement, and the lowest compressive strength was observed in samples with bottom ash. When the percentage change of compressive strengths at 90 days cement mortar composites incorporating SCMs were examined, the percentage change of compressive strength results of the GGBFS5, FA5, BA5 samples with 5% replacement ratio were found 0.4%, 0.5%, and 1.8% lower than the control sample (54.43 MPa), respectively. At 90 days old samples with a 5% replacement ratio, the highest compressive strength result was observed in samples with slag and the lowest compressive strength result was observed in samples with bottom ash replacement. All results were similar at 90 days samples at a 5% replacement ratio. The percentage change of compressive strength results of GGBFS20, FA20, BA20 samples with a 20% replacement ratio were found 1.0%, 2.2%, and 6.8% lower than the control sample, respectively. At 90 days old samples with a 20% replacement ratio, the highest compressive strength result was observed in samples with slag replacement, and the lowest compressive strength was observed in samples with bottom ash.
When the compressive strength results of cement mortar composites incorporating slag, fly ash, and bottom ash with similar Blaine fineness are explored, the compressive strength of the composites incorporating slag gave the highest results, followed by composites incorporating fly ash and the lowest results has been observed composites incorporating bottom ash. When looking at the chemical properties of these SCMs substituting into cement, it is necessary to evaluate the components of SiO2 and calcium oxide (CaO), providing the formation of C3S, C2S, which play a major role in the cement's binding property and the formation of its first and final strength. The use of SCMs with low calcium content generally decreases the mechanical strength of cement mortar composites at early ages when compared to cement mortar composites without additives. On the other hand, the use of SCMs with high-calcium content has a positive effect on strength development. The use of supplementary cementitious materials with low calcium content usually reduces the mechanical strength of cementitious composite systems at early ages when compared to cementitious composites without additives. On the other hand, the use of supplementary cementitious materials with high-calcium content has a positive effect on strength development [49–51]. Note that the calcium oxide (CaO) amounts of slag, fly ash and bottom ash are 39.60%, 16.89%, 15.35%, respectively and the amount of SiO2 in the chemical structure of these SCMs is 36.10%, 44.69%, and 42.35% for slag, fly ash and bottom ash, respectively. Although the material with the highest amount of SiO2 is fly ash, due to the low amount of calcium oxide (CaO), it could not contribute to the formation of C3S and C2S as much as slag. Therefore, the compressive strength results of composites with slag replacement are higher than composites incorporating fly ash and bottom ash replacement with similar Blaine fineness. The cementing ability of slag is very high, and it has the highest specific gravity, therefore it gave the best results. In addition, the fact that the SiO2 and CaO amounts of the composites incorporating fly ash were higher than the composites incorporating bottom ash, causing the strength results of the composites incorporating fly ash to be higher than the composites having bottom ash replacement. Note that the highest loss on ignition is 5.21, which is in bottom ash, so the lowest mechanical results are found in cement mortar composites incorporation bottom ash.
3.2. Microstructure analysis
The microstructural properties of cement mortar composites incorporating SCMs were examined at 7, 28, and 90 days and at 5% and 20% slag, fly ash, and bottom ash replacement ratio. Microstructures were examined with the help of a scanning electron microscope.
Cement mortar composites have hydration products such as calcium silicate hydrate (C-S-H), calcium hydroxide (CH), and ettringite (C-A-S-H) in their microstructure. Calcium silicate hydrate gels are the most important phase affecting the properties of cement paste with the progress of the hydration process. The morphological structure of C-S-H gel is that they gain a web-like structure from weak crystalline fibers. In the early stages of the hydration process, C-S-H gels develop into water-filled spaces by forming thin layers of low density. During the hydration process, C-S-H concentrates around the hydrated cement particles and covers all particles. CH morphology has a structure that varies structurally from a generally undefined shape to a cluster of large plaques. Compared to C-S-H gels, it contributes less to strength due to its low surface area.
Calcium silicate hydrate (C-S-H), calcium hydroxide (CH) developments of microstructure studies of cement mortar composites incorporating slag, fly ash, bottom ash samples, and control samples were investigated by scanning electron microscope at 2000× and 5000× magnifications.
3.2.1. Microstructure analysis of additive-free samples
7-day SEM images of the samples without additive are given in Fig. 4. It was observed in the images that C-S-H gels started to develop at 7-day-old samples at 2000× and 5000× magnifications and CH gels were formed. It is seen that the density of C-S-H in the samples started to increase and developed by covering the CH gels. CH gels reach 20 µm in size, while C-S-H gels with 10 µm dimensions are in the first development process in the early strength process.
SEM images of the samples at 28 days are given in Fig. 5. When the images are examined, it is seen that the C-S-H gels reach a web-like structure and cover the entire surface. The CH gels in the structure are covered with the web-like structure of C-S-H gels. C-S-H gels fill the gaps in the cement mortar composites during the hydration process. At the 2000× and 5000× images of the samples, the web-like structure of the C-S-H gels was seen in Fig. 5 to fill the void structure in the cement mortar composites. C-S-H gels observed at 5000× magnification at 7-day images of the samples at a size of 2 µm reach approximately ranges from 20 to 25 µm at 28-day samples and cover the entire structure.
SEM images of the samples at 90 days are given in Fig. 6. In the images, it is seen that the web-like structure of the developed C-S-H gels has completely developed, and the structures have increased to 30 µm and above and this web-like structure completely covers the microstructure of the cement mortar composites.
Developments in the microstructure of cement directly affect the strength and durability properties of cement mortar composites. The increase in the formation of dense C-S-H gels in the samples without additives due to the curing time causes the compressive strength to increase depending on the curing time.
3.2.2. Microstructure analysis of samples having GGBFS.
Microstructure analysis of the slag added samples were performed at 7-, 28- and 90-days curing times according to 5% and 20% replacement ratio. SEM images at 7-day samples with a 5% slag replacement rate at 2000× and 5000× magnification is given in Fig. 7. When the images are examined, it is seen that the web-like structure of the C-S-H gels is formed and begins to condense on the surface. The fibrous C-S-H gels that filled the gaps reached a size of approximately ranges from 5 to 10 µm.
SEM images of the samples with the same replacement ratio at 28-day at 2000× and 5000× magnification is given in Fig. 8. When these images are examined, it is seen that the density of C-S-H gels has increased compared to 7-day samples and completely covers the structure. C-S-H gels developed in the form of a web, completely filling the void structures and reached a size of approximately 20 µm.
SEM images obtained at 2000× and 5000× magnification at 90-day samples with a 5% slag replacement ratio are given in Fig. 9. When the images are evaluated, C-S-H gels are partially seen size of approximately ranges from 30 to 40 µm, and it is seen that the ettringite needles are concentrated and reached a size of approximately 50 µm.
SEM images of the samples with 20% slag replacement ratio at 7, 28, and 90 days at 2000× and 5000× magnification are given in Fig. 10, Fig. 11, and Fig. 12, respectively. When the SEM images were examined, it was observed that fibrous C-S-H gels developed and started to condense at 7-day-old samples. It has been observed that the web-like structures formed by the gels develop in a size of approximately ranges from 5 to 10 µm.
At the 28-day SEM images of the samples with the same replacement ratio, it was observed that the C-S-H gels covered the entire structure, developed in the voids and the web-like C-S-H structure developed and took the form of a cactus. The cactus shaped C-S-H phase has reached the size of about 20 µm. The development of the gels increased compared to 7-day samples.
When the 90-day SEM images were examined, it was seen that C-S-H gels and ettringite needles covered the entire structure. C-S-H gels and ettringite needles were observed to be range from 30 to 40 µm at 5000× magnification.
3.2.3. Microstructure analysis of samples having FA.
Microstructure studies were carried out at 7-, 28- and 90-days curing times according to 5% and 20% replacement ratio of fly ash substituting samples. SEM images at 7-day samples with a 5% fly ash replacement ratio, at 2000× and 5000× magnification, are given in Fig. 13. When the images are examined, it is seen that the web-like structure of the C-S-H gels started to form. The gels that started to fill the gaps reached a size of approximately 5 µm.
SEM images of the samples with the same replacement ratio at 28 days, at 2000× and 5000× magnification, are given in Fig. 14. When these images are examined, it is seen that the density of C-S-H gels has increased visibly compared to 7-day samples and started to cover the structure completely. C-S-H gels developed in web-like form, completely filling the voids. The web-like structure bonded and developed, reaching approximately 15 µm in size.
SEM images of the samples at 2000× and 5000× magnification at 90 days with a 5% fly ash replacement ratio are given in Fig. 15. When the images were examined, it was observed that the size of the C-S-H gels increased, and the web-like structure thickened and filled the voids. It was observed that the sizes of the C-S-H gels were in the range from 20 to 30 µm. He also observed ettringite needles of range from 20 to 30 µm in size.
SEM images at 7-day samples with 20% fly ash replacement ratio, at 2000× and 5000× magnification, are given in Fig. 16. When the images are examined, it is seen that the web-like structure of the C-S-H gels started to form. CH gels were also observed in the microstructure and it was observed that C-S-H gels started to cover the CH gels with a web-like structure. It was observed that the C-S-H gels that started to fill the gaps reached a size of approximately 10 µm and the CH gels were approximately 20 µm in size.
SEM images of the samples with the same replacement ratio at 2000× and 5000× magnification at 28 days are given in Fig. 17. When these images are examined, it is seen that the density of C-S-H gels has increased visibly compared to 7-day samples and started to cover the structure completely. C-S-H gels developed in web-like form, completely filling the voids. The web-like structure bonded together and developed and reached the size of approximately 20 µm.
SEM images of the samples at 2000× and 5000× magnification at 90 days with a 20% fly ash replacement ratio are given in Fig. 18. When the images were examined, it was observed that the size of the C-S-H gels increased, and the web-like structure thickened and filled the voids. It was observed that the dimensions of the web formed by C-S-H gels were approximately 30 µm.
3.2.4. Microstructure analysis of samples having BA.
Microstructure studies were carried out at 7-, 28- and 90-days curing times according to the 5% and 20% replacement ratio of the bottom ash- substituting samples. SEM images at 7-day samples, with a 5% bottom ash replacement ratio at 2000× and 5000× magnification, are given in Fig. 19. When the images are examined, it is seen that the web-like structure of the C-S-H gels started to form. It was observed that the sizes of the C-S-H gels that started to fill the gaps were in the range of approximately 5 µm.
SEM images of the samples with the same replacement ratio at 2000× and 5000× magnification at 28 days are given in Fig. 20. When these images are examined, it is seen that the density of C-S-H gels has increased visibly compared to 7-day samples and started to cover the structure completely. C-S-H gels developed in web-like form, completely filling the voids. The web-like structure bonded together and developed and reached the size of approximately 10 µm.
SEM images obtained at 2000× and 5000× magnification at 90-day samples with a 5% bottom ash replacement ratio are given in Fig. 21. When the images were examined, it was observed that the size of the C-S-H gels increased compared to the 7- and 28-day samples with the same replacement ratio and the web-like structure became denser. It was observed that the dimensions of the web formed by C-S-H gels were in the range of approximately ranges from 20 to 30 µm.
SEM images at 7-day samples with a 20% replacement ratio, taken at 2000× and 5000× magnification, are given in Fig. 22. When the images were examined, it was observed that the web-like structure of C-S-H gels started to form. CH gels were also seen in the microstructure, and it was observed that the web-like structure of C-S-H gels developed around the CH gels. It was observed that the C-S-H gels that started to fill the gaps reached a size of approximately 5 µm and that the CH gels were in the range of approximately ranges from 5 to 10 µm.
SEM images of the samples with the same replacement ratio at 2000× and 5000× magnification at 28 days are given in Fig. 23. When these images are examined, it is seen that the density of C-S-H gels increased at 7-day-old samples and started to cover the structure completely. It is seen that the cactus-shaped structures formed by the combination of C-S-H gels reach approximately 20 µm in size.
SEM images obtained at 2000× and 5000× magnification at 90-day samples with 20% bottom ash replacement ratio are given in Fig. 24. When the images were examined, it was observed that the sizes of the C-S-H gels increased compared to the 7- and 28-day old samples with the same replacement ratio, the web-like structure was thickened by intertwining and took the form of a cactus. It was observed that the dimensions of the web-like formed by the C-S-H gels were in the range of approximately ranges from 20 to 30 µm.
According to the microstructural analysis by the scanning electron microscope, it was found more C-S-H concentration and bigger C-S-H gels in cement mortar composites incorporating blast furnace slag. Recall that the calcium oxide (CaO) and SiO2 amounts of slag are 39.60% and 36.10, respectively. The high amount of calcium oxide (CaO) could contribute to the formation of C3S and C2S and thus C-S-H forming with the pozzolanic capability and the filler effect might result in high compactness through mechanical particle filling and further formation of calcium-silicate-hydrate (C-S-H) gels by using GGBS. Note that the highest loss on ignition is 5.21, which is in bottom ash, so the shortest C-S-H gels are found in cement mortar composites incorporating bottom ash. It seems that filler effect and pozzolanic capability resulted in high compactness through mechanical particle filling and further formation of calcium-silicate-hydrate (C-S-H) gels. These results were found to be consistent with compressive strength results.