Comparison of the Influence of Silica‐rich Supplementary Cementitious Materials on Cement Mortar Composites: Mechanical and Microstructural Assessment

The silica-rich supplementary cementitious materials (SCMs) are the key components of mechanical and microstructural properties. The use of SCMs results in improving the mechanical and microstructural properties and decreasing the environmental burden caused by cement production. In this regard, this paper reports a study to compare the influence of silica-rich supplementary cementitious materials (slag, fly ash, and bottom ash) having similar Blaine fineness on cement mortar composites in terms of mechanical and microstructural properties. The Blaine fineness of supplementary cementitious materials was chosen at cement Blaine fineness by using short grinding time due to minimize energy consumption and cost. Supplementary cementitious materials (slag, fly ash, and bottom ash) were ground at similar cement Blaine fineness (~ 3300 cm2/g) and then by replacing 5 % and 20 % with cement, the 7-, 28-, 90-day mechanical and microstructural properties of cement mortar composites incorporating SCMs were examined. As a result, it was observed that the compressive strength properties of cement mortar composites incorporating 20 % slag gave similar strength of control samples (without SCM) according to samples with fly ash and bottom ash having similar fineness and this will decrease the required amount of cement and grinding time for the target strength properties, thus the number of CO2 emitted to nature will also decrease and less energy consumption, and the cost-effectiveness will be ensured by shorter grinding time for target strength.


Introduction
Supplementary cementitious materials (SCMs), including fly ash, ground granulated blast furnace slag, bottom ash, silica fume, etc., are widely used in clinker to make Portland cement or added as a replacement for a portion of ordinary Portland cement in concrete [1][2][3][4][5][6]. This is due to the potential capability of supplementary cementitious materials to increase the properties of concrete through their filler effect, as well as pozzolanic reaction. Also, most of the SCMs are by-product materials; hence, the use of by-products prevents the destruction of the non-renewable resources by hindering the use of very limited natural materials and minimizes environmental problems by the disposal of waste [7,8]. Concrete is of great importance for the evaluation of industrial by-products. Currently, concrete is widely used with more than 10 billion tons produced annually in the world [9]. The amount of CO 2 gas released to nature during production increases. The cement industry is responsible for about 7 % of the total CO 2 emissions in the world. It has been shown that every ton of ordinary Portland cement manufactured emits almost one ton of CO 2 into the atmosphere [10,11]. One of the main methods of reducing this emission is to keep the properties of cement as a building material and to substitute industrial by-products with the cement. Their utilization in concrete can serve as an effective means of disposal. Moreover, their utilization as supplementary cementitious materials to partially replace cement could protect the natural resources needed to produce cement and thus can contribute to sustainable concrete manufacture.
Supplementary cementitious materials are defined as materials containing high levels of silica and alumina, which do not have binding properties on their own but have hydraulic binding properties when combined with water and calcium hydroxide (CH) [12,13]. The blast furnace slag, fly ash, bottom ash represents the majority of supplementary cementitious materials. The utilization of supplementary cementitious materials as a component of concrete happened to ordinary [14][15][16][17][18][19][20][21][22][23]. It has been shown in the literature that the use of supplementary cementitious materials as cement additive improves the mechanical and durability properties of concrete [24][25][26]. These improvements depend on the different properties of the supplementary cementitious materials. One of these properties is the Blaine fineness of supplementary cementitious materials. Research examining the effect of Blaine fineness of supplementary cementitious materials on cementitious composites is available in the literature.
Bentz et al. investigated the effect of fly ash Blaine fineness as a supplementary cementitious material in their studies. The compressive strength of the samples containing fly ash with higher Blaine fineness was higher than the reference mixture containing 100 % Portland cement [27,28]. Slanicka [29] explored the effect of utilization of fly ash as supplementary cementitious material in concrete with different Blaine fineness on mechanical properties. In their study, the Blaine fineness led to prominent differences in the mechanical properties of concrete. Zhang et al. [30] substituted 50 % of the ground blast furnace slag to cement having Blaine fineness 3460 and 8380 cm 2 /g. It was showed that ground granulated blast furnace slag having higher Blaine fineness has higher mechanical properties. Li and Zhao [31] produced high strength concrete by ground blast furnace slag and ground fly ash in different ratios and Blaine fineness adding them to cement as an additive. As a result, it has been shown that the composites in which ground blast furnace slag and ground fly ash gave positive effects both in the short term and in the long term. Monzo et al. [32] investigated the mechanical properties of cement mortar composites incorporating fly ash with different Blaine fineness at a 30 % replacement ratio. They stated that the mechanical properties increased with the increase of fly ash fineness. Sevim and Demir [33,34] and Filazi et al. [35] explore that the mechanical and durability properties of cement mortar composites with fly ash replacement with help of particle size distribution of fly ash. They stated that all properties increased with the increase of fly ash fineness with the help of particle size distribution. Karim et al. [36] showed that the strength development of alkali-activated binder-mortar is greatly influenced by the types and fineness of SCMs. Ma et al. [37] observed that the pozzolanic activity increases with increasing waste brick powder fineness. Alaskar et al. [38] showed that increased fineness of the silica fume blended cement increased the compressive strength of concrete. Shaikh and Arel [39] showed that the compressive strength of steel fiber reinforced concrete increase with the increase in fly ash fineness. Sun et al. [40] indicate that the compressive strength increases linearly with the increase of fly ash fineness. Maeijer et al. [41] showed that an increased fineness of fly ash contributes to better strength, durability, and workability. Hallet et al. [42] showed that an increased fineness of blast furnace slag did not yield significant compressive strength enhancement at early ages, but the increased reactivity for higher slag fineness led to more hydration products and consequently higher compressive strengths at 28 days. Khongpermgoson et al. [43] showed that grinding coal bottom ash improved its quality by achieving high fineness, and it increased strength.
Having carried out a thorough literature review, to date, existing studies only focused on the influence of different fineness of some kind of supplementary cementitious materials on cement-based composite properties. To address the above-mentioned research gap, this paper reports a study to compare the influence of silica-rich supplementary cementitious materials (slag, fly ash, and bottom ash) having similar Blaine fineness on cement mortar composites in terms of mechanical and microstructural properties. Since the high grinding time reveals high specific energy consumption and is not cost-effective, the Blaine fineness of supplementary cementitious materials was chosen at cement Blaine fineness by using the short grinding time to minimize energy consumption and cost. In this regard, supplementary cementitious materials (slag, fly ash, and bottom ash) were ground at cement Blaine fineness (~3300 cm 2 /g) and then by replacing 5 % and 20 % with cement, the 7-, 28-, 90-day mechanical and microstructural properties of cement mortar composites incorporating SCMs were examined. The findings of the present study contribute significantly to the understanding of how to improve mechanical and microstructural properties in the development of cement mortar composites using more SCM with the help were used. Bottom ash was calcined at 1000˚C to minimize the carbon content. The slag, fly ash, and bottom ash was ground in a ring mill at cement Blaine fineness (~3300 cm 2 / g). The physical and chemical properties of the OPC, GGBFS, FA, and BA are given in Table 1. Minor oxides were not considered in chemical oxide analysis.

Sand
The sand used in this study is the CEN reference sand specified in EN 196-1 [45] standard.

Methods
The Blaine fineness of supplementary cementitious materials was chosen at cement Blaine fineness by using short grinding time due to minimize energy consumption and cost. In this regard, supplementary cementitious materials (slag, fly ash, and bottom ash) were ground at cement Blaine fineness (3 300 cm 2 /g) and then by replacing 5 % and 20 % with cement, the 7-, 28-, 90-day mechanical and microstructural properties of cement mortar composites incorporating SCMs were examined.

Mixture Proportions
The sample coding and mixture design of the cement mortar composites incorporating SCMs are given in Table 2. To assess the hydration degree of the supplementary cementitious materials are used at replacement level 20 % [46][47][48], in this regard, a 20 % replacement ratio was used in this present study. Also, the effect of a 5 % replacement ratio was assessed in this study.

Test for Compressive Strength
The compressive strength tests were carried out complying with EN 196-1 [45]. The mixture ratios given in Table 2 were

Test for Microstructure Analysis
The microstructural properties of cement mortar composites incorporating SCMs kept in water curing were examined at 7, 28, and 90 days and 5 % and 20 % ground granulated blast furnace slag, fly ash, and bottom ash replacement ratio. Microstructures were examined with the help of a scanning electron microscope. For performing the microstructure surface inspection, specimens were applied gold coating with Sputter Coater. The specimens were investigated by scanning electron microscopy. Scanning Electron Microscope (SEM) image is obtained by collecting the effects of elastic and inelastic collisions between electrons and sample atoms during scanning of the electron beam accelerated by high voltage and focused on the sample on the sample surface, passing through the signal amplifiers and transferring it to the screen of the cathode ray's tube.

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 was explored and are given in Table 3 and illustrated in Figs. 1, 2 and 3.
When the percentage change of compressive strengths at 7 days cement mortar composites incorporating SCMs were examined (Fig. 1), 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 (Fig. 2), 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 (Fig. 3), 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 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 SiO 2 and calcium oxide (CaO), providing the formation of C 3 S, C 2 S, 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][50][51]. Note that the calcium oxide   ash, due to the low amount of calcium oxide (CaO), it could not contribute to the formation of C 3 S and C 2 S 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. Also, the fact that the SiO 2 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.
As a result, it was observed that the compressive strength properties of cement mortar composites incorporating 20 % slag gave similar strength of control samples (without SCM) according to samples with fly ash and bottom ash having similar fineness and this will decrease the required amount of cement and grinding time for the target strength properties, thus the number of CO 2 emitted to nature will also decrease and less energy consumption, and the cost-effectiveness will be ensured by shorter grinding time for target strength.

Microstructure Analysis
The microstructural properties of cement mortar composites incorporating SCMs were examined at 7, 28, and 90 days and 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),  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.

Microstructure Analysis of Additive-free Samples
7-day SEM images of the samples without additive are shown 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 shown 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  SEM images of the samples at 90 days are shown 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 weblike 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.

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 shown 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 shown in Fig. 8. When these images are examined, it is seen that the  SEM images obtained at 2000× and 5000× magnification at 90-day samples with a 5 % slag replacement ratio are shown in Fig. 9. When the images are evaluated, C-S-H gels are partially seen the 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 shown in Figs. 10, 11, and 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.

Microstructure Analysis of Samples Having FA
Microstructure studies were carried out at 7-, 28-and 90-days curing times according to the 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×   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 shown 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, filling the voids. The weblike 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 shown 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 shown 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 shown 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  SEM images of the samples at 2000× and 5000× magnification at 90 days with a 20 % fly ash replacement ratio are shown 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.

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 shown 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 shown 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, filling the voids. The weblike 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   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 shown 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 shown 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 shown 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 SiO 2 amounts of slag are 39.60 % and 36.10 %, respectively. The high amount of calcium oxide (CaO) could contribute to the formation of C 3 S and C 2 S 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.

Conclusions
This study investigated the effect of the microstructural and mechanical properties of cement mortar composites incorporating fly ash, slag, and bottom ash as supplementary cementitious materials with similar Blaine fineness. The fly ash, slag, and bottom ash were ground at a similar cement Blaine fineness (~3300 cm 2 /g), and then by replacing 5 % and 20 % with cement, the 7-, 28-, 90-day microstructural and compressive strength properties of cement mortar composites incorporating SCMs were examined. Following findings were drawn:  & It is considered that the differences in the compressive strength and microstructure of cement mortar composites incorporating slag, fly ash, and bottom ash with similar Blaine fineness caused by the differences in the chemical composition of the samples. Although the material with the highest amount of SiO 2 is fly ash, due to the low amount of calcium oxide (CaO), C 3 S could not contribute as much to the formation of C 2 S as slag. Therefore, the compressive strength of the cement mortar composites incorporating slag was found to be higher than the fly ash and bottom ash substituting samples. & Minimizing the grinding time, less energy consumption, and the cost-effective process was ensured by shorter grinding time for target cement mortar composite strength. & As a result of the microstructure analysis of the produced cement mortar samples, it was observed that the web-like structure of C-S-H gels became denser and filled the voids in the microstructure depending on the maturation period. & Microstructure analyses showed the reduced quantity of voids in the cement mortar composites incorporating SCMs, due to C-S-H phase formation.
As a result, it is likely to improve the mechanical and microstructural properties of the cement mortar composites using a more slag replacement ratio (20 %) instead of cement and other SCMs having similar fineness. This will decrease the required amount of cement for target properties by using SCMs type and less energy consumption, and the cost-effectiveness will be ensured by shorter grinding time for target strength. Also, depending on the lessen in the quantity of cement, the number of CO 2 emitted to nature will also decrease. These supplementary cementitious materials could be used in the construction sector and could much advantage the economy and environment.