Performance evaluation of fly ash-lime-gypsum-quarry dust (FALGQ) bricks for sustainable construction

4.1.1 Fly ash

Fly ash exhibits a specific gravity of 2.19. Table 1 provides the silica and calcium contents in the fly ash, with values of 73.3 and 1.5%, respectively. Figure 1 displays the SEM image of a fly ash particle.

Figure 1

SEM image of fly ash.

Figure 1 showcases the structures of both pure fly ash and lead fly ash as observed using SEM. Within the SEM image, several small hollow spherical particles, termed cenospheres are visible, ranging in diameter from 4 to 23 µm. The predominant constituents include amorphous silicon dioxide (SiO₂), exhibiting a spherical and smooth form, and crystalline SiO₂, characterized by sharp, pointed, and needle-like shapes. The spherical shape indicated that the material is pozzolanic in nature. Additionally, the materials consist mainly of aluminium oxide (Al₂O₃) and iron oxide (Fe₂O₃). Typically, fly ash is a heterogeneous mixture of glassy particles containing a variety of distinguishable crystalline phases, including quartz, mullite, and different iron oxides. A few agglomerate particles also visible in the SEM image of lead fly ash particles now have an elongated shape. This figure also depicts the potential for lead ultrafine particles to adhere to fly ash cenosphere surfaces.

The XRD pattern of fly ash, as shown in Figure 2, indicates that the chemical composition is alkaline. The major phases present in the fly ash samples consist of silicon (Si), oxygen (O), titanium (Ti), and aluminum (Al). XRD analysis reveals high quantities of silica, alumina, and iron oxides. The resulting mineral phases identified in the XRD peaks include crystalline phases of quartz, mullite, haematite, and calcium oxide. The peak mineral identified in the XRD analysis, as shown in Table 1, provides the chemical composition of fly ash, which matches the results of the PDF 01-089-1961 for the compounds SiO₂ and quartz.

Figure 2

XRD image of Fly ash.

4.1.2 Lime

The specific gravity of lime is found to be 2.7. The CaO in the powder form having particles lower than 600 microns is used.

The SEM images of lime in Figure 3 show that particles are irregular in shape. The surface morphology of lime appears rough, consisting of micro-pore and fibre likes structure. It is determined that there are vertical and transverse overlapping structure, and each layer had a surface area resembling a dense, fibre-like structure. Additionally, the surface structure was obtained to be multi-layered and rough elements of the raw power were detected through EDS including MgO and CaO. The small grain sizes likely provide greater surface area and increased reactivity. The particle size should correlate proportionally with the surface area.

Figure 3

SEM image of lime.

According to XRD data, lime, the natural mussel shell, mostly contains CaCO₃, and is found in PDF01-070-5490. Crystalline structure of lime powder were determined by XRD analysis, the peak obtained from the XRD patterns represents the calcite phases and are shown in Figure 4. The most severe peaks seen in the XRD are calcite of CaCO₃ at 2-tetra scale 35⁰. The result of XRD also confirms the chemical composition of lime given in Table 1. Almost same results are seen in the other research journals.

Figure 4

XRD image of lime.

4.1.3 Gypsum

The CaO content in gypsum is found to be 44.12% and is given in Table 1 and the specific gravity of gypsum is 2.31.

Gypsum powder results as a by-product in the fertilizer industry. Gypsum can be used as a substitute for calcite. The SEM image of gypsum particles is irregular in shape and is shown in Figure 5, which illustrates the morphology of gypsum crystals. A bassanite type crystal structure was observed across all gypsum samples. These clusters appear in the structure of every gypsum type. The morphology varies slightly, with smooth surface aggregate and filament like particles being the dominant features. Traces of gypsum crystals can be identified on the surface using an SEM. The crystal clusters reach sizes as large as 20 microns. It is indicated that this material well bonded with other pozzolanic material.

Figure 5

SEM image of gypsum.

Gypsum is analysed by XRD method and confirmed the phase using PDF00-021-0816 CaSO₄·2H₂O, PDF01-072-7532 Apatite were identified from the result of XRD. There is a slight difference in the peak width of gypsum shown in Figure 6. Qualitatively, it can be observed that the peak corresponding to the lattice plane has a smaller value compared to the other peak in the sample containing format, while the other peaks show the same width in both samples. This can be explained by changes in the morphology of the gypsum crystals, which lead to variations in the size of the coherently scattering domains in different crystallographic directions. Peak broadening is related to the mean apparent crystallite sizes of the phases.

Figure 6

XRD image of gypsum.

4.1.4 Quarry dust

It serves as a filler substance and possesses a significant concentration of silica (SiO₂). The specific gravity of quarry dust is found to be 2.57.

The mineral present in quarry dust is given in Table 1 and Figure 7 represents the morphology. The particles exhibit an angular shape with a coarse surface texture and are present in considerable quantities. Though particles are closely packed and a bonding exists, voids are also seen. The angular form of the quarry dust increases the friction thereby reducing the characteristic flow of the concrete. Nevertheless, the intertwining of the angles could enhance the strength properties of the concrete and brick.

Figure 7

SEM image of quarry dust.

The quarry dust is analysed using XRD method and the amorphous content was found to be 71% for CaCO₃ PDF01-071-3699, SIO₂ (Quartz) PDF 01-079-0910, Calcium magnesium carbonate (Dolomite) CaMg (CO₃)₂ PDF00-011-0078, iron oxide Fe₂O₃ PFD 01-076-4113 were identified in phase report in Figure 8. The crystalline structure of this material was identified as triclinic. The material showed peaks at 2θ values around 27° and 29°. Potassium aluminosilicates (sodium feldspar, albite) and calcium comprise a group of minerals called feldspars. Feldspars, together with quartz and mica, form the primary components of granitic rocks, which are believed to have originated from igneous processes [11].

Figure 8

XRD image of quarry dust.

5.1.1 Preparation of specimen

The material’s components are bundled by weight, the binder’s combination of gypsum and lime. Ball mill is initially used to combine the lime and gypsum. The fly ash is added and properly mixed with the binder. These ingredients are prepared and then blended in a rotating motion with the quarry dust. A water-to-binder ratio of 0.2 was deemed suitable for the brick powder moulding procedure. The dry mix is poured into the concrete mixer along with water, and the mixing process is built. The mixture is depressed to create a homogenous, partially wet mix shown in Figure 9a, which is then moved to a mortar-driven moulding machine shown in Figure 9b, for compacting the loose material. This compresses the mixture within the mould. When the electric mortar rotates at a speed of 1,720 rpm, a pressure of 16 MPa is applied, simultaneously exerting a compressive force on the integrated mould to minutely compress the material inside. As a result, the dimensions of the fly ash brick are achieved at 230 mm × 110 mm × 70 mm. Subsequently, the formed fly ash brick samples are manually removed from the mould without any observed damage to the bricks during the extraction process. The freshly compacted brick was then dried for 24 h, afterwards the sample was cured for the remaining 6 days (3 times per day) as shown in Figure 9c. The specimen was examined for additional work after curing.

Figure 9

(a–d) Preparation of specimen, (a) mixing of materials, (b) preparation of brick, (c) curing, and (d) compressive testing machine.

5.1.2 Compressive strength

The compressive strength of brick specimen after curing for 7th , 14th, 28th, and 56th days are experimentally determined for an average of the six specimen as given in Table 3. The compressive strength is found to vary between 1 and 3.7 MPa for the 7th day and between 3.8 and 12 MPa for the 56th day. The increase in strength on the 58th day when compared to the 7th day is due to the pozzolanic reaction of the binder material in the brick specimens. The M₄ is found to have a strength of 3.7 N·mm⁻² on the 7th day which is greater than the minimum brick strength required for structural application as per IS 3495 (Part- 1):1976. The compressive strength of M₄ is found to be higher among M₁ to M₆ specimens. Similarly, the compressive strength of M₉ is found to be greater when compared to the other specimens for M₇ to M₁₃.

The particle packing and binding in specimen M₉ is found to be greater which is indicated by the lower voids in the microstructure evaluation using SEM Figure 1. This is the reason for having greater strength for the M₉ specimen when compared to other specimens.

The findings suggested that substituting fly ash with quarry dust is advantageous up to 35% level. The combination of quarry dust and binder significantly influences the overall strength of the brick specimen. The binder covers the inert quarry dust and bonds together and provides better stiffness to the matrix. If the quarry dust content is more, the coverage of the binder may not be sufficient to develop an effective bond inside the matrix. From the results, it is evident that with the increase in proportion of quarry dust content, the density and water absorption increases from the tolerance, but strength reduces. The maximum strength is found for M₉, hence the proportion of constituent M₉ is taken as the best proportion among the various mix from M₁ to M₁₃ from Figures 10 and 11.

Figure 10

(a)–(f) Void and crack pattern of fly ash brick without quarry dust content. (a) M₁, (b) M₂, (c) M₃, (d) M₄, (e) M₅, and (f) M₆.

Figure 11

Effect of fly ash and quarry dust on the IRA of FLGQ bricks.

The most important property of any brick is its compressive strength. A graph is prepared for a period of 56 days commencing with results on the 7th, 14th, 28th, and 56th day, respectively, for 13 specimens. The results of testing the FLGQ brick after 7, 14, 28, and 56 days are shown in Table 3. For the 7th day, the value of compressive strength varies from 1.3 to 3.1 MPa. For the 14th day, the value of CS varies from 1.7 to 5.2 MPa. The corresponding value ranges between 2.1 and 7.05 MPa for the 28th-day observation and is between 3.2 and 12.0 MPa for the 56th day. Thus, M₉ is selected as an ideal specimen with a maximum 56th day compressive strength as 12 MPa.

From the graph in Figure 12, an addition of quarry dust percentage up to 30% in FLGQ brick resulted in first an increase in compressive strength and then a slight decrease in compressive strength was observed.

Figure 12

Variation in 7th, 14th, 28th, and 56th day compressive strengths for various quarry dust content.

The observed strength improvement can be attributed to the filling of gaps among quarry dust particles with binding material, resulting in a denser and better-connected structure known as the filler effect. The finer texture of quarry dust powder offers a larger specific surface area, causing water added to the mix to surround the particles, reducing workability as the concentration of quarry dust powder increases. This reduced workability during casting contributes to the lower compressive strength seen in mixes containing more than 30% quarry dust and fly ash. The sudden decline in strength might be mainly due to increased bonding between loose particles resulting from the addition of quarry dust. As the amount of quarry dust powder increases, the workability of the concrete decreases, leading to insufficient compactness [18]. The compactness of the concrete is inversely related to the brick’s porosity, and an increase in porosity can reduce the compressive strength.

5.1.3 Density

The average density of the saturated surface brick specimens is determined. The density of the brick specimen is found to vary between 1,193 and 1,224 kg·m⁻³ for specimens M₁ to M₇ as shown in Figure 13. The highest density is found for the specimen M₄. The water absorption of brick is an indication of voids connected to the surface. The water gets filled in the voids when immersed for 24 h. The water absorption is found to be lower for brick specimen M₄ when compared to specimens designated by M₁ to M₆. This is because the voids in the M₄ specimen is relatively lower and is also observed in the SEM image Figure 10d. Similarly, specimen M₉ is found to have lower water absorption when compared to specimens M₇ to M₁₃. This indicated that the surface-connected pores are lower in M₉, and it corroborates with the SEM results given in Figure 14c.

Figure 13

Effect of fly ash and quarry dust on the density of FALGQ bricks.

Figure 14

(a–g) Void and crack pattern of fly ash brick with quarry dust content. (a) M₇, (a) M₈, (c) M₉, (d) M₁₀, (e) M₁₁, (f) M₁₂, and (g) M₁₃.

5.1.4 Water absorption

Brick seldom encounters water exposure when utilized for internal purposes, like forming wall elements within a structure. The water absorption of brick holds little significance in this context since the material remains unaffected by freeze-thaw cycles. However, when brick is employed as an external component, as seen in locations like pedestrian pathways, it comes in direct contact with water. In such cases, water absorption becomes crucial due to the adverse effects of freeze-thaw phenomena. Despite the absence of a specified limit for water absorption capacity in the EN 771-1 code, certain countries, including Brazil, have established regulations. According to the Brazilian standard NBR 6480 [25], the water absorption value must not surpass 22%, as illustrated in Figure 15.

Figure 15

Effect of fly ash and quarry dust on the water absorption of FALGQ bricks.

The graph depicts the outcomes of a water absorption test conducted on specimens over 56th day. As the percentage addition of quarry dust increases, the rate of water absorption also increases (15.9–20.5%).

5.1.5 Efflorescence

Effloresce is the indication of leaching out salty crystalline in bricks. These crystals are leached out in the presence of moisture and left deposited at the surface resulting in patches at the surface. The percentage of the surface covered by crystalline deposits was determined based on visual observation. The presence of more patches indicates the presence of more salt crystals. The specimens M₁ to M₁₃ showed no effloresce at the surface, which indicates that the brick does not contain salty crystalline.

5.1.6 Split tensile strength

The tensile strength of a brick when subjected to splitting is a critical factor in evaluating the overall tensile resilience of masonry structures under different stress conditions. Split tensile strength of brick measures the resistance against tension, which is essential to ensure that it can withstand crack/failure forces under load. The brick is compressed between two steel bars which causes splitting of the brick that ranges between 0.21 and 1.44 MPa, Figure 16. The specimen M₄ exhibits a higher split tensile strength compared to specimens M₁ to M₆. This indicates that M₄ is having less defects in it. Similarly, specimen M₉ is found to have higher split tensile strength among specimens M₇ to M₁₃. This is due to the fact that the M₉ has fewer voids which is confirmed in the SEM image given in Figure 14m.

Figure 16

Effect of fly ash and quarry dust on the split tensile strength of FALGQ bricks.

5.1.7 Initial rate of absorption

The significance of brick absorption lies in its impact on water availability for the effective hydration of cement mortar. When brick exhibits high initial absorption, it adversely influences the water supply for hydration. In Figure 11, the initial absorption rates for brick specimens M₄ and M₉ are 0.42 and 0.28 gm·cm⁻²·min⁻¹, respectively. These brick specimens demonstrate a low initial absorption rate, ensuring that internal water in the mortar remains retained, preventing its loss to brick absorption. Consequently, the bond between the brick and mortar is expected to be quite robust following the cement’s hydration in the mortar.

6.2.1 Compressive strength

The findings of the compressive strength test for the FLGQ brick are showcased in Table 3. The maximum 56th day compressive strength of FLGQ brick obtained is 9.3 kN·mm⁻². This indicates that the brick exhibits commendable compressive strength and meets the criteria outlined in the reference [27].

The incorporation of quarry dust powder notably affects the compressive strength of FLGQ brick to some extent, primarily by filling inner voids. From the graph, it is clear that compressive strength is directly proportional to quarry dust content up to 30%. Among the various proportions of FLGQ bricks, only M₉ exhibited a 7th-day compressive strength greater than 3.5 MPa [28,29]. Since the minimum compressive strength of brick used in construction purpose is 3.5 MPa, M₉ can be used as a substitute for country-burnt clay bricks and hollow bricks. The superior strength values seen in the M₉ mix might be attributed to the fine texture and grading characteristics of quarry dust, gypsum, lime, and fly ash powder.

Bricks serving as an in-fill material do not require the strength of engineering bricks. Therefore, various mix ratios identified in this study can be effectively used as an infill material. Compressive strength of traditional cement bricks that are already on the market is 5–8 MPa whereas that of the proposed FLGQ brick is 12 MPa. Therefore, FLGQ brick is a green alternative to locally available cement brick where compressive strength is a prime concern. Previous research references selected for this study are included in Table 4, and the results from the 56th test are shown in Figure 17.

Figure 17

Effect of fly ash and quarry dust on the 56th-day compressive strength of FALGQ bricks.

6.2.2 Split tensile strength

Table 3 indicates the results of split tensile strength assessment conducted on the FLGQ brick. At 56th day, the brick exhibited a superior split tensile strength of 1.44 kN·mm⁻². The tensile strength observed in this study surpassed the average tensile strength of burnt clay and other materials examined in the existing literature and confirmed with IS 5816 [37].

6.3.1 Density of the bricks

Table 3 and Figure 13 displayed the average wet density of different proportions of FLGQ brick. The density of FLGQ bricks found in this study are relatively higher than that of fly ash bricks suggested by other researchers. The density of the proposed FLGQ brick (M9) is measured to be 1,529 kg/m³, which is sufficient to compare with the density of standard burnt clay bricks.

6.4.1 Water absorption

The durability of the brick underwent assessment through the analysis of its water absorption and initial absorption rate. Table 3 and Figure 15 depict the water absorption findings, demonstrating conformity to the IS 3495-2 standard, which specifies 20% less water absorption requirement for the burnt bricks. This confirms the acceptability of the current brick’s water absorption, meeting the necessary benchmarks for structural purposes.

The results detailing water absorption in FLGQ bricks are presented in this document. The minimum water absorption of FLGQ brick with 0% quarry dust is 30% and the minimum water absorption of FLGQ brick (M₉) with 30% quarry dust is 16.5%. Therefore, water absorption of FLGQ brick (M₉) conforms to the guidelines of IS 3495 – Part 2 [11,17], which specify that burnt bricks should have a water absorption rate of less than 20%.

6.4.2 Initial rate of absorption

Table 3 and Figure 11 shows the initial absorption rate of the brick. The bricks absorption coefficient at 56 days is 0.28 gm·cm⁻², which is within the acceptable range as per the ASTM C – 67, suggesting that its resistance to absorption is considered acceptable and exhibits lesser shrinkage as well as stronger bond when compared to burnt clay brick.

6.4.3 Efflorescence

No remarkable indication of efflorescence was detected and all the FLGQ bricks were conforming to IS specification. Hence, the bricks can be used for all types of constructions.

6.6.1 Regression analysis using prediction model for compressive strength

The relation between the variable can be estimated using regression analysis. The technique is mainly based on the modelling and the variables in this study, independent variables are fly ash percentage, quarry dust percentage, and age in days and dependent variable is compressive strength at 7th, 14th, 28th, and 56th days.

Predicted model compressive strength of brick

where C _s is the compressive strength, F _a is the percentage proportion of fly ash, Q _d is the percentage proportion of quarry dust, G _y is the percentage proportion of quarry dust, and A _g is the age in days.

Based on these findings, it is evident that the suggested model closely aligns with the experimental data, effectively forecasting the compressive strength of FLGQ Bricks. To facilitate easy comparison, both the experimental and predicted results have been graphically represented in Figure 18. Significantly, the proposed method produces results that show impressive consistency with the experimental findings gathered on the 28th and 56th days.

Figure 18

Comparison chart of experimental and predicted 7th, 14th, 28th, and 56th day compressive strength results of various mixes of M₁ to M₁₃.

6.6.2 Predicted split tensile strength

where S _t is the split tensile strength, F _a is the percentage proportion of fly ash, Q _d is the percentage proportion of quarry dust, and G _y is the percentage proportion of gypsum.

The proposed prediction equation can effectively determine the 56th day split tensile strength with an accuracy of 96.5%. It is evident that the split tensile strength value obtained for FLGQ brick maintains a consistent ratio with the compressive strength. This has been confirmed by the findings of previous authors, as shown in Table 6.