Optimization Using Central Composite Design of the Response Surface Methodology for the Compressive Strength of Alkali-Activated Material from Rice Husk Ash

Abstract

Alkali-activated materials are promising alternatives to cement. This study investigated the effects of the silica content, particle size, and replacement ratio of rice husk ash (RHA) on the compressive strength and the optimization of these parameters. Seventeen mixtures with different materials were tested to evaluate their compressive strengths. Three levels of particle size, silica content, and RHA replacement ratio were used. The effects of RHA characteristics on the compressive strength were investigated based on Archimedes porosity, pH, ignition loss, and X-ray diffraction. The experimental results reveal that the replacement ratio of RHA was p-values < 0.002, which affected the compressive strength compared with the particle size (p-values < 0.450) and silica content of the RHA (p-values < 0.017). It was confirmed that the optimum values of particle size, silica content, and replacement ratio of RHA were 50 µm, 90%, and 15 wt.%, respectively. After re-testing, the compressive strength of mortar made with the optimum values was 49.8 MPa. This increase in compressive strength was also found to be closely related to the porosity, pH, and ignition loss of the paste. It was confirmed that the replacement ratio of RHA increased with decreasing porosity and pH and increasing ignition loss, which was related to the formation of calcite and C-S-H.

1. Introduction

CO₂ emissions from cement manufacturing account for 8% of global CO₂ emissions [1]. This increase in emissions has become a major social problem worldwide. Alkali-activated materials (AAMs), also called geopolymers, comprise stimulants and industrial by-products, such as blast furnace slag (BFS), metakaolin, and fly ash without cement, and alkaline stimulants such as Na₂SiO₂ and NaOH. As this material does not include cement, it is a promising material to significantly reduce CO₂ emissions, preserve natural resources, and protect the environment [2]. Therefore, the potential to expand AAMs to construction materials to realize a low-carbon society has attracted significant research attention [3,4,5,6]. BFS and fly ash are not reactive, but their hydration can be promoted using stimulants such as Na₂SiO₂ and NaOH to cure them at temperatures of 60 °C or higher [4,7]. Moreover, Na₂SiO₂ is highly concentrated and needs to be diluted with water. The ease of use and reduced fluidity due to its high viscosity [8] are limitations of Na₂SiO₂ in applications. In addition, although NaOH is an effective stimulant, it is poisonous and requires careful handling. Thus, a safe and satisfactory stimulant for AAMs has not yet been identified.

In a previous study, basic research on AAMs using Na₂CO₃ and Ca(OH)₂ as substitute stimulants to Na₂SiO₂ and NaOH was conducted, and it was reported that the use of Na₂CO₃ with AAMs contributes to a compressive strength enhancement compared with using Ca(OH)₂. Moreover, practically acceptable strength at 28 d was confirmed [9]. Therefore, Na₂CO₃ can be used as a substitute for Na₂SiO₂ and NaOH [9,10]. RHA is an agricultural byproduct that is discharged in large quantities and contains lignin, cellulose, silica, and other components [11]. Silica is a promising replacement for cement. The hydration reaction and durability of cementitious materials containing RHA have been extensively studied for several decades [12,13,14,15,16]. However, only a few studies have been conducted on the applicability of RHA as a binder for AAMs without cement.

Recently, studies have been conducted on the mechanical properties and durability of AAMs that incorporate RHA. Pradhan et al. evaluated the effect of RHA replacement on the workability, compressive strength, and durability of GGBS-based AAC and reported that the fluidity decreased as the RHA replacement ratio increased, and the compressive strength was affected by the RHA replacement ratio [17]. Huo et al. reported that the addition of 10% RHA to AAM reduced water absorption and increased sulfuric acid attack resistance and carbonation resistance [18]. This improvement in the durability of AAMs using RHA is related to the specific surface area of RHA and its reaction with amorphous silica. Zhao et al. reported that when the RHA content was approximately 10%, the internal structure became denser and more C-S-H gels were generated, resulting in higher strength. It has been reported that RHA has a high specific surface area, and a small amount of RHA can fill the internal pores to make the internal structure of the concrete denser [19]. In addition, according to research on the performance of AAMs, including RHA, it is known that the replacement ratio of RHA can greatly improve the performance of AAMs [20,21]. However, there has been no systematic study on the performance of AAMs considering the silica content, particle size, and replacement ratio of RHA.

Despite the many studies mentioned above, there is a lack of information in the literature on how certain parameters such as the particle size, silica content, and replacement ratio of RHA as a binder quantitatively affect the compressive strength development of AAMs. Moreover, an optimal material design for AAMs using RHA has not been fully investigated. Experimental designing is used in various fields for material design and process optimization. It has the advantage of reducing the number of tests, quantitatively evaluating the effect of each factor on performance, and deriving the content of each optimized material [22,23]. This study provides an optimal material design method for AAMs considering the silica content, particle size, and replacement ratio of RHA with silica and examines the effects of the porosity, pH, ignition loss, and hydration products on the compressive strength of AAMs. This approach is important from both environmental and economic perspectives and contributes to expanding the development of AAMs as cement substitutes to realize a low-carbon society. Finally, the results obtained from this study provide valuable information for practical design codes in which an understanding of the performance of AAMs made with RHA is needed to make the concrete structures.

2. Materials and Methods

2.1. Raw Materials

BFS with a fineness of 4340 cm²/g, a SO₃ content of 2.14, and a density of 2.89 g/cm³ was used as the binder material in this study. Silica sand was used as the fine aggregate. The surface dry density and water absorption of the sand were 2.65 g/cm³ and 0.42%, respectively. To evaluate the effect of silica purity, two types of RHA with silica contents of 89.9 (RHA1) and 93.5% (RHA2) were purchased and used. Their chemical compositions are listed in

Table 1. Chemical compositions of the two types of RHA.

Materials	SiO2	Al2O3	CaO	P2O3	K2O	Fe2O3	CO2
RHA1	89.9	0.6	1.9	0.8	0.7	0.6	4.5
RHA2	93.5	0.2	0.4	0.3	0.3	0.2	4.2

, and the types of RHA used as the raw material are shown in Figure 1. In addition, to evaluate the effect of silica particles on the compressive strength of the mortar, each silica sample was crushed and passed through 45, 150, and 250 µm sieves used in the experiment. As an alkaline activator, Na₂CO₃ (NC, purity greater than 95.0%) was purchased from Hayashi Pure Chemical Ind. Ltd., Osaka, Japan. The chemical agents used in the experiment were an air-entraining and high-range water-reducing agent (HP-8, Polycarboxylate type) from Takemoto Oil and Fat Co., Ltd., Aichi, Japan and a defoamer (NO.21, Polycarboxylate salt) from Kao Global Chemicals, Co., Ltd., Tokyo, Japan. The air contents of all mortar samples using these chemical agents were adjusted and ranged between 3.0 and 6.0%.

2.2. Central Composite Design

In this study, we used central composite design (CCD), which is widely used for the optimization of material design with different factors, using Minitab Statistical Software Version 22.2.1.

Table 2. Level of each factor.

Level of Factor	Particle Size (µm)	Silica Contents (%)	Replacement Ratio (%)
−1	50	90 (RHA1)	0
0	150	92 (RHA1:RHA2 mixed)	7.5
1	250	94 (RHA2)	15.0

shows the experimental design for obtaining the three types of silica content, particle size, and replacement ratio of BFS using Minitab 16. In addition, silica with a purity of 92% was prepared by mixing 50% RHA1 and RHA2. From this design, 8 cube points, 1 center point in the cube, 6 axis points, and 2 center points in axial were obtained and a total of 17 experiments were employed in this work. Table 2 shows the experimental design for obtaining the three types of silica content, particle size, and replacement ratio of BFS. Each design was randomly selected.

2.3. Mixture Proportions and Specimen Preparation

The mixture proportions of the mortar samples are listed in

Table 3. Mixture designations.

Particle Size (µm)	Silica Contents (%)	Replacement Ratio (wt.%)
50	90	0
250	94	0
250	90	15
50	94	15
150	92	7.5
250	90	0
50	94	0
50	90	15
250	94	15
50	92	7.5
250	92	7.5
150	90	7.5
150	94	7.5
150	92	0
150	92	15
150	92	7.5
150	92	7.5

. Seventeen types of mixtures were obtained with respect to the central composite design. The water-to-binder ratio (W/B = water/BFS + NC + RHA) was 0.45 and an NC of 10 wt.% was set in the binder. Three types of RHAs were added to the mortar at replacement levels of 0, 7.5, and 15.0%. The fine aggregate content was 1.5 to binder. They were mixed and cast as 40 mm × 40 mm × 160 mm prisms to form the mortar test specimens. After 4 d, the samples were de-molded and cured at 20 °C in water for 28 d.

2.4. Testing Methods

2.4.1. Compressive Strength

Compressive strength tests were conducted to evaluate the effects of each material. Figure 2 shows the compressive strength testing machine. The materials were mixed twice for 2 min and then cast in a prism mold with dimensions of 40 mm × 40 mm × 160 mm. After casting, the mortar samples were sealed using a lap on the top and placed at 20 °C in the experimental room for 4 d. Then, the samples were de-molded and moved into water until 28 ages at 20 °C. The compressive strength was tested according to JIS R 5201.

2.4.2. pH Value

Paste samples without sand were prepared for this study. Each material was mixed for approximately 5 min using a hand mixer and then placed in a silicone mold with dimensions of 75 mm × 75 mm × 5 mm. Then, they were de-molded after 4 d and stored in a container at 20 °C in water. At 7, 21, and 28 d age, the samples were placed in an oven at 40 °C and dried for 3 d. Then, the paste was pulverized to less than 150 µm. The liquid-to-solid ratio was 5. Figure 3 shows us testing the pH of a sample. After mixing for approximately 2 min, the pH was measured using a pH meter (DKK-TOA CO., HM-30P, DKK-TOA Co., Tokyo, Japan) after 5 min.

2.4.3. Porosity

To assess the change in the porosity of the paste due to the addition of each material, the Archimedes porosity was measured. After manufacturing the paste in the same manner as that for the pH test, it was placed in water at 20 °C. Before testing, hydration of the paste was stopped using ethanol. Figure 4 describes the Archimedes porosity measurement method. All the samples investigated were dried at 40 °C for 3 d to dry the ethanol. Then, using a vacuum pump, the paste was immersed in water for 1 d to measure the underwater and surface dry weights.

2.4.4. Ignition Loss

The ignition loss was measured to evaluate the change in the hydration reaction with different mixtures. Figure 5 shows the electric furnace used in this study. The method of paste preparation was the same as that for the pH test (hydration of the paste was stopped using ethanol, and the paste samples were dried at 40 °C for 3 d). Then, the pastes were pulverized to less than 150 µm, and 1 to 2 g of paste was placed in a furnace for 60 min at 350 °C. The temperature ranged between 40 °C and 350 °C and the ignition loss was calculated using the formula (m40 − m350)/m40 × 100.

2.4.5. X-Ray Diffraction (XRD) Analysis

The above-mentioned samples were immersed in anhydrous ethanol for 24 h to terminate hydration, then dried in a drying oven and ground to pass through a 150 µm sieve. The phase composition of the samples was tested using a Smart Lab X-ray diffractometer (diffraction angle 2θ = 5–40°, total scanning time = 5 min).

3. Results and Discussion

3.1. Compressive Strength Results

Mortar samples were made based on the central composite design generated based on the particle size of RHA, silica content, and BFS replacement ratio, and the compressive strength was evaluated after curing in water for 28 d. The measured and predicted values of the compressive strength are listed in

Table 4. Compressive strength measurements and predictions using central composite design.

Factors			Compressive Strength (MPa)
Particle Size (µm)	Silica Contents (%)	Replacement Ratio (%)	Measured Value	Predicted Value	Difference (Meas. − Pred.)
50	90	0	42.5	46.9	−4.4
250	94	0	38.9	44.6	−5.7
250	90	15	49.4	55.2	−5.8
50	94	15	47.3	52.9	−5.6
150	92	7.5	37.7	44.3	−6.6
250	90	0	42.9	48.5	−5.6
50	94	0	39.4	44.8	−5.4
50	90	15	51.6	57.2	−5.6
250	94	15	42.4	49.2	−6.8
50	92	7.5	40.1	46.0	−5.9
250	92	7.5	42.2	45.0	−2.8
150	90	7.5	43.2	47.8	−4.6
150	94	7.5	39.7	43.8	−4.1
150	92	0	37.7	43.5	−5.8
150	92	15	48.1	51.0	−2.9
150	92	7.5	42.4	44.3	−1.9
150	92	7.5	41.5	44.3	−2.8

. Figure 6 shows a plot of the measured and predicted compressive strengths. Based on the results shown in Figure 6, the measured and predicted values of the compressive strength were confirmed to be slightly high. The coefficient of determination was confirmed to be 0.999, which suggests that the regression model by central composite design is reliable.

The regression analysis model by central composite design is given in Equation (1). Here, particle size represents the particles of RHA (50, 150, and 250), amount represents the silica content contained in RHA (90, 92, and 94%) based on the XRF analysis results, and quantity represents the RHA replacement ratio (0, 7.5, and 15.0%) substituted as a weight percent for BFS.

Compressive strength
− 68.5X₂ + 3.11X₃ + 0.000118X₁² + 0.370X₂² + 0.0521X₃²
− 0.00225X₁×X₂ − 0.001167X₁X₃ − 0.0350X₂X₃
where X₁ is the particle size (µm), X₂ is the silica content (%), and X₃ is the replacement ratio of RHA (%).

(1)

The results of the analysis of variance are shown in

Table 5. Analysis of variance.

Source	Sum of	Degree of	Mean	F	p-Value
	Squares	Freedom	Square	Value	Prob > F
Model	11	248.236	22.567	5.82	0.032
Blocks	2	5.352	2.676	0.69	0.544
Linear	3	190.438	63.479	16.36	0.005
Particle size	1	2.601	2.601	0.67	0.450
Silica contents	1	47.961	47.961	12.36	0.017
Replacement ratio	1	139.876	139.876	36.05	0.002
Square	3	33.687	11.229	2.89	0.141
size × size	1	3.102	3.102	0.8	0.412
contents × contents	1	4.882	4.882	1.26	0.313
ratio × ratio	1	19.142	19.142	4.93	0.077
2-Way Interaction	3	9.95	3.317	0.85	0.521
size × contents	1	1.62	1.62	0.42	0.547
size × ratio	1	6.125	6.125	1.58	0.264
Contents × ratio	1	2.205	2.205	0.57	0.485
Error	5	19.403	3.881
Lack-of-Fit	4	18.998	4.749	11.73	0.215
Pure Error	1	0.405	0.405
Total	16	267.639

. In general, the statistical significance of the model can be evaluated by a low p-value of 0.05 or less. The p-values of the RHA particle size, silica content, and replacement ratio of RHA on the compressive strength were 0.450, 0.017, and 0.002, respectively. These results imply that the silica content and replacement ratio of RHA are more dominant than the RHA particle size in terms of the compressive strength of the mortar.

3.2. Main Effects

The influence of each parameter on compressive strength is shown in Figure 7. It can be confirmed that the smaller the particle size of RHA, the lower the silica content, and the higher the replacement ratio of RHA to BFS, the higher the compressive strength. Considering the particle size of RHA, it is well known that a smaller powder size may result in a higher compressive strength of the hardened mortar. Despite the different particle sizes of the RHA, the compressive strength ranged from 39 to 41 MPa, implying that the particle size of the RHA, from 50 to 250 at 28 d age, does not significantly affect the compressive strength. However, the compressive strength of the mortar increased as the silica content decreased. This was an unusual observation that differed from the expected results. This is considered to be closely related to the crystalline silica content. Thus, XRD analysis was performed to determine the effect of the silica type on the decrease in compressive strength. Figure 8 shows the XRD patterns of the raw materials RHA1 and RHA2. From the figure, it can be observed that crystalline peaks with cristobalite, tridymite, and quartz (20–30°) were noticeable, and these diffraction peaks of the raw materials correspond to cristobalite, tridymite, and quartz, which means that the presence of these crystalline peaks may have caused the decrease in compressive strength.

3.3. Interaction Effects

The interaction effects of each factor on the compressive strength are represented as contour graphs in Figure 9. Figure 9a shows the effects of the RHA particle size and silica content on the compressive strength. The graph shows that the replacement ratio of RHA for BFS was fixed at 7.5%. As shown in Figure 9a, higher silica contents and a larger RHA particle size lead to lower compressive strength. The reason for the increase in compressive strength as the particle size of the RHA decreased is that the internal matrix in the mortar became physically denser when smaller RHA particles were used, and the pore volume decreased, resulting in the increase in compressive strength. In addition, it was found that the compressive strength decreased as the silica content increased. In general, as the content of amorphous silica in the RHA increases, the pozzolanic reaction is promoted, which increases the mechanical strength of the mortar. However, the results of this study were slightly different from general knowledge. The reason for this behavior is due to the fact that the compressive strength decreased because the RHA2 used in this study has more crystalline silica, which is not measured by XRF.

Figure 9b shows the relationship between the size and the replacement ratio of RHA and their combined effect on the compressive strength. From this result, it was confirmed that the lower the particle size of RHA and the higher the replacement ratio of RHA, the higher the compressive strength. As explained above, in addition to the densification of the internal structure of the mortar using small-particle-size RHA, the higher the replacement ratio of RHA, the more the pozzolanic reaction of the RHA is promoted. The densification and promotion of the pozzolanic reaction using these small particles increase the compressive strength.

Figure 9c shows the interaction between the silica content and RHA replacement ratio. From this result, it was confirmed that the lower the silica content and the higher the replacement ratio of RHA for BFS, the higher the compressive strength.

3.4. Optimization and Verification of Compressive Strength

Considering the particle size of RHA, silica content, and replacement ratio as parameters, the optimal parameters of compressive strength were obtained using the equation suggested by the central composite design. The optimal parameters for obtaining a high compressive strength are shown in Figure 10 and listed in

Table 6. Suggested optimum parameters.

Variable	Setting
Particle size (µm)	50
Silica contents (%)	90
Replacement ratio of RHA (wt.%)	15
Suggested value (MPa)	51.7
Measured value (MPa)	49.8

. From the results, the particle size, silica content of RHA, and replacement ratio for BFS were confirmed to be 50 µm, 90%, and 15.0%, respectively. Based on these optimal values, a compressive strength experiment was repeatedly performed, and the results were confirmed to be approximately identical to the measured results for the proposed compressive strength.

3.5. Discussion on the Enhancement of the Compressive Strength of AAC with RHA

Based on central composite design, the compressive strengths of mortar samples with different particle sizes, silica contents, and replacement ratios of RHA to BFS were measured, and the effect of each parameter on the compressive strength of the mortar was investigated. In addition, an optimal material design method for obtaining high compressive strength was proposed, and the verification results show that the measured and predicted values of the compressive strength were approximately equal, indicating that the proposed experimental design method has high reliability. In particular, the replacement ratio of BFS has a significant influence on the compressive strength of the mortar, and an increase in the replacement ratio can be considered to play an important role in the compressive strength of the mortar. In general, the compressive strength is closely related to the porosity, ignition loss, and hydration products of the hardened body. Therefore, to investigate the increase in compressive strength according to the replacement ratio of RHA in more detail, paste samples made with a particle size of 50 µm and an RHA1 replacement ratio of 0, 5, 10, and 15 wt.% to BFS with a water-to-binder ratio of 0.45 were prepared and measured for porosity and ignition loss, and the hydration products due to the use of RHA were analyzed through XRD analysis. In addition, a pH analysis was conducted. These investigations are important for understanding the role of RHA in AAMs.

Figure 11 shows the results of porosity measurements using the Archimedes method. As shown in the figure, the porosity of the samples decreased gradually as the replacement ratio of RHA1 increased, and the porosities of 0% and 15.0% RHA1 were measured as 27.0% and 16.9%, respectively. This indicates that the incorporation of RHA1 can contribute to a decrease in the porosity of the paste sample. The decrease in the porosity of these test specimens was related to the amount of hydrate in the cementitious materials. Therefore, the ignition loss from 40 to 350 °C was measured to investigate the relationship between the porosity and content of the hydration products in the paste sample. Figure 12 shows the ignition loss results for the paste sample. The ignition loss increased from 10.5 to 15.2% as the RHA replacement ratio increased, implying that the hydration of the AAC was accelerated by an increase in the RHA replacement ratio. In addition, the decrease in porosity and increase in ignition loss were mainly due to the pozzolanic reaction of the RHA. From the test results, it was concluded that the increase in the compressive strength with the RHA replacement ratio reported in Section 3.1 is due to the increase in hydration products with RHA, resulting in the densification of the pore structure in the AAC paste.

The pH measurement results for each paste sample are shown in Figure 13. It can be seen that the pH tends to decrease as the RHA replacement ratio increases, which may be due to the consumption of Na⁺ ions. The XRD results are presented in Figure 14. In previous studies, Yoshida et al. investigated the hydration reaction of BFS using a combination of NC and calcium nitrite using XRD analysis [24]. As a result, they observed that, in the case of the paste containing 8 wt.% NC in the BFS, peaks of mono-carbonate (11.6°, 23°), portlandite (18°), calcite (29°), and C-S-H (7°, 29°) were generated at an age of 28 days. In this study, in the case of the paste without RHA (RHA1; 0%, 28 d), a weak peak of mono-carbonate around 10 to 12° and strong peaks of calcite or C-S-H peaks at around 29° were identified. Except for the peak of C-S-H at 7°, the hydration products observed in this study showed almost similar trends to those in previous studies. From the figure, it can also be seen that, for the sample with the 15% RHA1-to-BFS ratio (RHA1; 15%, 28 d), the peak intensity at 29 to 30 θ was slightly higher compared with that for the sample without RHA (RHA1; 0%, 28 d). These peaks correspond to calcite and C-S-H. Therefore, when BFS is replaced by RHA, the types of hydration products do not change significantly, and calcite and C-S-H hydrates are generated. Therefore, these hydration products contribute to the mechanical strength of the mortar.

Based on the above-described results, incorporating RHA into BFS as a binder had a positive effect on the compressive strength development, given that (i) calcite and C-S-H were generated and the ignition loss increased, (ii) the pore structure from the generation of these minerals was densified, and (iii) the alkalinity in the liquid phase of the hardener was reduced.

4. Conclusions

The purpose of this study was to investigate the effects of particle size, silica content, and RHA replacement ratio on the compressive strength of AAMs comprising Na₂CO₃, RHA, and BFS. Based on these parameters, 17 batches of mortar were manufactured using central composite design, and the strength of the mortar was measured at 28 d. In addition, the pore ratio, ignition loss, and XRD results were analyzed to investigate the microstructure and pore structure of the paste. The pH values of the pore solutions were also measured. Based on the experimental results, the following conclusions were derived, which are meaningful for improving the design code of AAMs:

• It was found that the RHA replacement ratio in the mortar increased the compressive strength, indicating that the compressive strength is dependent on the ratio of RHA to BFS rather than the particle size and silica content. In addition, it was confirmed that the compressive strength decreased with an increase in the RHA silica content, which may be related to the presence of crystalline silica;
• Based on the analysis of variance results, the RHA replacement ratio resulted in a lower p-value in comparison with the particle size and silica content of RHA. The compressive strengths of the mortar samples were predicted using a regression equation. The optimum values for the replacement ratio, particle size, and silica content of RHA were suggested to be 15%, 50 µm, and 90%, respectively. Mortar samples were prepared based on the optimal particle size, silica content, and replacement ratio of RHA, and their compressive strengths were measured. Based on these results, the experimental and predicted values were confirmed to be approximately equal;
• From the results of the investigation of the microstructure and pore structure based on porosity, pH, ignition loss, and XRD analysis, it was confirmed that increasing the RHA replacement ratio in the paste sample decreased the porosity and pH and increased the ignition loss, which is related to the calcite and C-S-H in the paste.

5. Future Study

In this study, the quantitative evaluation of each hydration product and changes in the pore structure of the AAMs mixed with RHA were not fully investigated. Therefore, a quantitative evaluation of hydration products and an analysis of changes in the pore structure of AAMs partially containing RHA are required to clarify the effect of RHA on compressive strength development. Moreover, this study investigated the effects of particle size, silica content, and replacement ratio of RHA on the compressive strength. It was, however, confirmed that the compressive strength tended to decrease as the silica content increased. This is slightly different from general research knowledge; thus, additional investigation is necessary. There was a slight difference between the measured and predicted values of compressive strength, which is believed to be due to the small number of samples, and additional experiments are needed to further improve the compressive strength prediction model. Finally, this study focused on the compressive strength of AAMs containing RHA, but aspects of their durability, such as their elastic modulus, drying shrinkage, moisture resistance, and freeze–thaw resistance, were not investigated, and this requires additional research.