# Effect ofα: Phase Nano Al2O3 and Rice Husk Ash in Cement Mortar

Authors: Alexander Gladwin Alex; Zeyneb Kemal; Tsegay Gebrehiwet; Solomon Getahun
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

The study focused on the investigation of the influence ofα-phase nano Al2O3 (NA) and M-sand as a fine aggregate on the partial replacement of cement by micro rice husk ash (MRHA) to enhance the mechanical properties and durability of mortar and achieve an environmentally sustainable material. MRHA was added to the M-sand cement mortar (by partial replacement of cement) at varying concentrations of 0%, 5%, 10%, 15%, and 20% by weight of cement; and NA was added at the rate of 0%, 0.5%, 1%, 1.5%, and 2% by weight of cement. The results showed that the partial replacement of cement by MRHA (10%) improves the comprehensive and tensile strength by 7% and 6.9%, respectively, compared to the control. Moreover, the incorporation of NA in cement increased the comprehensive and tensile strengths by 15.5% and 41%, respectively. The optimal increment in the combination of MRHA and NA (MN) in the partial replacement of cement resulted in a 26.4% comprehensive strength and a 48.72% tensile strength compared to the control. The flowability of M-sand mortar containing MRHA and NA was observed to vary depending on the degree of dosage and the admixture. Our study concludes that the partial replacement of cement by the admixtures MRHA (10%) and NA (1%) in combination improved the strength and reduced water absorption when compared to the individual effects and control, suggesting the application of MRHA and NA in concrete technology.


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## Body

## 1. Introduction

The biggest challenge in the construction industry is to reduce the negative impact of ordinary portland cement (OPC) production on carbon emissions and energy consumption. During production, one ton of cement emits 600–900 kg of CO2 due to the decomposition of calcium carbonate into calcium oxide [1, 2]. On a global scale, anthropogenic carbon dioxide emissions from cement industries range between 5 and 8% [3–5]. To minimize the negative effects, the partial replacement of cement powder by siliceous or siliceous and aluminous materials can be considered [6]. Calcined clay is a potentially viable material for cement in emerging economies because of its low combustion temperature [7, 8]. In addition, recent studies point to the use of agricultural byproducts as partial replacements, which would also improve the economy and agricultural waste management [9]. The well-organized burnt husk chemical composition has more than 90% amorphous silica [10]. Therefore, various researchers have investigated the grinding techniques and microstructure of rice husk ash [11, 12]. The ashes from different rice straw particles have a high amount of SiO2 with K2O and CaO [13]. The partial replacement of cement by rice strew ash has improved the compressive strength and reduced the permeability [14], making MRHA, an important alternative in cement replacement.In terms of the durability of concrete structures, cement mortar should have adequate adhesion [15, 16]. The use of nanoparticles in concrete significantly modifies the cement composition [17–19]. The addition of nanofillers in cement-based components improves compactness due to their small size and filling ability, meanwhile improving the mechanical properties and crack propagation [20]. The addition of α-phase nanoalumina improves the mechanical property of M-sand cement mortar [21]. The inclusion of nanosilica improves compressive, tensile, flexural, and shear strength as well as modulus of elasticity [22]. The presence of nanofillers reduces the orientation of CH crystals and improves the hardness and rigidity of C–S–H gel (see reference [23]). The incorporation of nanofillers also reduces the porosity by decreasing the pore water from the C–S–H gel in cementitious compounds [24]. Nanomaterials also improve durability and resistance to water absorption [25]. Further, microsized fibers like hemp and polyvinyl alcohol are used along with nanomaterials were found to improve the strength of mortars, suggesting a combinatorial approach in concrete technology.Even though some research has studied the effects of MRHA and NA on cement mortar individually, only limited studies have explored the combination of MRHA and NA in enhancing the mechanical property, durability, and flowability. Therefore, this study focused on the mechanical properties of M-sand cement mortar with different percentage partial replacement combinations of microlevel MRHA and nano Al2O3.


## 2. Material and Method

The material used in this study is Coromandel OPC cement of 43 grade conforming to IS8112 with an initial and final setting time of 140 min and 355 min, respectively. Manufactured sand conforming to IS1542 with a specific gravity of 2.61, bulk density of 1.48, fines modules of 3.0 and grading zone II, MRHA with a particle size of 5–10 µm, and NA with 20–25 nm were used in the study.The proportion of 10 mixtures prepared for this study was presented in Table1. A water to binder ratio of 0.50 was used for the mortar mix. At the screening stage, the binder was replaced by 5%, 10%, 15%, 20% MRHA and 0.5%, 1%, 1.5%, 2% NA by using the trial and error method. The mixtures are placed into cubical and cylindrical molds using tamping rods as per ASTM C311-16 [26]. The samples were demolded, dried for 24 hours at room temperature, and water curing was carried out for up to 28 days to achieve the ultimate strength.Table 1
Mix proportion.
Sample IDMicro rice husk ash % (MRHA)Nano Al2O3% (NA)Water to binder ratioC-1000.50MHRA-1500.50MHRA-21000.50MHRA-31500.50MHRA-42000.50NA-100.50.50NA-2010.50NA-301.50.50NA-4020.50MN1010.50


### 2.1. Test Method

#### 2.1.1. Analysis of the Fresh Property

The fresh mortar workability has been tested immediately after mixing by using a standard flow table test in accordance with ASTM1437-16 [27]. The fresh mortar was placed into a cone mold with two layers of 25 mm thickness each, and the second layer was brought up to the top and compacted by using tampering rods. The flow spread diameter was measured after removing the mold, and the result was recorded from the average results of five measurements.


#### 2.1.2. Mechanical Properties

Compressive strength and tensile strength test were carried out as per ASTM C109-16 [28] and ASTM C469-16 [29], respectively. The 50 mm × 50 mm x 50 mm cubical specimens were tested at the 7th, 14th, and 28th days of curing. The compressive strength was taken as the average value of the strengths of the three tested specimens. The 100 mm × 200 mm cylindrical specimens were tested at the 7th, 14th, and 28th days after curing. For the split-tensile strength test, longitudinal stress was applied at a rate of 1 mm/min.


#### 2.1.3. Water Absorption

To study the durability of mortar samples, a capillary water absorption test was used as per ASTMC1403-15 [30]. The samples were placed inside a watertight bag for 28 days, and on the 28th day, the samples were taken out and placed inside an oven for 24 hours at a temperature of 100 0C. Then the samples were taken out from the oven and cooled down to room temperature. The initial weights of the samples were measured and recorded to 0.01 g accuracy, followed by the immersion of the samples in a container. The water uptakes were observed by measuring the weight of samples in the intervals of 15 min, 30 min, 1, 2, 3, 4, 6, 24, 48, 72, 96, 120, and 144 hours. The amount of water absorption was determined by using(1)At=WT−W0A×100.where At, WT, W0, and A are water uptake, specimen weight at a given time (T), initial weight of the sample, and cross-sectional area, respectively.


#### 2.1.4. Characterization of Materials

Material characterization, morphology, and size of samples were examined by using a scanning electron microscope (SEM) (JEOL-JSM-IT 200 with EDS). MRHA, NA, M-sand, and cement were placed on the sticky carbon tape and placed on a sputter coater. The samples were placed in a high-vacuum chamber and analyzed at a voltage of 15 kV. An EDS analysis was executed for MRHA and NA particles.


## 2.1. Test Method

### 2.1.1. Analysis of the Fresh Property

The fresh mortar workability has been tested immediately after mixing by using a standard flow table test in accordance with ASTM1437-16 [27]. The fresh mortar was placed into a cone mold with two layers of 25 mm thickness each, and the second layer was brought up to the top and compacted by using tampering rods. The flow spread diameter was measured after removing the mold, and the result was recorded from the average results of five measurements.


### 2.1.2. Mechanical Properties

Compressive strength and tensile strength test were carried out as per ASTM C109-16 [28] and ASTM C469-16 [29], respectively. The 50 mm × 50 mm x 50 mm cubical specimens were tested at the 7th, 14th, and 28th days of curing. The compressive strength was taken as the average value of the strengths of the three tested specimens. The 100 mm × 200 mm cylindrical specimens were tested at the 7th, 14th, and 28th days after curing. For the split-tensile strength test, longitudinal stress was applied at a rate of 1 mm/min.


### 2.1.3. Water Absorption

To study the durability of mortar samples, a capillary water absorption test was used as per ASTMC1403-15 [30]. The samples were placed inside a watertight bag for 28 days, and on the 28th day, the samples were taken out and placed inside an oven for 24 hours at a temperature of 100 0C. Then the samples were taken out from the oven and cooled down to room temperature. The initial weights of the samples were measured and recorded to 0.01 g accuracy, followed by the immersion of the samples in a container. The water uptakes were observed by measuring the weight of samples in the intervals of 15 min, 30 min, 1, 2, 3, 4, 6, 24, 48, 72, 96, 120, and 144 hours. The amount of water absorption was determined by using(1)At=WT−W0A×100.where At, WT, W0, and A are water uptake, specimen weight at a given time (T), initial weight of the sample, and cross-sectional area, respectively.


### 2.1.4. Characterization of Materials

Material characterization, morphology, and size of samples were examined by using a scanning electron microscope (SEM) (JEOL-JSM-IT 200 with EDS). MRHA, NA, M-sand, and cement were placed on the sticky carbon tape and placed on a sputter coater. The samples were placed in a high-vacuum chamber and analyzed at a voltage of 15 kV. An EDS analysis was executed for MRHA and NA particles.


## 2.1.1. Analysis of the Fresh Property

The fresh mortar workability has been tested immediately after mixing by using a standard flow table test in accordance with ASTM1437-16 [27]. The fresh mortar was placed into a cone mold with two layers of 25 mm thickness each, and the second layer was brought up to the top and compacted by using tampering rods. The flow spread diameter was measured after removing the mold, and the result was recorded from the average results of five measurements.


## 2.1.2. Mechanical Properties

Compressive strength and tensile strength test were carried out as per ASTM C109-16 [28] and ASTM C469-16 [29], respectively. The 50 mm × 50 mm x 50 mm cubical specimens were tested at the 7th, 14th, and 28th days of curing. The compressive strength was taken as the average value of the strengths of the three tested specimens. The 100 mm × 200 mm cylindrical specimens were tested at the 7th, 14th, and 28th days after curing. For the split-tensile strength test, longitudinal stress was applied at a rate of 1 mm/min.


## 2.1.3. Water Absorption

To study the durability of mortar samples, a capillary water absorption test was used as per ASTMC1403-15 [30]. The samples were placed inside a watertight bag for 28 days, and on the 28th day, the samples were taken out and placed inside an oven for 24 hours at a temperature of 100 0C. Then the samples were taken out from the oven and cooled down to room temperature. The initial weights of the samples were measured and recorded to 0.01 g accuracy, followed by the immersion of the samples in a container. The water uptakes were observed by measuring the weight of samples in the intervals of 15 min, 30 min, 1, 2, 3, 4, 6, 24, 48, 72, 96, 120, and 144 hours. The amount of water absorption was determined by using(1)At=WT−W0A×100.where At, WT, W0, and A are water uptake, specimen weight at a given time (T), initial weight of the sample, and cross-sectional area, respectively.


## 2.1.4. Characterization of Materials

Material characterization, morphology, and size of samples were examined by using a scanning electron microscope (SEM) (JEOL-JSM-IT 200 with EDS). MRHA, NA, M-sand, and cement were placed on the sticky carbon tape and placed on a sputter coater. The samples were placed in a high-vacuum chamber and analyzed at a voltage of 15 kV. An EDS analysis was executed for MRHA and NA particles.


## 3. Experimental Results

### 3.1. Characteristics of Micro, Nano, M-Sand, and Cement

The morphological size of MRHA and NA are shown in Figures1 and 2. The MRHA showed an amorphous structure with an irregular cubical shape and a size range of 5–10 µm. NA are found as clusters of rounded particles, which occur both as combined and as individual particles in the range between 20 and 25 nm. The morphological structure of M-sand shows an amorphous structure with cubical particles and cement shows an amorphous and bulky structure as shown in Figures 3 and 4. XRD patterns of MRHA and NA are shown in Figures 5 and 6, respectively.Figure 1
Morphology of micro MRHA.Figure 2
Morphology ofα-phase nano Al2O3.Figure 3
Morphology of M-sand.Figure 4
Morphology of OPC.Figure 5
XRD patterns of MRHA.Figure 6
XRDE patterns ofα-phase nano Al2O3.


### 3.2. Effect of Micro and Nanoparticles on Fresh and Hardened Properties of Mortar

The standard flow table test was used to measure the workability of mortar and the results are shown in Table2 and Figure 7. Both MRHA and NA have very minor effects on fresh properties. The addition of 5% MRHA increased the flow spread by 3.89% compared to the control mix, whereas 10% MRHA showed a 1.02% decrease compared to 5% MRHA, but the workability was within the acceptable limit. The NA-1 and NA-2 showed 2.12% and 2.83% of flow spread, compared to the control mix, whereas 1.5% addition showed a 3.10% decrease in flow spread compared to the 1% dosage. However, it can be observed that the workability of MRHA is higher than that of NA at all the tested concentrations.Table 2
Flowability and ultimate strength.
Sample IDFlow in mm28th day compressive strength (MPa)28th day tensile strength (MPa)C-128333.382.46MHRA-129434.142.55MHRA-229135.742.63MHRA-328631.242.31MHRA-427328.522.14NA-128935.283.21NA-229138.543.47NA-328227.421.64NA-427122.681.32Figure 7
Ultimate strength and slump.The strength of partial dosage specimens was tested by compressive and tensile strengths. The test results are shown in Table2 and Figure 7. The general strength increasing trend was observed for MRHA and NA up to 10% and 1%, respectively, beyond which showed a reduction in compressive strength. Initially, the compressive strength increased by 2.28% and 7.07% for MRHA with 5% and 10% dosage, respectively, compared to the control sample. A 10% sudden decrement of compressive strength was recorded compared to the control sample. This may be due to the insufficient availability of C–H to react with the high amount of available silica upon the addition of MRHA in the hydrated blended cement mix [31]. For NA, 5.7% and 15.45% strength increments were recorded for the dosages of 0.5% and 1%, respectively. Beyond the dosage of 1%, a decreasing trend was observed. However, NA dosage samples have a higher strength than MRHA samples.The split-tensile strength results are shown in Table2 and Figure 7. The optimum spit-tensile strength of 2.63 MPa (6.91% increase compared to control) was observed for 10% of MRHA and 3.47 MPa (41.05% increase compared to control) was observed for 1% of NA. The addition of NA gives more strength compared to MRHA. But beyond the optimum dosage, a sudden decreasing trend was observed in split-tensile strength. The decrement is 12% (MRHA) and 52.73% (NA) compared to the optimized samples. The addition of a large amount of nanoparticles produced low interfacial or interface properties and poor tensile strength [32]; meanwhile, MRHA addition has shown less strength reduction compared to NA. It was found that the addition of NA beyond the optimum value significantly affects the split-tensile strength. The optimum dosage for further study was derived as 10% and 1%, respectively, by MRHA and NA.


### 3.3. Effect of Optimum Combined Dosage of MRHA and NA on Fresh and Strength Development

Based on the screening test results, the optimum limit was set at 10% and 1% for MHRA and NA, respectively. The strength development of MN was examined at the durations of 7, 14, and 28 days. The result in Figure8 shows the strength development relevant to the optimum dosage of MRHA and NA. The compressive strength of MN samples increased by 86.20%, 45.72%, and 26.84% on the 7th, 14th, and 28th days, respectively. The results indicate that the combined addition of MRHA and NA particles improves the ultimate strength rather than the individual dosage. Meanwhile, MN particles also highly improved the early age strength growth.Figure 8
Compressive strength development.The split-tensile strength test results are represented in Figure9. It shows that the MN optimum dosage specimens scored higher in strength compared to the control and individual dosages of MRHA and NA. The overall strength was increased by 82.25%, 40.36%, and 48.72% on the 7th, 14th, and 28th days, respectively when compared to control. It was increased on the 7th and 14th days. The addition of MN particles improves the split-tensile strength to a considerable extent.Figure 9
Split-tensile strength development.


### 3.4. Water Absorption

The durability of mortar is mainly based on the capillary water intake. The control, optimum dosage samples of MRHA and NA absorbed more water compared to MN samples. The water absorption values are shown in Table3. Two phases of water absorption were found. The control samples absorbed more water compared to the optimum dosage (10% MRHA and 1% NA samples). Meanwhile, MN samples absorbed much less than optimum dosage samples in initial water uptake. After reaching the secondary water uptake stage (24 hr), the optimum MRHA and NA samples absorbed less water compared to the control sample. As a result, MN samples have a very low absorption value. The results indicate that the addition of micro and nanoparticles decreases the borehole size and reduces the water permeability. In particular, the combination of micro and nanoparticles shows better durability than the other samples.Table 3
Water absorption rate.
Absorption phaseTime (h)At∗ (g/100 cm2)ControlMHRA-2NA-2MNFirst phase00000.2523.4521.3621.0219.02.5032.0531.6129.6227.12141.7242.9640.2436.27259.6158.4253.2142.13478.2472.6368.3656.28686.5182.1679.4163.2924163.73157.23145.31121.72Second phase48169.23161.20152.60133.6372171.32164.72152.73134.2496172.16164.98152.96134.48120172.53265.06153.02134.69144172.96165.71153.08134.84∗At∗: water uptake.


## 3.1. Characteristics of Micro, Nano, M-Sand, and Cement

The morphological size of MRHA and NA are shown in Figures1 and 2. The MRHA showed an amorphous structure with an irregular cubical shape and a size range of 5–10 µm. NA are found as clusters of rounded particles, which occur both as combined and as individual particles in the range between 20 and 25 nm. The morphological structure of M-sand shows an amorphous structure with cubical particles and cement shows an amorphous and bulky structure as shown in Figures 3 and 4. XRD patterns of MRHA and NA are shown in Figures 5 and 6, respectively.Figure 1
Morphology of micro MRHA.Figure 2
Morphology ofα-phase nano Al2O3.Figure 3
Morphology of M-sand.Figure 4
Morphology of OPC.Figure 5
XRD patterns of MRHA.Figure 6
XRDE patterns ofα-phase nano Al2O3.


## 3.2. Effect of Micro and Nanoparticles on Fresh and Hardened Properties of Mortar

The standard flow table test was used to measure the workability of mortar and the results are shown in Table2 and Figure 7. Both MRHA and NA have very minor effects on fresh properties. The addition of 5% MRHA increased the flow spread by 3.89% compared to the control mix, whereas 10% MRHA showed a 1.02% decrease compared to 5% MRHA, but the workability was within the acceptable limit. The NA-1 and NA-2 showed 2.12% and 2.83% of flow spread, compared to the control mix, whereas 1.5% addition showed a 3.10% decrease in flow spread compared to the 1% dosage. However, it can be observed that the workability of MRHA is higher than that of NA at all the tested concentrations.Table 2
Flowability and ultimate strength.
Sample IDFlow in mm28th day compressive strength (MPa)28th day tensile strength (MPa)C-128333.382.46MHRA-129434.142.55MHRA-229135.742.63MHRA-328631.242.31MHRA-427328.522.14NA-128935.283.21NA-229138.543.47NA-328227.421.64NA-427122.681.32Figure 7
Ultimate strength and slump.The strength of partial dosage specimens was tested by compressive and tensile strengths. The test results are shown in Table2 and Figure 7. The general strength increasing trend was observed for MRHA and NA up to 10% and 1%, respectively, beyond which showed a reduction in compressive strength. Initially, the compressive strength increased by 2.28% and 7.07% for MRHA with 5% and 10% dosage, respectively, compared to the control sample. A 10% sudden decrement of compressive strength was recorded compared to the control sample. This may be due to the insufficient availability of C–H to react with the high amount of available silica upon the addition of MRHA in the hydrated blended cement mix [31]. For NA, 5.7% and 15.45% strength increments were recorded for the dosages of 0.5% and 1%, respectively. Beyond the dosage of 1%, a decreasing trend was observed. However, NA dosage samples have a higher strength than MRHA samples.The split-tensile strength results are shown in Table2 and Figure 7. The optimum spit-tensile strength of 2.63 MPa (6.91% increase compared to control) was observed for 10% of MRHA and 3.47 MPa (41.05% increase compared to control) was observed for 1% of NA. The addition of NA gives more strength compared to MRHA. But beyond the optimum dosage, a sudden decreasing trend was observed in split-tensile strength. The decrement is 12% (MRHA) and 52.73% (NA) compared to the optimized samples. The addition of a large amount of nanoparticles produced low interfacial or interface properties and poor tensile strength [32]; meanwhile, MRHA addition has shown less strength reduction compared to NA. It was found that the addition of NA beyond the optimum value significantly affects the split-tensile strength. The optimum dosage for further study was derived as 10% and 1%, respectively, by MRHA and NA.


## 3.3. Effect of Optimum Combined Dosage of MRHA and NA on Fresh and Strength Development

Based on the screening test results, the optimum limit was set at 10% and 1% for MHRA and NA, respectively. The strength development of MN was examined at the durations of 7, 14, and 28 days. The result in Figure8 shows the strength development relevant to the optimum dosage of MRHA and NA. The compressive strength of MN samples increased by 86.20%, 45.72%, and 26.84% on the 7th, 14th, and 28th days, respectively. The results indicate that the combined addition of MRHA and NA particles improves the ultimate strength rather than the individual dosage. Meanwhile, MN particles also highly improved the early age strength growth.Figure 8
Compressive strength development.The split-tensile strength test results are represented in Figure9. It shows that the MN optimum dosage specimens scored higher in strength compared to the control and individual dosages of MRHA and NA. The overall strength was increased by 82.25%, 40.36%, and 48.72% on the 7th, 14th, and 28th days, respectively when compared to control. It was increased on the 7th and 14th days. The addition of MN particles improves the split-tensile strength to a considerable extent.Figure 9
Split-tensile strength development.


## 3.4. Water Absorption

The durability of mortar is mainly based on the capillary water intake. The control, optimum dosage samples of MRHA and NA absorbed more water compared to MN samples. The water absorption values are shown in Table3. Two phases of water absorption were found. The control samples absorbed more water compared to the optimum dosage (10% MRHA and 1% NA samples). Meanwhile, MN samples absorbed much less than optimum dosage samples in initial water uptake. After reaching the secondary water uptake stage (24 hr), the optimum MRHA and NA samples absorbed less water compared to the control sample. As a result, MN samples have a very low absorption value. The results indicate that the addition of micro and nanoparticles decreases the borehole size and reduces the water permeability. In particular, the combination of micro and nanoparticles shows better durability than the other samples.Table 3
Water absorption rate.
Absorption phaseTime (h)At∗ (g/100 cm2)ControlMHRA-2NA-2MNFirst phase00000.2523.4521.3621.0219.02.5032.0531.6129.6227.12141.7242.9640.2436.27259.6158.4253.2142.13478.2472.6368.3656.28686.5182.1679.4163.2924163.73157.23145.31121.72Second phase48169.23161.20152.60133.6372171.32164.72152.73134.2496172.16164.98152.96134.48120172.53265.06153.02134.69144172.96165.71153.08134.84∗At∗: water uptake.


## 4. Discussion

### 4.1. Preliminary Study

The screening studies showed that the addition of MRHA and NA has a progressive effect on the mechanical properties of cement-based composites. The compressive strength was gradually increased up to 10% and 1% for MRHA and NA, respectively. The split-tensile strength improved significantly for 1% of the NA dosage samples. The specimens with NA have more strength compared to MRHA; hence, it is proved that the addition of 1% nanoparticles scored more strength compared to the addition of 10% of microparticles. The observed effect could be due to the effective filling property of NA compared to the MRHA. In addition, the high surface area of the nanoparticles makes the nucleated sites of the hydrated products present in concrete denser and more compact [32]. Further, the nanoparticles accelerated cement hydration thereby improving the early age strength [33].Moreover, it was observed that increasing NA dosage more than required would decrease the mechanical properties. This strength reduction was more than 1% in both compressive and tensile strength. The addition of nanoparticles up to the optimum limit improves the mechanical properties, adding too much number of nanoparticles causes agglomeration and avoids the growth of Ca(OH)2 instead of promoting [34]. Further, previous reports suggest that higher levels of NA (>2–3%) increase agglomeration, weak zone formation, and contribute to the decreasing strength [35]. But in this study, the strength reduction was observed to be more than 1% NA dosage, due to the insufficient level of C–H in the hydrated products to react with the increase in alumina content.The study of the flow table test Figure7 shows increasing flowability when increasing particle dosage up to the optimal limit; the optimal flowability recorded for MHRA-1 is 294 mm, whereas NA-2 recorded 291 mm, but the change was relatively small as well, within the acceptable limit. Meanwhile, the flowability of NA dosage samples was always less than that of MRHA dosage samples. It is due to the higher surface area of NA particles that directly affects the workability, which in turn indirectly increases the quantity of water. Even though the 20% MHRA and 2% NA dosage sample workability is also within the acceptable limit, it shows the strength reduction. However, the 10% MRHA and 1% NA dosage samples increased the mechanical properties without affecting the workability.


### 4.2. Strength Development of Optimum Samples

The optimum strength development was observed in the MRHA-2 sample. Both the compressive and split-tensile strength of specimens have increased during the 28-day curing period (Figure7). RHA with a high amount of reactive silica leads to the cement reaction. The addition of RHA to concrete shows a compressive strength increment due to the reaction of SiO2 with free calcium hydroxide, meanwhile reducing the internal air voids [36]. The optimum dosage of 1% of NA-2 samples recorded better mechanical strength compared to control and MRHA-2 specimens. It was observed that the strength development between the 7th and 28th day was gradually increased, indicating NA contributed to a gradually increasing pozzolanic reaction within the cement matrix as the curing time extended. However, the presence of 1% NA had a significant effect on early ages, which could be due to the high rate of reaction. However, the strength growth was decreased after 7 days, which may be due to the limited space for Ca(OH)2 crystal growth [37].


### 4.3. Strength Development of MN Samples

The cement mortar strength was investigated for the combinatorial effect (MN) of MRHA and NA at their optimum concentrations. The strength development of MN samples was shown in Figures8 and 9. Similar to the individual effects of micro and nanoadditive samples, the combinatorial effect also showed early age strength growth. The 7th day test results of MN samples were recorded at 54.01% higher than MRHA-2 samples and 17.85% higher than NA-2. The combination of MRHA and NA promotes cement hydration, filling ability, and less water absorption than individual additives. However, the split-tensile strength of MN samples did not exhibit significant changes compared to 1% NA, indicating the addition of MRHA does not have any effect on the split-tensile strength of the mortar.


## 4.1. Preliminary Study

The screening studies showed that the addition of MRHA and NA has a progressive effect on the mechanical properties of cement-based composites. The compressive strength was gradually increased up to 10% and 1% for MRHA and NA, respectively. The split-tensile strength improved significantly for 1% of the NA dosage samples. The specimens with NA have more strength compared to MRHA; hence, it is proved that the addition of 1% nanoparticles scored more strength compared to the addition of 10% of microparticles. The observed effect could be due to the effective filling property of NA compared to the MRHA. In addition, the high surface area of the nanoparticles makes the nucleated sites of the hydrated products present in concrete denser and more compact [32]. Further, the nanoparticles accelerated cement hydration thereby improving the early age strength [33].Moreover, it was observed that increasing NA dosage more than required would decrease the mechanical properties. This strength reduction was more than 1% in both compressive and tensile strength. The addition of nanoparticles up to the optimum limit improves the mechanical properties, adding too much number of nanoparticles causes agglomeration and avoids the growth of Ca(OH)2 instead of promoting [34]. Further, previous reports suggest that higher levels of NA (>2–3%) increase agglomeration, weak zone formation, and contribute to the decreasing strength [35]. But in this study, the strength reduction was observed to be more than 1% NA dosage, due to the insufficient level of C–H in the hydrated products to react with the increase in alumina content.The study of the flow table test Figure7 shows increasing flowability when increasing particle dosage up to the optimal limit; the optimal flowability recorded for MHRA-1 is 294 mm, whereas NA-2 recorded 291 mm, but the change was relatively small as well, within the acceptable limit. Meanwhile, the flowability of NA dosage samples was always less than that of MRHA dosage samples. It is due to the higher surface area of NA particles that directly affects the workability, which in turn indirectly increases the quantity of water. Even though the 20% MHRA and 2% NA dosage sample workability is also within the acceptable limit, it shows the strength reduction. However, the 10% MRHA and 1% NA dosage samples increased the mechanical properties without affecting the workability.


## 4.2. Strength Development of Optimum Samples

The optimum strength development was observed in the MRHA-2 sample. Both the compressive and split-tensile strength of specimens have increased during the 28-day curing period (Figure7). RHA with a high amount of reactive silica leads to the cement reaction. The addition of RHA to concrete shows a compressive strength increment due to the reaction of SiO2 with free calcium hydroxide, meanwhile reducing the internal air voids [36]. The optimum dosage of 1% of NA-2 samples recorded better mechanical strength compared to control and MRHA-2 specimens. It was observed that the strength development between the 7th and 28th day was gradually increased, indicating NA contributed to a gradually increasing pozzolanic reaction within the cement matrix as the curing time extended. However, the presence of 1% NA had a significant effect on early ages, which could be due to the high rate of reaction. However, the strength growth was decreased after 7 days, which may be due to the limited space for Ca(OH)2 crystal growth [37].


## 4.3. Strength Development of MN Samples

The cement mortar strength was investigated for the combinatorial effect (MN) of MRHA and NA at their optimum concentrations. The strength development of MN samples was shown in Figures8 and 9. Similar to the individual effects of micro and nanoadditive samples, the combinatorial effect also showed early age strength growth. The 7th day test results of MN samples were recorded at 54.01% higher than MRHA-2 samples and 17.85% higher than NA-2. The combination of MRHA and NA promotes cement hydration, filling ability, and less water absorption than individual additives. However, the split-tensile strength of MN samples did not exhibit significant changes compared to 1% NA, indicating the addition of MRHA does not have any effect on the split-tensile strength of the mortar.


## 5. Conclusions

The performance of M-sand cement mortar upon the introduction of micro RHA and nano alumina at different concentrations, individually and in combination, was evaluated through compressive and split-tensile strength test, table flow test, and capillary water observation. The results showed that the addition of MHRA and nano Al2O3 powders individually had different effects on fresh and mechanical properties. In general, MHRA particle dosage mixes due to their ultrafine particles directly increased the amount of water and showed better workability compared to nanoparticle (NA) dosage mixes. Meanwhile, NA, with its pozzolanic action, higher surface area, and better void filling ability, showed better strength improvement compared to MRHA dosage samples. However, the combination of MRHA (10%) and NA (1%) improved the strength and reduced water absorption of cement mortar compared to the individual effects and control. Our study concludes that the partial replacement of cement by micro and nanoparticles in combination is efficient in strength improvement, suggesting the application of MRHA and NA in concrete technology.



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*Source: 4335736-2022-03-10.xml*