Mechanical performance of geopolymer mortars from the incorporation of rice husk ash in metakaolin and sodium silicate

Abstract

The use of waste materials for the production of construction materials is a sustainable alternative to their disposal, as well as a way of reducing the environmental impact of producing the materials. Geopolymer is a promising building material with attractive properties. Rice husk ash (RHA) is a potentially polluting waste material that, due to its properties, can be used to produce geopolymers. This study investigates the hardening behaviour of geopolymer mortars, incorporating up to 20 wt.% rice husk ash in the geopolymer formulations. After 28 days of curing, the samples were tested for mechanical and physical properties. Statistical analysis of the results revealed significant differences between the samples. In addition, the results showed an increase in the mechanical strength of the geopolymer mortars, reaching a value of 43.29 MPa with the simultaneous replacement of 20% metakaolin and sodium silicate with rice husk ash. The highest compressive strength reached 47.75 MPa, corresponding to an increase of 52.4% compared with the reference mixture.

Key Words:
Geopolymer; Metakaolin; Rice Husk Ash; Sustainability; Construction Waste

INTRODUCTION

Portland cement, a fundamental component of many construction materials, is the second most widely used material globally, surpassed only by water¹. However, the Portland cement industry is the second-largest emitter of carbon dioxide, contributing approximately 5% of total global emissions². Despite its numerous advantages, Portland cement is prone to pathologies in aggressive environments, which can compromise its functionality and durability³.

To address the environmental impacts associated with Portland cement production, extensive research has focused on developing alternative materials that can partially or fully replace Portland cement in construction. Simultaneously, the use of waste materials in construction offers an additional environmental benefit, reducing waste disposal and minimizing the extraction of virgin raw materials⁴.

Geopolymers have emerged as a sustainable alternative to traditional cementitious materials. Developed by Joseph Davidovits in the 1970s, geopolymers are considered eco-friendly substitutes for cement-based materials. These inorganic polymers, formed in highly alkaline environments, exhibit remarkable properties such as high compressive strength, durability, low shrinkage, resistance to thermal and chemical attacks (e.g., fire, acids, and moisture), and low thermal conductivity^{5), (6), (7}.

The process of geopolymerization occurs at ambient or slightly elevated temperatures through a chemical reaction that produces polymeric bonds involving silicon (Si), oxygen (O), and aluminum (Al) atoms⁵. In geopolymer systems, sodium ions play a crucial role in the dissolution of aluminosilicate precursors and in the formation of geopolymeric gels. Highly alkaline environments promote the dissolution of Si and Al species from the precursor materials, enabling the formation of a three-dimensional aluminosilicate network.

Metakaolin (MK), a calcined clay rich in aluminosilicates, is commonly used as a precursor for geopolymer synthesis. Kaolin is a naturally occurring clay material composed mainly of the mineral kaolinite (Al₂Si₂O₅(OH)₄). When kaolinite-rich clays are calcined at temperatures between 500 and 900ºC, the hydroxyl groups are removed, producing an amorphous phase known as metakaolinite. The term metakaolin is commonly used to describe the calcined material as a whole and is widely employed as a reactive precursor in geopolymer synthesis.

The activation process requires a highly alkaline solution, typically sodium hydroxide, which facilitates the dissolution of raw material bonds and initiates an exothermic polycondensation process, resulting in rapid hardening. This reaction forms a highly reactive gel with a three-dimensional silico-aluminate structure^{8), (9}.

Although geopolymers are inherently more environmentally friendly than Portland cement, their environmental advantages are further enhanced when industrial waste is utilized as a raw material. When a waste material is added to a system, its characteristics directly influence the properties of the final material¹⁰. In addition to replacing Portland cement in civil construction, geopolymers with the addition of waste have the potential for several applications, such as heavy metal adsorbents¹¹, artificial corals¹², and catalysts¹³. In Brazil, the food industry, including rice production, poses a significant environmental challenge due to its waste byproducts¹⁴.

Rice husk, a byproduct of rice milling, is often used for energy generation. When burned, it produces rice husk ash (RHA), accounting for approximately 25% of its mass. Improper disposal of this ash can have adverse environmental impacts¹⁵. The physicochemical characteristics of RHA can vary according to the conditions of the raw material and the production parameters¹⁶. While the chemical composition of RHA varies depending on the ecological nature of the planting soil and the use of fertilizers, the physical characteristics can be affected by the temperature of the production process^{17), (16}. However, in the construction industry, RHA is highly valued for its silica-rich composition, making it suitable for use in concrete and other materials^{18), (19}. For geopolymer-based construction materials, RHA is particularly advantageous due to its high content of amorphous SiO₂ (85-90 wt%), which dissolves readily in alkaline solutions^{20), (16}. In addition to amorphous silica, other elements can be found in significant quantities in the composition of RHA, such as K₂O and CaO^{21), (16}.

Sodium silicate, commonly used as an alkaline activator in geopolymer systems, is typically produced by the high-temperature fusion of silica (SiO₂) and sodium carbonate (Na₂CO₃) at temperatures ranging from 1100 to 1400ºC. This process is energy intensive and contributes to CO₂ emissions. Therefore, strategies that reduce the consumption of commercial sodium silicate, such as the incorporation of silica-rich residues like rice husk ash, may contribute to improving the environmental performance of geopolymer binders.

These characteristics of RHA allow it to be used as a partial substitute for metakaolin in geopolymer compositions since the amorphous silica present in RHA contributes to the formation of geopolymer chains. In addition to contributing to geopolymer chains, amorphous silica, in the presence of NaOH, can replace commercial sodium silicate solution²². In this way, RHA has the potential to reduce two commercial geopolymer components, metakaolin and sodium silicate.

Knowing the potential of RHA to reduce the use of commercial raw materials in geopolymer production, this study investigates the feasibility of partially substituting both metakaolin (MK) and sodium silicate (SS) with rice husk ash (RHA) in geopolymer mortars. While previous research has primarily focused on the replacement of individual components or on the use of RHA as a precursor or additive, limited studies have explored the combined substitution of both MK and SS with RHA and its implications for mechanical performance and sustainability. To address this gap, the present work evaluates the effects of substituting 20% of MK, SS, and both simultaneously with RHA. The mixtures were activated using sodium silicate and sodium hydroxide solutions at different molarities (6M, 8M, and 10M), and were characterized in both fresh and hardened states. By analyzing mechanical strength, water absorption, bulk density, and porosity, the study aims to identify the most efficient and environmentally beneficial formulation. This approach not only contributes to waste valorization and the reduction of industrial inputs in construction materials but also advances the development of sustainable binders by offering insights into the synergistic role of RHA as a dual substitute. The findings are expected to support the broader adoption of agricultural waste in geopolymer technology, particularly in contexts where economic and ecological efficiency are critical.

MATERIALS AND METHODS

Materials

For the experimental program, metakaolin (MK) supplied by Metacaulim do Brasil S.A. was used as the source of aluminosilicates. Approximately 90% of the solids in the metakaolin had a particle size of less than 44 µm. Rice husk ash (RHA), used as a partial substitute for the geopolymer components, was sourced from Silcca Nobre. The RHA had an average particle size of 4.208 µm and a specific surface area of 21.142 m²/kg. Table I presents the chemical composition of both metakaolin and rice husk ash.

Table I
Chemical weight composition of metakaolin (MK) and rice husk ash (RHA).

Oxides (wt.%)	MK	RHA
Al 2 O 3	32.6	0.18
CaO	0.08	0.59
Fe 2 O 3	2.26	0.06
K 2 O	2.13	2.51
MgO	0.59	0.42
MnO	<0.05	0.32
Na 2 O	<0.05	0.05
P 2 O 5	0.06	0.56
SiO 2	57.12	90.65
TiO 2	1.51	-
Loss on ignition (%)	2.62	4.01

Figure 1 shows the mineralogical characterization of MK and RHA. The analysis revealed that MK exhibits a predominance of amorphous phases, resulting from the dehydroxylation of kaolinite, which enhances its high reactivity in alkaline media and promotes the formation of the three-dimensional aluminosilicate network that characterizes geopolymers. In contrast, the RHA analyzed displayed intense quartz peaks, indicating a predominance of crystalline silica with low solubility, whose contribution to the geopolymerization process is limited, acting mainly as an inert or filler phase.

Figure 1
Mineralogical characterization of MK and RHA.

Sodium hydroxide (HS) and sodium silicate (SS) were employed as alkaline activators. Sodium hydroxide, produced by Realtec, was used at molar concentrations of 6, 8, and 10 M. Sodium silicate was supplied by Quimidrol and had a silicon oxide (SiO₂) to sodium oxide (Na₂O₃) weight ratio ranging from 2.10 to 2.20.

The sand used for mortar production was sieved, with the fraction retained on the 150 µm sieve being utilized. Consequently, the fine aggregate used in the specimens is classified as fine sand. The mass quantity of sand added to the mortar was determined based on the consistency index, using the slump test table to achieve workability comparable to that of commercial mortar.

Methods

The experimental program was divided into two stages. The first stage involved defining the compositions, preparing the mixtures for specimen molding, and determining the curing time. The second stage consisted of performing characterization essays on the produced samples.

Production of mixtures and molding of test specimens

The main objective of the formulations was to analyze the addition or replacement of the precursor metakaolin (MK) and/or sodium silicate (SS) with 20% by mass of rice husk ash (RHA), evaluating the behavior of the resulting geopolymer mortar. Previous studies have proposed the synthesis of alternative sodium silicate solutions by reacting RHA with NaOH²³. In the present work, however, RHA was directly incorporated into the geopolymer mixture in order to evaluate its combined physical and chemical effects on the geopolymer matrix. A reference sample was established based on existing compositions from the literature. Subsequently, formulations incorporating RHA as a substitute for MK and SS were defined, as shown in Tables II and III. The sodium hydroxide solution was prepared 24 h in advance, as it is an exothermic reaction and needs time to cool down. The sodium hydroxide molarity was varied between 6, 8, and 10 M for each composition.

Table II
Mass quantity of material for each composition.

Sample (g)	MK (g)	RHA (g)	SS (g)	HS (g)	Sand (g)
1	624	0	420	156	360
2	499.2	124.8	420	156	360
3	624	84	336	156	360
4	499.2	208.8	336	156	360

Table III
Composition details.

Sample	Mixtures formulation (wt.%)	Solid/Liquid
	MK/SS/RHA
1	50/50/0	1.083
2	40/50/10	1.083
3	50/40/10	1.439
4	40/40/10+10	1.439

For the geopolymer cement used in this study, the molar ratio of SiO₂/Al₂O₃ was maintained within the stipulated range of 3.3 to 4.5. All the water in the mixture was sourced from the sodium hydroxide and sodium silicate solutions. Consequently, adjustments to the solid-to-liquid ratio proportionally altered the amounts of these materials. While the addition of RHA required an increase in water content, the silica present in the sodium silicate partially compensated for potential losses in the material’s mechanical properties.

The preparation of the mixtures was divided into three stages: I - Add the activator solutions (Sodium hydroxide solution+Sodium silicate) to the automatic mortar mixing for 5 min; II - At the end of this time, add the precursor(s) (Metakaolin+RHA, depending on the formulation) mixing manually for 1 min, to improve the homogenization of the material, and then place in the mortar and mix for 10 min; III - add the fine aggregate, mixing 1 min manually and 1 min in the mortar.

After the mixing process, the amount of material for the fresh essays is removed, and the rest is taken to the molds for prismatic specimens, measuring 4 cm x 4 cm x 4 cm, with removable walls, wrapped in plastic film to facilitate the demolding process. After 24 h, the specimens are demolded and allowed to cure for 28 days for the hardening essays.

Essays in the hardened state

In the hardened state, the mortars had their physical and mechanical properties assessed. The physical properties evaluated were apparent porosity (AP), bulk density (BD), and water absorption (WA). The mechanical property evaluated was compressive strength.

For the physical properties, the procedures described in ABNT 9779²⁴ and ABNT 16661²⁵ were followed. The masses of the specimens were measured in three situations. The dry mass (Ms) was measured after the specimens had been dried for three days in an oven at a temperature of 80ºC. The saturated mass with dry surface (Msss) of the specimens was checked after three days immersed in water. The immersed mass (Mi) was measured with the specimens immersed in water. The following equations were used to calculate the properties:

$WA\left(\%\right)=\frac{M_{SSS}-M_S}{S}*100$ (1)

$BD\left(g/c{m}^3\right)=\frac{M_S}{M_{SSS}-M_i}$ (2)

$AP\left(\%\right)=\frac{M_{SSS}-M_S}{M_{SSS}-M_i}*100$ (3)

RESULTS AND DISCUSSION

Figure 2 illustrates the compressive strength test results. According to the literature, mechanical strength performance is closely linked to the Si/Al ratio in the geopolymer^{26), (27}. The results demonstrate the feasibility of partially replacing sodium silicate and metakaolin with rice husk ash (RHA) while identifying the optimal molar concentration for strength enhancement. Notably, starting with the reference mixture (Formulation 1) - composed of 100% metakaolin and sodium silicate - significant variations in mechanical strength were observed. It is important to note that formulation 1 contains sodium originating from both sodium hydroxide and sodium silicate. The presence of sodium ions plays a fundamental role in geopolymerization by promoting the dissolution of aluminosilicate species and enabling the formation of the geopolymeric gel network. Although the total sodium content may influence reaction kinetics, the alkaline activator composition was kept consistent for each molarity level, indicating that the differences observed between mixtures are mainly associated with the incorporation of RHA.

Figure 2
Compressive strength of the samples at 28 days of curing for 4 different mixture formulations and molarities (6 to 10 M).

When 20% of the sodium silicate was replaced with RHA (Formulation 3), the compressive strength increased across all tested molarities (10M: +52.4%, 8M: +30.8%, and 6M: +99.8%). The sodium silicate used in this study presents a SiO₂/Na₂O ratio between 2.10 and 2.20, according to the supplier specifications. In contrast, the rice husk ash used in the mixtures contains approximately 90 wt.% SiO₂, as shown in Table I. Although the silica in sodium silicate is present in soluble form and directly participates in geopolymerization, the high silica content of RHA may contribute to microstructural densification and improved particle packing, which can partially explain the mechanical behavior observed in mixtures containing RHA. A similar trend was evident in the mixture where both metakaolin and sodium silicate were substituted with 20% RHA (Formulation 4), with even greater increases in strength (10M: +38.2%, 8M: +4.6%, and 6M: +187%). Conversely, when metakaolin alone was replaced by 20% RHA (Formulation 2), a decrease in mechanical strength was observed at all molarities tested (10M: -26.1%, 8M: -44.3%, and 6M: -14.5%).

The mixture containing RHA with a reduced metakaolin content presented lower mechanical strength. This behavior can be attributed to the high reactivity of metakaolin as a source of aluminosilicate. The incorporation of RHA in the formulation results in a reduction of the reactive silica and alumina provided by metakaolin. Based on the XRD results (Figure 1), the substitution of MK by RHA reduced the mechanical strength due to the predominantly crystalline nature of RHA and its lower overall reactivity when compared to metakaolin.

In contrast, the replacement of sodium silicate by RHA resulted in an increase in compressive strength. Although RHA was previously characterized as mainly crystalline and presenting low solubility, this strength enhancement should not be interpreted as a full chemical replacement of soluble silicate. Instead, the observed behavior is mainly associated with a physical filler effect, in which RHA particles contribute to improved particle packing, reduced porosity, and matrix densification. Additionally, it is assumed that RHA may contain a minor fraction of amorphous or disordered silica, as well as reactive surface sites, which can partially dissolve under highly alkaline conditions, providing a secondary contribution of soluble silica to the system.

The Solid/Liquid (S/L) ratio varied between the reference mixtures (1.083) and the RHA-containing formulations (1.439), as shown in Table III. This adjustment was required to ensure adequate workability, since RHA presents a higher specific surface area and water demand compared to commercial sodium silicate.

It is well established that variations in liquid content can influence porosity, degree of reaction, and matrix densification, which may affect compressive strength. However, the increase in mechanical performance observed in mixtures containing RHA cannot be solely attributed to the higher S/L ratio. This is evidenced by comparisons between formulations produced with the same sodium hydroxide molarity and identical S/L ratios, where mixtures incorporating RHA exhibited significantly higher compressive strength than the reference formulation.

Therefore, although the increase in S/L ratio may contribute to changes in microstructure, the strength enhancement is mainly associated with the physical filler effect of RHA, improved particle packing, and a secondary contribution from the partial dissolution of reactive silica under highly alkaline conditions.

In other studies, the opposite behavior was observed. SOMNA et al.²⁸ observed that replacing fly ash with RHA increased mechanical strength, while NANA et al.²⁹ reported that substituting metakaolin with a combination of fly ash and RHA also enhanced mechanical strength. Conversely, substituting fly ash with RHA improves mechanical strength due to the higher content of amorphous silica in RHA compared to fly ash. In these cases, the addition of RHA increases the reactive silica in the system, resulting in a geopolymer network with a greater proportion of Si-O bonds and, consequently, enhanced mechanical strength³⁰.

When using RHA as an alkaline activator, BOUZON et al.³¹ noted no significant changes in mechanical strength. However, KAMSEU et al.³² observed that combining commercial sodium silicate with an alkaline activator based on RHA can improve mechanical resistance. These findings reinforce that the contribution of RHA depends not only on its mineralogical composition but also on its physical characteristics, such as particle size and surface area, which influence packing efficiency and microstructural development. The use of RHA in the activator solution increases the amount of Si in the system. The Si-O-Si bond is stronger than bonds involving Al. Thus, the increase in the amount of silicon promoted by the addition of RHA can increase mechanical resistance^{33), (34}.

It can be observed that the highest compressive strength results are with the highest molarity of sodium hydroxide (10M). KAUR et al.³⁵ and KISHORE et al.³⁶ also found the same pattern of mechanical behavior, in which increasing the molarity of sodium hydroxide results in higher mechanical strength values. This phenomenon occurs because in more concentrated alkaline solutions, the dissolution of Si and Al is more effective³⁷. With more aluminosilicate dissolved in the solution, there is greater freedom for geopolymer chains to form and the material matrix becomes more homogeneous.

Table IV presents an ANOVA at a 95% confidence level, indicating that the results varied significantly depending on the mixture. Furthermore, Table V identifies which compositions differed significantly, denoted by asterisks. As shown in column 1, the compositions 1-8M, 1-10M, and 4-8M did not differ significantly from each other.

Table IV
Analysis of variance of compressive strength results at 28 days.

	SS	Degr. of Freedom	MS	F	p
Intercept	41723.14	1	41723.14	5837.935	0.00
Compositions	11872.94	11	1079.36	151.025	0.00
Error	343.05	48	7.15

Table V
Tukey HSD test of compressive strength results at 28 days for 4 different mixture formulations and molarities (6 to 10 M).

Sample	Compressive Strength Mean	1	2	3	4	5	6	7
2-6M	5.35		*
1-6M	6.27		*
3-6M	12.53						*
4-6M	17.99			*
2-8M	18.07			*
2-10M	23.14							*
1-10M	31.32	*
1-8M	32.89	*
4-8M	33.39	*
3-8M	42.77				*
4-10M	44.96				*	*
3-10M	47.75					*

Based on the results in Tables IV and V, among the mixes with the highest compressive strength (4-10M: 44.97 MPa and 3-10M: 47.75 MPa), the formulation with the lowest environmental impact would be 4-10M. This formulation contains the highest proportion of RHA residue, making it the most suitable choice from a sustainability perspective.

The compressive strength values obtained in this study, reaching up to 47.75 MPa, fall within the range commonly reported for geopolymer mortars used in construction applications. Previous studies have reported compressive strength values for metakaolin-based geopolymer mortars typically ranging between 20 and 60 MPa depending on mixture composition and curing conditions. Therefore, the mechanical performance observed in this study is consistent with values reported in the literature.

The results for water absorption, bulk density, and apparent porosity are presented in Figures 3, 4, and 5, respectively. It is evident that bulk density follows an inverse trend compared to porosity and water absorption, consistent with observations reported in previous studies^{38), (39}.

Figure 3
Water absorption of the samples at 28 days of curing for 4 different mixture formulations and molarities (6 to 10 M).

Figure 4
Bulk density of the samples at 28 days of curing for 4 different mixture formulations and molarities (6 to 10 M).

Figure 5
Apparent porosity of the samples at 28 days of curing for 4 different mixture formulations and molarities (6 to 10 M).

Table VI shows the ANOVA tables (95% confidence level) for the results of the physical properties. It is statistically proven that the results varied significantly according to the mixture developed.

Table VI
Analysis of variance of water absorption (%), bulk density (g/cm³) and apparent porosity (%) results at 28 days.

Water Absorption (%)		SS	Degr. of Freedom	MS	F	p
	Compositions	456.54	11	41.50	321.45	0.00
	Error	6.20	48	0.13
Bulk Density (g/cm3)		SS	Degr. of Freedom	MS	F	p
	Compositions	2.564	11	233	98.9	0.00
	Error	113	48	2
Apparent Porosity (%)		SS	Degr. of Freedom	MS	F	p
	Compositions	969.86	11	88.17	258.3	0.00
	Error	16.38	48	0.34

Figure 4 illustrates that the physical properties were affected by the type of RHA substitution. Generally, the addition of RHA increased the bulk density of the samples. According to the XRD results (Figure 1), RHA contains a significant crystalline quartz fraction, which is less reactive but contributes to particle packing; therefore, in both substitutions, the addition of RHA reduced the amount of amorphous reactive phase but enhanced the filler effect, leading to improved particle packing and, consequently, higher bulk density. The opposite behavior has already been observed in other types of geopolymers, in which the addition of RHA reduces the density of the material⁴⁰. In fact, it can be seen from the reference mix (formulation 1) that there were significant changes in water absorption for the three molarities tested (formulation 3) when substituting 20% metakaolin for RHA. For example: -15.36% (10M), -6.76% (8M) and -12.3% (6M). The same trend was also observed for the other mixtures, with the variation for formulation 4 being -33.1% (10M), -19.8% (8M) and -46.25% (6M). Also, for formulation 2 -17.9% (10M), -39.1% (8M) and -37.4% (6M).

With regard to apparent porosity, the same trend was observed: formulation 3 -11.2% (10M), -4.5% (8M) and -4.6% (6M), formulation 4 -27.6% (10M), -15.5% (8M) and -37.4% (6M) and formulation 2 -15.5% (10M), -34.8% (8M) and -29.2% (6M). Regarding apparent porosity, the addition of RHA can cause different effects. NANA et al.²⁹ observed a similar behavior for two of her compositions, in which the addition of RHA decreased the apparent porosity. ZEYAD et al.⁴¹ observed an opposite behavior, the addition of RHA increased the apparent porosity. KAMSEU et al.³² found that the use of RHA in the alkaline activator can reduce porosity.

In the case of bulk density, the behavior was the opposite, as expected: formulation 3 had a variation of+4.7% (10M),+1.8% (8M) and+2.9% (6M), formulation 4 of+8.2% (10M),+4.71% (8M) and+10.0% (6M) and for formulation 2 of+2.4% (10M),+6.5% (8M) and+7.1% (6M). The higher density obtained for the mortars with RHA is justified by the fact that the presence of RHA in metakaolin-based geopolymers provides oligomers that contribute to greater densification of the gel structure²⁹. NASRUDDIN et al.⁴² and TCHAKOUTÉ et al.⁴³ realized that the presence of RHA makes the geopolymer structure denser. While NANA et al.²⁹ observed a decrease in density with an increase in the amount of RHA in the composition and ZEYAD et al.⁴¹ did not notice significant changes in density with the incorporation of RHA.

From a sustainability perspective, formulation 4 represents the most advantageous mixture. This formulation incorporates the highest proportion of RHA while maintaining high compressive strength values, indicating that agricultural waste can be effectively valorized without compromising mechanical performance.

Tables VII, VIII and IX show which values are considered equal using the asterisk symbolism.

Table VII
Tukey HSD test of water absorption.

Sample	Water Absorption Mean	1	2	3	4	5	6	7	8	9
4-6M	9.30				*
2-8M	10.39	*
2-6M	10.79	*
4-10M	11.53					*
4-8M	13.80		*
2-10M	14.22		*
3-10M	14.71						*
3-6M	15.20							*
3-8M	16.02								*
1-6M	16.99									*
1-10M	17.52			*
1-8M	17.52			*

Table VIII
Tukey HSD test of bulk density.

Sample	Bulk Density Mean	1	2	3	4	5	6	7
1-6M	1.66		*
1-8M	1.66		*
1-10M	1.69						*
3-8M	1.73	*
2-10M	1.74	*
3-6M	1.75	*
3-10M	1.78			*
4-8M	1.78			*
2-8M	1.80				*
2-6M	1.82				*	*
4-10M	1.84					*
4-6M	1.87							*

Table IX
Tukey HSD test of apparent porosity.

Sample	Apparent Porosity Mean	1	2	3	4	5	6	7	8
4-6M	17.44					*
2-8M	18.85						*
2-6M	19.67							*
4-10M	21.15								*
4-8M	24.61	*
2-10M	24.80	*
3-10M	26.18		*
3-6M	26.55		*
3-8M	27.84			*
1-6M	28.17			*
1-8M	29.21				*
1-10M	29.36				*

The trends described above can be demonstrated mathematically by means of graphs/equations that relate the physical properties to each other, as shown in Figures 6, 7 and 8. The relationships between water absorption, bulk density, and apparent porosity reflect the pore structure of the geopolymer mortars. In general, an inverse relationship between water absorption and bulk density can be observed, indicating that mixtures with a denser microstructure tend to present lower water absorption values. Similarly, reductions in apparent porosity are associated with higher density and lower water uptake. These trends are consistent with the densification of the geopolymer matrix promoted by the incorporation of RHA, which contributes to improved particle packing and reduced pore connectivity.

Figure 6
Relationship between physical properties of water absorption and apparent porosity.

Figure 7
Relationship between physical properties of water absorption and bulk density.

Figure 8
Relationship between physical properties of apparent porosity and bulk density.

CONCLUSION

The use of rice husk ash to produce geopolymers is a sustainable alternative that can also contribute to the material’s physical and mechanical properties.

The replacement of sodium silicate and the joint replacement of sodium silicate and metakaolin with rice husk ash proved to be effective in increasing compressive strength. When compared to the mixture with no added ash, the mixture with partial substitution of metakaolin and sodium silicate and a molar concentration of 6M (4-6M) showed a 99.8% increase in mechanical strength. The density of the material also increased with the addition of ash, causing the geopolymer to have lower water absorption and porosity.

In conclusion, rice husk ash can be considered a promising partial substitute for metakaolin and sodium silicate, resulting in increased mechanical strength, higher density, and reduced water absorption and porosity. Among the investigated mixtures, formulation 4 can be considered the most balanced composition from both mechanical and sustainability perspectives. This formulation incorporates the highest proportion of rice husk ash while still presenting high compressive strength values, demonstrating the potential of this agricultural residue as a viable component in geopolymer mortars.

ACKNOWLEDGEMENTS

The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES) and the National Council for Scientific and Technological Development (CNPq), agencies of the Brazilian Federal Government, for their financial support of this research.

DATA AVAILABILITY

The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.

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• FUNDING
  This research was supported by scholarships provided by the National Council for Scientific and Technological Development (CNPq), Brazil.

Edited by

• AE:
  Daniel Zanetti de Florio

Publication Dates

• Publication in this collection
  27 Apr 2026
• Date of issue
  2026

History

• Received
  29 Sept 2025
• Reviewed
  23 Jan 2026
• Reviewed
  07 Mar 2026
• Accepted
  19 Mar 2026