Strength Development of PPC Concrete with Rice Husk Ash: Optimal Replacement Levels for Sustainable Construction

Abstract

The construction industry is a major contributor to greenhouse gas emissions, with cement production responsible for 8–10% of global CO₂ output. This study investigates the use of rice husk ash (RHA) as a partial cement replacement in Portland Pozzolana Cement (PPC) concrete, which already contains fly ash. Five replacement levels (5%, 7.5%, 10%, 12.5%, and 15% by weight of PPC) were tested for compressive and flexural strength (modulus of rupture, MOR) at 3, 7, 28, and 56 days. An M20-grade mix, designed in accordance with Saudi Building Code (SBC) provisions, was adopted to ensure practical applicability. Results showed that moderate RHA contents (5–7.5%) enhanced strength, with maximum compressive (37.62 MPa) and flexural (5.47 MPa) strengths recorded at 7.5% RHA after 56 days, representing 3.5% and 9.6% improvements over the control, respectively. All RHA mixes exhibited reduced early-age strength due to delayed pozzolanic activity, whereas higher replacements (≥12.5%) caused strength loss from excessive cement dilution. The novelty of this study lies in demonstrating the synergistic effect of RHA with PPC—an underexplored dual-pozzolan system—and identifying optimal replacement levels for sustainable construction. The findings highlight RHA as a viable supplementary cementitious material that reduces clinker use and carbon emissions while maintaining or improving long-term concrete performance.

1. Introduction

Cement production accounts for nearly 8–10% of global CO₂ emissions, making it one of the most carbon-intensive industries [1,2]. To mitigate its environmental impact, supplementary cementitious materials (SCMs) are widely adopted as partial replacements for cement, thereby reducing clinker consumption and associated emissions [3]. Among these, rice husk ash (RHA) is of particular interest due to its high amorphous silica content and strong pozzolanic activity [4,5].

Extensive studies on ordinary Portland cement (OPC) systems have shown that RHA enhances compressive and flexural strength [6,7]. Researchers have also examined RHA combined with carbon nanotubes and bio-cementation techniques, reporting significant improvements in strength and durability [8,9]. Other innovative approaches include electrochemical impregnation and carbonate control processes (EICCPs), further demonstrating the versatility of RHA in concrete technology [10].

The dual-pozzolan effect of RHA with other mineral admixtures has been found to improve hydration and microstructural development [11]. Optimal replacement levels of 5–15% have been suggested in OPC and blended cement systems [12,13]. However, the durability performance of RHA concretes under aggressive environments, as well as their behavior in PPC systems, remains insufficiently explored [14,15,16,17,18,19,20,21].

This study addresses these gaps through a systematic investigation of PPC concretes incorporating RHA at five replacement levels (5%, 7.5%, 10%, 12.5%, and 15%). Compressive and flexural strengths were evaluated at 3, 7, 28, and 56 days of curing. The objectives were to

• Determine the optimal RHA replacement level for maximum strength in PPC systems.
• Compare strength development trends in RHA–PPC concretes with those reported for OPC–RHA mixes.
• Provide practical recommendations for the sustainable use of RHA in PPC-based construction.

By focusing on the underexplored dual-pozzolan system of PPC and RHA, this study contributes new insights into optimizing mix designs for both sustainability and structural performance.

2. Materials and Methods

2.1. Materials

Aggregates complied with ASTM C33 [22]. The pozzolanic material (RHA) satisfied ASTM C618 [23]. Materials selection aligns with waste/by-product utilization guidance in the literature [24] and (where applicable) air-entrainment requirements per ASTM C260 [25]. All provisions referenced below are consistent with the Saudi Building Code (SBC 301/304/305) [26].

2.1.1. Cement

Portland pozzolana cement (PPC) was used (see overall SBC alignment above [26]).

2.1.2. Fine Aggregate

Natural river sand (Zone II, FM ≈ 2.78); grading and quality per ASTM C33 [22].

2.1.3. Coarse Aggregate

Crushed granite (20 mm max size, SG ≈ 2.70, water absorption < 0.5%); grading per ASTM C33 [22].

2.1.4. Rice Husk Ash (RHA)

Produced by controlled burning (600–700 °C) and sieved to 75 µm; chemical requirements met ASTM C618 [23].

Table 1. Chemical composition of rice husk ash (RHA) by XRF analysis.

Component	Percentage by Mass (%)
SiO2	85.20
Al2O3	0.45
Fe2O3	0.25
CaO	2.30
MgO	1.10
K2O	1.85
Na2O	0.25
Loss on Ignition (LOI)	4.60

summarizes he chemical composition of rice husk ash (RHA), determined by XRF analysis, confirmed compliance with ASTM C618 [23] and was consistent with earlier reported values [7,27].

2.1.5. Chemical Admixture

High-range water-reducing admixture (≈1% of binder) conforming to ASTM C494 [28]; usage consistent with SBC 305 [26].

2.1.6. Water

Potable water conforming was used, consistent with SBC 304 [26].

2.2. Mix Design

Concrete was proportioned for M20 grade (target 27.6 MPa, w/c = 0.50). Proportioning followed SBC 305 [26]. Workability was measured by the ASTM C143 slump test [29]. Specimen making and curing followed ASTM C192 [30]. Compressive and flexural tests followed ASTM C39 (cylinders/cubes per local practice) and ASTM C78 (third-point loading) respectively [22,31].

Table 2. Proportions for M20-grade concrete mix design.

Material	Quantity (kg/m3)
Cement (PPC)	383
Fine Aggregate (Sand)	594
Coarse Aggregate (Gravel)	1356
Water	191.61
RHA (Rice Husk Ash)	0, 19.15, 28.72, 38.30, 47.87, 57.45 (5%, 7.5%, 10%, 12.5%, 15% of cement)
Superplasticizer (Conplast SP430A2)	3.83 (1% of cement)

presents the mix proportions for the M20-grade PPC concrete with varying RHA replacement levels.

Mix Proportions

The target mean strength of 27.6 N/mm² was chosen to provide a margin of safety, ensuring the concrete’s performance under a variety of real-world conditions. The mix proportions were optimized based on previous studies and recommendations from Ganesan et al. [6], which observed that replacement levels of 5–15% RHA provided the best results for mechanical performance in OPC-based systems.

To achieve a homogeneous mix, all dry ingredients—Portland Pozzolana Cement (PPC), fine aggregate, coarse aggregate, and rice husk ash (RHA)—were mixed in a pan mixer. This ensured uniform distribution of the cementitious material and aggregates. The process involved adding water in batches, with mixing continuing for 5 min to achieve the desired consistency. After mixing, the concrete was assessed for workability using the slump test, as per ASTM C143 [29]. Casting and compaction were carried out following SBC 301: Seismic Design Provisions and SBC 304: Special Structural Applications [26].

The mix was prepared for M20-grade concrete with an adequate workability for the required strength, following the guidelines from SBC 305: Concrete Mix Proportioning [26]. The water-to-cement ratio was consistently kept at 0.5 to ensure uniform hydration and adequate bonding of the aggregates. Conplast SP430A2 superplasticizer was used at 1% by weight of cement to improve workability and ensure that the water-to-cement ratio remained constant, despite the inclusion of RHA.

2.3. Mixing and Curing

Mixing was carried out in a laboratory pan mixer. Cubes (150 × 150 × 150 mm) were cast for compressive strength, and beams (100 × 100 × 500 mm) for flexural strength. Specimens were compacted using tamping rods and vibration tables.

After 24 h, specimens were demoulded and water-cured at 27 ± 3 °C until testing ages (3, 7, 28, and 56 days), as per ASTM C39 [22].

Curing Duration and Testing

The specimens were tested at 3, 7, 28, and 56 days of curing ages to assess both early-age strength and long-term performance. This allowed the study to evaluate the impact of different RHA replacement levels on the strength development curves at both early and later stages of curing.

2.4. Mixing, Casting, Compaction, and Curing

Preparation of concrete specimens was carried out strictly in accordance with relevant Indian Standards to ensure that results were repeatable and reliable. Proper mixing, casting, and curing are necessary in order to produce uniform specimens whose strength characteristics are representative of the intended mix.

2.4.1. Mixing Process, Casting and Compaction

Batching of the materials was in weight, as recommended by Standard Specification for Ready-Mixed Concrete to achieve high precision in proportioning. Aggregates were brought to surface-saturated dry (SSD) condition prior to mixing to avoid inadvertent alteration of the effective water–cement ratio.

The mixing sequence was as follows:

• The coarse aggregates were placed in the pan mixer initially, followed by fine aggregates and cementitious materials (PPC and RHA).
• Dry mixing was carried out for 2 min to ensure uniform distribution of cementitious powders in aggregates.
• Approximately 70% of the total water, pre-mixed with high-range water-reducing admixture (Conplast SP430A2), was slowly added.
• Water was added during the last 3 min of wet mixing for ensuring the desired workability without segregation.

The use of a superplasticizer was necessary in RHA mixes because of the high specific surface area and porous character of RHA, which has a tendency to enhance water demand [2,3]. Dosage with caution prevented overdosage, which would have caused undue retardation or bleeding.

Concrete was filled in two equal layers of thickness in the moulds. These were compacted either manually, by 35 even strokes with a standard tamping rod (16 mm diameter × 600 mm length), or mechanically, by a vibrating table as per ASTM C192 [30], until air bubbles stopped appearing on the surface. Mechanical compaction was chosen to achieve a higher density and eliminate entrapped air, which is especially important in RHA mixes since it has a tendency to trap more air voids.

2.4.2. Mould Preparation

Steel moulds were used to cast cubes (150 × 150 × 150 mm) for a compressive strength test and beams (100 × 100 × 500 mm) for a flexural strength test, as per ASTM C39 + ASTM C78 [22,31]. The inner surface of moulds was coated with a thin layer of mineral oil to demould easily without damaging specimen edges. Moulds were placed on a solid, horizontal surface to avoid distortion.

2.4.3. Demoulding and Curing

The specimens were demoulded after 24 ± 2 h of casting and were at once submerged in a curing tank containing drinking water at 27 ± 2 °C, as required in ASTM C39 + ASTM C78 [22,31]. Curing periods of 3, 7, 28, and 56 days were selected to capture early-age, normal-age, and long-term strength developments. The selected periods also allowed investigation of the retarding pozzolanic reaction of RHA, which usually plays a greater role in strength gain beyond 28 days [6].

2.5. Test Procedures

Table 3. Summary of test parameters for compressive and flexural strength evaluation.

Test Type	Specimen Size (mm)	Standard Followed	Loading Arrangement	Loading Rate
Compressive Strength	150 × 150 × 150	ASTM C39 + ASTM C78 [22,31]	Axial loading	140 kg/cm2/min (constant)
Flexural Strength	100 × 100 × 500	ASTM C39 + ASTM C78 [22,31]	Third-point loading	As per ASTM C39 guidelines [22]

consolidates the specimen dimensions, standards, and loading conditions adopted for compressive and flexural strength testing. The experimental program included both compressive and flexural strength testing of concrete specimens, in accordance with the relevant ASTM. All specimens were cast, cured, and tested under controlled laboratory conditions to ensure reproducibility of results. The procedures outlined below were followed to maintain consistency and reliability of measurements.

All specimens were prepared in compliance with SBC 304, SBC 305: Concrete Mix Proportioning [26], and relevant testing standards to ensure reproducibility. The batching sequence began with coarse aggregates, followed by fine aggregates, cementitious materials (PPC and RHA), and finally water containing the pre-measured dosage of Conplast SP430A2 high-range water-reducing admixture (ASTM C494 Type G-2019 [28]). All dry constituents were first blended for 2 min in a pan mixer to ensure uniform distribution, after which water with admixture was added gradually, and mixing was continued for 3 additional minutes to achieve homogeneity without segregation.

Cube moulds (150 × 150 × 150 mm) for compressive strength tests and prism moulds (100 × 100 × 500 mm) for flexural strength tests were prepared in accordance ASTM C39 + ASTM C78 [22,31]. Mould interiors were coated with a thin layer of mineral oil to prevent adhesion. Fresh concrete was placed in two layers, each compacted using either 35 strokes of a standard tamping rod (ASTM C192 [30]) for hand compaction or a vibrating table ASTM C192 [30]) until no surface air bubbles were visible.

Specimens were demoulded after 24 ± 2 h and immersed in clean potable water (SBC 304 [26] at 27 ± 2 °C. Curing durations of 3, 7, 28, and 56 days were adopted to capture early-age, standard, and long-term performance. This regimen enabled evaluation of both the initial hydration phase and the later pozzolanic reactions of RHA in PPC systems.

2.5.1. Compressive Strength Test

Cube specimens were tested using a 2000 kN compression testing machine (CTM) at a loading rate of 140 kg/cm²/min (ASTM C39) [22]. Strength was calculated from maximum load and cross-sectional area. The average of the three specimens was recorded.

Each specimen was placed centrally between the platens, ensuring uniform contact. The load was applied continuously and without shock at a controlled rate of 140 kg/cm²/min until failure occurred. The maximum load was recorded, and the compressive strength is calculated using Equation (1):

$$f_c={\displaystyle \frac{P}{A}}$$

(1)

where $f_c$ = compressive strength (kg/cm²);

P = maximum applied load (kg);

A = cross-sectional area of specimen (cm²).

The average of three specimens was taken as the representative strength value for each test age and replacement percentage.

2.5.2. Flexural Strength Test

Beam specimens were tested using third-point loading as per ASTM C39 [22]. The modulus of rupture (MOR) was calculated from fracture location and applied load. Load was applied at a constant rate until fracture, and the modulus of rupture (MOR) ($f_b$) was calculated using the standard formula based on fracture location:

If $a$ is greater than 20.0 cm for a 15.0 cm specimen, or greater than 13.3 cm for a 10.0 cm specimen, Equation (2) was used:

$$f_b={\displaystyle \frac{P·L}{b·d^2}}$$

(2)

If $a$ is between 17.0 cm and 20.0 cm for a 15.0 cm specimen, or between 11.0 cm and 13.3 cm for a 10.0 cm specimen, Equation (3) was used:

$$f_b={\displaystyle \frac{3P·a}{b·d^2}}$$

(3)

where $f_b$ = modulus of rupture (kg/cm²);

P = maximum applied load (kg);

L = span length (cm);

b = width of specimen (cm);

d = depth of specimen at the point of failure (cm);

a = distance from line of fracture to nearest support (cm).

2.5.3. Summary of Test Parameters

Table 3 consolidates dimensions, standards, and loading conditions for compressive and flexural strength tests.

The following section presents the detailed mix proportioning used in the experimental program, including rationale for RHA levels.

2.6. Mix Proportioning and Rationale for RHA Concrete

To evaluate the effects of varying cement replacement with RHA (5%, 7.5%, 10%, 12.5%, 15%), the study maintained constant amounts of binder during rice husk ash (RHA) concrete mix proportioning.

Table 4. Mix proportions of RHA concrete at different replacement levels.

Replacement %	Cement (kg/m3)	RHA (kg/m3)	Fine Aggregate (kg/m3)	Coarse Aggregate (kg/m3)	Water (L/m3)	Superplasticizer (L/m3)
0% (Control)	382.99	0	594	1394	191.61	4.50
5%	363.85	19.15	594	1394	191.61	4.36
7.5%	354.27	28.72	594	1394	191.61	4.25
10%	344.70	38.30	594	1394	191.61	4.13
12.5%	335.13	47.80	594	1394	191.61	4.02
15%	325.55	57.45	594	1394	191.61	3.90

demonstrates the systematic changes made to cement content, while RHA content increased and the remaining ingredients of fine aggregate (594 kg/m³), coarse aggregate (1394 kg/m³), and water (191.61 L) and superplasticizer dosage remained consistent. The higher levels of RHA replacement lead directly to reduced cement material quantities. At 5% replacement level, the cement content stood at 363.85 kg/m³ whereas it decreased to 325.55 kg/m³ at 15% replacement. The controlled decrease in cement level meets sustainability standards by combining RHA pozzolanic material, which enhances concrete durability benefits alongside environmental conservation. As RHA content increases in the mix, the dosage of superplasticizer requires a slight reduction. When the RHA replacement reaches 5%, the superplasticizer dosage stands at 4.36 L/m³, and it drops to 3.90 L/m³ at 15% RHA content. The water characteristics together with flow of concrete mixtures are influenced by RHA when it is included as an ingredient. The small particles in RHA generate additional surface area which affects admixture requirements to achieve proper workability. Water content stays at 191.61 L/m³ because the water–cement ratio is established at 0.5. The uniform water content level allows researchers to attribute concrete changes to RHA content adjustments instead of water adjustments. RHA contributes to secondary stone action because its high silica percentage and pozzolanic activity enable improved long-term strength and durability. The results of mix proportioning testing show RHA work as a suitable cement replacement which maintains the basic concrete mixture ingredients. However, as the RHA percentage increases beyond 10–12.5%, there may be concerns regarding strength reduction due to excessive cement replacement. Hence, the optimum RHA replacement level must be determined by mechanical strength and durability testing to ensure that performance does not get impacted.

Therefore, the application of rice husk ash as a substitute for cement offers a cost-effective and sustainable approach to concrete mixed designs. The variations in reported mix proportions indicate a balance between cement reduction, workability modification, and material performance, which makes RHA an alternative solution in green construction.

3. Results

This section presents the compressive and flexural strength results of PPC concrete with varying RHA replacement levels (0%, 5%, 7.5%, 10%, 12.5%, 15%) at curing ages of 3, 7, 28, and 56 days.

3.1. Compressive Strength

Compressive strength increased steadily with curing age. The optimum was at 7.5% RHA, reaching 37.6 MPa at 56 days, 3.5% higher than the control [6]. At replacement levels above 10%, strength declined due to dilution effects [32,33].

Table 5. Compressive strength of control concrete in N/mm².

Grade of Concrete	3 days	7 days	28 days	56 days
M20	14.51	20.58	30.3	36.36

and Figure 1 indicate the compressive strength of concrete specimens at varied RHA replacement levels (0%, 5%, 7.5%, 10%, 12.5%, and 15%) with curing ages of 3, 7, 28, and 56 days. There is a trend: compressive strength increases with RHA replacement at up to 7.5% but decreases at higher replacement levels.

At 3 curing days, all mixes containing RHA were less compressive than the control mix. The explanation is due to the slow pozzolanic reactivity of RHA during early hydration stages, as its high amorphous silica content requires time to react with the released calcium hydroxide (CH) during cement hydration Zain et al. [27]. However, with greater curing to 7 and 28 days, pozzolanic activity becomes more pronounced, and the development of greater amounts of calcium silicate hydrate (C–S–H) gel takes place, leading to improving microstructure and strength of the hardened concrete [6,7].

The maximum compressive strength was achieved at 7.5% RHA replacement level, with a value of 37.62 MPa at 56 days, the highest by 3.5% when compared to the control mix (36.36 MPa). The strength gain at 28 days was approximately 8.3% when compared to the control, confirming the synergistic benefit of the pozzolanic reaction and filler effect of RHA [18]. Fine RHA particles enhance the packing density of the cementitious matrix and reduce porosity, resulting in better mechanical performance.

But with more than 10% replacement of RHA, compressive strength decreased gradually with all curing ages. At 15% RHA, the compressive strength at 28 days decreased by 12.5% compared to the 7.5% mix. This is largely due to the dilution effect where increased RHA content replaces excessive cement, hence reducing the proportion of initial clinker used for early hydration. Moreover, increased RHA content can enhance water demand and reduce matrix cohesiveness, consequently impairing strength development [10,34].

These findings are in line with previous studies, which established maximum mechanical performance in the range of 5–10% RHA, followed by weakening at higher levels [6,18]. Further, the sustained increase in strength between 28 and 56 days indicates the delayed pozzolanic activity of RHA, suggesting that it is a suitable material for long-term sustainable applications of concrete.

The results confirm that 5–7.5% RHA replacement provides optimum compressive strength, consistent with dual-pozzolanic behavior.

Accordingly, incorporating rice husk ash as a partial cement replacement offers a sustainable and cost-effective approach in concrete mix design. The observed variations in mixed proportions demonstrate a balance between cement reduction, workability adjustments, and material performance, making RHA a viable alternative in eco-friendly construction practices.

3.1.1. Control Concrete (CC)

In this research, M20-grade control concrete compressive strength development at ages ranging from 3 to 7 days to 28 days to 56 days is checked. Figure 2 shows compressive strength at different ages of control concrete in days. Table 5 and Figure 3 confirm how concrete compressive strength increases when curing time is extended. The hydration process works continuously because it produces calcium silicate hydrate (C-S-H) gel that causes strength development. The compressive strength measurements of control concrete at various stages of development are given in Table 5.

• At 3 days of curing, the concrete reaches 14.51 MPa strength, which represents 47% of its 30-day strength value.
• The material strength measures 20.58 MPa at 7 days, reaching 67% of the target 28-day strength.
• Standard 30.3 MPa concrete strength is reached after 28 days of hydration.
• The compressive strength of the material rises to 36.36 MPa during the 56-day period where it equals 1.2 times its 28-day strength.

Table 5 demonstrates that concrete compressive strength shows a direct relation to the 28-day strength measurements. Results indicate that strong strength development occurs quickly during the initial week because concrete reaches 67% of its 28-day strength point at day 7 and then shows reduced strength advancement afterwards. Compressive strength rose rapidly up to 28 days and then increased more gradually up to 56 days. C-S-H gel formation reaches its maximum during primary cement hydration, thus resulting in this fast increase. At 28 days, the gain in strength slows down with the hydration process slowing down as there is only 20% additional development in strength between 28 and 56 days. These findings refer to the requirement of proper curing for at least 28 days to ensure the concrete attains its specified strength. The continuing gain in strength after 28 days confirms that long-term hydration results in durability and improved mechanical properties. Such a characteristic is important for structural application, wherein strength and durability with greater long-term values are crucial.

As a result, control concrete of M20 grade has its compressive strength increase immensely with age, with most of the strength being attained in 28 days, while further gain in strength continues at a decreasing rate up to 56 days. Proper curing and hydration are necessary in the formation of desired mechanical properties and maintaining long-term concrete structure performance. The compressive strength development of RHA–PPC concrete is presented in Figure 1. The results indicate that strength increased with curing age for all mixes, with optimal performance at 7.5% replacement. At early ages (3 and 7 days), RHA mixes exhibited slightly lower strength than the control due to delayed pozzolanic activity. However, by 28 and 56 days, the pozzolanic reaction contributed to improved strength, with the 7.5% mix achieving 37.62 MPa at 56 days, representing a 3.5% increase compared to the control. The numerical values of control concrete across curing ages are summarized in Table 5 for reference.

3.1.2. Rice Husk Ash (RHA) Concrete

• Effect of age on Compressive Strength of Concrete

M20-grade RHA concrete was tested for compressive strength at 3–56 days with cement replacement levels of 5–15%. The study shows that concrete aging leads to higher compressive strength. However, strength development depends on the RHA replacement ratio as shown in Table 5 and

Table 6. Age-to-28-day compressive strength ratios for control concrete (dimensionless).

Grade of Concrete	3 days	7 days	28 days	56 days
M20	0.47	0.67	1	1.2

and Figure 3.

During the initial timeframe (3–7 days), RHA concrete demonstrates weaker compressive power than concrete with no RHA replacement. The compressive strength reduction reaches its highest level when RHA replaces 15% of cement due to a decrease of 63.40% in 3 days. RHA acts as a pozzolanic material; thus, its strength development needs more time to occur because cement hydrates more rapidly. The strength improvement for 15% RHA replacement becomes especially pronounced between days 3 and 7 as exhibited in Table 5, resulting in an 82.65% strength increase.

The analysis in

Table 7. Peak compressive strength values of RHA concretes at 3, 7, 28, and 56 days.

Age in Days	0%	5% RHA	7.5% RHA	10% RHA	12.5% RHA	15% RHA
3	14.51	12.96	13.32	12.7	10.7	8.88
7	20.58	19.3	19.7	18.96	18.58	16.22
28	30.3	31.5	31	30	30.14	21
56	36.36	35.84	37.62	36.15	32.88	25.88

displays compressive strength changes for various RHA substitution levels throughout 3, 7, 28, and 56 days of the observation period. The findings indicate that 2-day old RHA at 5% shows maximum strength improvements, but higher replacement amounts (≥10%) weaken the concrete structure in the initial stages.

The strength enhancement of RHA concrete reaches its maximum point between 7 and 28 days when 5% RHA replacement results in 5% higher strength than control concrete. An ideal pozzolanic response happens at this replacement level which leads to extended-term strength improvement. Beyond 10% RHA replacement, the strength development decreases until reaching 15% RHA where compressive strength demonstrates a considerable decrease compared to control concrete at −44.28%. The replacement of cement with 5% RHA leads to the maximum strength gain (63.21%) in this time period according to

Table 8. Comparison of compressive strength gains in RHA concretes relative to the control mix.

Percentage Replacement	3 Days (%)	7 Days (%)	28 Days (%)	56 Days (%)
0–5%	−11.95	−6.63	5	−1.45
0–7.5%	−8.93	−4.46	2.31	3.46
0–10%	−14.25	−8.54	−1	−0.58
0–12.5%	−35.6	−10.76	−0.53	−2.27
0–15%	−63.4	−26.88	−44.28	−40.49

, which makes it the top choice as a cement substitution material. The percentage changes in compressive strength across different curing intervals and replacement levels are summarized in

Table 9. Percentage change in compressive strength of RHA concretes across curing ages.

CRL	% Increase Between 3 Days–7 Days	%Increase Between 7 Days–28 Days	% Increase Between 28 Days–56 Days
0%	41.83	47.23	20
5%	48.91	63.21	13.77
7.5%	47.89	57.36	21.35
10%	49.29	58.22	20.5
12.5%	42.41	62.22	17.94
15%	82.65	54.13	23.23

, providing comparative insight into the strength development trends.

The period from 28 to 56 days diminishes strength gain for all replacement percentages due to the combined effects of hydration reactions and pozzolanic activity. The strength development pattern for control concrete remains constant at 20% throughout time, yet 7.5% RHA concrete displays a small 3.46% improvement over control concrete at day 56, indicating its durability potential. Extreme RHA addition of 12.5% and above leads to major concrete strength reduction so that replacement with 15% RHA results in a strength level decrease of 40.49% when measured at 56 days compared to the control concrete. Figure 4 shows the effect of age on compressive strength of concrete with respect to different % replacement of rice husk ash.

The best range for RHA content replacement that provides strength improvement ranges from 5% to 7.5%. Having RHA as cement replacement in the specified percentages leads to increased strength development at extended ages, which makes it an environmentally sound alternative for cement partial substitution. The concrete matrix becomes weaker as replacement levels exceed 10% RHA because substantial RHA contents damage the concrete material structure. Table 7 demonstrates that 5% replacement of RHA achieves optimal sustainability together with mechanical performance, which qualifies it for use in eco-friendly concrete applications.

Effect of percentage replacement of cement with rice husk ash (RHA) on compressive strength of concrete: The substitution of cement with RHA throughout M20-grade concrete samples leads to measurements of compressive strength shown in Figure 3 and Figure 4 at various curing durations. Figure 5 shows the effect of rice husk ash percentage on compressive strength of concrete. The evaluation of compressive strength through different RHA replacement levels takes place in Figure 6 under 3, 7, 28, and 56-day curing conditions. It represents the effect of % replacement of rice husk ash on compressive strength with respect to water binder ratio for M20-grade concrete.

When concrete ages to three or seven days, its compressive strength decreases when RHA content increases above 0%. The slow pozzolanic reaction of RHA causes the downward trend displayed in the curves because RHA takes longer to influence strength development in comparison to ordinary cement hydration.

The test results from Figure 4 show that concrete with 5% and 7.5% RHA replacement reaches higher compressive strength than concrete without RHA (0% RHA). The pozzolanic reaction of RHA begins substantially increasing concrete strength at this development stage. An excessive amount of RHA in concrete leads to reduced strength since it decreases the effective cementitious material that supports the concrete matrix.

An analysis of various replacement percentages and their relation to curing age is given in Figure 5 through bar charts for clear visual comprehension. The maximum compressive strength at 56 days occurs when the RHA replacement ranges from 5% to 7.5%. However, strength drops substantially when using 15% RHA which prevents building applications that need high durability. The research data indicates RHA demonstrates value as a cement mixture component between 5% and 7.5%, allowing improved long-lasting compressive strength. The substitutive content of RHA above 10% produces negative effects on concrete properties that subsequently diminish strength measures.

3.2. Flexural Strength

Flexural strength results are shown in Figure 6. Similarly to compressive strength, moderate RHA replacement (5–7.5%) improved long-term performance, while higher dosages (>10%) reduced strength. Flexural strength peaked at 7.5% RHA with 5.47 MPa at 56 days, 9.6% higher than the control [6,7]. At higher replacement levels (≥10%), flexural strength decreased [33,35] in

Table 10. Flexural strength of control concrete in N/mm².

Curing Period	3 days	7 days	28 days	56 days
M20	1.01	1.17	4.21	4.95

to supplement the graphical results. Table 10 and Figure 6 summarize the values of flexural strength for concrete specimens with various RHA replacement percentages at 7, 28, and 56 days’ curing times. The general trend was in line with compressive strength: increasing the content of RHA up to 7.5% increased flexural strength, and beyond that, it decreased.

At 7 days, flexural strength of RHA blends was slightly lower than the control, particularly at higher replacement levels (10–15%). This is because RHA’s early-age pozzolanic reaction is limited, and it is less effective in strengthening the cementitious matrix at an early stage of hydration Zain et al. [27]. However, at 28 and 56 days, 5–7.5% RHA blends were superior to the control mix.

The highest flexural strength was achieved at 7.5% RHA with a value at 56 days of 5.47 MPa, an increase of 9.6% from the control value of 4.99 MPa. This improvement is due to the formation of more C–S–H gel by pozzolanic reactions, which improves the internal bond and resistance to tensile stress [6,7]. In addition, the filler effect of highly dispersed RHA particles contributes to enhanced matrix compaction and crack resistance.

Above 10% RHA, the flexural strength started decreasing. The 28-day strength at 15% replacement decreased by about 11% from the optimal 7.5% mix. It is a result of excessive cement dilution under cementing for a strong matrix [32].

Increased RHA content also tends to disturb the continuity of the matrix and enhance porosity, hence making the material too weak to withstand bending stresses.

These findings are in line with similar studies like [18,34], which indicated the same flexural strength improvement at 5–10% RHA and decrease afterwards. The trend supports the need to maximize the amount of RHA in the ratio to counteract the undesirable consequences of extreme cement replacement in the background of idealized pozzolanic reaction and particle refinement.

This rise in long-term flexural strength also indicates RHA’s capability to contribute towards durability-related mechanical response, particularly in PPC concrete systems where it is synergistic with the pozzolanic activity of fly ash.

Concrete depends on flexural strength to resist different types of bending and tensile forces. The connection between compressive and tensile strength exists but remains nonproportional. The relation between concrete flexural strength and compressive strength changes according to the concrete’s total strength level. The strengthening connection between compressive strength and tensile strength develops gradually after the point when compressive strength reaches higher levels. Laboratory observation shows that, when incorporating rice husk ash (RHA) into pozzolanic materials, concrete can experience changes in its tensile behavior together with flexural properties. Figure 7 illustrates flexural strength vs. age in days of control concrete.

The flexural strength results for M20-grade control concrete according to curing duration: The flexural strength demonstrates a major increase within 28 days before showing a slower growth from 28 to 56 days. The strength development during cement hydration shows similar behavior since initial gains are rapid but slow down as the reactions progress through time.

The flexural strength of control concrete increased from 1.01 N/mm² at 3 days to 4.95 N/mm² at 56 days according to Table 10. The data in Figure 6 shows that the flexural strength grows steadily over time. The test results show that concrete strengthens through additional curing time as cement hydration reactions persist.

A detailed study of M20-grade RHA concrete flexural strength exists in

Table 11. Flexural strength of rice husk ash concrete in N/mm².

Curing Period	3 Days	7 Days	28 Days	56 Days
5%	1.22	1.36	3.62	4.21
7.5%	1.44	1.62	3.84	4.62
10%	1.34	1.41	2.75	3.29
12.5%	1.22	1.44	2.24	2.76
15%	1.04	1.25	2.08	2.35

across different curing periods along with various replacement levels (See Figure 7). The test results show maximum strength development through pozzolanic reactions takes place in the time period between 7 and 28 days. Past day 28, the strength growth becomes slower yet stays comparable to standard concrete behavior. After 28 days of curing, there exists a minimal variation in flexural strength between various levels of RHA replacement. The strength enhancing effect of RHA manages to reach stability after this curing period ends. The flexural strength performance of RHA concrete improves at longer curing periods (56 days) when using replacement quantities between 5 and 7.5%. The long-term pozzolanic activity contributes to refined concrete microstructures and improved strength because of its ability to strengthen concrete. According to Table 11, the flexural strength of concrete containing 7.5% RHA replacement reaches 4.62 N/mm² at 56 days of age and remains near the control concrete strength value of 4.95 N/mm². The strength properties of concrete diminish as the replacement percentage of RHA exceeds 10%. The tensile strength declines when excessive RHA content hinders cementitious matrix bonding capacity and produces inferior bond properties. Figure 8 presents the effect of rice husk ash percentage on the flexural strength of concrete. It shows that the flexural strength decreases by increasing the rice husk ash percentage.

Flexural strength measurements recorded from control concrete and RHA concrete can be found in

Table 12. Flexural strength of control and rice husk ash concrete in N/mm².

Curing Period	3 Days	7 Days	28 Days	56 Days
0%	1.01	1.17	4.21	4.95
5%	1.22	1.36	3.62	4.21
7.5%	1.44	1.62	3.84	4.62
10%	1.34	1.41	2.75	3.29
12.5%	1.22	1.44	2.24	2.76

based on curing time variations. Flexural strength measurements showed that concrete specimens made with RHA replacement ratios between 5% and 7.5% demonstrated higher strength values during the period of 28 to 56 days. The use of high replacement ratios exceeding 10% leads to reduced flexural strength because the mix lacks enough cementitious material.

Table 13. Compressive and flexural strength of control concrete and rice husk ash concrete for 28-day period.

Strength Type	Compressive Strength in N/mm2									Flexural Strength in N/mm2
Percentage Replacement	0%	5%	7.5%	10%	12.5%	15%	0%	5%	7.5%	10%	12.5%	15%
Control Concrete	30.3	-	-	-	-	-	4.21	-	-	-	-	-
Rice Husk ash Concrete		31.5	31	30	30.14	25	-	3.62	3.84	2.75	2.24	2.08

provides an extensive comparison between concrete specimens regarding their flexural and compressive strengths at 28 days. An increase in compressive strength occurs at all RHA replacement levels of up to 12.5% yet flexural strength shows deterioration after 7.5% replacement. This suggests that while RHA can enhance compressive strength to some extent, its effect on flexural strength is more sensitive to higher replacement levels.

The flexural strength of concrete depends on the levels of cement replacement with RHA documented in the results. A moderate level between 5% and 7.5% results in the best compressive and flexural strength ratio because it enables pozzolanic activity along with enhanced microstructure. At RHA replacement levels exceeding 10%, there is a negative effect on flexural strength which decreases structural application suitability because structural components require good tensile or flexural performance.

4. Discussion

4.1. Strength Development

The results confirm that RHA enhances both compressive and flexural strength when used at moderate replacement levels (5–7.5%). Early-age strength reduction was observed in all RHA concretes due to the slower reactivity of amorphous silica [6,7]. At later ages, secondary hydration reactions generated additional C–S–H, improving matrix density and strength [6,12,18,27].

4.2. Comparison with Literature

Findings are consistent with those of Chindaprasirt et al. [32], Singh et al. [33], and Nazari & Toufigh [36].

Lee et al. [13] reported enhanced carbonation resistance with RHA, while Ma et al. [14] linked variability in performance to silica content and loss on ignition. These observations explain the mixed results in the literature and support the need for standardized processing of RHA [19,20,21].

Compared with ordinary Portland cement systems, PPC–RHA concretes performed better. The dual-pozzolan effect of RHA and fly ash in PPC improved long-term strength, confirming results from an earlier work by Kumar and Singh [12]. This synergy is one of the main novelties of this study.

4.3. Sustainability Implications

Recycling RHA supports CO₂ reduction and circular economy [18,24]. Replacing 7.5% of PPC with RHA reduces clinker demand, which can lower CO₂ emissions by approximately 60–70 kg per cubic meter of concrete, while simultaneously recycling agricultural waste. Given that cement production contributes ~0.9 tons of CO₂ per ton of clinker [2], the use of RHA can contribute substantially to carbon reduction, while recycling agricultural waste. This makes RHA–PPC concrete a promising material for sustainable construction in regions where rice husk is abundant.

4.4. Limitations

This study focused only on compressive and flexural strength under standard curing. Durability aspects such as chloride penetration, sulfate resistance, carbonation, and corrosion behavior were not evaluated. Microstructural analysis (e.g., SEM, XRD) was also not included. Future work should address these aspects to strengthen the application of RHA in PPC concretes. Microstructural validation and long-term service-life modeling remain for future study [34,37,38].

5. Conclusions

This study demonstrates that incorporating rice husk ash (RHA) in Portland Pozzolana Cement (PPC) concrete can enhance structural performance while contributing to sustainability. At early ages (3–7 days), RHA concretes exhibited lower strength due to delayed pozzolanic activity, but at later ages (28–56 days), mixes containing 5–7.5% RHA achieved superior compressive and flexural strength compared to the control, with optimum performance observed at 7.5% replacement. When RHA exceeded 10%, strength declined, indicating that excessive cement dilution compromises matrix integrity. These findings highlight the dual-pozzolan synergy between fly ash in PPC and RHA and support a recommended replacement range of 5–7.5% as a balance between structural efficiency and carbon footprint reduction.

Future investigations should extend beyond mechanical performance to address durability concerns such as sulfate attack, chloride penetration, carbonation, and reinforcement corrosion. Microstructural validation through SEM and XRD will be critical to confirm the mechanisms underlying the observed strength gains. Furthermore, field-scale trials under aggressive environmental conditions and optimized mix designs for higher-strength applications are needed to establish practical guidelines. Collectively, such efforts will strengthen the case for RHA as a viable supplementary cementitious material in modern sustainable construction.