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TECHNICAL PAPER

5.8 X-ray diffraction (XRD) reducing the reliance on natural aggregates and promoting the
use of more eco-friendly materials. While the slight reduction
The X-ray diffraction (XRD) analysis revealed the crystalline in compressive strength at a 40 % replacement level might be
structures of mix A (M30) and mix B (M50) concretes, as well as permissible for certain applications, it is important to carefully
their variants with 40 % pumice replacement (M30-PA40 and consider the structural requirements and desired performance
M50-PA40). Mix A showed prominent peaks between 25° and outcomes when determining the optimal mix proportions.
35° 2θ, indicating well-crystallized phases like C-S-H and calcium
hydroxide. In M30-PA40, pumice addition increased background The results of the split tensile strength tests revealed a
noise and reduced peak sharpness, suggesting more significant reduction in strength for both M30 and M50 concrete
amorphous phases and dilution of cementitious phases. Mix B mixes when 40 % of the coarse aggregate was replaced with
featured a strong quartz peak around 26° 2θ, typical of higher- pumice. The M30 concrete initially exhibited a split tensile
strength concrete, with a simple mineralogical composition strength of 3.54 MPa, which decreased to 1.42 MPa for M30-
mainly involving quartz and standard hydration products. PA40 with 40 % pumice replacement, indicating a 59.9 %
M50-PA40 retained clear peaks for crystalline phases like C-S-H, reduction. Similarly, the M50 concrete’s split tensile strength
portlandite, and quartz, despite pumice addition. dropped from 3.82 MPa to 1.84 MPa for M50-PA40 with the
same level of replacement, representing a 51.8 % reduction. The
substantial decrease in split tensile strength observed in both
6. RESULTS AND DISCUSSIONS
M30 and M50 mixes with pumice replacement highlights the
material’s impact on the tensile properties of concrete. The M30
In compressive strength test, the results showed that both mix showed a slightly higher percentage reduction compared
M30 and M50 concrete mixes experienced a reduction in to the M50 mix, suggesting that the tensile strength of lower-
compressive strength with increasing levels of pumice coarse grade concrete may be more sensitive to the replacement of
aggregate replacement. For the M30 mix, the compressive traditional coarse aggregates with pumice. Despite the potential
strength decreased from 33.63 MPa in the control sample benefits of using pumice, such as reduced weight and improved
M30 to 25.04 MPa for M30-PA40 at 40 % pumice replacement, thermal properties, the significant reduction in tensile strength
representing a 25.5 % reduction. Similarly, the M50 mix saw a raises concerns about its suitability in structural applications
decrease from 53.19 MPa to 45.04 MPa for M50-PA40 at the where tensile performance is critical. However, the reduction
same 40 % replacement level, a 15.3 % reduction. While the in tensile strength suggested that pumice-enhanced concrete
20 % replacement level resulted in a less significant strength may require supplementation with additional materials, such as
reduction, the 40 % replacement still maintained a relatively reinforcing fibers, to ensure that tensile strength meets structural
high compressive strength, particularly in the M50 mix, requirements. These considerations are crucial for optimizing
suggesting that it could be considered an optimal dosage the balance between sustainability and performance in concrete
for certain applications where a balance between strength applications.
and the benefits of pumice aggregate is desired. However,
beyond 40 % replacement, the strength reduction became The modulus of rupture results showed a notable reduction
more pronounced, indicating that higher levels of pumice for both M30 and M50 concrete mixes with pumice replacing
replacement could compromise the structural integrity of the 40 % of the coarse aggregate. The modulus of rupture for the
concrete. Thus, while 20 % replacement is ideal for minimizing M30 mix decreased from 3.03 MPa in the control sample to
strength loss, 40 % replacement could still be considered 2.58 MPa for M30-PA40, marking a reduction of about 14.85 %.
optimal if a slight reduction in strength is permissible. On a Similarly, the M50 mix’s modulus of rupture fell from 3.29 MPa to
commercial scale, using pumice as a partial replacement for 2.70 MPa for M50-PA40, a reduction of approximately 17.93 %.
coarse aggregates can offer several advantages. For instance, This decline was attributed to the porous and weaker nature
pumice is a lightweight material that can reduce the overall of pumice compared to traditional aggregates, which lowered
weight of concrete structures, leading to cost savings in the tensile strength of the concrete. The greater reduction
transportation and construction. Additionally, the lower density in the M50 mix indicated that higher-strength concretes are
of pumice can improve the thermal insulation properties of the more sensitive to pumice’s weakening effects. Despite this, the
concrete, making it more energy-efficient. An example of its modulus of rupture with 40 % pumice replacement remained
application could be in the construction of high-rise buildings, relatively high, suggesting its viability for applications where
where the reduced weight of the concrete would lessen the load slight tensile strength reduction is acceptable. Incorporating
on the foundation and support structures, potentially allowing fibers such as steel, polypropylene, or glass could enhance
for slimmer columns and beams, which can increase usable tensile properties and crack resistance, compensating for the
space and reduce material costs. Furthermore, using pumice strength loss due to pumice replacement. In terms of structural
can contribute to the sustainability of construction practices by behaviour, concrete with pumice aggregate can be beneficial in

64 THE INDIAN CONCRETE JOURNAL | FEBRUARY 2026