PSI - Issue 71

Yash Rathore et al. / Procedia Structural Integrity 71 (2025) 401–408

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The improvement in compressive strength of concrete with RHA, and MK, is primarily due to their pozzolanic properties and their ability to refine the concrete microstructure. RHA reacts with calcium hydroxide formed during cement hydration, producing additional CSH gel that fills capillary voids, reduces porosity, and enhances compressive strength (Chopra et al., 2015). However, beyond 20% replacement of OPC with RHA, excess silica can remain unreacted due to insufficient calcium hydroxide, leading to reduced strength. Similarly, MK promotes the formation of calcium silicate hydrate (CSH), calcium aluminate hydrates (CAH), and calcium alumino-silicate hydrates (CASH), densifying the interfacial transition zone (ITZ) and further boosting mechanical strength. However, excessive MK can deplete calcium hydroxide, slow down cement hydration, and hinder CSH formation (Homayoonmehr et al., 2021). Additionally, the inclusion of M-sand with both rough and fine particles enhances the ITZ and packing density of concrete, further contributing to improved mechanical strength (Dobiszewska et al., 2023; Gupta et al., 2019). 4.2. Flexural Strength Fig.3(b) shows the relationship between the flexural strength of concrete at 28 days and various concrete mixes. An increase in MK content initially leads to a rise in flexural strength, reaching its peak at 15% MK content with a 29.93% improvement during the 28-day curing period in comparison to CC mix. However, beyond 15% MK content, the flexural strength of the M40 grade concrete starts to decrease. The variation in the flexural strength of concrete can be explained by the same factors discussed in the compressive strength section. 4.3. Durability Sulphuric acid has a significant impact on the performance of concrete. Fig. 4(a) and Fig. 4(b) demonstrate percentage changes in compressive strength and weight of the mixes due to acid exposure with respect to CC mix. Among the different mixes, the one with 10% MK replacing OPC in the CC mix, combined with 20% RHA and 100% Deccan basalt M-sand as fine aggregate (R20MK10), shows the least susceptibility to acid attack, both in terms of weight loss and strength reduction. This mix effectively reduces these effects, showing only an 11.13% decrease in compressive strength and a 6.84% weight loss, thereby offering protection against acid attack. This superior performance of R20MK10 can be attributed to an optimal balance between pozzolanic reactivity and alumina content. While MK enhances durability by refining the microstructure, higher MK content (as in R20MK15) introduces excess alumina, which, under sulfuric acid exposure, may react with sulfate ions to form Fig.3. Strength results for different concrete mixes (a) Compressive Strength; (b) Flexural Strength.

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