Construction Potential of Rice Husk Ash as Eco-Friendly Cementitious Material in a Low-Water Demand for Self-Compacting Concrete

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Self-compacting concrete (SCC) is a special mix design that offers advantages, such as high flowability and better compaction. Combining it with natural pozzolans through partial replacement influences considerably the filling and passing abilities, and segregation resistance, while generating high-strength performance. A low water-to-binder ratio (w/b) leads to a dense, non-porous, and durable SCC, but its desired workability is difficult to obtain. Hence, the addition of chemical admixtures, such as superplasticizers (SP) and air-entraining agents (AEA) may resolve this issue. Moreover, the incorporation of rice husk ash (RHA) as supplementary cementitious material (SCM) in an alkali environment promotes enhanced stability and workability, as well as avoiding any bleeding or segregation problems. RHA also improves the concrete's microstructure owing to its highly reactive fine silica. However, there is limited evidence reported on the effects of RHA on SCC's overall flowability, pozzolanic reactions, and microstructural analyses. Here the synergistic effects of RHA on the rheological performance, strength development, pozzolanic activities, and microstructural characterization of fresh and hardened SCC were reported. The study showed that while the filling ability, passing ability, and segregation resistance of fresh mixture all conform to the threshold values, the recorded compressive strength of the hardened samples was highest in SC-09 at 90-day cured samples. The mix proportion of this sample includes a low w/b ratio of 0.35 and 15% RHA replacement, with optimized rheological performance and pozzolanic activities. It was observed in the sample's microstructure that the silica particles chemically reacted with CH, promoting CSH gel products. The disappearance of dicalcium silicate (C2S) and tricalcium silicate (C3S) in diffractograms was observed and replaced by CSH gel, which plays a key role in strength formation. IR spectral bands at ∼765 and ∼464 cm−1 indicated the amorphous silica phase of the RHA-SCC samples, while the spectral band relation of RHA and the samples showed a high degree of reaction of RHA upon mixing. The surface morphology images proved a highly dense matrix in 15% RHA replacement. Observed pores and cracks were lessen compared to samples with 0–10% RHA, which verified the construction potential of RHA in an optimized amount. While the sulfate and seawater performance tests indicated a low mass loss and high relative strength occurred to samples with 15% RHA replacement when immersed to both solutions, signifying that the addition of RHA in concrete leads to better chemical attack resistance compared to OPC alone due to the highly reactive silica of RHA.