Comparative evaluation of cement stabilized well graded sand using rice husk ash, fly ash, and recycled plastic fibers for subgrade applications

Abstract This study evaluates the strength and compaction behavior of well-graded sand stabilized with cement and waste-derived materials for subgrade applications. Rice husk ash (RHA), fly ash (FA), and recycled plastic fibers (RPF) were combined with cement to investigate their effectiveness as sustainable stabilizers. Few studies have done a direct comparative analysis of these materials under the same conditions, especially in the case of subgrade applications. The natural soil was defined as well-graded sand (SW) with its uniformity coefficient 6.76 and coefficient of curvature 1.43. Standard Proctor compaction and UCS tests were conducted on cement only (10% and 20%) and blended mixes (5%+5% and 10%+10%) at curing periods of 7, 14, and 28 days. The findings indicated that cement-only mixes had best maximum dry density (1830 kg/m 3 with 24% optimum moisture content (OMC) whereas fly ash mixes showed better strength performance. In comparison to cement-treated specimens, RHA showed moderate improvement at 5% content (406 kPa), but higher replacement levels (10%) resulted in severe strength reduction (55 kPa) due to increased water demand and poor particle packing indicating that RHA is less effective than fly ash for sandy soil stabilization. Similarly, RPF mixes exhibited the lowest density (as low as 1410 kg/m³) and inadequate compressive strength, making them unsuitable for subgrade applications compared to cement and fly ash treatments. The 5% cement + 5% fly ash mix achieved a 28-day UCS of 841 kPa (211% improvement over 10% cement) with only a 5.7% reduction in maximum dry density (MDD) (1650 kg/m³ vs. 1750 kg/m³), demonstrating an excellent balance between strength and compaction efficiency. Although cement only mixes achieved the highest maximum dry density (1830 kg/m³), the 5% cement + 5% fly ash mix is identified as the optimum blend based on the best balance of strength, compaction performance, and sustainability by reducing cement consumption by 50%. This mix offers a viable and sustainable solution for road subgrade stabilization in regions with limited resources, where waste materials are abundant but conventional stabilizers are costly and environmentally damaging.