**1. Introduction**

Generally, a bituminous mix is composed of aggregate, mastic (bitumen mixed with filler), and air voids [1]. Fillers are fine material that mostly passes through a 75-µm sieve and occupies 2–10% of the total mixture's aggregate weight [2]. The critical roles of filler are to steady the mix by filling the holes in the aggregate skeleton and to enhance the viscoelastic properties of the mastic that glues the aggregate together [3]. It is commonly acknowledged that the performance of bituminous mixes is greatly affected by the overall characteristics of the filler [3].

Conventionally, stone dust produced during the process of stone crushing was used as filler in a bituminous mix. Presently, cement and hydrated lime (HL) are used as fillers in place of stone dust, since they deliver better binding and antistripping properties in the mix [4]. In comparison with cement, HL acts as an active filler and enhances

**Citation:** Mistry, R.; Roy, T.K.; Aldagari, S.; Fini, E.H. Replacing Lime with Rice Husk Ash to Reduce Carbon Footprint of Bituminous Mixtures. *C* **2023**, *9*, 37. https:// doi.org/10.3390/c9020037

Academic Editors: Indra Neel Pulidindi, Pankaj Sharma and Aharon Gedanken

Received: 26 December 2022 Revised: 8 March 2023 Accepted: 10 March 2023 Published: 27 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

adhesion by altering the aggregate's surface chemistry and reacting with acid components in bitumen [5]. These conventionally used fillers are acquired by mining, and their constant use in bituminous mixes has led to their shortage in different parts of the world. Regular extraction for control fillers also causes the loss of riparian vegetation, damage in waterretentive strata, lowering of the groundwater level, and disturbance of the remaining ecosystem [6]. There are additional negative impacts on the environment: every ton of cement production emits 0.73–0.99 tons of CO2, and every ton of lime production emits 0.2–0.45 tons of CO<sup>2</sup> [7].

The use of available waste as an eco-friendly replacement for conventional fillers will improve sustainability in building roads and create a way to lessen the carbon footprint throughout the world. However, before applying such wastes, it is essential to check the technical and economic viability of the resulting bituminous mix.

Rice is a primary food source for more than 100 countries and is consumed as an essential food by nearly half of the world's population. In 2014, the global rice harvesting area was 162.72 million ha, and 741.48 million tons of rice were produced. Asia alone produced more than 90% of the rice; India contributed 21.20% of the total output [8]. In the production of milled rice, huge quantities of rice husk are generated as by-products. For instance, India produced 104.8 million tons of rice in 2015 [9]. Theoretically, in 2015, India generated 20.96 million tons of rice husk, which caused a problem for the rice-grinding industry. Due to the inherent properties of rice husk (hard surface, poor nutritional value, high silica content, fair resistance to bacterial decomposition), it is left unused or burned in a boiler to generate steam or electricity. Additionally, the burning of rice husk produces another source of pollutants called rice husk ash (RHA). The volume of this waste generated every year (5.58 million tons in 2019) is so large that the safe disposal of this waste powder has become a severe problem for the rice-milling industry [10]. Usually, RHA is discarded in nearby landfills, resulting in severe environmental pollution and contamination of water resources [10].

To overcome these problems, investigators have made many attempts to use RHA as added building materials to reduce the contamination effect and total construction cost. For instance, RHA has been used as a pozzolanic material in the construction industry [11]. From previous studies, it is known that RHA can be grinded finer and become more active because of the metastable state of the surface structure [12]. As a result, the activated fine nature of RHA makes it a potential filler/modifier in other materials, such as a bituminous binder or mix [12]. However, very few studies deal with RHA's use in a bituminous mix. The application of RHA was examined as a partial replacement for the standard filler, i.e., limestone powder [4]. One study investigated the applicability of RHA combined with brick powder and waste glass dust as a filler in a bituminous mix; in that study, RHA did not show any noteworthy performance compared to the other fillers [13]. The result of another study showed that RHA could boost the visco-elastic properties of a bituminous binder. Furthermore, RHA upgraded the elastic response of the modified binder [12]. Yet, nearly all these research works are limited to only a few properties of the bituminous mixes, or the studies applied RHA in an arbitrary percentage. Various studies also established that RHA could be used as filler in a bituminous mix, but tests of RHA's effectiveness against stripping acquired conflicting results [13]. Additionally, up to now, no study has investigated how modification with RHA affects the physical and morphological properties of the resultant bituminous mastic. Thus, further investigation is needed on the usefulness of RHA as a filler in a bituminous mix.

### *Objectives*

This research has four objectives:


#### **2. Materials and Methods**

#### *2.1. Materials*

Aggregates of the basalt type were gathered from a Pakur quarry (Birbhum district, West Bengal, India) and used in this study. Table 1 provides a description of the aggregates' attributes. To suit the continuous aggregate gradation of dense bituminous macadam (DBM) grade-II established by Indian standard specification [2], the collected coarse and fine aggregates were sieved. The results are shown in Figure 1.

**Table 1.** Evaluated properties of studied aggregates.


**Figure 1.** Adopted gradation of DBM grading II mix.

A VG30 grade of bitumen was used as a binder. The bitumen came from Haldia Petrochemicals (Purba Medinipur district, West Bengal, India) and is typically used in the hot-mix asphalt industry in India. The binder had a penetration value of 57, a kinematic viscosity (135 ◦C cSt) of 368, an SG of 1.045, and a softening point of 49 ◦C.

HL collected from a local market of Shibpur (Howrah district, West Bengal, India) was used as the conventional filler. RHA was collected from a rice mill in the Purba Burdwan district of West Bengal, India. Oven-dried fillers were used in this investigation after sieving through a 0.075 mm sieve.

#### *2.2. Test for Filler Characterization*

The physical and chemical features of each filler were estimated according to associated specifications. Specific gravities of fillers were determined using a pycnometer as per ASTM D854 [14]. The particle size distribution test was conducted as per ASTM D422 [15], and the curves were plotted according to the guidelines. The fineness modulus (FM) and uniformity coefficient (UC) were used to examine the curves. The specific surface area (SSA) was evaluated by Blaine's air permeability test using IS 4031 Part 2 [16]. Scanning electron microscopy (SEM) imaging methods were used to observe that the fillers' particle shape and surface texture conformed to ASTM E 986–04 [17].

The quantity of harmful clays in the fillers was estimated by the methylene blue values (MBV) according to ASTM C837-09 [18]. X-ray diffraction (XRD) analysis was used to identify the prevalent minerals present in the fillers; the XRD was conducted using a Rigaku D/Max-IIIC diffractometer operated with CuKα radiation over a range of 5–40◦ (2θ = 10◦ to 80◦ ) at a rate of 0.2◦ min−<sup>1</sup> . X-ray fluorescence spectroscopy was used to determine the chemical composition of each filler as per ASTM E1621-13 [19]. The organic content of each filler was quantified using a Loss-On-Ignition (LOI) test.
