Effect of wetting–drying cycles on compressive strength and microstructure of recycled asphalt pavement – Fly ash geopolymer

https://doi.org/10.1016/j.conbuildmat.2017.03.243Get rights and content

Highlights

  • Effect of w–d cycles (C) on the strength and microstructural changes of RAP-FA blend and RAP-FA geopolymer.

  • Micro-structure analyzed using XRD and SEM.

  • For C < 6, the w–d cycles stimulate the chemical reaction and hence strength improvement.

  • For C > 6, the significant macro- and micro-cracks developed during w–d cycles cause strength reduction.

  • UCS after w–d cycles of RAP-FA geopolymers and RAP-FA blends were compared with road authorities’ requirements.

Abstract

The usage of recycled asphalt pavement (RAP) and fly ash (FA) in pavement applications contributes to the sustainable usage of such waste by-products. Although RAP-FA geopolymer and RAP-FA blend without liquid alkaline activator have been proven as a pavement material based on strength and leachate requirement, the durability of these by-products when exposed to an aggressive environment has not been investigated to date. This research investigates the effect of wetting–drying (w–d) cycles on the strength and microstructural changes of RAP-FA blend and RAP-FA geopolymer. The strength characteristics of these materials were determined by unconfined compression strength (UCS) test. The micro-structure of the compound pavement material was also analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Test results show that the UCS of RAP-FA blend increases with increasing the number of wetting–drying (w–d) cycles (C), reaching its peak at 6 w–d cycles. The XRD and SEM analyses indicate that the increased UCS of RAP-FA blend is due to stimulation of the chemical reaction between the high amount of Calcium in RAP and the high amount of Silica and Alumina in FA during w–d cycles leading to production of more Calcium (Aluminate) Silicate Hydrate [C–(A)-S-H]. For C > 6, the significant macro- and micro-cracks developed during w–d cycles cause strength reduction. For RAP-FA geopolymer, geopolymerization products [Sodium Alumino-Silicate Hydrate, N-A-S-H] co-existed with C-(A)-S-H results in increased UCS within the first 6 w–d cycles. The macro- and micro-cracks when C > 6 cause strength reduction of RAP-FA geopolymers. A better durability performance is observed when RAP-FA geopolymers are prepared with higher NaOH content that can be attributed to formation of a stable cross-linked alumino-silicate polymer structure. The outcome from this research confirms the viability of using RAP-FA blends and RAP-FA geopolymer as alternative sustainable pavement materials.

Introduction

Sustainable infrastructure is a key strategic initiative in many developed and developing countries. Research on the usage of alternative sustainable materials is at the forefront of many governments, researchers, and pavement industries worldwide [1]. The usage of waste by-products in civil infrastructure enables a more durable alternative to quarried materials resulting in conservation of natural resources, decreased energy use, and reduced greenhouse gas emission. In recent years, extensive research works on innovative and environmentally friendly solutions have resulted in the applications of green technologies in pavement construction, which have led to more efficient use of natural resources and recycled materials [2].

Several geotechnical researchers have evaluated recycled waste materials as an alternative construction material in civil infrastructure applications. Surplus clay and fly ash (FA) were used for developing a sustainable lightweight cellular cemented construction material [3]. Waste carpet fibers were used to increase the strength and reduce the swelling pressure of expansive soils [4], [5]. Calcium carbide residue (CCR), a waste by-product of the acetylene gas production process, has been established as a green soil stabilizer [6], [7], [8], to develop non-bearing masonry units [9], and stabilized subgrade materials [10], additives of grouting materials used in tunneling and groundwater sealing [11], [12], [13].

From a geoenvironmental perspective, FA based geopolymer is an environmentally friendly additive for improving the mechanical and durability characteristics of problematic soils [14], [15], [16]. Geopolymer using FA and CCR as alkali activator is a low-carbon method to stabilize clayey soil [17], [18]. Water treatment sludge, FA, and rice husk ash have furthermore been used to manufacture sustainable geopolymer masonry units [19], [20], [21], [22]. The strength and durability of sludge-geopolymer masonry units were found to be significantly higher than those of sludge-cement masonry units.

Meanwhile, roads are a central component of many nation’s infrastructure and present a wide array of opportunities for the usage of vast quantities of recycled materials. Recycled asphalt pavement (RAP), is obtained from spent asphalt extracted from roads that have reached the end of their design life [23], [24]. RAP contains asphalt binder (3–7%) and aggregates (93–97%) by weight [25], and is an ideal recycled material for reuse in pavement applications. RAP often exhibits low strength and stiffness performances, hence chemical stabilization of RAP is used extensively for developing bound pavement base/sub-base material [26], [27]. An evaluation of FA-stabilized RAP as pavement base/sub-base material has been investigated by Saride et al. [28] whom reported that the unconfined compression strength (UCS) and resilient modulus (MR) properties can be improved by FA replacement. However, the 7-day UCS of RAP was reported to be lower than the strength requirement specified for pavement base materials. Further studies on the mechanical and microstructural properties of a stabilized RAP, virgin aggregate (VA) and FA blend as a pavement base/sub-base [26], [29] indicated that RAP:VA = 80:20 with 40% FA replacement satisfied the strength, stiffness, and California Bearing Ratio requirements for low volume roads. Mohammadinia et al. [30] explored a sustainable stabilization solution for RAP by using FA and blast furnace slag geopolymers and reported that the 7-day strength of geopolymer stabilized RAP could meet pavement subbase specification requirements.

Hoy et al. [31], [32] have evaluated the strength development and leachate characteristics of RAP-FA geopolymers and RAP-FA blends as sustainable stabilized pavement base/sub-base materials, in which up to 80% RAP was used as aggregates. Liquid alkaline activator (L), a mixture of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH), was used to activate the alumino-silicate FA to produce FA-geopolymer binder, while RAP, FA, and water (RAP-FA blend) were mixed as a control material. The authors reported that both the RAP-FA blends and RAP-FA geopolymers could be used in pavement base applications as the strength requirements met the specifications of the Department of Highways, Thailand. Furthermore, the waste by-products were found to pose no significant environmental and leaching hazards into soil and ground water resources, and geopolymer stabilization was also found to reduce the leaching of heavy metals from RAP-FA mixture significantly.

Besides strength and environmental requirements as investigated previously, the durability of RAP-FA blends and RAP-FA geopolymers under severe climatic conditions is a crucial parameter when used in road construction applications. The study on durability of RAP-FA blends and RAP-FA geopolymers is however still in its infancy. Dempsey and Thompson [33] defined durability as the ability of the materials to retain their stability and integrity and to maintain adequate long-term residual strength to provide sufficient resistance to climate conditions.

Cyclic wetting–drying (w–d) test, simulates weather changes over a geological age, and is considered to be one of the most appropriate simulation that can induce damage to pavement materials [34], [35]. The durability study against w–d cycles of the chemically treated RAP with VA have been reported by Game (2009) [36] and indicated that the strength loss was approximately 10–15% on an average for all the mixes studied after 14 cycles of w–d test. The authors reported that weight loss and strength drop of the mixture composed of RAP:VA = 75:25 with 2% cement were low even after 14 w–d cycles, while higher volumetric change and lower strength were observed when the same RAP mixed with 7% FA after 7 w–d cycles. In addition, Kampala et al. [37] have investigated the influence of w–d cycle on the durability of CCR-FA stabilized clays as a pavement application to ascertain its serviceability. It is concluded that the optimal CCR and FA contents were found at about 7 and 20%, respectively. The excessive FA contents cause the strength reduction. Furthermore, although the input of FA can enhance the pozzolanic reaction, the strength of the CCR stabilized clay reduced significantly with the number of w–d cycles. In recent year, the study on the green material by using FA activated NaOH to treat the RAP and VA mixture demonstrated that the assessment of durability is highly important when secondary material like RAP was used in the pavement applications [38]. The study concluded that the strength of the mixes RAP:VA = 60:40 + 4%NaOH + (20% or 30%FA) was found to be higher than the minimum strength requirements specified by Indian road congress even after 12 cycles of w–d test. Al-Obaydi et al. [39] and Al-Zubaydi [40] indicated that the cyclic w–d cycles cause crack propagation, resulting in severe effects on the engineering properties of the materials, particularly in terms of their residual strength and stability.

This research attempts to study the durability of RAP-FA blends and RAP-FA geopolymers when subjected to cyclic wetting–drying tests. The change in the strength and physical properties of both RAP-FA blends and RAP-FA geopolymers at various cyclic w–d cycles were examined using UCS and weight loss tests, while the mineralogical and microstructural changes were examined by the application of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses to explain their strength development. The outcome of this research will enable the development of construction guidelines for researchers, pavement engineers and end-users in assessing suitable RAP-FA blends and RAP-FA geopolymers in future road construction applications.

Section snippets

Materials

In this research, RAP samples were collected from a mill asphalt pavement stockpile in Nakhon Ratchasima province, Thailand. A cold milling machine was used to remove the existing old asphalt pavement for resurfacing. In Thailand, the used cold in-place recycling machine has a narrow tooth spacing milling drum with a lower speed and the milled pavement thickness is approximately 20–25 mm. The maximum size of RAP aggregate studied was approximately 10 mm as the maximum aggregates in asphalt

Unconfined compression strength (UCS)

Fig. 4 summarizes the UCS results of RAP + 20%FA blend and RAP + 20%FA geopolymer at all NaOH/Na2SiO3 ratios (100:0–50:50) at the curing times of 7 days and 28 days. It clearly indicates that the UCS values of both the RAP + 20%FA blend and RAP + 20%FA geopolymer increase with curing time. This is notably similar to previous studies on strength development of cement-stabilized RAP [54], [55] and FA-stabilized RAP [26], [29]. The standard for strength requirement for FA geopolymer stabilized base material

Conclusions

The present study investigated the durability of the RAP-FA blend and RAP-FA geopolymer as a sustainable pavement material. The outcome of this research is to promote the use of recycled waste material in road construction, with economic and environmental benefits. The following conclusions can be drawn from this study:

When subjected to w–d cycles, the UCS of RAP + 20%FA blend increases with increasing the number of w–d cycles (C) up to 6 cycles and then decreases. The XRD and SEM analyses

Acknowledgements

The first author is grateful to a financial support from Suranaree University of Technology under SUT-Ph.D. program for his Ph.D. studies. This work was financially supported by the Thailand Research Fund under the TRF Senior Research Scholar program Grant No. RTA5980005, Suranaree University of Technology and the Higher Education Research Promotion and National Research University Project of Thailand, Office of Higher Education Commission.

References (77)

  • C. Suksiripattanapong et al.

    Compressive strength development in fly ash geopolymer masonry units manufactured from water treatment sludge

    Constr. Build. Mater.

    (2015)
  • C. Suksiripattanapong et al.

    Unit weight, strength and microstructure of water treatment sludge-fly ash geopolymer lightweight cellular geopolymer

    Constr. Build. Mater.

    (2015)
  • E. Nimwinya et al.

    A sustainable calcined water treatment sludge and rice husk ash geopolymer

    J. Clean. Prod.

    (2016)
  • J. Han et al.

    Sustainable roadway construction using recycled aggregates with geosynthetics

    Sustainable Cities Soc.

    (2015)
  • M. Hoy et al.

    Strength development of Recycled Asphalt Pavement – fly ash geopolymer as a road construction material

    Constr. Build. Mater.

    (2016)
  • D. Avirneni et al.

    Durability and long term performance of geopolymer stabilized reclaimed asphalt pavement base courses

    Constr. Build. Mater.

    (2016)
  • S. Pangdaeng et al.

    Influence of curing conditions on properties of high calcium fly ash geopolymer containing Portland cement as additive

    Mater. Des.

    (2014)
  • K. Somna et al.

    NaOH-activated ground fly ash geopolymer cured at ambient temperature

    Fuel

    (2011)
  • P. Chindaprasirt et al.

    Comparative study on the characteristics of fly ash and bottom ash geopolymers

    Waste Manage.

    (2009)
  • N. Yarbaşı et al.

    Modification of the geotechnical properties, as influenced by freeze–thaw, of granular soils with waste additives

    Cold Reg. Sci. Technol.

    (2007)
  • A. Aldaood et al.

    Impact of wetting–drying cycles on the microstructure and mechanical properties of lime-stabilized gypseous soils

    Eng. Geol.

    (2014)
  • Y.J. Du et al.

    Effect of acid rain pH on leaching behavior of cement stabilized lead-contaminated soil

    J. Hazard. Mater.

    (2014)
  • F. Brue et al.

    Effect of temperature on the water retention properties of two high performance concretes

    Cem. Concr. Res.

    (2012)
  • Y. Wang et al.

    Initial temperature-dependence of strength development and self-desiccation in cemented paste backfill that contains sodium silicate

    Cement Concr. Compos.

    (2016)
  • M. Jooss et al.

    Permeability and diffusivity of concrete as function of temperature

    Cem. Concr. Res.

    (2002)
  • E. Drouet et al.

    Temperature influence on water transport in hardened cement pastes

    Cem. Concr. Res.

    (2015)
  • O. Cuisinier et al.

    Quantification of the effects of nitrates, phosphates and chlorides on soil stabilization with lime and cement

    Eng. Geol.

    (2011)
  • N. Cristelo et al.

    Effect of calcium content on soil stabilisation with alkaline activation

    Constr. Build. Mater.

    (2012)
  • S. Hanjitsuwan et al.

    Effects of NaOH concentrations on physical and electrical properties of high calcium fly ash geopolymer paste

    Cem. Concr. Compos.

    (2014)
  • M. Wu et al.

    A study of the water vapor sorption isotherms of hardened cement pastes: Possible pore structure changes at low relative humidity and the impact of temperature on isotherms

    Cem. Concr. Res.

    (2014)
  • E. Celik et al.

    Effects of ground granulated blastfurnace slag (GGBS) on the swelling properties of lime-stabilized sulfate-bearing soils

    Eng. Geol.

    (2013)
  • M.N. Rahmat et al.

    Effects of mellowing sulfate-bearing clay soil stabilized with wastepaper sludge ash for road construction

    Eng. Geol.

    (2011)
  • W. Chen et al.

    Alkali binding in hydrated Portland cement paste

    Cem. Concr. Res.

    (2010)
  • P. Chindaprasirt et al.

    Workability and strength of coarse high calcium fly ash geopolymer

    Cem. Concr. Compos.

    (2007)
  • T. Bakharev

    Durability of geopolymer materials in sodium and magnesium sulfate solutions

    Cem. Concr. Res.

    (2005)
  • T. Bakharev

    Resistance of geopolymer materials to acid attack

    Cem. Concr. Res.

    (2005)
  • M. Mirzababaei et al.

    Unconfined compression strength of reinforced clays with carpet waste fibers

    J. Geotech. Geoenviron. Eng.

    (2013)
  • M. Mirzababaei et al.

    Impact of carpet waste fibre addition on swelling properties of compacted clays

    Geotech. Geol. Eng.

    (2013)
  • Cited by (150)

    View all citing articles on Scopus
    View full text