Abstract
Traditional Hot mix asphalt (HMA) is widely used for the construction of bituminous
pavements due to its durability and flexibility properties. HMA is prepared at temperatures
between 165 °C and 200 °C, where both the asphalt binder and aggregates are heated up to
ensure proper coating of aggregates and provide adequate workability of the HMA. The
production of HMA is associated with significant environmental impacts, such as heavy carbon
dioxide emissions and high mixing temperatures, which create undesirable working conditions
during placing. There is a growing need for sustainable alternatives to traditional asphalt mixes
and developing bituminous binders that can be mixed at temperatures lower than the traditional
HMA production temperatures.
Most South African roads have developed frequent and severe pavement distress over the past
few years; the increase in pavement damage could result from delayed maintenance, rapidly
increasing traffic volumes, and roads having reached their design life. Currently, the
government is spending a lot of money on repairing roads. As a result, preventative
maintenance is delayed and not done to appropriate standards. It has become necessary to
explore asphalt binder modification techniques to help overcome these issues. Warm mix
asphalt has captured the attention of many researchers due to its production temperature being
lower than Hot Mix Asphalt, which can result in a reduced number of gases released during
production, allow for better handling of the binder on-site during the application, reduced
energy consumption, and allow for a quick opening to traffic.
The current study investigates the effect of fly ash/ground granulated blast furnace slag
geopolymer additive on the asphalt binder's physical and rheological properties. The
geopolymer was prepared in the lab using fly ash partially replaced with ground granulated
blast furnace slag and activated with an alkaline solution of sodium hydroxide and sodium
silicate. The effect of the activator-to-binder ratio, sodium silicate-to-sodium hydroxide ratio,
the sodium hydroxide solution's molarity, and the addition of slag on the geopolymer's
compressive strength was evaluated. The geopolymer was added into 50/70 penetration grade
and 70/100 penetration grade asphalt binder at different dosages of 3%, 6%, 9%, and 15% by
mass of the asphalt binder respectively.
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The softening point, ductility, penetration, and storage stability of geopolymer-modified
asphalt binder (GMAB) samples were investigated. The results indicate that the GMAB had
improved physical properties compared to the control asphalt binder. The rotational viscometer
test was utilized to assess the viscosity levels of GMAB, revealing a correlation between higher
geopolymer dosage and increased binder viscosity. Adding 6% of the FA/GGBS geopolymer
dosage recorded the highest viscosity of 1.91 and 1.794 Pa.s at 120 °C for bitumen 50/70 and
70/100, respectively. The outcomes remained within the acceptable limits outlined by
Superpave requirements, with a maximum viscosity of 3.0 Pa.s, indicating satisfactory
workability. The storage stability results indicated that the geopolymer was well spread in the
binder since all mixes did not exceed the separation index limit of 15%. The optimum
geopolymer-modified asphalt binder was a 9% FA/GGBS geopolymer-modified binder, having
improved physical and rheological properties compared to the control asphalt binder. All
GMAB samples indicated better rutting resistance after being subjected to short term curing
where all the samples recorder rutting parameter above the 2.2 kPa Superpave specification.
The asphalt binder 50/70 with 15% geopolymer dosage recorded the highest G*/sin δ of 14.356
kPa at 58 °C and bitumen 70/100 recorded the highest G*/sin δ of 10.648 kPa with 9%
geopolymer dosage at the same temperature after being cured for 90 minutes. Overall, the study
concluded that using geopolymers as asphalt binder additives is a sustainable and novel way to
improve the performance of flexible pavements. Adopting this new warm mix asphalt
technique will also lead to timely repairs on pavement defects, lower the rate of repairs, and
help reduce the cost associated with the frequent need to repair pavement defects.