pH Level of Pore Solution in Alkali-Activated Fly-Ash Geopolymer Concrete and Its Effect on ASR of Aggregates with Different Silicate Contents

Durability has always been a major concern in concrete-based infrastructures. Alkali–silica reaction (ASR) is the result of a chemical reaction between hydroxyl ions in pore water and silica from aggregates and cement of the concrete matrix. Geopolymer is a type of alkaline reactivated binder which can be synthesized by a polycondensation reaction of geopolymeric precursors and alkali polysilicates. Due to the alkaline solution brought in by alkali polysilicates, it is intuitive that a higher alkaline concentration in pore solutions of geopolymer concrete will adversely affect long-term performance. This study researched the preceding hypothesis and reports the finding of an experimental investigation of alkali–silica reaction between reactive aggregates and the geopolymer matrix. Specimens were prepared using Class C fly ash with a high CaO content and three types of aggregates (granite, carbonate, and gravel), each with different alkaline activator ratios and ${\mathrm{Na}}_2\mathrm{O}$ doses. Each aggregate also had different percentages of silicate and aluminum content. Mechanical testing of potential resistivity of the aggregates was performed via length change. Pore solution of the hardened geopolymer concrete was extracted and the pH value of the pore solution was determined. Results suggest that the extent of ASR reaction based on the presence of all three types of aggregates in fly ash–based geopolymer concrete is substantially smaller than that of ordinary portland cement (OPC)-based concrete. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) tests also revealed that the amount ASR gel formed in geopolymer is less than that formed in ordinary portland cement concrete. Hence, utilizing ASR vulnerable aggregates in the production of geopolymer concrete might be permissible.