ASR formation of reactive chert in conducting model experiments at highly alkaline and temperature conditions

• ASR products (C–S–H and C–Na–S–H) are formed that promotes the reactivity of chert.
• Silica dissolution of chert (ICP-AES) can be determined and provide ion concentration data for the simulation of PHREEQC program.
• The Q3 sites (29Si-NMR) tend to give the increase in the degree of polymerization of C–S–H, to produce ASR gel, like C–Na–S–H.
• The simulation by PHREEQC tends to give predictions of C–S–H, C–Na–S–H and Na–S–H that supports the sequence of ASR into 4 steps.

A chemical model is presented here for investigation of the alkali–silica reaction (ASR) of reactive aggregates. The ASR formation was studied with experiments involving aggregates (Yoro- and Seto-chert) and Ca(OH)2 with alkalinity under accelerated conditions at 80 °C. The role of Ca on ASR formation was also investigated by varying the contents of Ca(OH)2 in the reacting system. The investigation employed Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), X-ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive X-ray Analysis (SEM/EDX), and 29Si Nuclear Magnetic Resonance (29Si-NMR) analysis. The experiments demonstrated there are two types of ASR which are produced (i) tobermorite type C–S–H due to the reaction between dissolved Si and Ca ions and (ii) alkali calcium silicate hydrate (C–Na–S–H) from the reactions among dissolved Si, Ca and Na ions. It was found that by addition of small content of Ca(OH)2, the XRD peaks of C–S–H located at 29.3° 2θ shift to 30.0° 2θ and the spectra of 29Si-NMR displayed the peaks at −79 ppm (Q1 sites), −85 ppm (Q2 sites) and −95 ppm (Q3 sites). These results indicated the formation of ASR of C–Na–S–H. Correlating these experimental results with simulation by PHREEQC program leads to insight into the mechanism of ASR-affected structures. The sequence of ASR have confirmed, in particular the ASR formation in the presence of Ca(OH)2. The C–S–H and C–Na/K–S–H presented here ensure that tobermorite type C–S–H is the first likely structures as analog ASR product when Ca ions are present. After Ca ions almost consumed, C–Na/K–S–H is readily formed due to the available source of alkali.