Aggregate reactivity to the alkali-silica reaction (ASR) in ground aggregate-cement pastes
- Publisher:
- Academica Greifswald
- Publication Type:
- Conference Proceeding
- Citation:
- CEEC-TAC5 & MEDICTA 2019, 2019, pp. 185 - 185 (1)
- Issue Date:
- 2019-08-27
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A range of standard accelerated test methods for the screening of aggregates for susceptibility to the alkali-silica reaction (ASR) which causes deleterious cracking in concrete structures are available worldwide and two standard test have been recently adopted in Australia (AS 1141.60.1 (Accelerated mortar bar test (AMBT)) and AS 1141.60.2 (concrete prism test (CPT))). These accelerated test methods are empirical and based on expansion measurement correlated to field performance. The mechanism of deleterious ASR resulting in cracking involves two processes; the chemical processes involved in the formation of the expansive ASR gel and the mechanical action of the ASR gel of the concrete in crack formation. Expansion tests, although empirical in nature are important as they probe the mechanical potential of the reactivity of aggregates. The chemical processes involved in the phase development are also important as they provide the gel responsible for cracking and understanding these processes canlead to more effective methods of mitigation of ASR as well as alternative methods for the screening of aggregates for reactivity to ASR. This paper focusses on correlating reactivity of aggregates determined using the standard test methods with phase development in paste tests using ground aggregate-cement pastes aged under accelerated conditions. Two aggregates are investigated in this study, a micro-diorite (CPT non-reactive) and a greywacke (CPT reactive) which have been selected because of their relative reactivity to standard test methods. Both contain quartz as the phase potentially reactive to ASR. The aggregates were initially fine ground in a ring mill in order to make paste specimens using a general purpose Portland cement. Pastes specimens were prepared using a 3 to 1 aggregate to cement ratio with a water to cement ratio of 0.7. Pastes were initially hardened for 24 before stripping from the moulds and aging in alkali media (1 M NaOH) at elevated temperature (40, 60 and 80°C) for periods up to 84 days. Specimens were recovered, crushed and dried in a vacuum oven for 24 hours at 105°C prior to grinding in the ring mill and characterising using XRD, TG and FTIR for phase analysis. Phase development with age based on the calcium hydroxide OH stretch in the FTIR, the decomposition step in the TG and the (101) peak in the XRD and the (101) quartz peak in the XRD is reported. Characterisation for the full aging period will be reported at CEEC-TAC5 – MMEDICTA 2019; however, results to date indicate that for these quartz containing aggregates, the relative reactivity can be correlated to the quartz reactivity and the calcium hydroxide consumption.
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