Abstract
Durability of in situ shotcrete under external sulfate attack was investigated, taking into consideration the addition of mineral admixtures, along with the presence of chloride ions. Three water-to-binder ratios (w/b), i.e., 0.35, 0.45, and 0.55, and two types of supplementary cementitious materials (SCMs), namely, fly ash (FA) and silica fume (SF), were considered in the current study. Two different laboratorial immersion regimes (continuously full immersion and partial immersion with cycling temperature and relative humidity) were carried out to induce chemical/physical sulfate attack. Results show that loss of strength was the typical feature of chemical sulfate attack on shotcrete, while surface spalling dominated in deterioration caused by physical sulfate attack. The presence of chloride ions can globally mitigate these deteriorations. Meanwhile, the lower w/b ratio proved to be efficient in increasing the resistance to both sulfate attacks. Adding fly ash (FA) in shotcrete mixtures enhanced the long-term performance but invited massive white efflorescence on surface layer under partial-immersion exposure condition. Silica fume (SF) admixture can compensate the undesired reduction of early-age strength caused by FA addition, but make these specimens more susceptible to sulfate attack. Mercury intrusion porosimetry (MIP) analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD) tests reveal that these consequences were strongly related to the refinement of microstructure resulted from pozzolanic reactions and hydration kinetics.
Highlights
Durability of cement-based materials, no matter emerging or conventional ones, has become a major issue in structural engineering over the last few decades [1, 2]
For partial immersion with cycling temperature and RH, physical sulfate attack in the form of surface spalling is the main type of deterioration
(ii) Shotcrete specimens incorporating 10% fly ash (FA) as partial replacement of OPC achieved adequate resistance to chemical sulfate attack. Such specimens suffered from surface deterioration under the exposure of partial immersion with cycling temperature and RH
Summary
Durability of cement-based materials, no matter emerging or conventional ones, has become a major issue in structural engineering over the last few decades [1, 2]. Shotcrete durability is generally overlooked due to the reliable performance benefited from its technology of high speed jet, high early-age strength and the addition of the accelerator, which differs from conventional concrete materials [3] They have the risk to be eroded by contaminated groundwater under harsh ambient environments, such as shotcrete liners of a tunnel underneath a refuse landfill. A laboratory-scale accelerated corrosion test on shotcrete specimens (100 mm cube) exposed to sodium sulfate solutions was performed, taking into account the presence of chloride ions. The deterioration mechanism of shotcrete specimens exposed to Na2SO4 solution in the presence of Cl− was preliminarily explored, by means of different analytical techniques and evaluation indicators
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