Degradation phenomenon of cementitious materials under sulphuric acid attack

Abstract

Sulphuric acid corrosion of Ordinary Portland Cement (OPC) based materials has attracted significant attention in recent decades, both as a result of its widespread occurrence in aggressive environments and the substantial economic impact of the resulting damage. At the same time, the development of alkali-activated (AA) cementitious materials that have been reported to have considerably improved durability-related properties has significantly increased. Alkali-activated cementitious materials are expected to have higher sulphuric acid resistance than that of OPC based materials and investigations on understanding the behaviour of OPC based and AA cementitious materials under sulphuric attack therefore of great research interest. The agglomeration of a global database of test results is complicated by the differing test methodologies employed in each study which makes it very difficult to compare the relative results of studies performed as part of septate experimental campaigns. To better utilise the results of existing and new experimental studies and to be able to compare the results across different types of material there is a need to assess and understand the results obtained from a range of test methodologies applies to for a range of cementitious composite materials. This thesis contains a series of journal papers in which the methodologies commonly applied in the literature have been further assessed and the degradation mechanisms under sulphuric acid attack of both OPC based and AA cementitious materials at the paste, mortar, and concrete levels have been investigated. In the assessment of test methodologies of OPC and AA concrete under sulphuric acid attack, the behaviour of concretes with different exposure regimes (continuous immersion, brushing, and wetting and drying cycling) and chemical concentrations (1% and 3% w/w) have been investigated such that the influence of test methodology on degradation rate and microstructural performance can be identified. The results show that the OPC and AA concrete subjected to increased acid concentration displays the most rapid degradation. The indicators of susceptibility to corrosion (acid consumption, change of mass, length, compressive strength, and cross-section) are also discussed based on their potential advantages and limitations as well as application to both OPC based and AA cementitious materials. The results show that there is no single universal indicator of resistance to sulphuric acid that could be applied as a simple test technique for both materials. Through these assessments of the test methods and measurement technique, it shows that a test procedure to indicate the susceptibility of concretes can be performed within a reasonable time frame for standard testing. In the investigation of degradation mechanism of OPC based and AA cementitious materials under sulphuric acid attack, the physical, mechanical properties, and the chemical and microstructural characteristics of cementitious materials have been analysed over time. A multiscale approach has been established to understanding how cementitious materials respond to the external acidic environment in terms of scale of manufacture (paste to mortar to concrete) and in levels of observation from microscopic to macroscopic measurements. The investigation of paste behaviour shows a clear chemical response of OPC and AA paste exposed to sulphuric acid. The contradictory degradation mechanism of OPC and AA paste identified in the previous studies have been investigated. The investigation of mortar shows that the presence of fine aggregate and interfacial transition zone between the paste and fine aggregate causes change in porosity of the mortar, affecting the resistance of mortar to sulphuric acid. Finally, the investigation of concrete shows that the chemical composition of coarse aggregate has significant effects on the degradation mechanism.