The durability of inorganic polymer cements

© 2008 Dr. Redmond Richard LloydInorganic polymer cements have the potential to provide a substantial reduction in global greenhouse gas emissions by replacing Portland cement in concrete. Several decades of research has advanced the field to the point where commercial implementation of the technology is imminent. In order for this to proceed, some understanding of the ability of inorganic polymer cements to provide durable, long lasting materials must be obtained. The objective of this thesis is to examine some aspects of the durability of IPC. Previous studies on various aspects of the durability of IPC are examined, with reference to known durability issues in Portland cement concrete. From this, some specific aspects in need of investigation are identified, including the importance of microstructure and porosity. The microstructure of IPC synthesised from fly ash and mixtures of fly ash and blast-furnace slag is examined, and the fate of various components of the ash and slag determined. The effect of the composition of the activating solution on microstructural development is also examined, and a mechanism describing the effect of soluble silicate on microstructural development proposed. The porosity and size distribution of the pores in IPC are determined, and linked to compositional factors. Examination of the distribution and morphology of pores is achieved by intrusion of Wood's metal. This technique is applied to IPC for the first time, and a novel method for achieving high-pressure intrusion is described. The functional aspects of the pore system are also determined for the first time, using a leaching test to determine the ionic permeability of the IPC paste. From this, important compositional factors are identified which will help produce IPC capable of protecting embedded steel reinforcement from corrosion. The effect of acids on IPC is determined, and it is demonstrated that some test methods currently in use provide highly misleading results. One corollary of this is that IPC is not as resistant to acidic environments as is frequently claimed. A new model for the process of acidic corrosion of IPC is presented, which helps to explain the discrepancy between apparent and actual performance of IPC binders in various tests. The effect of a broad range of mix design and processing parameters on acid corrosion of IPC at a pH of 1 is examined, and the factors which contribute most to acid resistance are identified. This will provide a basis for future endeavours to produce acid resistant concrete products. The conversion of amorphous metakaolin-based IPC into crystalline zeolitic phases, accompanied by dramatic strength loss, is identified for the first time. The process of strength loss is found to be mechanistically similar to that of conversion in calcium aluminate cements, i.e. formation of a metastable phase, which converts to a denser, more stable phase with time. The densification procedure is accompanied by a decrease in the volume of the reaction products, an increase in the volume of large pores and a significant reduction in strength. A recommendation is made against the use of metakaolin-based IPC because of the deleterious effect of crystallisation. Crystallisation is also examined in fly ash-based and mixed fly ash and blast-furnace slag-based IPC. In fly ash based systems crystallisation does occur, but to a much lower degree than in metakaolin-based systems. Crystallisation is not accompanied by a major loss of strength. The presence of blast-furnace slag prevents crystallisation in IPC systems. Finally, some conclusions regarding the durability of IPC are made. The importance of testing the durability of IPC formulations is highlighted, as is the need to apply accelerated tests carefully and interpret results with consideration of the differences in chemistry and microstructure of Portland and inorganic polymer cements