Microstructure and microanalysis of portland cement pastes with high w/c ratios

This study examines the influence of an increasing water-to-cement (w/c) ratio on the hydration mechanism of portland cement. Portland cement pastes at three w/c ratios of 0.5, 0.8 and 1.2 were prepared and cured in hermetically sealed vials at 40 °C for 14 days. Compressive strength, bulk electrical resistivity and bound water measurements were conducted. Thermogravimetric analysis was performed on hydrated pastes. Hydrated microstructures of fractured surfaces of dried pastes representing the extreme w/c ratios of 0.5 and 1.2 were investigated using scanning electron microscopy for morphology and energy dispersive X-ray spectroscopy for chemical composition and elemental mapping of hydration products. The results indicate that an increase in the w/c ratio benefits the hydration of portland cement; however, the advantageous magnitude is limited beyond a certain w/c ratio. The microstructural characteristics of pastes and the types of hydration products formed are highly governed by the w/c ratio. The effects of w/c ratio on the morphology and composition of hydration products, particularly calcium silicate hydrate and calcium hydroxide, are discussed.