Liwu Mo

Surface modification of fly ashes with carbide slag and its effect on compressive strength and autogenous shrinkage of blended cement pastes

Surfaces of grade III fly ashes were modified through mixing with carbide slag and calcining at 850 °C for 1 h. Mineralogical compositions and surface morphology of fly ashes before and after modification were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Effect of surface-modified fly ashes on compressive strength and autogenous shrinkage of blended cement pastes was investigated. Microstructures of cement pastes were examined by backscattered electron (BSE) imaging and mercury intrusion porosimetry (MIP). The experimental results showed that β-C2S was formed on the surfaces of fly ashes after modification. Hydration of β-C2S on the surface-modified fly ashes densified interface zone and enhanced bond strength between particles of fly ashes and hydrated clinkers. In addition, surface modification of fly ashes tended to decrease total porosity and 10–50 nm pores of cement pastes. Surface modification of fly ashes increased compressive strength and reduced autogenous shrinkage of cement pastes.

Potential Approach to Evaluating Soundness of Concrete Containing MgO-Based Expansive Agent

This paper will discuss how MgO-based expansive agent (MEA) has increasingly been used to compensate for the effects of thermal shrinkage of concrete in China. Long-term soundness of concrete containing MEA attracts much attention; however, the autoclave test, which is used to assess the soundness of cement caused by excessive intrinsic dead-burnt MgO, seems to be inapplicable to the reactive lightly burnt MEA due to its harsh and unreliable autoclaving condition. This study seeks a reasonable accelerated approach for ascertaining the relevant soundness of concrete made with this admixture. The effects of temperature on accelerating the hydration of MEA are investigated by curing cement pastes containing three types of MEA at 25, 40, 60, and 80°C (77, 104, 140, and 176°F) in water, respectively. Results of the expansion measurement of cement pastes and X-ray diffraction analysis on the hydration products of MEA indicate that the hydration of MEA is accelerated most effectively at 80°C (176°F), and it is fully reacted within a short period. This provides a new possible accelerated method. Soundness of concrete prisms containing MEA is investigated under the accelerated condition of 80°C (176°F) and the feasibility of this new approach is discussed.

Mechanical properties and microstructure of blended cement containing modified quartz tailing

The amount of inert quartz tailing used in concrete construction is limited due to the low strength development of cementitious materials that may be caused by the quartz tailing. We manage to increase the strength of blended cement by modifying quartz tailing through solid-phase reaction of quartz tailing with carbide slag at high temperature. The mineral composition and morphology of the modified quartz tailing were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical properties and microstructure of blended cement mortars containing modified quartz tailing were investigated. Results showed that the strengths of blended cement mortars containing modified quartz tailing were close to those of the corresponding blended cement mortars containing quartz tailing at early age of 3 d, but increased significantly to be similar to that of plain Portland cement mortars at late ages of 90 d. This is attributed to the microstructure densification and the enhancement of interface between quartz tailing and cement paste due to the hydration of β-C2S surface layer on modified quartz tailing.