Ahmad Ardani

Enhancing High Volume Fly Ash Concretes Using Fine Limestone Powder

One of the primary approaches to producing more sustainable concretes consists of replacing 50% or more of the portland cement in a conventional concrete with fly ash, producing a so-called high volume fly ash (HVFA) concrete. While these mixtures typically perform admirably in the long term, they sometimes suffer from early-age performance issues including binder/admixture incompatibilities, delayed setting times, low early-age strengths, and a heightened sensitivity to curing conditions. Recent investigations have indicated that the replacement of a portion of the fly ash in these concrete mixtures by a suitably fine limestone powder can mitigate these early-age problems. The current study investigates the production of concrete mixtures where either 40% or 60% of the portland cement is replaced by fly ash (Class C or Class F) and limestone powder, on a volumetric basis. The mixtures are characterized based on measurement of their fresh properties, heat release, setting times, strength development, rapid chloride penetrability metrics and surface resistivity. The limestone powder not only accelerates the early age reactions of the cement and fly ash, but also provides significant benefits at ages of 28 d and beyond for both mechanical and transport properties.

The Effect of Nano Materials on HVFA Mixtures

The interest in more sustainable concrete mixtures with the supplementary cementitious materials like fly ash has increased significantly in the past few years. Nevertheless, the early age properties development (setting and strength gain) of these mixtures still remains a challenge and, in most cases, prevents their efficient use in practice. In this paper, nano-aluminosilicates were used to improve early age properties of high volume fly ash (HVFA) mixtures. This paper presents results of several different paste and mortar mixtures where 60 % (by volume) of the portland cement was replaced by fly ash (Class C or Class F) and 1 % of amorphous nano-aluminosilicates with different silicon-aluminum ratios. Paste mixtures were characterized by measuring heat release and setting times and the compressive strength was evaluated in mortar mixtures in order to determine the optimal silicon-aluminum ratio for the cementitious materials used in the study.

Effects of Nanomaterials on the Hydration Kinetics and Rheology of Portland Cement Pastes

In this paper, effects of nanomaterials on the hydration kinetics and rheology of ordinary Portland cement pastes were investigated. Three nanomaterials, nano-limestone, nano-silica, and nano-clay (a highly purified magnesium aluminosilicate), were added to a cement paste at the levels of 0.0 %, 0.5 %, 1.0 %, and 1.5 % (by mass) of cement. The heat of cement hydration of the paste was measured using isothermal calorimetry. Rheological behavior of the paste was characterized using a rotational rheometer. The rheology measurements were performed at 10, 30, 60, 90, and 120u2009min after the cement was mixed with water. Set times of the paste were measured according to ASTM C191 [Standard Test Methods for Time of Setting of Hydraulic Cement by Vicat Needle, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA]. The experimental results indicate that the addition of nano-limestone and nano-silica accelerated cement early hydration, the maximum heat flow increased, whereas the time to reach the heat-flow peaks decreased. The initial and final set times were also reduced. These effects were enhanced with increased nano-addition level. The addition of nano-clay also significantly increased the intensity of the heat flow peaks, and, especially, the peak corresponding to the renewed reaction of the aluminate phase. Addition of these nanomaterials generally increased yield stress and viscosity of the cement paste, especially after 60u2009min when cement hydration started to accelerate. Nano-clay considerably influenced the rheological behavior of the cement paste. Significantly higher shear stresses were required to initiate the flow.