Sulapha Peethamparan

Physicochemical Behavior of Cement Kiln Dust-Treated Kaolinite Clay

The physicochemical responses induced in kaolinite clay treated with 25% of a high free lime content cement kiln dust (CKD) were investigated. Atterberg limits, unconfined compressive strength, and stiffness of the compacted, CKD-treated kaolinite were measured as functions of the curing period. These properties were compared with those of the untreated clay and of the clay treated with quicklime, so as to determine the comparative extent of enhancement induced by the CKD treatment. The CKD-treated clay developed significantly higher strength than quicklime-treated clay containing the same amount of lime. The remarkable enhancement in clay properties observed suggests significant potential for use of some CKDs as soil stabilizers. The mineralogi-cal and morphologic changes induced in the clay by the CKD addition were characterized using X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray analyses, so as to help provide an understanding of the CKD-clay interaction mechanism. These examinations indicated that the preexisting stacks of kaolinite clay particles were disaggregated and that the individual clay flakes had absorbed calcium hydroxide that had been previously generated by the hydration of quicklime in the CKD. However, direct evidences for production of calcium silicate hydrate (C-S-H) by pozzolanic reaction between the clay and the calcium hydroxide was not found, despite high likelihood that this had occurred. The same basic interaction mechanisms appeared to operate in both the CKD-treated and the lime-treated clays. The 90-day cured specimens were found to be well cemented and not affected by water immersion.

Microstructural and Mineralogical Characterization of Cement Kiln Dust–Activated Fly Ash Binder

A clinker-free binder for making sustainable concrete was developed using cement kiln dust (CKD) and Class F fly ash. The CKD-activated fly ash binder developed a compressive strength of approximately 30 MPa after 48 h of elevated temperature curing. The mineralogical composition of the developed clinker-free binder was determined with the help of thermogravimetric and X-ray analysis. Microstructure of the hardened binder was examined to identify phases using a scanning electron microscope equipped with an energy dispersive X-ray detector. Various reaction products, such as calcium aluminosilicate hydrate (C-A-S-H), ettringite (Aft), and calcium silicate hydrate, were identified in the hardened clinker-free binder. The major contribution to strength development is attributed to the C-A-S-H gel, which was found extensively as a ground mass in the hardened CKD-activated fly ash system.

Effect of Specimen Size and Curing Condition on the Compressive Strength of Alkali-Activated Concrete

Alkali-activated concrete is a rapidly emerging sustainable alternative to portland cement concrete. The compressive strength behavior of alkali-activated concrete has been reported by various studies to a limited extent, but these discussions have been based on minimal evidence. Furthermore, although it is known that specimen size has a distinct effect on the apparent compressive strength of concrete, this effect has yet to be modeled for alkali-activated concrete. This paper presents the results of a comprehensive study of the effects of curing condition (i.e., moist-cured at ambient temperature for 28 days or heat-cured at 50çC for 48 h) and specimen size on the compressive strength of sodium silicate–activated fly ash and slag cement concrete. The heat-cured strength of alkali-activated slag cement concrete was linearly related to the moist-cured strength; the former was about 5% greater than the latter. Heat curing also improved the strength of alkali-activated fly ash concrete, although the effect was greatly magnified for lower-strength mixtures and was much less significant at higher strengths. Existing size effect laws employed for portland cement concrete proved reasonably accurate in describing the effect of specimen size on the apparent strength of alkali-activated slag cement concrete. However, these existing models greatly underestimated the size effect in alkali-activated fly ash concrete; the authors suggest that this finding was the result of significant microcracking in the alkali-activated fly ash concrete.

Novel Cementitious Binder Incorporating Cement Kiln Dust: Strength and Durability

Fresh and hardened properties of a suite of cementitious binders with cement kiln dust (CKD) as the main binding component (70% by weight) are evaluated in this study. Two CKDs with different chemical and physical properties were used in formulating CKD-fly ash (FA) and CKD-slag mixtures without portland cement. The setting time, workability, and strength development of these pastes were evaluated first and the best-performing binders were then used as a component in making concrete. The strength and durability of such concrete is also evaluated. The CKD (I)-slag combination outperformed all other mixtures with respect to the strength and durability performance under various curing conditions. The elevated temperature curing was found to be essential for the property development of the CKD-FA binder. The performance of CKD containing mortar mixtures with respect to the delayed ettringite formation (DEF) and the alkali-silica reaction (ASR) was also good. The CKD that exhibited better performance contained 5% free lime, 3% Na2Oeq alkali, and 10.6% SO3 with an average particle size of 4 mm (0.00016 in.).

The Role of Nano Silica in Modifying the Early Age Hydration Kinetics of Binders Containing High Volume Fly Ashes

This paper outlines the development of several sustainable binders made with 70 % replacement of ordinary portland cement (OPC) with class F and C fly ashes. The early age strength and hydration kinetics of high-volume fly ash (HVFA) binders formulated with water/binder mass ratio of 0.3 were evaluated. Additions of nano-silica (NS) were used to modify the early-age hydration kinetics, microstructure, and strength development of HVFA binders. Nano-silica-modified HVFA binders developed 1-day compressive strengths nearly double those of un-modified HVFA binders. Modification with nano-silica promoted faster nucleation and higher heat of hydration over un-modified binders. The 28-day compressive strength and density of the microstructure at late age were also improved by nano-silica modification.

STRUCTURE AND PROPERTIES OF NaOH ACTIVATED CEMENT FREE BINDER (CFB) CONCRETES

Increasing emphasis on sustainability of the built environment has resulted in attempts to drastically reduce the cement consumption in concrete and replace it with waste/recycled materials. This study reports the development of concretes with cement free binders (CFB) and the evaluation of their properties. A Class F fly ash and a ground granulated blast furnace slag (GGBFS) are used as the binding materials. The activating agent used in this study is sodium hydroxide (NaOH), at concentrations ranging from 6 M to 10 M. The optimal temperature and curing duration required to achieve desirable compressive strengths of CFB concretes are reported. The influence of the binding material and the concentration of the activator on the compressive strength and porosity of the CFB concretes are studied. The compressive strength of CFB concretes with fly ash as the binding material increases with increase of activating solution concentration but for CFB concretes with GGFBS as the binder, activation with 8 M NaOH is seen to result in the highest compressive strength. The strength-porosity relationship of CFB concretes shows an exponential trend, similar to that of conventional cement based materials. Microstructure and the phase composition of the reaction products are also discussed.