Piyush Chaunsali

Abstract: Calcium sulfoaluminate (CSA) cements have lower carbon footprint than that of portland cement, which makes them a suitable alternative as a sustainable cementitious binder. Early-age expansion of CSA cements can be exploited to induce compressive stress in restrained concrete which can later counteract tensile stress developed during drying shrinkage, thus enhancing the resistance against shrinkage cracking. However, a proper understanding of the expansion behavior is critical to eliminate any risk related to expansion-induced cracking. This study examines the expansion and hydration characteristics of various ordinary portland cement (OPC)-CSA blends. Early-age expansion of paste samples was monitored. The increase in CSA cement content increased the extent of expansion. Samples having the highest CSA content (30% by mass) exhibited excessive expansion which led to their cracking. Quantitative X-ray diffraction, pore solution extraction, porosity, tensile strength, and dynamic modulus tests were performed to monitor the physico-chemical changes in OPC-CSA blends. It was shown that the ettringite supersaturation in the investigated systems gave rise to the crystallization stress, responsible for the expansion. Thermodynamic models enabled a reasonable prediction of tensile failure, particularly in the blends with the higher CSA content.

Abstract: The present study aims at examining the physico-chemical factors influencing the expansion characteristics of OPC–CSA blend in the presence of mineral admixtures. Three different admixtures: Class ‘F’ fly ash (‘F’FA), Class ‘C’ fly ash (‘C’FA) and silica fume (SF) were used as 15%, 15% and 5% replacement of total cementitious binder. Longitudinal expansion of cement pastes prepared at w/cm – 0.44 showed that the Class ‘F’FA increased the expansion whereas the Class ‘C’FA and SF reduced the expansion. The pore solution of the OPC–CSA cement pastes was extracted at different ages to monitor the concentration of various ionic species. The saturation level of ettringite was determined using a geochemical modeling program (GEMS). Furthermore, an upper bound of crystallization stress was estimated. The expansion behavior in the presence of Class ‘F’FA and SF was found to be influenced by the changes in the stiffness, whereas the expansion of the Class ‘C’FA-based mixture was governed by faster hydration of ye'elimite.

Abstract: Characterization of a nontraditional binding material containing cement kiln dust (CKD) and ground granulated blast furnace slag (GGBFS) is discussed in this paper. Significant compressive strength was obtained for a CKD-GGBFS blend with 70% CKD and 30% GGBFS at a water-to-binder ratio of 0.40 after 2 days of curing at elevated temperature. Similar strength was also obtained for the samples subjected to normal moisture curing over a period of 28 days. The compressive strength increased with additional moist curing in both the cases. The microstructural and the mineralogical examinations show that the strength development was mainly due to the formation of calcium silicate hydrate (C-S-H). In addition to normal C-S-H, aluminum and magnesium incorporated C-S-H phases were also present in the CKD-GGBFS blends. The formation of ettringite appears to be a contributing factor in early age strength development of CKD-GGBFS binder.

Abstract: Cement kiln dust (CKD), a by-product of the cement industry, contains significant amounts of alkali, free lime, chloride and sulfate. Wide variation reported in the chemical composition of CKDs limits their potential application as a sustainable binder component in concrete. In the current study, the performance of two different CKDs as components in a novel binder is evaluated. Several binders are developed by blending CKDs with fly ash or slag. Binders with 70% CKD were prepared at a water-to-binder ratio of 0.4, and heat-cured at 75 °C to accelerate the strength development. The hydration progress was monitored using X-ray diffraction, and morphological examination was performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ettringite and calcium aluminosilicate hydrate (C-A-S-H) were identified as the main hydration products in the hardened binder system. Strength development of CKD-based binder was found to be significantly influenced by its free lime and sulfate contents.

Abstract: While the incineration of biomass residues is gaining traction as a globally available source of renewable energy, the resulting ash is often landfilled, resulting in the disposal of what could otherwise be used in value-added products. This research focuses on the beneficial use of predominantly rice husk and sugarcane bagasse-based mixed biomass ashes, obtained from two paper mills in northern India. A cementitious binder was formulated from biomass ash, clay, and hydrated lime (70:20:10 by mass, respectively) using 2M NaOH solution at a liquid-to-solid mass ratio of 0.40. Compressive strength of the biomass ash binder increased linearly with compaction pressure, indicating the role of packing density. Between the two mixed biomass ashes used in this study, the one with higher amorphous content resulted in a binder with higher strength and denser reaction product. Multi-faceted characterization of the biomass ash binder indicated the presence of aluminum-substituted calcium silicate hydrate, mainly derived from the pozzolanic reaction.

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Abstract: Due to their good performance and environmental friendliness, fly ash-based construction materials have great potential as alternatives to ordinary Portland cement. To realize sustainable development and beneficial use of fly ash in the construction industry, this paper presents a comprehensive review of relevant literature to evaluate the properties and performance of fly ash, with a particular focus on recent advances in characterization, compositional understanding, hydration mechanism, activation approaches, durability and sustainability of fly ash as a construction material. Several key aspects governing the performance of fly ash, including chemical composition, activator type and hydrates evolution in concrete, are highlighted. Finally, the important needs, pertinent to the optimal and broad utilization of fly ash as an integral part of sustainable construction materials, are identified for further research and development, where large-scale application studies, further classification of fly ash, advanced characterization tools and technology transfer to biomass fly ash are recommended.

Abstract: Introduction: Dissolution and precipitation of feldspar is an important reaction in many environmental systems, as plagioclase feldspar is the most common mineral in the crust. However, the mechanism of dissolution of feldspar has remained in dispute despite twenty or more years of analysis of feldspar dissolution. We are pursuing an approach toward better understanding of feldspar dissolution by investigating reactivity of aluminosilicate glasses of variable composition. In particular, to investigate the effects of Al/Si ratio on plagioclase dissolution without complications of varying Na/Ca content or exsolution, three glasses with varying Al/Si ratio (albite, jadeite, nepheline) were synthesized. Glass powders were then prepared and dissolved at various values of pH in batch reactors under ambient conditions. Solution chemistry was analyzed by inductively coupled plasma -optical emission spectroscopy or -mass spectrometry. In addition, alteration of the glass surface was investigated using Xray photoelectron spectroscopy, secondary ion mass spectrometry, and Fourier transform infrared spectroscopy. Dissolution results: Many similarities in dissolution behavior between plagioclase crystals and this suite of glasses were observed: 1) dissolution was slowest at near-neutral pH and increased under acid and basic conditions, 2) dissolution rate at all pH values increased with increasing Al/Si ratio, 3) the pH dependence of dissolution was higher for the phase with Al/Si = 1 than the phase with Al/Si = 0.3, 4) after acid leaching, the extent of Al-depletion of the altered surface increased with increasing bulk Al/Si ratio from Al/Si = 0.3 (albite) to 0.5 (jadeite), but then decreased in nepheline (Al/Si = 1.0), which dissolved stoichiometrically with respect to Al, and 5) little to no Al depletion of the surface of any glass occurred at pH > 7. However, in contrast with some observations for plagioclase dissolution, log(rate) increased almost linearly with Al content, and n, the slope of the log(rate)pH curve at low pH, varied smoothly from albite to jadeite to nepheline (n = 0.3, 0.6, 1.0 respectively). At high pH, the slope of this curve, m, did not differ between glasses (m = -0.4 + 0.1). Mechanistic interpretation: These results are consistent with an identical mechanism controlling dissolution of nepheline, albite, and jadeite glass, although no Si-rich layer can develop on nepheline because of the lack of Si-O-Si linkages. Such a conclusion is consistent with a transition state for these aluminosilicates at high pH consisting of a deprotonated Q3 hydroxyl group (where Qv refers to an x atom in a tetrahedral site with v bridging oxygens) or a five-coordinate Si site after nucleophilic attack by OH, and, at low pH, a protonated Q4OQ4. At low pH, we infer that Q4OQ4 Al linkages are ratelimiting because they are presumed to hydrolyze more slowly than Qv OQw Si (v,w < 3). According to this model, dissolution rate increases from albite to jadeite to nepheline because hydrolysis of Al-O-Si linkages becomes more energetically favorable as the number of Al atoms per Si tetrahedral increases, a phenomenon documented by geometry optimizations using ab initio methods (for example, Figure 1). However, a model wherein Q4OQ4 Si linkages are faster to hydrolyze than lower connectivity linkages between Si atoms (Qv OQw , v,w < 3) may also explain aspects of this data.

Abstract: Clayey soils in Syria cover a total area of more than 20,000 km 2 of the country, most of which are located in the southwestern region. In many places of the country, the clayey soils caused severe damage to infrastructures. Extensive studies have been carried out on the stabilization of clayey soils using lime. Syria is rich in both lime and natural pozzolana. However, few works have been conducted to investigate the influence of adding natural pozzolana on the geotechnical properties of lime-treated clayey soils. The aim of this paper is to understand the effect of adding natural pozzolana on some geotechnical properties of lime-stabilized clayey soils. Natural pozzolana and lime are added to soil within the range of 0%–20% and 0%–8%, respectively. Consistency, compaction, California bearing ratio (CBR) and linear shrinkage properties are particularly investigated. The test results show that the investigated properties of lime-treated clayey soils can be considerably enhanced when the natural pozzolana is added as a stabilizing agent. Analysis results of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) show significant changes in the microstructure of the treated clayey soil. A better flocculation of clayey particles and further formation of cementing materials in the natural pozzolana-lime-treated clayey soil are clearly observed.