Mridul Garg

Cementitious binder from fly ash and other industrial wastes

Abstract In this paper, investigations were undertaken to formulate cementitious binder by judicious blending of fly ash with Portland cement as well as by admixing fly ash with calcined phosphogypsum, fluorogypsum, lime sludge, and chemical activators of different finenesses. The effect of addition of calcined clay in these types of binders was studied. Data showed that cementitious binders of high compressive strength and water retentivity can be produced. The strength of masonry mortars increased with the addition of chemical activators. The strength development of binders takes place through formation of ettringite, C-S-H, and C4AH13. The binders are eminently suitable for partial replacement (up to 25%) of the cement in concrete without any detrimental affect on the strength. The results showed that fly ash can be used in the range from 45% to 70% in formulating these binders along with other industrial wastes to help in mitigating environmental pollution.

Relationship between mechanical properties and porosity of water-resistant gypsum binder

The paper describes an experimental investigation involving pore structure of blended gypsum binder. The pore structure was investigated by using mercury porosimeter. A relationship between structure and mechanical properties has been established. It has been found that the principal factor of porosity governs the development of structure and strength of the blended gypsum binder. The pore size distribution was characterised by a reduction in the volume of the pores whose diameters are larger than 500 A° and an increase in the volume of those smaller diameters. An interpretation of the development in strength vis-a-vis porosity of the blended gypsum binder is presented.

Phosphogypsum — Fly ash cementitious binder — Its hydration and strength development

Abstract The paper deals with the formulation of a cementitious binder based on calcined phosphogypsum, flyash, hydrated lime and portland cement. Strength properties and hydration of the cementitious binder studied at room temperature and at 50 °C in over 90% R.H are presented. It was found that the compressive strength of the cementitious binder was remarkably enhanced at 50 °C than at 27 °C. The hydration of the cementitious binder as studied by differential thermal analysis and scanning electron microscopy showed that the early age strength in the cementitious binder was due to the hardening of calcined gypsum and the hydration of portland cement while later age strength development was ascribed to the formation of ettringite and CSH.

Making of anhydrite cement from waste gypsum

The paper presents the results of research on the utilization of phosphogypsum produced as the waste of phosphoric acid manufacture. Phosphogypsum is a fine powder with high calcium sulfate content. The phosphatic and fluoride impurities present in phosphogypsum cannot be removed completely either by washing or chemical treatments. However, phosphogypsum, when heated at elevated temperature, produced an anhydrite and the impurities become inert. The formation of anhydrite cement was examined by microscopy and X-ray diffraction. Data showed that a stable anhydrite can be produced by heating phosphogypsum at 1000°C. The effects of different chemicals on setting and hardening of anhydrite cement and its hydration characteristics were studied. Results show that with the use of sodium sulfate and ferrous sulfate activators, maximum attainment of strength can be achieved. A correlation was established between hydration and chemically combined water. Microscopic studies revealed that formation of euhedral prismatic and rhombic shaped gypsum crystals govern high strength development in the anhydrite cement. Manufacture of anhydrite cement from phosphogypsum is recommended because of its lower energy requirements than the traditional building materials.

Activation of gypsum anhydrite-slag mixtures

Gypsum anhydrite-slag mixtures were produced by blending anhydrite with granulated blast furnace slag, Ca(OH)[sub 2] and small amounts of Na[sub 2]SO[sub 4] [center dot] 10H[sub 2]O and FeSO[sub 4] [center dot] 7H[sub 2]O as activators. A mechanism of conversion of anhydrite into gypsum through transient double salts in presence of activators is suggested. Activation of granulated slag with the gypsum anhydrite and Ca(OH)[sub 2] to form ettringite and tobermorite has been discussed. The correlation between gain in strength and hydration products is reported.

Perlite-based building materials — a review of current applications

Abstract Perlite has been extensively used as a lightweight aggregate material in concrete or mortar. In expanded form perlite offers thermal insulation, fire resistance and other desirable properties when used in portland cement- or gypsum-based plaster. The authors review current applications, with particular reference to lightweight fibre-reinforced composites based on cement/gypsum plaster and perlite binder. The availability of local supplies of perlite has promoted interest in its use in India but progress has been hampered by a lack of local knowledge and Standards. A brief description is given of an ongoing research project examining the effect of litemix and fellite perlite aggregate on the properties of a plain gypsum plaster and a new water-resistant gypsum binder reinforced with glass fibre.

Retarding action of various chemicals on setting and hardening characteristics of gypsum plaster at different pH

Abstract Gypsum plaster sets quickly due to its natural process of crystallization. For commercial applications, the retardation of plaster to a desired level is required. The effect of various chemicals as retarders on the setting time, compressive strength and microstructure of the gypsum plaster was investigated at pH 4 to 12 adjusted by the addition of Ca(OH) 2 or HCl to gypsum plaster. It was found that retardation factor has no direct relation with the compressive strength of gypsum plaster but pH is certainly related to the strength factor. Maximum compressive strength of the plaster was obtained at pH 7.0. The morphology of the hardened gypsum plaster was found to change according to variation in the pH of the aqueous phase as well as the nature of retarder added.

An improved process for the purification of phosphogypsum

Phosphogypsum, a by-product of the phosphoric acid industry, is contaminated with the impurities of P 2 O 5 , F, organic matter etc. These impurities impair the strength development of calcined products. For effective and proper utilization, a purification process for phosphogypsum by wet sieving through a 300 micron sieve has been developed. Based on the wet sieving, the laboratory trials were conducted using hydrocyclone. The findings of the purification trials and proposed pilot plant along with a list of uses and applications of the purified material are presented in this paper.

Purifying phosphogypsum for cement manufacture

Abstract Phosphogypsum available as a by-product from the wet process phosphoric acid industry in India contains various impurities which, when used in cement manufacture, impair the strength of the cement. Such methods as simple washing with water or wet sieving of phosphogypsum, heating to hemihydrate-anhydrite stage followed by washing or neutralization with lime, and treating phosphogypsum with a mixture of sulphuric acid and silica or hot aqueous ammonium sulphate solutions have been suggested for reducing the amount of impurities or making them ineffective. In the investigation reported here, treatment of phosphogypsum with aqueous ammonium hydroxide solutions was tried to purify phosphogypsum and render it suitable for cement manufacture. The treatment converts phosphatic and fluoride impurities into water-soluble ammoniated compounds which can be removed with a stream of water. The results of chemical and physical tests and differential thermal analysis of the phosphogypsum with and without the ammonium hydroxide treatment confirmed the efficacy of the treatment for purifying phosphogypsum. The treated phosphogypsum was found to have comparatively smaller amounts of impurities that the untreated material. The cement produced was found to have strength characteristics similar to those produced using mineral gypsum.

Some aspects of the durability of a phosphogypsum-lime-fly ash binder

Abstract This paper deals with the effect of curing temperature on the durability of a cementitious binder based on calcined phosphogypsum, fly ash and lime. The durability of the binder was studied by its performance in water and accelerated ageing, i.e. alternate wetting and drying as well as heating and cooling cycles at temperatures from 27 °C to 60 °C. The results indicate that the strength of the binder decreased and loss in weight increased with increases in cycles and exposure temperature. The variation in strength and weight loss was assessed by differential thermal analysis and scanning electron microscopy. The maximum fall in strength occurs at 60 °C.