Cement Energy and Environment

.. Zinc Industry By-Product "Jarosite" as Set Controller in OPC S K Agarwal, S Harsh, S K Chaturvedi, Ashwani Pahuja National Council for Cement and Building Materials, Ballabgarh, Haryana, India Key Words: Jarosite, mineral gypsum, set controller, sulphate, solubility, setting time Abstract In the manufacture of ordinary Portland cement, the clinker is inter-ground with mineral gypsum which provides sulphate ions to regulate hydration of calcium aluminate (C3A) phase resulting in retardation of cement setting. The presence of mineral phases such as natrojarosite, anhydrite and bassenite in 'jarosite' showed its utilization potential in controlling cement setting similar to mineral gypsum. Investigations on ordinary Portland cement samples containing different doses of jarosite in place of mineral gypsum showed higher rate of availabil ity of sulphate ions during early period of hydration resulting increase in setting time inspite of lower S03 contents. The delay in setting time also showed marginal reduction in 1-day compressive strength whereas 3, 7 and 28-days compressive strength values were found to be comparable to control cement. The optimized cement sample was found to be resistant to chloride and sulphatic environmental conditions up to the period of 6 months. 1.0 Introduction In the manufacture of ordinary Portland cement, mineral gypsum is identified as the most effective source of sulphate needed to arrest rapid hydration of CJA to prevent the flash setting of cement on addition of water by formation of more or less impermeable temporary coating of amorphous ettringite around the unreacted C3A particles. During clinker grinding, the mineral gypsum is added to regulate setting of hydrated cement where, it is essential to regulate the supply of available sulphate at the start of cement hydration so that only ettringite and no mono– sulphate or secondary gypsum formation takes place (Locher et.al , 1980). However in cement containing highly reactive C3A, the availability of sulphate ion concentration from mineral gypsum used during hydration may not be sufficient due to its lesser solubility and in such cases, it is desirable that an appropriate proportion of the mineral gypsum gets dehydrated to hemi-hydrate during clinker grinding under the influence of mill temperature. Hemi-hydrate is able to dissolve rapidly providing needed sulphate concentration immediately after the water is added in cement. In Jarosite, the presence of mineral phases such as natrojarosite, anhydrite and bassenite indicates its suitability for its use in place of mineral gypsum on account of sulphate as well as alkali contents. The present paper highlights the use of jarosite vis-a-vis mineral gypsum as set controller in regulating cement setting along with their performance characteristics and resistance to chloride and sulphatic environmental conditions. 2.0 Experimentation The chemical analysis of jarosite and other materials such as plant OPC clinker and mineral gypsum was carried out according to the procedures described in relevant Indian standards. The availability of sulphate ions from jarosite and mineral gypsum was studied by treatment for 10, 20, 30, 60 and 120 minutes in distilled water (pH=?) and alkaline water (pH=1 0). The mineralogy of the samples was studied using X– ray diffractometer (XRD) [Rigaku International, Japan, D-MAX 2200V/PC] operated at 40 KV and 30 mA at a scan speed of 1°/min using CuKa. radiation (A=1.5405A).The heavy/toxic elements in jarosite were determined using Inductive Coupled Plasma-Optical Emission Spectrophotometer (ICP– OES) [Model: VISTA-MPX, Varian Make]. The leachability of heavy elements in jarosite was carried out as per ASTM method D 5233-1992, an agitated extraction test similar with Toxicity Characteristics Leaching Procedure (TCLP). In order to investigate the effect of j arosite as part/total replacement of mineral gypsum on the setting behaviour and other physical properties of ordinary Portland cement (OPC), different cement samples were prepared by using plant clinker and 19