Technology profile No. 1: Blended cements*

* Submitted by the Central Building Research Institute (CBRI), Roorkee, India.

Due to rising fuel prices, energy shortages and fast depleting natural resources, the manufacture of blended cements using materials with latent hydraulicity and intrinsic energy value offers good scope for increasing cement output with minimum inputs.

The Central Building Research Institute (CBRI) has carried out extensive Research and Development work over the past three decades on the production of blended cements using industrial wastes such as blast furnace slags, fly ashes, by-product gypsum, lime sludges etc. which are available in India. Over 60 per cent of the total cement produced at present in the country, consists of blended cements such as Portland pozzolana cement (PPC), Portland slag cement (PSC), super sulphated cement (SSC), pozzolana mixtures and masonry cement.

Blended cements are ordinary Portland cement (OPC) with which pozzolanic materials such as calcined clays, shales or fly-ash are blended by intergrinding with a Portland cement clinker and gypsum. A thorough mix is obtained by intergrinding, but, except for this, the same effects can be obtained by adding the pozzolana separately subsequent to grinding. The pozzolana content in PPC can range from 10 to 25 per cent by weight. PPC is recommended for use in place of ordinary Portland cement (OPC) in all types of construction works particularly in low rise and low-cost housing construction. It is common experience that as compared to ordinary Portland cement (OPC), the PPC has a longer setting time and slower rate of strength development. It has also been observed that to obtain a similar grade of concrete, the quantity of PPC required is more than that of OPC. To overcome these deficiencies in the use of PPC vis-a-vis OPC, the use of 0.75 per cent by weight of admixture R(Na₂SO₄ + Na₂CO₃) was found effective in enhancing the rate of strength development in PPC containing 15 per cent fly-ash or 20 per cent calcined clay in the production of precast concrete building units.

Portland slag cement (PSC) is made by intergrinding Portland cement clinker, granulated blast furnace slag and gypsum, the proportion of slag not exceeding 65 per cent by weight of the mixture. Hydrated lime released from the hydration of Portland cement combines with the granulated slag and acts as starter. However, the further hydration of the slag is direct and does not depend on the combination with lime. PSC is rather similar to OPC requirements for fineness, setting time, soundness and strength. But in actual practice, the fineness of PSC is kept higher. Work done at CBRI shows that PSC can be successfully produced using high manganese slags (MnO up to 6.75 percent) and Portland cement clinker in the proportion of 50:50 with four per cent gypsum by weight without exhibiting unsoundness.

Super sulphated cement (SSC) is a sulphate-resisting hydraulic cement produced by intergrinding or thoroughly blending a mixture of ground granulated slag (80-85 per cent) gypsum anhydrite (10-15 per cent) with a small quantity of cement clinker or Portland cement (2.5 per cent) which act as activators. Work done at CBRI has shown that SSC can be produced using 70 per cent slag, 20 per cent phosphogypsum anhydrate and 10 per cent by weight cement clinker. For producing one ton of SSC, it has been estimated that heat consumption has to be 900 MJ against 5860 and 3350 MJ for the Portland cement clinker produced by wet and dry processes respectively.

It has also been observed that the blended cements PPC and PSC can be used wherever OPC is usable under normal conditions. These cements produce less heat of hydration and offer greater resistance to sulphate attack than OPC. However, under marine and highly industrial environments, extra measures such as increase in cement content and/or increase of the cover thickness of reinforcement bars are suggested to overcome enhanced susceptibility to corrosion of steel reinforcement. SSC possesses greater resistance to aggressive conditions and is recommended for use in marine works, concrete construction in sulphate bearing soils, concrete sewers carrying industrial effluents and in conditions involving exposure to high concentration of sulphates and weak solutions of mineral acids (PH value down to 3.5). PSC and SSC cements, also possess good sulphate resistance and can be used in construction in situations where sulphate attack is likely. Lime pozzolana mixtures and masonry cement have been found suitable for use in place of ordinary Portland cement, in masonry mortars and plasters for simple construction works.