Aligning with the global developments, Sri Lanka Standards Institution (SLSI) revised two available SLS standards, namely SLS107 for Ordinary Portland Cement (OPC) and SLS515 for Masonry Cement in 2002 and 2003, respectively. Then, with the European Union’s introduction of EN197-1:2000 for the new category of cement called "Common Cements" in 2000, the SLSI considered it opportune to develop new Sri Lankan standards aligning with this latest international standard. Consequently, SLSI introduced in 2003 two SLS standards for two categories of "Common Cements":
* SLS1247:2003 for Blended Hydraulic Cements
* SLS1253:2003 for Portland Limestone Cement
Both these cements - also called "composite cements" - are highly recommended for concrete applications although they can also be used as general-purpose cements. When used for concrete, these cements improve both fresh state properties such as workability and hardened state properties such as long-term strength and durability.
SLS1247:2003 for Blended
Hydraulic Cements
A blended hydraulic cement is a blend of Ordinary Portland Cement (OPC) and pozzolanic material (such as fly ash) / blast furnace slag. It is manufactured either by intergrinding Portland cement clinker, pozzolanic material/slag and gypsum or by blending pozzolanic material/slag with Ordinary Portland Cement.
This cement is of two types, depending on the main constituent used for blending with Portland cement or Portland cement clinker. The main constituents of Type I (Portland pozzolanic cement) are clinker (65 to 94% by mass) and pozzolanic material (6 to 35%). The pozzolanic materials allowed by the standard are natural pozzolana, paddy husk ash, fly ash and burnt shale. The main constituents of Type ll (Portland slag cement) are clinker (65 to 94%) and blast furnace slag (6 to 35%).
SLS1253:2003 for Portland
Limestone Cement
Portland Limestone Cement is manufactured either by intergrinding Portland cement clinker with limestone or by blending finely ground limestone with OPC. The main constituents of Portland Limestone Cement are clinker (85 to 94% by mass) and limestone (6 to 15%). The calcium carbonate content of the limestone should be greater than 75%.
Specification parameters and their effect on cement quality
The specification parameters given in the two standards (i.e., SLS1247:2003 and SLS1253:2003), their effect on cement quality and the values specified in the two standards are given below:
Compressive strength
Compressive strength at the age of 3 days is important with respect to pre-cast applications and early removal of moulds (formwork). Compressive strength at 28 days is important with respect to safety, economy and durability of the structure. Both standards specify that compressive strength at 3 days should be greater than 20MPa and that at 28 days should be between 40 and 62.5MPa. These requirements are identical to the values given in SLS107:2001, the SLS standard for OPC.
Setting time
Initial setting time allows adequate time to work with mortar or concrete while final setting time avoids abnormal retardation. Both standards require that the initial setting time be greater than 45 minutes and the final setting time be less than 10 hours.
Soundness
Two tests on soundness - Le Chatelier and autoclave tests - ensure the limited presence of MgO and free CaO, which cause unsoundness in cement. The hydroxides they produce by slow reaction with water cause expansion in the hardened cement paste, resulting in its disruption. Both standards specify that the Le Chatelier value be less than or equal to 10mm and the autoclave value be less than 0.8%.
Chemical requirements
Presence of SO3 and chlorides should be limited as they contribute to late ettringite formation and corrosion of steel reinforcement respectively. Both standards specify that the SO3 content be less than 3.5% and chloride content be less than 0.1%.
Fineness
Fineness of cement particles affects strength development and workability. SLS1247:2003 does not specify fineness while SLS1253:2003 specifies that the fineness value be greater than 3300 square centimeters per kilogram.
Loss on ignition
Loss on ignition indicates the presence of CaCO3, CaSO4 and water, enabling the checking of the percentage addition of calcareous main or minor constituents and aeration of cement. SLS1247:2003 does not specify loss on ignition while SLS1253:2003 specifies that loss on ignition be less than 7.5%. Loss on ignition is however not an application parameter.
Good for the construction industry
Now that the SLS standards on composite cements are in place, the Sri Lankan construction industry can reap the benefits offered by these new special application cements by way of enhanced concrete construction.
