Diffusivity of chloride and sulphate ions into mortar cubes made using ordinary portland and Portland pozzolana cements
Mutitu, Daniel Karanja
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Cement is subject to degradation by aggressive media found in the environment. Durability tests are therefore necessary for any cementitious material in a given aggressive environment. This study investigated chloride and sulphate diffusivity in mortar made from selected Kenyan cements. The test cements included three brands of Ordinary Portland Cements (OPC) and three brands of Portland Pozzolana Cements (PPC). The test cements were used to make mortar prisms at different water / cement ratios (w/c) of 0.55, 0.60 and 0.70. Compressive strength was determined at the 7th and 28th day of curing. A laboratory method on electrically induced ion transport through concrete was first developed in United States of America in the late 1970‘s. Laboratory determined migration diffusion coefficient, Dmig. obtained through this method were converted to Apparent diffusion coefficient, Dapp. These laboratory determined Dapp, values showed significant correlation with the Dapp values obtained through the natural exposure of the same aggressive media. This method takes a shorter time than the natural method of ion transport. In Kenya, a majority of cementitious structures are exposed to marine, industrial or sewerage effluents. These media contain significant levels of Cl- and SO42-. The ingress of these aggressive ions exposes the cementitious structures/buildings to great risk of failure with time. There is need therefore to subject Kenyan Portland cements to aggressive ions to test their durability. The objectives in this research were to investigate the ingress of Cl- and SO42-, their diffusion coefficients and their effect separately on compressive strength development in the OPC and PPC mortars. In this research, mortars were subjected to laboratory prepared 3.5 % by mass of NaCl solution and 3.5 % by mass of Na2SO4 solution under accelerated ion migration test method for a period of thirty six (36) hours using a 12V DC power source. The compressive strength before subjecting to aggressive media was found to increase with curing duration as well as on decreasing w/c ratio. Compressive strength for these mortars at all w/c ratios was found to increase after the ingress of the ions. Per cent gain in compressive strength with Cl- across all OPC test cements was in the range 11.374 % to 25.162 % for w/c 0.55 and 0.70 respectively. Per cent gain in compressive strength with Cl- across all PPC test cements was in the range 9.145 % to 18.874 % for w/c 0.55 and 0.70 respectively. Per cent gain in compressive strength with SO42- across all OPC test cements was in the range 7.150 % to 17.393 % for w/c ratio 0.55 and 0.70 respectively. Per cent gain in compressive strength with SO42- across all PPC test cements was in the range 4.773 % to 11.381 % for w/c 0.55 to 0.70 respectively. These results indicate that, when Cl- and SO42- separately ingress into a cementitious structure without exceeding deterioration inducing-limit levels, they are compressive strength accelerators. After subjecting the mortar cubes to the aggressive media, they were sliced into ten millimetres (10 mm) width slice, along the 100 mm length of the mortar. The cores from the slices were then analyzed for Cl- and SO42- contents. From these results, Dapp was approximated from solutions to Fick‘s 2nd law using the error function. Across all OPC test cements Cl- Dapp for mortar of w/c 0.55, 0.60 and 0.70 were in the range 4.1704 x 10-10 m2/s to 4.9017 x 10-10 m2/s. Across all PPC test cements Cl- Dapp for mortar of w/c 0.55, 0.60 and 0.70 was in the range 3.6207 x 10-10 m2/s to 4.6971 x 10-10 m2/s. Across all OPC test cements SO42- Dapp for mortar of w/c 0.50, 0.60 and 0.70 was in the range 6.3845 x 10-11 m2/s to 9.7434 x 10-11 m2/s. Across all PPC test cements SO42- Dapp for mortar of w/c 0.55, 0.60 and 0.70 was in the range 5.6260 x 10-11 m2/s to 7.1951 x 10-11 m2/s. It would therefore be recommended that in areas prone to Cl- and SO42- media, PPC with a lower w/c ratio be used for construction, as it might show better protection against concrete / mortar properties deterioration and resistance against Cl- and SO42- ingress compared to OPC.