Effect of metakaolin and foundry sand on the near surface characteristics of concrete Rafat Siddique a,⇑ , El-Hadj Kadri b a Department of Civil Engineering, Thapar University, Patiala 147 004, Punjab, India b University of Cergy Pontoise, Rue Eragny Neuville sur Oise, Cergy Pontoise 95031, France article info Article history: Received 8 December 2010 Received in revised form 5 February 2011 Accepted 1 March 2011 Available online 25 March 2011 Keywords: Concrete Compressive strength Foundry sand Initial surface absorption Metakaolin Near surface characteristics Sorptivity Water absorption abstract This paper deals with the effect of foundry sand (FS) and metakaolin (MK) on the near surface character- istics of concrete. A control concrete having cement content 450 kg/m 3 and w/c of 0.45 was designed. Cement was replaced with three percentages (5%, 10%, and 15%) of metakaolin weight, and fine aggregates were replaced with 20% foundry sand. Tests were conducted for initial surface absorption, sorptivity, water absorption and compressive strength at the ages of 35, 56, and 84 days. Test results indicated that with the increase in MK content from 5% to 15%, there was a decrease in the initial surface absorption, decrease in the sorptivity till 10% metakaolin replacement. But at 15% MK replacement an increase in sorptivity was observed. All mixtures showed low water absorption charac- teristic i.e. less than 10%. Compressive strength shared an inverse relation with sorptivity. Higher MK replacements of 15% are not helpful in improving inner core durability, even though it helps in improving surface durability characteristics. Inclusion of foundry sand resulted in reduction in compressive strength. This aspect cements the findings that addition of FS causes permeability of concrete to increase causing in an increase in sorptivity and water absorption of concrete. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Concrete durability is one of the most important parameters in the design of structures. Concrete durability is related to (i) speci- fication of materials; (ii) design process; (iii) workmanship; (iv) environmental effects; (v) accidents; and (vi) repairs. Deterioration can occur due to either external factors or due to internal causes within the concrete itself. The various actions can be mechanical, chemical or physical. The deterioration of reinforced concrete usu- ally involves movement of aggressive gases and/or liquids from the surrounding environment into the concrete through the near-sur- face concrete, followed by physical and/or chemical actions in its internal structure. The near-surface concrete is highly heterogeneous in nature, due to the relative movement of cement paste and aggregates during the compaction of fresh concrete and bleeding of mix water during the early stages of hydration of cement. As a consequence, there ex- ists a porosity gradient in the near-surface concrete. The transport of the various aggressive substances into the concrete depends on the quality of the near-surface concrete and its characteristics. In order to reduce the porosity and make the micro-structure of the concrete dense, use of supplementary materials has been very encouraging. Various materials in this category such ash, fly ash, sil- ica fume, rice husk ash and GGBS, and metakaolin have been used in cement/mortar and concrete to enhance their durability. There has been an increased utilization of metakaolin (MK) in concrete as par- tial substitution or addition of cement. Due to its high pozzolanic activity, the inclusion of MK greatly improves the mechanical and durability properties of concrete. Many researchers have studied its effect on mechanical properties and durability of concrete. But very less research has been carried out to study its effect on the near surface characteristics of concrete, which include, initial surface absorption, sorptivity and water absorption. In the present study, influence of foundry sand and metakaolin on the near surface characteristics of concrete was carried out. Foundry sand is high quality silica sand with uniform physical characteristics. It is a by-product of ferrous and non-ferrous metal casting industries, where sand has been used for centuries as a molding material because of its thermal conductivity. Foundries successfully recycle and reuse the sand many times in a foundry. When the sand can no longer be reused in the foundry, it is re- moved from the foundry and is termed as foundry sand. Used foun- dry sand can be reused in various applications as an alternative to sending it to landfill, and reuse options are well established in Eng- land, Europe and North America. Reuse options include cement manufacture, asphalt, concrete, bricks and free-flow fill for certain construction applications. 2. Literature review Metakaolin (MK) has been known to enhance strength [1] and pore-refinement [2] at relatively early ages. A relationship between strength and sorptivity has been demonstrated by Gopolan [3] for 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.03.012 ⇑ Corresponding author. Fax: +91 175 2393005/2364498. E-mail address: siddique_66@yahoo.com (R. Siddique). Construction and Building Materials 25 (2011) 3257–3266 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat