Microstructure of cement paste subject to early carbonation curing Vahid Rostami a , Yixin Shao a, ⁎, Andrew J. Boyd a , Zhen He b a Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 b School of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, China abstract article info Article history: Received 16 July 2011 Accepted 21 September 2011 Keywords: Carbonation (C) Microstructure (B) Curing (A) Compressive strength (C) Calcium–Silicate–Hydrate (B) Microstructure of Ordinary Portland Cement paste subjected to early age carbonation curing was studied to examine the effect of early carbonation on performance of paste at different ages. The study was intended to understand the mechanism of concrete carbonation at early age through the microstructure development of its cement paste. Early carbonation was carried out after 18-hour initial controlled air curing. The micro- structure characterized by XRD, TGA, 29 Si NMR and SEM was correlated to strength gain, CO 2 uptake and pH change. It was found that early carbonation could accelerate early strength while allowing subsequent hydration. The short term carbonation created a microstructure with more strength-contributing solids than conventional hydration. Calcium hydroxide was converted to calcium carbonates, and calcium–silicate–hydrate became inter- mingled with carbonates, generating an amorphous calcium–silicate–hydrocarbonate binding phase. Carbon- ation modified C–S–H retained its original gel structure. The re-hydration procedure applied after carbonation was essential in increasing late strength and durability. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Concrete is well known to be reactive with carbon dioxide. When this carbonation reaction occurs in freshly cast cement and concrete, the process has been shown to offer improved mechanical [1–3] and durability [4] properties. It is believed that carbonation of fresh dical- cium silicate (C 2 S) and triclacium silicate (C 3 S) pastes is an accelerated curing process, with the governing reactions shown by Eqs. (1)–(2). The carbonation reaction products are a hybrid of calcium–silicate–hydrate (C–S–H) and calcium carbonate (CaCO 3 ). High early strength can be obtained within a few minutes to a few hours [1, 3]. C 3 S þ 3-x ð ÞCO 2 þ yH 2 O→C x SH y þ 3-x ð ÞCaCO 3 ð1Þ C 2 S þ 2-x ð ÞCO 2 þ yH 2 O→C x SH y þ 2-x ð ÞCaCO 3 ð2Þ The carbonation reaction could also occur in mature concrete dur- ing service (Eqs. (3)–(4)). Atmospheric carbon dioxide reacts with hydration products, such as calcium hydroxide (CH) and calcium sil- icate hydrate (C–S–H), forming CaCO 3 and silica gel: Ca OH ð Þ 2 þ CO 2 →CaCO 3 þ H 2 O ð3Þ C–S–H þ 2CO 2 →SiO 2 þ 2CaCO 3 þ H 2 O ð4Þ Extensive investigations into weathering carbonation of hydration products have suggested that the reactions of CH and C–S–H with car- bon dioxide are dominant in atmospheric carbonation [5, 6]. It was found that atmospheric carbonation of well hydrated tricalcium sili- cate for over two years has led to complete carbonation and decalci- fication of C–S–H and formation of silica gel and partial carbonation of CH [5]. A 100-day carbonation of 28-day hydrated Ordinary Port- land Cement (OPC) using natural weathering carbonation lowered the Ca/Si of the C–S–H. Carbonation of the same duration performed on the same hydrated OPC paste using 10% and 100% pure CO 2 , decal- cified the C–S–H completely [6]. Carbonation of mature concrete exposed to atmospheric carbon dioxide is not a desired chemical reaction. It generates carbonation shrinkage and reduces the pH value of the pore solution in concrete, leading to restrained concrete cracking and carbonation-induced steel corrosion [7]. However carbonation at early ages can be benefi- cial, acting as an accelerated curing technique [1, 3] that can consider- ably improve the durability performance of concrete due to the elimination of CH [4]. A recent study showed that two-hour carbon- ation curing of concrete after 18 h of pre-setting can effectively re- place steam curing for precast concrete production [8]. Concrete carbonated on this manner has comparable strength to steam-cured concrete and exhibits an enhanced resistance to permeation, sulfate attack, and freeze–thaw damage. Subsequent hydration after early carbonation contributes significantly to late strength gain and main- tains concrete alkalinity above the threshold value. The early carbon- ation of precast concrete could reduce the carbonation shrinkage of assembled concrete structures in service [9]. In addition to the technical advantages, early carbonation curing is a CO 2 sequestration process that Cement and Concrete Research 42 (2012) 186–193 ⁎ Corresponding author. Tel.: + 1 514 398 6674; fax: + 1 514 398 7361. E-mail address: yixin.shao@mcgill.ca (Y. Shao). 0008-8846/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2011.09.010 Contents lists available at SciVerse ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp