Influence of the initial moisture content on the carbonation degree and performance of fiber-cement composites

Abstract

The objective of this work is to show the effect of the initial moisture content of fiber-cement composites on their carbonation degree (CD), mechanical and physical properties after their exposition to CO2 atmosphere, and after accelerated ageing cycles. Fiber-cement composites produced by Hatschek process and submitted to carbonation at early age for 10 h of exposition to 15% of CO2. Part of the composites was previously submitted to different drying times (20, 40, 60 and 120 min). The CD was determined using thermogravimetric measurements. Mechanical (flexural) and physical properties were determined at 10 days of cure and after 200 soak and dry ageing cycles. As expected, as higher as the time of drying, the higher was the removing of moisture from the composites. Accelerated carbonation caused the decrease of Aft, C-S-H, Afm and Ca(OH)2; and increased the content of CaCO3. The higher time of drying (120 min) led to the lower content of Ca(OH)2 and the higher content of CaCO3. Fiber-cement composites without drying and with 40 min of drying presented moisture between 19% and 21% and led to average CD between 28 and 39%; while those fiber-cements dried for 120 min (14% of moisture) presented an average CD of 44%. Higher CDs led to the significant decrease of water absorption (WA) and apparent porosity (AP) of the composites, while caused the increase of bulk density (BD) of the fiber-cement composites. LOP and MOE values increased with carbonation, while they were less affected by the previous drying before carbonation. 200 ageing cycles caused increase of LOP, MOR, MOE and BD of all the composites, while decreased toughness, WA and AP. Therefore, the control of the initial moisture and free pores of the fiber-cement composites seems to be a strategy to improve the efficiency of the accelerated carbonation of fiber-cement composites.