Carbonation of Alkali-Activated Slag Mortar
Open Access
- Author:
- Ghahramani, Sara
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 21, 2017
- Committee Members:
- Aleksandra Z Radlinska, Dissertation Advisor/Co-Advisor
Aleksandra Z Radlinska, Committee Chair/Co-Chair
Farshad Rajabipour, Committee Member
Parisa Shokouhi, Committee Member
Zuleima T Karpyn, Outside Member - Keywords:
- alkali-activated slag
AAS
carbonation
non-destructive testing
NDT
shrinkage
mortar durability - Abstract:
- Sustainability is an important global issue nowadays, and it is known that production of portland cement is highly energy-intensive (embodied energy of OPC production is 5.3 MJ/kg OPC [1]) with a significant contribution to greenhouse gas emissions (0.97 ton CO2/ton OPC [1]). While the absolute value of the embodied energy of OPC appears to be small, considering the large global consumption and production of OPC (annual world production of cement is 109 tons [2]) the negative environmental impact of OPC becomes evident. Therefore, many researchers have focused on development of Portland cement substitutes to produce a binder with a lower environmental impact. Alkali-activated binders, such as alkali-activated slag or fly ash (AAS or AAF) have shown to be promising alternative materials in fitting this purpose and can provide similar mechanical and fresh properties to portland cement. Slag and fly ash are industrial by-products, and their utilization in concrete, besides the added environmental values, will result in a more durable concrete in most aggressive environments compared to ordinary portland cement (OPC) [3]. Use of alkali activated slag binder and its potential to replace portland cement requires improvement in the knowledge of the durability aspects of this material. Carbonation of alkali-activated slag concrete is known to be higher than ordinary portland cement and can influence the long-term performance (durability) of this binder. Nevertheless, there are limited numbers of research focused on this issue. Carbonation is an important consideration in concrete durability, since it can promote corrosion of embedded reinforcement and disintegration of concrete matrix. The goal of this study is to investigate the mechanism of AAS carbonation in order to provide the means needed to develop a high performance and durable AAS concrete. It was found that the effect of carbonation depends to a large extent on the type of the activator. Carbonation rate of sodium silicate-activated slag was almost twice the carbonation rate of sodium hydroxide-activated slag (sodium silicate and sodium hydroxide are two commonly used alkaline activators). Additionally, while carbonated sodium silicate activated slag lost half of its strength during carbonation, the compressive strength of sodium hydroxide activated slag did not decrease after accelerated carbonation. The results were consistent with the observations in microanalysis techniques. When sodium hydroxide was used as the activator, carbonation products (calcite crystals) formed densely in the binder. For sodium silicate activated slag, deposition of crystalline calcium carbonate due to carbonation was not significant. The results also showed that natural carbonation in air softens AAS binder and leads to a higher shrinkage for AAS paste. Utilizing an NDT method to monitor carbonation progress in OPC and AAS, it was observed that microcracks develop during carbonation of AAS binder caused by C-A-S-H decalcification. The results showed that nonlinearity of sodium hydroxide activated slag uniformly increased by 90% during carbonation. For sodium silicate activated slag nonlinearity increased monotonically by 85% until carbonation reached its half, after which, the internal damage was beyond micro- damage measurement range of the implemented NDT method. In OPC, carbonation decreased the nonlinearity by 38% overall, due to transformation of portlandite to calcium carbonate.