The degradation mechanisms of alkali-activated fly ash/slag blend cements exposed to sulphuric acid
Introduction
Geopolymer is a class of alkali aluminosilicates produced by reaction of solid aluminosilicate precursors with alkaline activator [1], and today, ‘geopolymer’ usually refers to a wide range of alkali-activated cements (AACs), including alkali-activated blast furnace slag (AAS), alkali-activated fly ash (AAFA), alkali-activated metakaolin (AAM) and products that are made from blends of these raw materials [2], [3]. Geopolymers have been extensively studied because of their unique properties, such as better acid resistance. This is a very interesting performance in comparison with ordinary Portland cement (OPC) based concretes, which are ready to degrade under acidic conditions (exposed to acidic solutions, for example, industrial waste water with low pH [4], sewer [5], [6] etc). Geopolymers have the potential to manufacture acid resistant concrete products that can be applied for treatment water tank, sewer pipe and many others under the rigid conditions.
Regarding to the mechanisms of excellent acid resistant performance of the geopolymers, there are different theories and explanations, which are sometimes conflicting. Bakharev [7] studied the compressive strength changes of AAFAs derived from various activators including sodium silicate, sodium hydroxide and sodium carbonate, and compared with OPC pastes after sulphuric acid attack. The results indicated that the acid resistance of geopolymer is better than OPC pastes. The worse performance of OPC specimens in the acidic environment should be related to the chemical decomposition with high calcium content, and larger size pores. In other words, the low calcium and less porous structure of AAFAs are accounting for their better performance. In addition, Vladimir [8] reported that the hydrochloric acid resistance of sodium hydroxide activated slag was better than OPC samples, attributing to the lower calcium content in slag comparing with OPC and the difference of the formed hydration products. It also seems that the calcium aluminosilicate (C-(A)-S-H) gel phase containing lower Ca/Si ratios and atomic ordering [9] generated by AAS results in a good performance of hydrochloric acid resistance. Lee and Lee [10] investigated the sulphuric acid resistance of the sodium silicate activated fly ash geopolymers incorporated with 10–50% slag. Their results indicated the acid resistance of Al-substituted sodium calcium silicate hydrates N-C-(A)-S-H produced by AAFA incorporated 30% slag was worse than C-S-H generated by the hydration of OPC. This could be attributed to the combination of decalcification and dealumination phenomenon for the N-C-(A)-S-H gel in acid attack. This is somehow conflicted with the above mentioned.
In an early research by Allahverdi and Skvara [11], [12], sodium silicate activated 50% fly ash/50% slag samples was immersed into nitric acid with pH equaling to 1, 2 and 3 for 60 days, and their results suggested the free calcium and sodium in the formed geopolymer network was negative to acid resistance. It was explained that “the exchange reaction of calcium and sodium with hydronium ions results in loss of them, further a degradation of gels”. In addition, dealumination could occur due to the attack of acid prone on Si-O-Al bonds, causing the changes of composition and structure of aluminosilicte network. Lloyd et al. [13] investigated some factors that affected the acid corrosion extent of geopolymers using corroded depth as an index. Their results indicated that (1) at the same soluble silicate or alkali content, increasing the alkali and soluble silicate content could decrease the corroded depth of fly ash based geopolymer; (2) with the slag content increasing, the acid resistance of the binder increased and the corroded depth of AAS was only one third in comparison to AAFA. This was due to the low permeability and porosity of the slag incorporating binder, and in addition the blocking effect of the newly formed gypsum produced during the reaction between the sulphuric ions and the calcium leached from gels [14], [15]. Bernal et al. [16] systematically investigated the performance of AAS and OPC specimens exposed to hydrochloric, nitric and sulphuric acids at pH 3.0, and acetic acid (CH3COOH) at pH 4.5. Their results indicated that compressive strength for both AAS and OPC samples all maintained well; but the resistance of AAS samples immersed in acetic acid was better than OPC due to their lower permeability, as well as the lower CaO/SiO2 ratio. However, it still remains unclear which are the important factors bringing the excellent acid anticorrosion properties, i.e. low permeability or the inherent stable nature of AACs gels under acidic conditions.
The aim of present work is to understand the acid resistance mechanisms of alkali activated fly ash/slag blend cements, which represent the most promising type of geopolymer that have been used in several demonstration constructions [17], [18], and are possibly used in a range of applications where acid is present. There are various assessment methods for the determination of acid resistance of geopolymers, such as weight loss [10], [19], change of compressive strength [7], [20], [21] and corroded depth [13], [20], [22]. These methods can provide useful information from a particular angle, but may also lead to inconsistent conclusions. This study rather than examining these ‘apparent’ properties but investigated the alternations of mineralogical/chemical composition and pore structure after acid exposure. Different corroded layers are compared to understand the degradation profile of geopolymer matrix exposed to acid. The correlation between compressive strength and pore structure and microstructure developed were discussed as well.
Section snippets
Materials
Fly ash was obtained from Huaneng Power Station (Jiangsu, China) and granulated blast furnace slag was purchased from Nangang Jiahua Co, Ltd (Jiangsu, China). The oxide compositions of the fly ash and slag, as determined by X-ray fluorescence (XRF), were given in Table 1. According to ASTM C618-08 standard, this fly ash can be identified as F Class. Table 1 also presents the physical properties of fly ash and slag. The particle size distribution of the two materials was tested by a laser
Compressive strength
The compressive strength development of the AACs samples cured at ambient condition after 28 days and after the following 28 days sulphuric acid attack is shown in Fig. 4. The compressive strength increases along with the content of slag addition and reaches the maximum at 75% slag. The strength of F70S30 after 28 days ambient curing (65.5 MPa) was almost six times as large as that of F100 (11.8 MPa). This can be explained that F70S30 samples had a lower porosity and higher reactivity in
Conclusions
In order to study the changes of the geopolymer binders produced from sodium silicate activating fly ash/slag cements after sulphuric acid corrosion, mineralogical compositions, pore structure and microstructure of the corroded and the uncorroded parts in the binders after sulphuric acid ponding test are characterized, along with the compressive strength. The influence of sulphuric acid corrosion on the mineralogical properties of 100% fly ash binder is negligible, and gypsum is identified in
Conflict of interest
The authors declared that they have no conflicts of interest to this work.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
The authors thank the financial support of the Jiangsu Higher Education Institutions for Priority Academic Program Development (PAPD). The work by Yang is supported by the National Natural Science Foundation of China (No. 51702278) and the Collaborative Innovation Center for Ecological Building Materials of Jiangsu Province (CP201506). The technical support provided by Chen Yue from the Modern Analysis and Testing Centre, Nanjing Tech University is also acknowledged. The participation of Z.
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