Deterioration of basalt-based reinforcement in corrosive environments: experimental monitoring and mathematical models

The basalt-based reinforcing elements are considered as alternative to conventional steel-based reinforcing elements (rebars, wires). The motivation to use basalt-based elements (fibers, composite bars, meshes etc.) is better corrosion resistance of basalt fibers especially in sea-water environment, compared to carbon steel. Nevertheless, it does not mean that basalt fibers are 100% corrosion resistant. The basalt fibers are produced from silicate melt of proper composition, i.e. the basalt fibers are vulnerable to both acid and alkaline hydrolysis, as well as other silicates do. When basalt fibers are used as reinforcement in concrete, the alkaline hydrolysis will become an important issue. The present paper deals with experimental observation of basalt fibers in alkaline environment of Simulated Pore Solution. The fibers deterioration was monitored by their mass loss and SEM microscopy. Jander’s model was used to describe mathematically the kinetics of the fibers alkaline hydrolysis. The results revealed that a corrosion products layer is formed on the fibers surface in this environment. The composition of this layer corresponds to N-A-S-H and C-A-S-H phases known from alkali-activated aluminosilicates or hydrated Portland cement.


Introduction
Basalt fibers are used, among other applications, also as reinforcement element of concrete structures; individual fibers may be used as dispersed reinforcement, or their ropes can be part of Basalt Fiber Reinforced Polymer (BFRP) bars [1].Basalt fiber are produced from molten siliceous rock (not necessarily only from "real basalt") by technology derived from glass fibers production [2].The most important parameter of the mineral batch to be used for basalt fibers (BF) production is its modulus of acidity Ma (see equation.(1)) which controls the melting point and viscosity.The common range of batch and fibers composition is provided in table 1.

(
) ( ) ( ) ( ) Igneous rocks, including basalt, are giving the impression of being a rigid and stable matter, but in fact, basalt is taking part in several (geo)chemical processes.First of all, one has to realize that basalt may be composed from a range of silicate minerals (in a range of proportions); the most abundant components are plagioclase (containing Ca-Na feldspars) and clinopyroxene (e.g.diopside), while feldspathoid (e.g.nepheline or analcime), olivine and quartz may be present as well.Besides the crystalline minerals, basalt usually contains also a glassy phase, due to rapid solidification of the initial magma.The produced basalt fibers are, due to rapid cooling, fully amorphous.Nevertheless, the chemical corrosion of basalt fibers may be inspired by geochemical studies performed with intention to understand the basalt rock weathering, important step of geology cycles.Oelkers and Gislason [3] described the basaltic glass hydrolysis by a three-step mechanism.The mechanism is valid for both acid, neutral or alkaline pH, but in the following, the equations are written in form relevant for alkaline environment since it is the most likely situation in case of basalt elements used as reinforcement in concrete.
The entire process of interaction of basalt (or another silicate material) with alkaline media can be called "alkaline hydrolysis".The first step (equation ( 2)) may be understood as release of univalent or divalent cation M n+ (i.e.Na + , K + , Ca 2+ , Mg 2+ ) from the solid rock matrix to the solution; the solid state is in the following marked by lower index s.The vacant position in the solid is filled by hydrogen cation H + .In the second step (see equation ( 3)), the Aluminum atoms in the rock are replaced by hydrogen while Al(OH) 4-anions are generated into the solution.Finally, in the 3 rd step, SiO4 tetrahedrons are evolved in form of (SiO4) 4-silicate anions as shown in equation ( 4).
( ) The equations ( 2)-( 4) represent fundamental mechanisms of an aluminosilicate dissolution but do not describe the rate of the dissolution process.Gislason and Oelkers in the subsequent work [4] measured experimentally the basalt glass dissolution rate and concluded that: the first step (equation ( 2)) is very fast while the Si detachment being relatively slow.They proposed kinetic model for "far from equilibrium" conditions and for pH range 1-11, where the dissolution rate is controlled by activity of H + (i.e. by pH) and Al 3+ in the corrosive solution.Nevertheless, this model seems to be not applicable in conditions prevailing when BF are part of concrete.Since the corrosion study of fibers embedded in concrete is experimentally difficult, most of scholars work with fibers in a corroding solution (alkaline, acid, salt etc.).Here, the fast depletion of glass modifying elements (cations) was observed as well [5], but the presence of Ca 2+ cations in corroding solution (inevitable in concrete pore solution) is changing the corrosion mechanism compared to alkaline environment based just on Na + species [6].Formation of a kind of protective layer was reported [7].This layer was identified as C-S-H hydration products by Wang et al. [8], who compared corrosion mechanism of BF in NaOH and "hydrating cement solution" (i.e.water saturated by Ca(OH)2).The outer diffusion of OH -from solution, as well as the surface reaction (alkaline etching) were both identified as rate controlling steps.The experimental kinetic results were modelled by zero-order and contracting cylinder model.Förster et al. [9] studied dissolution of BF by means of analysis of the surface layers by EDX and XPS; they concluded that dissolution kinetics depends on the fibers composition.
So far, the corrosion of basalt fibers was studied mainly experimentally [1].Nevertheless, the need to predict the lifetime of concrete structures reinforced by range of elements based on BF implies the necessity to model the corrosionand related mechanical propertiesin time [10].The present paper aims to introduce the possibility to describe the BF corrosion by Jander's equation, known from modelling of cementing binder systems [11].

Experimental
The experimental program was based on dissolution of chopped basalt fibers of chemical composition given in table 1.When the composition is compared with "typical ranges" published in [1] one observes that studied BF are very rich in Al2O3 and MgO and belong to medium-acid fibers.The dissolution experiment was performed in "simulated pore solution" which intended to imitate the environment prevailing in concrete.It was prepared by dissolution of 2.6 g of Ca(OH)2, 3.4 g of KOH and 8.3 g NaOH in 1 L of deionized water. 1 g of fibers was contacted with 100 mL of the solution in orbital shaker at 25 °C at 120 rpm.After the given time fibers were filtered out, rinsed with water and dried.The mass loss was the principal measured value.The 28-days fibers were analyzed also by SEM microscope Phenom XL.

Results and Discussion
The mass loss (ML) of fibers was observed for 28 days (672 hours).In sake of numerical treatment, the was expressed by means of degree of conversion α given by Eq. ( 5), where m0 stands for initial mass of BF sample and mt stands for mass in the respective time.( The dissolution experiment results (figure 1) show that the BF conversion is the fastest in the beginning and gradually slows down.Obviously, the conditions prevailing in the dissolution test are not comparable with those in real concrete, but the chosen experimental arrangement enable to describe the corrosion of fibers itself, without limits given by external transport of corrosive environment (or its corrosive species) to the vicinity of a fiber surface.The experimentally obtained dissolution data were fitted by help of Jander's model (equation 6) [11,12], where k [day -1 ] is a rate constant and N [-] order of reaction.This model was developed for system where a solid reactant reacts with another component.The value of N correspond to the rate determining step; for N < 1, the rate of process is controlled by dissolution of reactant or nucleation of new (solid) product.For N > 1, the rate is controlled by diffusion of reactants through the layer of reaction product(s).The calculation of reaction parameters k and N is done be means of linearized form of the model (equation 7).The parameters obtained for the experimental data were k = 8.7×10 -11 day -1 and N = 4.It indicates the reaction control by diffusion through a reaction product layer.
( ) ( ) The corroded basalt fibers were examined by SEM microscopy (figure 2).The "as received" BF (a) are smooth while the corroded fibers (b) are covered by layer of corrosion products.The EDS analysis revealed that the corroded layer iscompared to the bulk of fibersenriched on Ca and Na; it might indicate that the surface of fibers is covered by reaction products corresponding to N-A-S-H and C-A-S-H, i.e. those which are obtained in alkaline activation of basaltic fibers (as waste mineral wool) [13].It is in accordance with assumption that basalt fibers are undergoing the alkaline hydrolysis which deliberates silicate anions (Equation 3) and Al(OH) 4-species (equation 2) which are subsequently available for polycondensation to the above mentioned N-A-S-H and C-A-S-H structures.This mechanism corresponds not only to alkaline activation but also to "more common" pozzolanic reaction which occurs also in the interface between basalt powder and cement paste [14].

Conclusions
The corrosion of commercial basalt fibers was studied by means of their dissolution in "simulated pore solution" for 28 days.The progress of dissolution was monitored by help of their mass loss.The obtained experimental data were fitted by Jander's equation, whose parameters (order of reaction and rate constant) were calculated.The value of reaction order indicates that the rate determining step is diffusion of corrosive environment through the layer of corrosion product formed on the surface of fibers.The nature of this corrosion layer is probably, assumed by help of SEM, similar to the products of aluminosilicate alkaline activation.It would correspond also to mechanism of fibers hydrolysis where (SiO4) 4-and Al 3+ are detached from the fiber and may undergo polycondensation in the alkaline environment.

Figure 1 .
Figure 1.Degree of dissolution of basalt fibers in "simulated pore solution" and the model based on Jander's equation.

Figure 2 .
Figure 2. SEM micrographs of basalt fibers "as received" (a) and after 28 days of dissolution in "simulated pore solution" (b).