Article pubs.acs.org/IECR
Morphological Differentiation and Crystal Growth Form of Friedel’s Salt Originated from Pozzolan and Portland Cement Rafael Talero* “Eduardo Torroja” Institute for Construction Science-CSIC, Serrano Galvache 4, 28033 Madrid, Spain
Lidia Trusilewicz Escuela Técnica Superior de Ingeniería y Diseño Industrial, Technical University of Madrid, Ronda de Valencia 3, 28012 Madrid, Spain S Supporting Information *
ABSTRACT: This paper has a scientific−technical character and deals with morphological comparison of Friedel’s salt (Fs), which is a compound that is originated from two routes: C3A phase of Portland cement (PC) and Al2O3r− (reactive alumina) of pozzolans. The reaction courses are contrasted and the role of reactive components is discussed in detail. The results show a great difference in morphology of the Fs, relative to the function of its origin, which is directly related to its very different formation rate: Friedel’s salt of rapid formation (Fs-rf) or Friedel’s salt of slow formation (Fs-lf). The reactive alumina (Al2O3r−) component of pozzolans is found to be involved in the direct, nondirect, and, above all, indirect stimulation of the C3A phase of ordinary Portland cement (OPC) saline hydration, because of its high, fast, and early pozzolanic activity. For this reason, its pozzolanic activity is justifiably classified more as specif ic than generic, intensifying the formation of Fs from the C3A origin of OPC.
1. INTRODUCTION The present work represents part of a research project that is actually underway, whose primordial objective is to prevent/ hinder the access of chlorides to steel reinforcements of reinforced concrete exposed to marine environments and/or thaw salts. If chlorides gain access to these reinforcements, the materials can suffer electrochemical corrosion with increases in the volume involved. It causes the steel reinforcement to detach from the concrete, inclusively provoking its rupture, because of the decline in mechanical strength occurs in a progressive way. Taking into special consideration that practically the entire hotel infrastructure that is maintained by the Spanish tourism industry is spread over 8000 km, important matters such as the attack of chlorides to such constructions must be addressed and, most of all, prevented. The use of pozzolanic additions in Portland cements (PCs) is a long-standing practice.1−7 In general, the pozzolanic reactivity of such materials is closely associated with the amorphous and/or vitreous nature of their reactive main components structure: reactive silica (SiO2r−) and reactive alumina (Al2O3r−).7,8 Particularly, the reactive alumina of pozzolans is defined as a fraction of aluminum oxide (Al2O3r−) of tetra-coordinated or penta-coordinated aluminum organized in a χ-alumina-like structure, as reported by Trusilewicz et al.9 Both components are capable of reacting with PC hydration products (portlandite or calcium hydroxide solely in this very case) to give stable compounds of hydraulic properties. The SiO2r− gives rise to a C−S−H gel, subsequent transformed into tobermorites, and silanol groups (Si−OH) that were later converted to hydrated silicic acid, reacting with Ca2+ and OH− ions, respectively. In turn, Al2O3r− can form several hydrated calcium aluminates, © 2012 American Chemical Society
particularly Stratling’s compound and, if sulfates are present, ettringite10−15 and/or AFm phase16−18 is formed, but if chlorides are present, Friedel’s salt (Fs) is precipitated.19 Detailed studies about Fs have been published.20−22 On the other hand, Talero et al.19 have shown that pozzolanic additions also form Fs when exposed to chloride solution (3.75% NaCl), like the C3A phase from ordinary Portland cement (OPC) origin. This study, which occurred previous to the present one, has dealt with chloride attack on PC−pozzolanic additions blends and, more specifically, with the kinetic differentiation of Fs formed from these two different origins. The XRD comparative and semiquantitative analysis has shown that the formation rate for Fs from reactive alumina (Al2O3r−) origin present in pozzolans, must be higher than the formation rate for Fs originating from C3A origin present in OPC. Furthermore, Talero et al. have also demonstrated that, in the formation of Fs, the topochemical (TPQ) mechanism with prior dissolution must prevail over the through-solution (TS) mechanism. Nevertheless, disagreement between researchers about the effect caused by pozzolans on the kinetic chemistry of Fs has converted this new study into a definitive behavioral hypothesis, thanks to the new and additional reasons and proofs stated below. Therefore, given the lack of consensus on this issue, the previous XRD study was again used in the current study, in conjunction with SEM-EDS analysis. Moreover, a new line of reasoning was deployed in order to prove finally that the Received: Revised: Accepted: Published: 12517
March 26, 2012 August 3, 2012 August 15, 2012 August 15, 2012 dx.doi.org/10.1021/ie301671z | Ind. Eng. Chem. Res. 2012, 51, 12517−12529
Industrial & Engineering Chemistry Research
Article
determined using the Bogue formulas,32 which gave the following values: For P1 (OPC): 51% C3S, 16% C2S, 14% C3A, and 5% C4AF For PY6 (SRPC): 79% C3S, 2% C2S, ∼0% C3A, and 10% C4AF On the other hand, the reactive silica (SiO2r−) and alumina (Al2O3r−) contents of the pozzolanic additions (SF, D, A, C, M1, M0), determined gravimetrically according to UNE Standard 80-225-9333 and the Florentin method,8 respectively, are as follows: 88.5, 89.0, 37.5, 39.0, 38.5, and 48.5 wt %, and 0.0, 0.0, 8.0, 11.5, 15.0, and 29.0 wt %, respectively. In addition, the Strength Activity Index (IP) after 28 days of mortar curing was achieved according to ASTM Standard C 311.26 The results are present in Figure 1. As it can be observed,
formation rate of Fs from Al2O3r− origin present in pozzolans is greater than the formation rate of Fs from C3A origin present in OPC. The new approach is based especially on the Fs microstructure observations and demonstrates the imperfections and/or smaller size of the Fs of rapid formation (Fs-rf) crystals. In addition, the respective inter-relation or interconnection during the formation process also is discussed.
2. OBJECTIVES The objectives sought in this research were, primarily, as follows: First of all, to verify conclusively if the formation rate of Friedel’s salt (Fs) from Al2O3r− origin of pozzolans (or Fs-rf) is greater than the formation rate of Fs from C3A origin of OPC (or Fs-lf) and then to prove it by SEMEDS analysis. Second, to bear out that the Fs crystals from Al2O3r− origin are more imperfect and/or smaller than Fs crystals from C3A origin, and to support it by statistical considerations. This objective is a consequence of the first one. Third, to determine the similarities and/or differences between the morphology of the Fs crystals from both Al2O3r− and C3A origins, as well as point out their interdependent formation. 3. EXPERIMENTAL SECTION 3.1. Materials and Methodology. 3.1.1. Sample Preparation. The experimental part of this new approach have been described in detail by Talero et al. in the preceding, alreadypublished work.19 Two PCs were selected, both with opposite mineralogical composition: PC P1 (with 14% C3A) and PC PY6 (with ∼0% C3A). Different blends or POZC were elaborated with a following gamut of pozzolanic additions (Z pozzolan), siliceous or siliceous and aluminous in nature, according to ASTM Standard C 618-95a:23 artificial and vitreous type (silica fume, SF), natural and vitreous of organic origin (diatomite, D), two Spanish natural pozzolans (A (Ciudad Real) and C (Islas Canarias)), and an activated clay (metakaolin) doped with 50% quartz and undoped (denoted as metakaolins M1 and M0, respectively). Distilled water, which is used as mixing water for the preparation of pure and blended cement pastes, was applied. The 100/00 ratio denotes a pure PC. The samples were immersed into 3.75% NaCl solution (saline hydration) up to different ages: 1, 7, 14, 21, 28, 60, 90, 270, and 730 days. 3.1.2. Cement Chemistry Procedures Applied to Raw Materials. The standard procedures applied for physicochemical description of the raw materials, both PC and Z pozzolans, respond to the requirements of EN Standard 197-1,24 which protects and includes all the building materials used in European Union. This, especially and particularly, was of great importance in the present research during the materials selection (mainly in the choice of pozzolanic additions). Hence, the selected physicochemical parameters of the materials, PCs and pozzolans, obtained using standard procedures EN Standard 196-225 and ASTM Standard C 311,26 respectively, can be found in ref 19, as well as the rest of the physicochemical parameters, including grinding soundness,27 real density,28 setting times,29 mechanical strengths,30 and pozzolanicity (as determined using the Frattini test31). According to these results, the potential compositions of the PCs were
Figure 1. Strength Activity Index (IP) of the selected pozzolans, by ASTM Standard C311.
the IP values for D and C pozzolans are situated significantly below 75%, which is set as a lower limit required by ASTM Standard C 618-95a for the pozzolanic materials.23 The same results can be seen for A pozzolan, but with different formers. The reason for D pozzolan can be seen later, and the reason for C pozzolan is its high Na2Oeq content, 9.04% (ASTM Standard C 150 requires a Na2O content of max. 0.60% for low-alkali cements). The best achievements are noticed for silica fume (SF) and metakaolins (M0, M1), which lightly surpass the required limit. According to the water demand of the PC blended mortars (Figure 1), it remains on an equal level for all of the pozzolans, except for diatomite (D), being a direct consequence of its frustules-like morphology that provokes the absorption of water molecules into the interior of its empty, perforated sheaths. 3.1.3. Saline Hydration and Data Collection. The basis for the data collection was the XRD results obtained for the PC/Z blends samples subjected to the presence of Cl− ions (3.75% NaCl solution) up to different ages: 1, 7, 14, 21, 28, 60, 90, 270, and 730 days. In order to achieve the technical−scientific purposes of this articlethe technical aspects most of allthe XRD data were 12518
dx.doi.org/10.1021/ie301671z | Ind. Eng. Chem. Res. 2012, 51, 12517−12529
Industrial & Engineering Chemistry Research
Article
Table 1. Classification of the Pozzolans in Function of Fs Amount Precipitated in the Blends (Starting with Minimum Value): Family Series 1 age (days)
PC P1/pozzolan “Z” and PC PY6/pozzolan “Z” blend series
1 7 14 21 28 60 90 270 730
PY6/SF = PY6/D = P1/SF = P1/D = P1/A < PY6/A < PY6/C < P1 < PY6/M1 < P1/C < PY6 < P1/M1 < PY6/M0 < P1/M0 PY6/SF = PY6/D = P1/SF < PY6/M1 < PY6/A < PY6/C < PY6 < P1/D < P1 < P1/A < P1/C < PY6/M0 < P1/M1 < P1/M0 PY6/SF = PY6/D < PY6/A < PY6 < P1/D < PY6/C < P1/SF < P1/A< PY6/M1 < P1 < PY6/M0 < P1/C < P1/M1 < P1/M0 PY6/SF = PY6/D < PY6 < PY6/A < P1/A < PY6/C < P1/SF < P1/D < P1 < P1/C < PY6/M1 < P1/M1 < PY6/M0 < P1/M0 PY6/SF < PY6/D < PY6 < PY6/A < P1/D < PY6/C < P1/SF < P1/A < P1 < P1/C < PY6/M1 < PY6/M0 < P1/M1 < P1/M0 PY6/SF < PY6/D < PY6 < PY6/A < P1/D < PY6/C < P1/SF < P1/A < P1 < PY6/M1 < P1/C < PY6/M0 < P1/M1 < P1/M0 PY6/SF < PY6/D < PY6 < PY6/A < PY6/C < P1/SF < P1/D < P1/A < P1 < PY6/M1 < PY6/M0 < P1/C < P1/M1 < P1/M0 PY6/D < PY6/SF < PY6/A < PY6/C < PY6 < P1/A < P1/SF < P1/D < PY6/M1 < P1 < PY6/M0 < P1/C < P1/M1 < P1/M0 PY6/SF < PY6/D < PY6/A < PY6 < PY6/C < P1/SF < P1/D < P1/A < P1 < PY6/M1 < P1/C < PY6/M0 < P1/M1 < P1/M0
classification of the pozzolans, as a function of Al2O3r− content (%)
SF = D < A < C < M1 < M0 0%
0%
8.0%
11.5%
15.0%
29.0%
Table 2. Classification of the Pozzolans, as a Function of the Amount of Fs Precipitated in the Blends (Starting with Minimum Value): Family Series 2 and 3 age (days)
PC P1•/pozzolan “Z” blend series
PC PY6*/pozzolan “Z” blend series
•
•
1
* M1 < M0 SF = D = A < C
