STRUCTURE FORMATION OF HARDENING CEMENT PASTES AT FREEZING

Transcription

1 STRUCTURE FORMATION OF HARDENING CEMENT PASTES AT FREEZING M. Sanitsky, H. Sobol, U. Marushchak National University Lviv Polytechnic, Ukraine Abstract Peculiarities of frost influence taking place in clinker minerals and Portland cement gel pastes are investigated. Relationship between the crystal structure clinker minerals, character of the ice sublimation and H 2 O and СО 2 ablimation of the air, strength kinetic, degree of hydration has been established. The influence of the storage time and new kind of anti-freeze admixtures on the temperature of ice formation beginning and extension strain of mortars is given. Keywords: cement minerals, ablimation and sublimation, cement pastes, ice formation, hardening at freezing, expansive deformation, low temperature dilatometry. 1 Introduction As it well established that the freezing temperature of pore water in cement pastes, mortars and concretes can be considerable lower than 0 C as one portion of water is an alkali solution, while another one is physically bound water in the nano-structure (water in gel structure of hydration phases and microcapillaries) [1, 2]. In freshly freezed concrete greater portion of water is situated in macropores in free state and converts into ice at the temperatures of C. As a result, irreversible concrete destruction, which cannot be eliminated during thawing, takes place. Fresh freezed concrete strength, which is hardening for several day on frost and then up to 28-day under normal conditions is 30-40% lower than graded, whereas its durability and water proofness diminishes dozens of times. The most dangerous thing for a concrete is its freezing period which coincudes with the final setting of cement. Upon reaching critical (minimum necessary) strength 5 MPa, the possibility to resist the destructive influence of negative temperatures becomes possible [3]. At monolithic concreting in winter period deicing admixtures are widely used for regulation of concrete mixture and concrete properties. The peculiarities of processes of 37

2 cement pastes structure formation in the presence of deicing admixtures with allowance for of destructive action gypsum dihydrate have been established by authors [4]. Due to the effect of alkali-bearing compounds on the time of ordinary Portland cement setting they can be divided into two groups. The first group consists sodium salts such as chloride, nitrate, nitrite not affecting strongly on time of setting. The second group consists sodium carbonate and silicate and potassium salts. Sodium rodanide belongs to the new types deicing salts, which do not contain chlorine. In this paper peculiarities of cement pastes structure formation processes at deep freezing and in the presence of deicing admixtures are described. 2 Materials and Methods of Investigation Ordinary Portland cement and clinker minerals (C 3 S, β-c 2 S, C 3 A, C 6 A 2 F, C 4 AF, C 6 AF 2 ) were used. New kind of alkali-containing anti-freeze admixtures as NaCNS and Na 2 S 2 O 3 were included in cement compositions. A number of physicochemical analysis (methods X-ray diffractometry, IR-spectroscopic and electron microscopy) were used for investigation of hydration processes of cement compositions. Physical-mechanical tests of cements and mortars were carried out according to usual procedures. The temperature of beginning of ice formation and cement-sand mortar extension strain was determined by the method of low-temperature dilatometry. To do this special cylinder-shaped form was used, consisting of separate rings allowing fresh mortar to deform freely to one direction. The investigation was carried out in cooling chamber with the rate of cooling 20 C/hour down to the temperature -30 C. 3 Results and Discussion It has been established that partial recrystallization of high sulphate form of calcium hydrosulphoaluminate into low sulphate form takes place at cooling concrete but phase composition of other hydrates doesn t practically change. Under freezing, the main changes in hardened cement paste are caused by crystallization of ice and its modification transformation. This leads to the considerable porosity growth of the hardened cement paste. According to the kinetics of strength increase on the frost Portland cement clinker minerals can be subdivided into two groups (table 1). The first group includes calcium silicates, which harden on frost slowly, the second group includes C 3 A and calcium alumoferrite, hardening of which increases quick enough. Thus, at cooling silicates hardening slows down sharply, while the minerals of the second group are not so greatly influenced by negative temperatures (up to -15 C). Moreover, some of them (C 3 A, 38

3 C 6 A 2 F) even increase their strength. Likewise kinetics of monomineral pastes strength hydration degree changes with the lowering of temperature (table 2). In 1-day age of hardening at -15 C, hydration degree of C 3 S decreases five times in comparison with normal conditions; while in C 3 A and calcium alumoferrites its change is not substantial. Table 1: Influence of hardening temperature on monomineral pastes compressive strength Mineral W/C Hardening temperature, Compressive strength of monomineral pastes, MPa, in age, days С * С 3 S 0, ,0 52,0 55,0 56,0 59,0 79, ,0 14,0 26,0 44, ,0 7,0 22,0 30,0 β-с 2 S 0, ,0 2,0 4,0 33,0 56, , ,0 19,0 С 3 А 0, ,6 0,7 0,8 1,3 6,4 7,5-15 4,2 5,0 5,5 13,5 17,0 29,0 С 6 А 2 F 0, ,0 32,0 36,1 41,0 53,0 55,0 0 31,0 31,5 36,5 42,7 52, ,1 31,2 37,5 43,1 53,3 54,4 C 4 AF 0, ,5 60,0 61,1 67,5 69,2 73,0 0 42,5 54,0 55,3 57,2 59, ,0 53,0 54,6 55,8 58,3 59,1 C 6 AF 2 0, ,0 37,5 40,2 40,8 39,7 39,5 0 29,2 30,4 35,7 37,1 39, ,0 26,0 33,0 36,7 39,2 39,8 * days at temperature 15 C, than at ambient temperatures Table 2: Hydration degree of cement minerals Mineral At 20 C in age, days At -15 C in age, days * C 3 S C 3 A C 6 A 2 F C 4 AF C 6 AF Based on monomineral pastes termogravimetric analysis data (fig. 1) it has been determined, that under negative temperature conditions, processes of ice sublimation and ablimation play an important role in cement pastes structure formation. In 1-day age C 3 S paste the total amount of water at -15 C lowers from 42% (initial W/C) only to 35% 39

4 whereas under normal conditions it reaches the minimum value of 19,8%, with further increase. On frost resulting from ice sublimation the contents of water contently decreases to 28 days and reaches 9,8%. Such a small amount of water in C 3 S paste which hardens on frost is not sufficient for hydration of the general mass of not get reacted mineral, that s why with age H 2 O and CO 2 of air sublimation i.e. ablimation process is being observed. Fig. 1. Loss on ignition of monomineral pastes: at the temperature 20 C at the temperature - 15 C The process of pastes hardening increase based on calcium silicates may be divided into two periods. During the first period mainly processes of ice sublimation take place at that time paste is characterized by low almost zero strength. Quantity of free water in a paste insufficient for the further development of hydration process is characteristic of the second period. Thus, the process of water molecules ablimation, inside a paste from air takes place leading to hydrolysis acceleration conditioning paste strength increase. Increased paste porosity as well as water vapor and carbon dioxide of air assists sublimation i.e. carbonization of paste takes place. During C 3 A hydration the processes of sublimation and ablimation are revealed as well. For C 3 A paste, hydrated at ambient temperature, up to 7-days more intensive mass decrease, than at negative ones, is being observed. Especially big difference is being 40

5 observed at 1-day age of hardening. Thus, change of C 3 A paste mass, which is hardening under normal conditions accounts to 19,3%, whereas at negative temperatures it amounts only to 5%. This is because of a considerable increase of temperature by heating the mass and evaporating of moisture during hydration under normal conditions. At 1-day C 3 A hardening age at 20 C, the ratio of H 2 O/C 3 A in amounts to 34,2% and at -15 C - 58,2% (at the initial contents of water in a paste 67%). Increasing hardening time on frost up to three months, the ratio H 2 O/C 3 A in C 3 A paste first decreases and then increases again because of the overbalance of ablimation process. According to X-ray diffraction and thermal analysis data the main products of C 3 A hydration on frost are polyaqueous hexagonal hydrates of C 4 AH 19 type, which later transform to C 4 AH 13. Evolving water is used for further hydration of mineral. At negative temperatures the process of ice sublimation from C 3 A paste slows down owing to combining of great amount of water into hydrated formations. That s why minimum water quantity in C 3 A paste at -15 C equals 33%, whereas in C 3 S - 9,8% only. In case of C 3 A hydration on frost strong carbonization of calcium hydroaluminates is being observed. It proceeds step by step with formation of calcium hydrocarboaluminates of alternative composition, followed by formation of vaterite, gibbsite and at the age of two years - calcite. When calcium is present in the alumoferrite composition and is replaced by the part of Al 3+ into more and less active Fe 3+ ions, the hydration process slows down. In such cases, with the increase of Fe 2 O 3 contents in calcium alumoferrites the intensity of heat evolution decreases. There is a direct dependence between the quantity of the evolved heat and the strength growth of calcium alumoferrites paste on frost. Thus, the strength of C 6 AF 2, C 4 AF as compared with C 6 AF 2, C 4 AF increases 4-5 times and, C 4 AF - C 6 AF times. Phase contents of calcium alumoferrite paste is in many ways analogous in the phase contents of C 3 A paste, but differs in the fact that solid solutions of calcium hydroaluminate ferrites and ferrum hydroxides are being formed. The difference in kinetics of clinker minerals hardening on frost has to do with their crystalline structure and hydration peculiarities at negative temperatures calcium silicates are characterized by retarding proton formation processes as well as polycondensation of SiO 4 -tetrahedrons in solid phase, which start developing only during ablimation period under the action of carbon dioxide of the air [6]. But, hydration of aluminate and aluminoferrite phases of Portland cement caused by protonization of all oxigen anions and the transition of Al 3+ and Fe 3+ from tetrahedral coordination into octahedral doesn t change so sharply with the decrease of temperature. To decrease the destructive action of sublimation process under negative temperatures conditions it is necessary to combine the largest quantity of water into instable hydration phase as well as to provide the long term moderate heat evolution and the presence of non freezing liquid phase. 41

6 Processes of ice sublimation and ablimation also have a considerable influence on Portland cement hardening at negative temperatures. Thus, up to fortnight of hardening at 15 C decrease of W/C from 25% to 18,6% is being observed, i.e. ice sublimation process takes place. Further hardening on frost is accompanied by increase of H 2 O and CO 2 amount resulting from their ablimation by cement stone. Total porosity of Portland cement stone which has been hardening at 15 C in 24 hour makes up 0,192 cm 3 /g and up to 28 days changes unsubstantionally (0,187 cm 3 /g), while under normal conditions, in 1 days and 28 days in equals, respectively 0,105 and 0,049 cm 3 /g. During hardening on frost, basic volume of pores is divided into capillary and large-capillary pores and at normal temperature into micro-capillary. According to the law-temperature dilatometry data during cooling of Portland cement paste (W/C=0,25), at 1 C, expansion by 1,5% takes place, amount of ice makes up 98%. According to the quantitative X-ray diffraction analyses data, hydration degree at -15 C after 28-days makes up 5% only, while under normal condition - 75%. The line of gypsum dihydrate with d/n=0,756 nm testifies to the fact that the process of its binding in ettringite flows much slowly than under normal conditions of hardening. To have more complete idea about the character of hydration process of ablimation containing phases in Portland cement at freezing and thawing with the help of X-ray diffraction analyses ettringite formation kinetics at different temperatures has been determined. To obtain such a result C 3 A, gypsum and water have been mixed in stoichiometric ratio, necessary for ettringite formation. Obtained patterns were kept in cuvette at temperature 20, 3, -3 and -15 C. According to the investigation results, at negative temperatures formation of ettringite practically stops, while at positive temperatures in flows rather intensively. According to the X-ray diffraction data of patterns with were thawed after being kept 1-day at -15 C, intensity of ettringite lines increases already in 30 min after thawing i.e. interaction between C 3 A and gypsum accelerates sharply. For hardening intensification and decrease of destructive phenomena processes of Portland cement early structure formation are to be accelerated as well as the amount of nonfreezing liquid phase is to be increased. This can be achieved by introducing hardening accelerators with antifreezing properties. Using of sodium rodanide admixture of Portland cement in the quantity up 4 mass.% does not substantially influence the setting beginning of binder. At the same time, sodium tiosulphate admixture accelerates the setting beginning of binder, which is of a considerable importance when using Portland cement compositions on frost. Application of Na 2 S 2 O 3 and NaСNS admixtures in the composition cement-sand mortar provides hardening of patterns at -10 C. Antifreezing properties of sodium tiosulphate and rodanide are determined by their anions structure peculiarities. Sodium sulphate may be considered as crystalline chemical analogue of sodium tiosulphate and sodium nitrite - of rodanide. As the dimensions 42

7 of S 2 O 3 2- and СNS - anions are greater than those of their analogues, they are more active and salt of their anions - more soluble, which facilitates the making the binding hydration processes more active and antifreezing action of admixtures. The eutectic temperature of NaNO 2 solution is 19,6 C (limiting concentration of solution is 28%). But it may be using up to 15 C, because concrete hardened slowly on the frost. It is significant that NaCNS is characterized by increasing solubility and the beginning freezing temperature of 40-% water solution is 24 C. According to X-ray diffractometry the using NaCNS admixture causes increasing of the hydration degree after 1 day on 1,5-2 times and it does not interact with gypsum dihydrate. It has be determined that beginning freezing temperature of cement-sand mortar based on the ordinary Portland cement (composition 1:2,W/C=0,4) makes -2 C, expansion by 1,4% takes place. At the same time, the increasing of Portland cement Blaine specific surface from 300 to 400 m 2 /kg is caused the decreasing of ice formation beginning temperature to 6 C. According to investigation results (table 3) keeping of cement-sand mortar for 2 hours provides the decrease of extension strain by 31% (keeping at +5 C) and 56% (keeping at +20 C) as well as decrease of temperature of the beginning of deformation by 3-4 C. Application of 4 mass. % sodium rodanide in this condition allowed to decrease the temperature of the beginning of deformation of freshly formed cement-sand mortar by C, while expansion strain, being observed hereby, decrease 0,46-0,61%. Table 3: The temperature of the ice formation beginning and expansion strain of cementsand mortar (composition 1:2) Time of keeping, h Without admixture 4 mass.% NaСNS T b, С l/l, % T b, С l/l, % 0 h -4 1,40-8 0,61 2 h at +5 С -7 0, ,59 2 h at +20 С -8 0, ,46 T b temperature of expansion strain beginning in cooling chamber 4 Conclusion It is shown that under freezing (up to 150 C) of hardened cement pastes the main changes are caused by crystallization of ice in pore solution and its modication transformation. According to the gravimetric data two periods are being observed at concrete hardening in frost: first the mass reduction is taking place because of ice sublimation, and with a flow of time, the mass grows as a result of the reverse process - ablimation of H 2 O and CO 2 of the air. It has been shown that protonizating process of clinker minerals crystal 43

8 structure has a decisive influence on hydration and the kinetic of hardening in the frost. It has been established that on frost strength growth of calcium silicates proceeds very slowly, while during ablimation period it accelerates. Sublimation and ablimation processes do not have such a considerable influence on the strength of C 3 A and calcium aluminoferrite. It has been shown that in the process of freshly prepared Portland cement paste freezing the initial destruction of its structure takes place because of the intensive ice formation and later at thawing as a result of its partial compacting followed by repeated expansion at ettringite crystallization. As a result of gypsum dihydrate action destruction process develop in Portland cement on the one hand, at freezing because of aluminate containing phase hydration blocking by protective membrane which forms at battering and on the other hand, at thawing as a result of ettringite formation, which facilitates additional cement grains partition off. That s why the main requirement at cements hardening on frost is to present the liquid phase freezing and to create proper conditions for cement hydration before concrete reaches the definite (critical) strength. Using deicing admixtures, which accelerate setting time and provide cement pastes structure formation processes in negative temperature condition, allows to decrease time of reaching critical strength by concrete and considerable reduce expansion strain at freezing. References 1. Setzer, M.J. and Auberg, R. Proceedings of the Intern. RILEM Workshop on Frost Resistance of Concrete (Essen, 1997). 2. Stark, J. Frost resistance with and without deicing salt - a purely physical problem? Proceedings of the Intern. RILEM Workshop on Frost Resistance of Concrete (Essen, 1997) Mironov, S.A. and Lagojda, A.V. Concretes hardening at frost (Stroyizdat, Moscow, 1975). 4. Sanitsky, M.A., Melnyk, V.M., Loza, M.Z. and Shichnenko, I.V. Testing of freezethaw resistance portland cement compositions by low temperature dilatometry, Proceedings of the Intern. RILEM Workshop on Frost Resistance of Concrete (Essen, ). 5. Sanitsky, M.A., Sobol, H.S. and Shevchyk, G.Ya. Higt frost durability concrete based rapid-hardening gypsum-free portlandcement compositions (Ibausil 12, Tagungsbericht, band2, 1994) Sanitsky, M.A. Some questions of cement minerals crystalochemistry (Kyiv 1996). 44