- Email: [email protected]
The Properties of Cement Pastes and Mortars Processed with Some Heavy Metal Nitrates Containing Solutions
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 108 (2015) 72 – 79
7th Scientific-Technical Conference Material Problems in Civil Engineering (MATBUD’2015)
The properties of cement pastes and mortars processed with some heavy metal nitrates containing solutions Nocuń-Wczelik Wiesławaa,*, Trybalska Barbaraa, Dziub Sylwiaa a
AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
Abstract The effect of some heavy metal nitrates on the course of early hydration process and the properties of hardened cement mortars was investigated. Heavy metal compounds were introduced to the hydrating cement composites in soluble form of nitrates. The results of tests allow to make a comprehensive assessment of the impact of lead, cadmium and nickel nitrate added in amounts of 1% to 5% by weight of cement. The calorimetric measurements together with conductometric, shrinkage, compressive strength measurements and microscopic observations were carried out. These salts modify the rate of cement hydration at early age. The hydration process is particularly strongly retarded in the presence of Pb compounds. As a consequence, the compressive strength at early age is very low. In case of nitrates there is no additional products formed. However, one can observe that the properties of pastes admixtured with nitrates decline from those found in case of reference The calcium silicate phase becomes less fibrous but more compact, with very small individual particles, as the nucleation from the liquid phase is disturbed. There are the accompanying cations incorporated in this product. © 2015 The Authors. Published by Elsevier Ltd. © 2015 Theand Authors. Published by Elsevier Ltd. This an open access article under BY-NC-ND license Conference Material Selection peer-review under responsibility of isorganizing committee of thethe 7thCC Scientific-Technical (http://creativecommons.org/licenses/by-nc-nd/4.0/). Civilresponsibility Engineering.of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering Problems inunder Peer-review Keywords: Cement hydration; heavy metals; calorimetry; compressive strength; microstructure; shrinkage
1. Introduction Cement hydration is a heterogeneous process in which the nearly amorphous calcium silicate hydrates are
* Corresponding author. Tel.: +48 12 617 2476. E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering
doi:10.1016/j.proeng.2015.06.121
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
formed, accompanied by crystalline calcium aluminosulfate phase (ettringite) and calcium hydroxide. Basic properties of cement pastes are attributed to these calcium silicates, known as C–S–H gel. The formation of hydrated phases leads to the reduction of distance between the grains and the viscosity of paste is enhanced. At the next stage of this process the rigid skeleton structure is built and this structure reveals ability to transfer some load as it has been presented in some fundamental works by Taylor [1] or Kurdowski [2]. Calcium sulfoaluminate growing from the surface of cement grains to the space between them, soon after mixing with water (up to 1 – 2 hours) occurs in the form of strong fibres; this phase is responsible for the loss of fluidity and setting. The heavy metal compounds are the components of many industrial wastes and by-products from the combustion process. They can be transferred from the alternative fuels or supplementary raw materials in the production of cementitious materials. According to Glasser [3] they are immobilized (stabilized) in hydrating cement paste, due to the physical and chemical properties of cement matrix; therefore a significant reduction of leaching (up to ppm level) is guaranteed. The gel-like C-S-H phase of unstable chemical composition and disordered structure is in equilibrium with alkaline pore solution and, as a consequence, the precipitation of hardly soluble compounds (mainly hydroxides) is thus promoted. This C-S-H shows, owing to the high internal surface, very good absorbability as has been reported in some works by Nocuń-Wczelik [4, 5]. Low permeability and porosity (compact microstructure) makes the transport of toxic liquids impossible. The replacement and incorporation of foreign ions is promoted by numerous defects in C-S-H structure. All the factors mentioned above are of the synergic character [3]. Heavy metal compounds are not inert as far as the setting and hardening are concerned. The studies on the effect of heavy metal compounds on cement hydration started many years ago and showed a considerable retardation of setting in many cases, as it has been reported by Rossetti and Medici [6], Fernandez-Olmo, et al. [7], and Mellado et al. [8]. Therefore the solidification/stabilization of the waste is disturbed. It relates particularly to Pb and Zn which have strong retarding effect as one could find in some works by Nocuń-Wczelik and Łój [9, 10] or Bochenek and Kurdowski [11]. This was observed and reported for the first time in the sixties of XXc. by Lieber [12]. Because of the complex character of wastes from one side and the mixtures used in solidification/stabilization from the other side, the effectiveness of the process must be experimentally verified in each case and there is a need to study the mutual relation in some simplified systems. When the admixtures in the form of soluble inorganic salts are added to the process water they affects the hydration of cement, entering the complex reactions with both calcium silicates, calcium aluminate and aluminoferrite from cement in the liquid phase. This liquid phase itself is highly alkaline (almost entirely this is the saturated calcium hydroxide solution). Some soluble salts accelerate the hydration process because of the strong interaction with silicate components of cements (dissolution of silicate sub-structure) but the other ones, particularly those forming the poorly soluble hydroxides and sulfates, can retard the hydration significantly. The majority of works dedicated to the interaction between the heavy metal compounds and cement mixtures has a main goal to determine precisely the so-called immobilization potential of stabilizing material (leaching tests) (Gineys et al. [13]; Mellado et al. [8]). The microstructure, as one of the factors affecting the process is not reported frequently. However, because in the presence of various salts the composition of the liquid phase filling the space among the hydrating cement grains is altered, the nucleation and precipitation of the calcium silicate hydrates is generally disturbed. Undoubtedly, the new, heavy metal bearing modified product is produced; this statement relates mainly to the C-S-H. These forms are relatively poorly shaped as compared to those formed in cement paste with no admixtures as it has been reported by Nocuń-Wczelik et al. [14] and Nocuń-Wczelik, Trybalska [15]. The foreign ions can be included in C-S-H structure [13, 15]. 2. Experimental 2.1. Materials and methods
73
74
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
The standard portland cement type CEM I 42,5 N commercially available was used as a basic component of expansive binders. The specific surface was approximately 3200 cm2/g (as measured by standard Blaine method). The chemical composition is given in table 1. The solutions of lead, cadmium and nickel nitrates were produced from analytically pure reagents and added, dissolved with water as 5%, 2% or 1% by mass of cement, at assumed water to cement ratio. Table 1. Composition of basic cement. Component
CaO
SiO2
Al2O3
Fe2O3
MgO
SO3
Loss on ignition
Percentage [wt. %]
66.0
23.0
4.7
2.4
1.2
1.2
0.1
The microcalorimetry was applied as a basic method for the estimation of hydration progress. Heat evolution measurements were carried out by use of so-called differential microcalorimeter on the pastes produced at w/c ratio 0.5 using 5g cement specimen. The starting temperature was kept constant at 25oC. As it is commonly known in cement chemistry, the calorimetric measurements give the possibility to follow the kinetic changes occurring in the hydrating mixture produced at standard water to cement ratio in a continuous way from the beginning of the process. The heat evolved vs. time curves reflect very precisely the progress of complex phenomena and reactions with water, accompanying the increasing content of hydration product. After the initial intensive growth during the first minutes (adsorption and surface dissolution) the heat/products increase steadily; the increase of total heat evolved values becomes very slow after 2 -3 days when the layer of products is formed on cement grains and the process is diffusion controlled (according to the standards, the values after 41h from the processing with water are measured and evaluated). Every retarding admixture brings about the reduction of heat evolved value increase throughout some period of time at early age of hardening (it means even up to 2 days). The retardation of hydration is observed particularly when the hydration takes place in the presence of heavy metals – Pb and Zn. Some additional information can be obtained from the electric conductivity measurements. They can be done in the hydrating cement suspensions produced at w/c ratio 100. They give the information about the dissolution/precipitation phenomena. The effect of heavy metal nitrates on the strength development was measured on the cylindrical samples produced as standard mortars. The plastic shrinkage measurements are the method of continuous investigation of volume changes occurring during the hydration process. The measurements were performed with help of computer aided registration (see fig. 1); the measuring system, with one movable wall of every mould was installed by the author (W. N.-W.).
Fig. 1. Continuous measurements of plastic shrinkage.
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
3. Results and Discussion The example set of calorimetric curves, illustrating the heat evolution vs. time for the process occurring in highly concentrated solutions is shown as fig. 2; hydration in less concentrated solution is shown in fig. 3. The calorimetric curves reflect well the effects attributed to the action of nitrate anions and accompanying cations. It is clearly seen that the hydration process is very strongly retarded in the presence of PbNO 3. The socalled “dormant period” is extended; it means that in these pastes the content of hydration products is low and there is no strong microstructure and, as a consequence, the setting does not occur for a long time after processing with water. As one can see in figs 2 an accelerating effect of nickel and cadmium nitrate is well visible; the effects can be attributed to the dissolution of cement phases at early age in a solution of certain ionic force; later, the heat evolution curves do not decline specially from that for the reference. The effect of cadmium and nickel salt at 1% concentration (fig.3) is less pronounced.
Fig. 2. Heat evolution of cement hydrated in 5% Pb, Ni and Cd nitrate solutions.
Fig. 3. Heat evolution of cement hydrated in 1% Pb, Ni and Cd nitrate solutions.
75
76
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
Fig. 4. Conductivity vs. time for cement suspensions (w/c=100) hydrated in Pb, Ni and Cd nitrate solutions (at 2% admixture by mass of cement).
As one can see in fig.4 the conductivity (it means the concentrations) vs. time curves of ions in the liquid phase differ in the presence of Pb, Ni and Cd nitrates. First of all the environment of hydrating cement has strongly buffering effect and an adsorption of additive occurs at very early age (the conductivities of solutions became very similar during 2 – 5 minutes). Subsequently a “competition mechanism” between the dissolution and precipitation can be noticed . The process in the liquid phase seems to be slightly accelerated in case of Ni nitrate but retarded in case of Pb nitrate – the lowest conductivity values presumably can be attributed to the hampered dissolution together with the formation of impermeable layer; the maximum point is visible after 3 days.
Fig. 5. Compressive strength of mortars produced without admixture (1) and with 1% (2, 3, 4) or 2% (5, 6, 7) Pb, Ni and Cd nitrate solutions.
The presence of nitrates has an impact on the compressive strength of mortars (fig.5). The early compressive strength can be even higher in the presence of nitrates of low concentration, as it is in case of 1%, 2%Ni(NO 3)2 and 1% Pb(NO3)2. However, the retarding effect visible on the calorimetric curves is compatible with the early strength lowering. The relations observed are complex and there is no simple explanation. The results of volume changes measurements (fig. 6) reveal that the hydration of mortars occurs with significant shrinkage in the presence of Ni(NO3)2 while at Cd(NO3)2 the slight expansion is observed.
Sample lenhth [mm]
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
96 95.5 95 94.5 94 93.5 93 92.5 92 91.5 91
Cd(NO3)3 Pb(NO3)3
cement "0" Ni(NO3)3
0
5000
10000 15000 20000 25000 30000 35000 Time [min]
Fig. 6. Heat evolution of cement hydrated in 5% Pb, Ni and Cd nitrate solutions.
The shrinkage of reference mortar is 0,8% after 3 days and 2.2% after 20 days and stabilizes. The mortar with Cd(NO3)2 is in fact shrinkage – less one (0.5% expansion). The highest shrinkage is observed in the presence of Ni(NO3)2 and attains 3,4% after 20 days. The shrinkage on the level 0.3% found for the mortars with Pb(NO3)2 is very low; at early age this is related to the retarding effect of this admixture. The observations of microstructure reveal that the morphology of pastes changes in the presence of inorganic salts used as admixtures. The hydration products reflect very well the kinetics of dissolution/precipitation reactions, controlled by the process occurring in the liquid phase, enriched with admixtures. At the presence of heavy metal compounds the nucleation of the products is generally hindered (see fig. 6) and formation of calcium silicate well shaped forms is disturbed. The compact C-S-H unlike the C-S-H type IV according to Diamond classification can be observed. In some cases the C-S-H fibres become very thin and short (fig. 7 upper left; sample with Cd(NO3)2) or short and looking like the C-S-H type III irregular small grains. The identification of products was done by EDS analysis. The C-S-H formed “through solution” reveals the presence of the other elements present initially in the liquid phase derived from the admixtures; these are cadmium, nickel or potassium.
77
78
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
Fig. 7. Microstructure of cement pastes hydrated with 5% Cd (upper left), Ni (upper right) and Pb (bottom) nitrate admixtures.
4. Summary • The rate of heat evolution during hydration (hydration kinetics), as well as the morphology and composition of hydration products are strongly affected by the heavy metal nitrates. • The properties of hydration products – shrinkage, strength and morphology reflect the kinetics of dissolution/precipitation reactions, controlled by the process occurring in the liquid phase, enriched with soluble admixtures. • The hydration process is strongly retarded in the presence of Pb nitrate; this retarding effect well visible on the calorimetric curves is compatible with the early strength lowering, as well as the conductivity and shrinkage measurements results. • The process in the liquid phase seems to be slightly accelerated in case of Ni nitrate, as it results from the calorimetric, conductivity and shrinkage measurements. • The early compressive strength can be even higher in the presence of nitrates of low concentration, as it is in case of 1%, 2%Ni(NO3)2 and 1% Pb(NO3)2. • The mortar with Cd(NO3)2 reveals very low shrinkage (0.5% expansion). The highest shrinkage is observed in the presence of Ni(NO3)2 and attains 3.4% after 20 days. Acknowledgements The financial support from the Faculty of Material Science and Ceramics, University of Science and Technology AGH in Cracow, Poland is greatly acknowledged (grant No 11.11.160.415). References [1] [2] [3] [4] [5] [6] [7] [8]
Taylor HFW. Cement Chemistry, Thomas Telford Publishing, London 1997. Kurdowski W. Cement and concrete chemistry. Springer (ed) 2014. Glasser FP. Immobilisation potential of cementitious materials, Environmental aspects of construction with waste material. Elsevier Science Publishers, Amsterdam, 1994. Nocuń-Wczelik W. Struktura i właściwości uwodnionych krzemianów wapniowych, Polski Biuletyn Ceramiczny 1999, Ceramika 59, Kraków. (in Polish) Nocuń-Wczelik W. Immobilizacja metali ciężkich przez fazę C-S-H, Cement-Wapno-Beton 1997; II/LXIV:188-191. (in Polish) Rossetti VW, Medici F. Inertization of toxic metals in cement matrices: effects on hydration, setting and hardening. Cem Concr Res 1995; 25:1147–52. Fernandez-Olmo I, Chacon E, Irabien A. Influence of lead, zinc, iron (III) and chromium (III) oxides on the setting time and strength development of Portland cement. Cem Concr Res 2001; 31:1213–9. Mellado A, Borrachero MV, Soriano L, Paya J, Monzo J. Immobilization of Zn(II) in Portland cement pastes. Determination of microstructure and leaching performance. J Therm Anal Calorim 2013; 112:1377–138.
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79 [9] [10] [11] [12] [13] [14] [15]
Nocuń-Wczelik W, Łój G. 2006. Effect of PbO on setting and hardening of Portland cement. Cement-Wapno-Beton 2006; XI/LXXIII, 285-290. Nocuń-Wczelik W, Łój G. Effect of PbO on setting and hardening of alite. Cement-Wapno-Beton 2006; XI/LXXIII: 343 – 350. Bochenek A, Kurdowski W. 2013. Influence of zinc phase on the properties of Portland cement. Cement Wapno Beton 2013; XVIII/LXXX:52-57. Lieber W. The influence of lead and zinc compounds on the hydration of Portland cement. In: 5th international symposium on the chemistry of cement, Tokyo 1968, vol. 2: 444-454. Gineys N, Aouad G, Damidot D. Managing trace elements in Portland cement-Part I: Interactions between cement paste and heavy metals added during mixing as soluble salts. Cem Concr Compos, 2010; 32:563–70. Nocuń-Wczelik W, Trybalska B, Rakowska A. 1994. The microstructure of C-S-H formed in the presence of heavy metal compounds. Polski Biuletyn Ceramiczny 1994; Ceramika 46: 289-292. (in Polish) Nocuń-Wczelik W, Trybalska B. Scanning electron microscopy in the studies of hydrated cementitious materials microstructure formed in the presence of some heavy metals containing admixtures. Solid State Phenomena 2015; 231:145-153.
79
ScienceDirect Procedia Engineering 108 (2015) 72 – 79
7th Scientific-Technical Conference Material Problems in Civil Engineering (MATBUD’2015)
The properties of cement pastes and mortars processed with some heavy metal nitrates containing solutions Nocuń-Wczelik Wiesławaa,*, Trybalska Barbaraa, Dziub Sylwiaa a
AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
Abstract The effect of some heavy metal nitrates on the course of early hydration process and the properties of hardened cement mortars was investigated. Heavy metal compounds were introduced to the hydrating cement composites in soluble form of nitrates. The results of tests allow to make a comprehensive assessment of the impact of lead, cadmium and nickel nitrate added in amounts of 1% to 5% by weight of cement. The calorimetric measurements together with conductometric, shrinkage, compressive strength measurements and microscopic observations were carried out. These salts modify the rate of cement hydration at early age. The hydration process is particularly strongly retarded in the presence of Pb compounds. As a consequence, the compressive strength at early age is very low. In case of nitrates there is no additional products formed. However, one can observe that the properties of pastes admixtured with nitrates decline from those found in case of reference The calcium silicate phase becomes less fibrous but more compact, with very small individual particles, as the nucleation from the liquid phase is disturbed. There are the accompanying cations incorporated in this product. © 2015 The Authors. Published by Elsevier Ltd. © 2015 Theand Authors. Published by Elsevier Ltd. This an open access article under BY-NC-ND license Conference Material Selection peer-review under responsibility of isorganizing committee of thethe 7thCC Scientific-Technical (http://creativecommons.org/licenses/by-nc-nd/4.0/). Civilresponsibility Engineering.of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering Problems inunder Peer-review Keywords: Cement hydration; heavy metals; calorimetry; compressive strength; microstructure; shrinkage
1. Introduction Cement hydration is a heterogeneous process in which the nearly amorphous calcium silicate hydrates are
* Corresponding author. Tel.: +48 12 617 2476. E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering
doi:10.1016/j.proeng.2015.06.121
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
formed, accompanied by crystalline calcium aluminosulfate phase (ettringite) and calcium hydroxide. Basic properties of cement pastes are attributed to these calcium silicates, known as C–S–H gel. The formation of hydrated phases leads to the reduction of distance between the grains and the viscosity of paste is enhanced. At the next stage of this process the rigid skeleton structure is built and this structure reveals ability to transfer some load as it has been presented in some fundamental works by Taylor [1] or Kurdowski [2]. Calcium sulfoaluminate growing from the surface of cement grains to the space between them, soon after mixing with water (up to 1 – 2 hours) occurs in the form of strong fibres; this phase is responsible for the loss of fluidity and setting. The heavy metal compounds are the components of many industrial wastes and by-products from the combustion process. They can be transferred from the alternative fuels or supplementary raw materials in the production of cementitious materials. According to Glasser [3] they are immobilized (stabilized) in hydrating cement paste, due to the physical and chemical properties of cement matrix; therefore a significant reduction of leaching (up to ppm level) is guaranteed. The gel-like C-S-H phase of unstable chemical composition and disordered structure is in equilibrium with alkaline pore solution and, as a consequence, the precipitation of hardly soluble compounds (mainly hydroxides) is thus promoted. This C-S-H shows, owing to the high internal surface, very good absorbability as has been reported in some works by Nocuń-Wczelik [4, 5]. Low permeability and porosity (compact microstructure) makes the transport of toxic liquids impossible. The replacement and incorporation of foreign ions is promoted by numerous defects in C-S-H structure. All the factors mentioned above are of the synergic character [3]. Heavy metal compounds are not inert as far as the setting and hardening are concerned. The studies on the effect of heavy metal compounds on cement hydration started many years ago and showed a considerable retardation of setting in many cases, as it has been reported by Rossetti and Medici [6], Fernandez-Olmo, et al. [7], and Mellado et al. [8]. Therefore the solidification/stabilization of the waste is disturbed. It relates particularly to Pb and Zn which have strong retarding effect as one could find in some works by Nocuń-Wczelik and Łój [9, 10] or Bochenek and Kurdowski [11]. This was observed and reported for the first time in the sixties of XXc. by Lieber [12]. Because of the complex character of wastes from one side and the mixtures used in solidification/stabilization from the other side, the effectiveness of the process must be experimentally verified in each case and there is a need to study the mutual relation in some simplified systems. When the admixtures in the form of soluble inorganic salts are added to the process water they affects the hydration of cement, entering the complex reactions with both calcium silicates, calcium aluminate and aluminoferrite from cement in the liquid phase. This liquid phase itself is highly alkaline (almost entirely this is the saturated calcium hydroxide solution). Some soluble salts accelerate the hydration process because of the strong interaction with silicate components of cements (dissolution of silicate sub-structure) but the other ones, particularly those forming the poorly soluble hydroxides and sulfates, can retard the hydration significantly. The majority of works dedicated to the interaction between the heavy metal compounds and cement mixtures has a main goal to determine precisely the so-called immobilization potential of stabilizing material (leaching tests) (Gineys et al. [13]; Mellado et al. [8]). The microstructure, as one of the factors affecting the process is not reported frequently. However, because in the presence of various salts the composition of the liquid phase filling the space among the hydrating cement grains is altered, the nucleation and precipitation of the calcium silicate hydrates is generally disturbed. Undoubtedly, the new, heavy metal bearing modified product is produced; this statement relates mainly to the C-S-H. These forms are relatively poorly shaped as compared to those formed in cement paste with no admixtures as it has been reported by Nocuń-Wczelik et al. [14] and Nocuń-Wczelik, Trybalska [15]. The foreign ions can be included in C-S-H structure [13, 15]. 2. Experimental 2.1. Materials and methods
73
74
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
The standard portland cement type CEM I 42,5 N commercially available was used as a basic component of expansive binders. The specific surface was approximately 3200 cm2/g (as measured by standard Blaine method). The chemical composition is given in table 1. The solutions of lead, cadmium and nickel nitrates were produced from analytically pure reagents and added, dissolved with water as 5%, 2% or 1% by mass of cement, at assumed water to cement ratio. Table 1. Composition of basic cement. Component
CaO
SiO2
Al2O3
Fe2O3
MgO
SO3
Loss on ignition
Percentage [wt. %]
66.0
23.0
4.7
2.4
1.2
1.2
0.1
The microcalorimetry was applied as a basic method for the estimation of hydration progress. Heat evolution measurements were carried out by use of so-called differential microcalorimeter on the pastes produced at w/c ratio 0.5 using 5g cement specimen. The starting temperature was kept constant at 25oC. As it is commonly known in cement chemistry, the calorimetric measurements give the possibility to follow the kinetic changes occurring in the hydrating mixture produced at standard water to cement ratio in a continuous way from the beginning of the process. The heat evolved vs. time curves reflect very precisely the progress of complex phenomena and reactions with water, accompanying the increasing content of hydration product. After the initial intensive growth during the first minutes (adsorption and surface dissolution) the heat/products increase steadily; the increase of total heat evolved values becomes very slow after 2 -3 days when the layer of products is formed on cement grains and the process is diffusion controlled (according to the standards, the values after 41h from the processing with water are measured and evaluated). Every retarding admixture brings about the reduction of heat evolved value increase throughout some period of time at early age of hardening (it means even up to 2 days). The retardation of hydration is observed particularly when the hydration takes place in the presence of heavy metals – Pb and Zn. Some additional information can be obtained from the electric conductivity measurements. They can be done in the hydrating cement suspensions produced at w/c ratio 100. They give the information about the dissolution/precipitation phenomena. The effect of heavy metal nitrates on the strength development was measured on the cylindrical samples produced as standard mortars. The plastic shrinkage measurements are the method of continuous investigation of volume changes occurring during the hydration process. The measurements were performed with help of computer aided registration (see fig. 1); the measuring system, with one movable wall of every mould was installed by the author (W. N.-W.).
Fig. 1. Continuous measurements of plastic shrinkage.
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
3. Results and Discussion The example set of calorimetric curves, illustrating the heat evolution vs. time for the process occurring in highly concentrated solutions is shown as fig. 2; hydration in less concentrated solution is shown in fig. 3. The calorimetric curves reflect well the effects attributed to the action of nitrate anions and accompanying cations. It is clearly seen that the hydration process is very strongly retarded in the presence of PbNO 3. The socalled “dormant period” is extended; it means that in these pastes the content of hydration products is low and there is no strong microstructure and, as a consequence, the setting does not occur for a long time after processing with water. As one can see in figs 2 an accelerating effect of nickel and cadmium nitrate is well visible; the effects can be attributed to the dissolution of cement phases at early age in a solution of certain ionic force; later, the heat evolution curves do not decline specially from that for the reference. The effect of cadmium and nickel salt at 1% concentration (fig.3) is less pronounced.
Fig. 2. Heat evolution of cement hydrated in 5% Pb, Ni and Cd nitrate solutions.
Fig. 3. Heat evolution of cement hydrated in 1% Pb, Ni and Cd nitrate solutions.
75
76
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
Fig. 4. Conductivity vs. time for cement suspensions (w/c=100) hydrated in Pb, Ni and Cd nitrate solutions (at 2% admixture by mass of cement).
As one can see in fig.4 the conductivity (it means the concentrations) vs. time curves of ions in the liquid phase differ in the presence of Pb, Ni and Cd nitrates. First of all the environment of hydrating cement has strongly buffering effect and an adsorption of additive occurs at very early age (the conductivities of solutions became very similar during 2 – 5 minutes). Subsequently a “competition mechanism” between the dissolution and precipitation can be noticed . The process in the liquid phase seems to be slightly accelerated in case of Ni nitrate but retarded in case of Pb nitrate – the lowest conductivity values presumably can be attributed to the hampered dissolution together with the formation of impermeable layer; the maximum point is visible after 3 days.
Fig. 5. Compressive strength of mortars produced without admixture (1) and with 1% (2, 3, 4) or 2% (5, 6, 7) Pb, Ni and Cd nitrate solutions.
The presence of nitrates has an impact on the compressive strength of mortars (fig.5). The early compressive strength can be even higher in the presence of nitrates of low concentration, as it is in case of 1%, 2%Ni(NO 3)2 and 1% Pb(NO3)2. However, the retarding effect visible on the calorimetric curves is compatible with the early strength lowering. The relations observed are complex and there is no simple explanation. The results of volume changes measurements (fig. 6) reveal that the hydration of mortars occurs with significant shrinkage in the presence of Ni(NO3)2 while at Cd(NO3)2 the slight expansion is observed.
Sample lenhth [mm]
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
96 95.5 95 94.5 94 93.5 93 92.5 92 91.5 91
Cd(NO3)3 Pb(NO3)3
cement "0" Ni(NO3)3
0
5000
10000 15000 20000 25000 30000 35000 Time [min]
Fig. 6. Heat evolution of cement hydrated in 5% Pb, Ni and Cd nitrate solutions.
The shrinkage of reference mortar is 0,8% after 3 days and 2.2% after 20 days and stabilizes. The mortar with Cd(NO3)2 is in fact shrinkage – less one (0.5% expansion). The highest shrinkage is observed in the presence of Ni(NO3)2 and attains 3,4% after 20 days. The shrinkage on the level 0.3% found for the mortars with Pb(NO3)2 is very low; at early age this is related to the retarding effect of this admixture. The observations of microstructure reveal that the morphology of pastes changes in the presence of inorganic salts used as admixtures. The hydration products reflect very well the kinetics of dissolution/precipitation reactions, controlled by the process occurring in the liquid phase, enriched with admixtures. At the presence of heavy metal compounds the nucleation of the products is generally hindered (see fig. 6) and formation of calcium silicate well shaped forms is disturbed. The compact C-S-H unlike the C-S-H type IV according to Diamond classification can be observed. In some cases the C-S-H fibres become very thin and short (fig. 7 upper left; sample with Cd(NO3)2) or short and looking like the C-S-H type III irregular small grains. The identification of products was done by EDS analysis. The C-S-H formed “through solution” reveals the presence of the other elements present initially in the liquid phase derived from the admixtures; these are cadmium, nickel or potassium.
77
78
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79
Fig. 7. Microstructure of cement pastes hydrated with 5% Cd (upper left), Ni (upper right) and Pb (bottom) nitrate admixtures.
4. Summary • The rate of heat evolution during hydration (hydration kinetics), as well as the morphology and composition of hydration products are strongly affected by the heavy metal nitrates. • The properties of hydration products – shrinkage, strength and morphology reflect the kinetics of dissolution/precipitation reactions, controlled by the process occurring in the liquid phase, enriched with soluble admixtures. • The hydration process is strongly retarded in the presence of Pb nitrate; this retarding effect well visible on the calorimetric curves is compatible with the early strength lowering, as well as the conductivity and shrinkage measurements results. • The process in the liquid phase seems to be slightly accelerated in case of Ni nitrate, as it results from the calorimetric, conductivity and shrinkage measurements. • The early compressive strength can be even higher in the presence of nitrates of low concentration, as it is in case of 1%, 2%Ni(NO3)2 and 1% Pb(NO3)2. • The mortar with Cd(NO3)2 reveals very low shrinkage (0.5% expansion). The highest shrinkage is observed in the presence of Ni(NO3)2 and attains 3.4% after 20 days. Acknowledgements The financial support from the Faculty of Material Science and Ceramics, University of Science and Technology AGH in Cracow, Poland is greatly acknowledged (grant No 11.11.160.415). References [1] [2] [3] [4] [5] [6] [7] [8]
Taylor HFW. Cement Chemistry, Thomas Telford Publishing, London 1997. Kurdowski W. Cement and concrete chemistry. Springer (ed) 2014. Glasser FP. Immobilisation potential of cementitious materials, Environmental aspects of construction with waste material. Elsevier Science Publishers, Amsterdam, 1994. Nocuń-Wczelik W. Struktura i właściwości uwodnionych krzemianów wapniowych, Polski Biuletyn Ceramiczny 1999, Ceramika 59, Kraków. (in Polish) Nocuń-Wczelik W. Immobilizacja metali ciężkich przez fazę C-S-H, Cement-Wapno-Beton 1997; II/LXIV:188-191. (in Polish) Rossetti VW, Medici F. Inertization of toxic metals in cement matrices: effects on hydration, setting and hardening. Cem Concr Res 1995; 25:1147–52. Fernandez-Olmo I, Chacon E, Irabien A. Influence of lead, zinc, iron (III) and chromium (III) oxides on the setting time and strength development of Portland cement. Cem Concr Res 2001; 31:1213–9. Mellado A, Borrachero MV, Soriano L, Paya J, Monzo J. Immobilization of Zn(II) in Portland cement pastes. Determination of microstructure and leaching performance. J Therm Anal Calorim 2013; 112:1377–138.
Nocuń-Wczelik Wiesława et al. / Procedia Engineering 108 (2015) 72 – 79 [9] [10] [11] [12] [13] [14] [15]
Nocuń-Wczelik W, Łój G. 2006. Effect of PbO on setting and hardening of Portland cement. Cement-Wapno-Beton 2006; XI/LXXIII, 285-290. Nocuń-Wczelik W, Łój G. Effect of PbO on setting and hardening of alite. Cement-Wapno-Beton 2006; XI/LXXIII: 343 – 350. Bochenek A, Kurdowski W. 2013. Influence of zinc phase on the properties of Portland cement. Cement Wapno Beton 2013; XVIII/LXXX:52-57. Lieber W. The influence of lead and zinc compounds on the hydration of Portland cement. In: 5th international symposium on the chemistry of cement, Tokyo 1968, vol. 2: 444-454. Gineys N, Aouad G, Damidot D. Managing trace elements in Portland cement-Part I: Interactions between cement paste and heavy metals added during mixing as soluble salts. Cem Concr Compos, 2010; 32:563–70. Nocuń-Wczelik W, Trybalska B, Rakowska A. 1994. The microstructure of C-S-H formed in the presence of heavy metal compounds. Polski Biuletyn Ceramiczny 1994; Ceramika 46: 289-292. (in Polish) Nocuń-Wczelik W, Trybalska B. Scanning electron microscopy in the studies of hydrated cementitious materials microstructure formed in the presence of some heavy metals containing admixtures. Solid State Phenomena 2015; 231:145-153.
79