Revista Ingeniería de Construcción Vol.24 N°3, Diciembre de 2009 PAG. 213-232

Properties and uses of alkali cements

Ana Fernández-Jiménez*¹, Ángel Palomo*

* Instituto de Ciencias de la Construcción Eduardo Torroja (CSIC) Madrid, ESPAÑA

Corresponding author:

In this paper are presented some of the technologic properties of cementitious material elaborated by alkali activation of aluminosilicates are presented. More specifically it is about the properties of alkali activated fly ash concrete and mortar (without Portland cement). So depending on the type of alkali activator that is used and after a previous thermal curing, the resulting material will show an interesting list of properties and features that includes: high initial mechanical strengths (under flexure and compression), low drying shrinkage, and a very good matrix-steel bonding, as well as an excellent strength to acid attack and an excellent behaviour when is exposed to fire. That is the reason why these new cements, due to their magnificent and durable technologic features as well as its easy adaptability to the existing installations in the precast industry, can be used in a variety of applications, for example: elaboration of railroad cross ties, blocks for building or paving, and so others.

Keywords: Alkali cements, mechanical strengths, durability, applications

1. Introduction

The construction materials based on ordinary Portland cement (OPC), mainly concrete, are the most used materials in the world. Nevertheless, OPC industry, due to its high production level in global terms, must face certain problems of social transcendence. Economic-energetic (the use of fossil fuels that each time become more expensive a scarce) and ecologic (1 T. Cement ≡ 1.5 T Raw Materials ≡ 0.8 T.C02) problems. Thus, it is estimated that 6- 7 % of total C02 atmosphere emissions, at a worldwide scale, is caused by the cement industry (Scrivener y Kirkpatrick 2007).

In contrast, we have to bear in mind that Portland cement based on concrete present some difficulties of durability (Lea's 1998; Johansen et al.,1995) (sulfate attack, reinforcement corrosión, alkali- aggregate reaction, low resistance to fire, and so others), that makes that 40-50% of the construction industry budget it is destined to repairs. All of that makes the search of cementitious alternatives, that could reduce C0₂ atmosphere emissions and also improve some of the features in which cases Portland cement shows deficiencies, this is one of the scientific community main objectives which has been working for years to develop materials and technologies that allow to advance further in a more sustainable construction industry. Such is the case of alkali cements (Shi et al., 2006; Duxson et al., 2007).

The alkali activation of silicoaluminate materials of partial configuration or totally amorphous or glassy (in fine división state), with strongly alkali dissolutions, and after a short period of soft thermal curing (50- 100°°C), allows to obtain a material with good cementitious properties (Duxson et al., 2007; Skvara et al., 2005; Fernández-Jiménez and Palomo 2005a). In this paper we will focus on class F fly ash alkali activation, which comes from thermal power stations that use coal as fuel. The more than 1 billion tons of ashes produced worldwide yearly, are a good argument to discuss about its competitiveness as base material in the manufacture of a new generation of alkali cements.

There are many the papers that bring data about the mechanical strength evolution of alkali activated fly ash, pastes and mortars [Shi et al., 2006; Duxson et al., 2007; Skavara et al., 2005; Fernández-Jiménez and Palomo 2005a; Palomo et al., 1999). Even though there are very few papers that refer to the manufacture of concretes exempted of Portland cement in which the binder material is alkali activated fly ash. The first papers correspond to A.Fernández-Jiménez and A.Palomo (2006a; 2007a) and Hardjito et al (2002). These papers show how the properties of alkali activated fly ashes are influenced, the same as conventional concretes, by a set of factors related to mixture dosage and curing conditions.

Mortars and concrete elaborated with Alkali activated fly ash allow for developing very high mechanical strength at early ages (1 day), strengths that continué progressing slowly as time goes by. They also present interesting physical-mechanical properties as low drying shrinkage, excellent matrix-steel bonding (pull-out test), and others (Hardjito et al., 2002; Fernández-Jiménez et al., 2006a; Palomo et al., 2007a). Another important aspect to highlight about these materials is their durability, and that is why in this paper it is also shown some of the results obtained on their behavior when exposed to chemical attack of aggressive elements (sea water, sulfate attack, HCI acid attack), as well as high temperatures, (Fernández-Jiménez et al., 2007b; Allahverdi and Skvara 2005; Bakharev 2005; Fernández-Jiménez et al., 2008).

Finally, the objective we try to achieve is to state the potential of these materials from its technologic properties perspective, as well as their durable behavior, thus in the near future it can be used in different uses inside the construction industry and especially in the precast industry.

2. Materials

In the present paper it was used a Class F fly ash from Spain (according to ASTM C618-03 classification) and a commercial cement (CEM I 52.5 R). The material chemical compositions, as well as it specific surface, are shown in Table 1. You can observe than while Portland cement is comprised basically of silicon and calcium oxides, fly ash is compresed of aluminum and silica oxides.

In alkali activation of fly ashes, two dissolutions were used: N= NaOH 8M y W= mixture of 15% of sodium silicate + 85% NaOH 12.5M (SiO₂/Na₂O=0.16 module). To prepare the dissolutions were used: NaOH (de 98% purity lentils, PANREAC) y bulk sodium silicate (8.2% Na₂0, 27% Si0₂ y 64.8% H₂0, d=1.38 g/cc).

Table 1. Chemical composition in (%) and specific surface of Portland cement and fly ash

3. Results and discussion

3.1 Technological properties

Mechanical strength to flexure and compression were determined as technological properties, as well as steel bonding through the pull-out test in alkali activated fly ash concretes (without OPQ. While drying shrinkage valúes were determined in mortars. In this research, Portland cement based concretes and mortars were also elaborated as a reference system. Below are presented the obtained results.

3.1.1 Mechanical Strength

In Table 2 is shown the tested concretes nomenclature, mixture proportions and curing conditions used in their elaboration. The compressive strength valúes were determined in cubic specimens of 15x15x15 cm and the flexural strength in prismatic specimens of 15x10x70cm. In all cases, concrete was compacted by vibration (15-20 seconds). In Figure 1 are presented the strength valúes, both to flexure and compression and their evolution with time.

The results presented in Figure 1 show how this type of concrete develops high compressive strength at initial ages; so valúes in the order of 45 MPa are obtained in 1 day. This is higher than those obtained for OPC based conventional concrete. These mechanical strength valúes are similar to those obtained for high strength OPC concretes. Another remarkable fact is that strengths keep increasing over the time, but in a more gradual way. It is also convenient to emphasize, as in the case of fly ash when using the W activating dissolution (soluble silicon from the sodium silicate used) it can be achieved an evident improvement of the mechanical strength development; though it has to be mentioned that in these cases the workability of pastes was reduced.

Table 2. Nomenclature, dosage and curing conditions of tested concretes

In relation with flexure strength (see Figure 1 (b) which only shows results for alkaline concretes), it can be concluded that, as occurs with compression strength, this concretes develop high early flexure strength which continué to increase with time.

Figure 1 Mechanical strengths of concretes, (a) to compression in cubic specimens (15x15x15 cm); (b) to flexure in prismatic specimens (15x10x70 cm).

3.1.2 Bonding test (pull out)

According to the test standardized by the RILEM/CEB/FIP, pull-out tests were conducted with the objective of studying the bonding between concrete and steel, using the same previous mixtures (H-FA-N, H-FAW y H-CE-A, see Table 2).

Bonding is the stress transfer phenomenon between steel and concrete. This bonding makes possible to combine the good behaviour of concrete in front of compression and the high tensile strength of steel in reinforced concrete.

When a bar embedded in a concrete matrix is subjected to a tensile strength, the stress transfer from steel to concrete is produced through angled compressive strengths that are originated at the ridges, depending on itsx angle. The radial component of this compressive strength is balanced with a tensile ring that appears in the concrete surrounding the bar. That causes internal longitudinal cracks. If a confining reinforcement does not exist these cracks go through the cover and a failure is produced in the concrete surface, failure by splitting. Even though if the steel bar is in good confining conditions, the failure will appear tearing bar in relation to the concrete surrounding, failure by pull-out.

In this "Pull-out" test the bonding length of the bar was located in the center of a prismatic specimen 20x20x20 cm. The bonding length of the bar depends on its diameter and must fulfil the (I > 5Φ) rule. On this test 160 mm-diameter bars were used, that is why the bonding zone was equal to 16 cm. To avoid the bonding on unwanted zones of the steel bar, plástic handles are placed on the ends. The B500-SD steel bars had a total length of 70 cm. The objective is to measure the bar displacement on the opposite end to that where the load is applied, passive end, in relation to the surface of the concrete specimen.

The load is applied on the longest end (see Figure 2) using a jack with 156 KN of capacity, at a speed of 72 N/seg until máximum load. During the test, the relative displacement of the bar is registered, in relation to the concrete in the opposite side where the load is applied, by means of three displacement transducer (see Figure 2 (a)).The specimen is placed over a 5 mm thick rubber sheet and at the same time over a 10 mm steel sheet.

The achieved results for different concretes appear in Table 3. The stress of local bonding on MPa is calen la ted as the load applied in the test divided by the bonding surface of the bar. Being Q the applied load (N),Φ the nominal diameter of the bar (mm) and l_D the bonding length (mm).

In Figure 3 are represented the diagrams of the average value of local bonding stress τ(N/mm²), in function of the slipping of the upper end of the bar,δ (mm), for tested specimens. These figures show the relative slipping between the embedded bar and the concrete that surrounds it when applying a growing stress on the end. The last point of the curve is the rupture point of bonding from which the slipping is produced.

Figure 2. (a) Pull-out test equipment; (b) H-FA-W concrete breaks the matrix; (c) H-FA-N concrete, breaks the 16 mm diameter steel bar.

Table 3. Results achieved from the Pull-Out test (RILEM/CEB/FIP 20x20x20 cm moulds)

As can be noticed on the graphics, in relation to 1 6 mm diameter bars, in alkali activated fly ash concretes (H-FA-N y H-FA-W) the failure is produced by matrix rupture and in OPC concrete (H-CE-A) by slipping.

Alkali activated fly ash concretes present some behaviour differences depending on the used activator. Thus in concretes activated by W dissolution, the matrix breaks (Figure2.b) at a máximum stress of ≡ 12 MPa.Meanwhile the concrete specimens with N dissolution reach valúes of 17MPa.

In this last case we have to remark that in the third of the three tested specimens the steel bar got broken without the matrix cracking (Figure2.c).That gives us an idea of the excellent steel/matrix bonding presented by these systems. Anyway in both cases, in both N dissolution and W one, the mínimum 9.70N/mm valué demanded by Spanish Structural Concrete Design Code to fulfil the BEAM TEST is more than achieved.

Figure 3. Local bonding/ slipping curves for 16 mm diameter bars (a) OPC concretes; (b) activated fly ash concretes (without OPC) with N an W dissolutions

3.1.3 Drying Shrinkage

To determine the drying shrinkage valúes, mortar specimens were prepared, following the specifications of standard ASTM C 806-87 (2.5x2.5x23 cm prismatic moulds) with the dosage and curing conditions shown in Table 4. After the initial curing, the specimens were demoulded and stored in the laboratory at 21 °C and 30-50% of relative humidity. Shrinkage measures were made within 1, 3, 7, 14, 28 days, etc. The achieved results are shown in Figure 4.

Table 4. Nomenclature, dosage and curing conditions of tested mortars

In Figure 4 is clearly observed that mortars elaborated with fly ash (without OPC) show very low drying shrinkage valúes, noticeable lower to those of mortars elaborated with Portland cement. Cement mortars, especially those cured at room temperature, experiment a higher drying shrinkage, reaching valúes around the 0.09% within 70 days. This is mainly because of free water loss due to drying. While alkali activated ash mortars (with dissolution N and W), in both cases the shrinkage valúes are lower than 0.025% within 90 days. These results indícate that alkali activated ash mortars and concretes present a high dimensional stability.

Figure 4. Drying shrinkage in mortars for Portland cement and alkali activated fly ash mortars.

3.2 Durability in Front of a Chemical Attack

The durability of materials is closely related to their mineralogical and microstructural composition. Most of the durability problems in OPC concretes, mortars and pastes are related, in one way or another, with the presence of calcium. However, in cements based on alkali activation of fly ash, the main reaction product formed is a moisturized aluminosilicate (N-A-S-H gel without calcium) with a tridimensional structure, and considered as a pre zeolitic gel (Palomo et al. 2004a; Fernández-Jiménez et al., 2006b) clearly different to the C-S-H gel formed in a OPC paste. Thus, there is not doubt that the durable behaviour of OPC and alkali cements will be clearly different.

3.2.1 Resistance to Aggressive Dissolutions Chemical Attack Sea Water and Sodium Sulfate (4.4% Na₂S0₄)

Mortars were elaborated in the same conditions shown in Table 4 for M-FA-N and M-FA-W compositions to determine the behaviour of these materials when exposed to sulfates and sea water attacks. With these mortars, 4x4x16 cm. prismatic specimens were made. After demoulded, the prisms were completely immersed into the corresponding aggressive environments [L= laboratory conditions (reference system); M= Sea water (ASTM D 1141-90); S= Sodium sulfate dissolution (4.4% Na₂S0₄)]. The materials were extracted from the aggressive environment at the following test ages 7, 28, 56, 90,180, 270 and 365 days) and mechanically tested under flexure and compression (see Figure 5) according to the Spanish standard UNE-80-101-88).

The results in Figure 5 show that as by general standard there is not significant deterioration of materials, though some fluctuations are observed in the mechanical strength, especially at initial ages. Mortars made of dissolution W present better strength valúes than those activated with N dissolution. However, in both cases, compressive mechanical strengths increase in relation with the time, independently of the environment where they have been immersed.

Figure 5. Mechanical compressive strengths in (a) M-FA-N; (b) M-FA-W alkali activated fly ash mortars

The visual observation of the immersed mortars into different aggressive environments does not show superficial signs of deterioration after a year. However, a more detailed micro structural study showed the existence of some alterations.

Figure 6 shows the morphology presented in M-FA-W samples after a year immersed in M and S dissolutions. The pictures show the typical aspect of alkali activated fly ash pastes and mortars (Fernández-Jiménez et al., 2005b; Palomo et al., 2004b; Duxson et al., 2005) where it is formed a sodic silicoaluminate, N-A-S-H gel reaction, responsible of the material cementation properties (see Figure 6, Point 1). This kind of micro structure is only interrupted by no reacted ash particles presence (see Figure 6, Point 2) or by the presence of the marks left by the reacted ashes.

In the particular case of sea water exposure, it is observed the presence of a gel richer in silicon and that also contains magnesium ions (Figure 6 (a) Point 3). While in mortars immersed in the sodium sulfate dissolution, a sodium sulfate formation is detected (see Figure 6 (b), Point 4). However, in both cases this degradation products are only detected in a specific way and it appears either inside the non reactionated ashes or in the hollows left by these after reacting.

Figure 6. M-FA-W Mortar SEM micrograph after 365 days immersed on dissolutions of a) M= Sea Water; (b) S= sodium sulfate; P1= cementitious matrix N-A-S-H gel; P2= unreacted ashes; P3= gel with Mg; P4= possible sodium sulfate

3.2.2 Acid Attack strength (HCI 0.1 N)

A dynamic test was made, to determine the behaviour of these materials in front of an attack in acidic environment (HCI 0.1 N dissolution, pH= 1.5), based on the ANS 16.1 lixiviation method (relation between acidic dissolution volume/ surface of the specimens=10) (Conner 1990). The test was made in cubic mortar specimens (3x3x3cm) prepared under the same conditions than those specified in Table 3 (M-FA-N, M-FA-W and M-CE-A mortars (reference system)). In this case, acidic dissolution renewed at the ages of 1, 2, 3, 7, 28, 56, and 90 days to always guarantee an acid environment strong enough. At the test ages of 7, 28, 56 and 90 days, weight fluctuations and mechanical strengths were determined.

In Figure 7 the results achieved from mechanical compressive strength are presented. These results show that though all studied mortars present a reduction in strength valúes, these are inferior in the ash mortars than in the OPC ones. In alkali activated fly ash, with N dissolution as well as with W dissolution, at 90days of exposure, mechanical compressive strengths have decrease about 23-25%, while in M-CE-A mortars this valué is almost double, 47%.

A visual exam to the specimens exposed to acidic dissolutions show that M-FA-N and M-FA-W specimens present a good physical aspect at 90 days. However, M-CE-A specimens show a severe decay after 56 days of immersion, and an evident color change is observed, as well as mass loss at the edges (see Figure 8). Thus weight loss at 90 days are about 2.5, 4.2 y 9.8% for M-FA-N, M-FA-W and M-CE-A, respectively.

Figure 7. mechanical strength of AAFAand OPC mortars immerse in HCI 0.1 N dissolution

Figure 8. Physical aspect of mortar specimens immerse during 90 days in a CIH 0.1 M dissolution (a) M-FA-W mortars; (b) M-CE-A mortar (reference system)

In Figure 9 are presented some micrographs of fly ash activated through W dissolution with and without immersing in acidic dissolution. At 90 days is observed that in both cases a compact matrix is obtained, formed by gel N-A-S-H. In both matrices, there is evidence of minor presence for crystalline phases of zeolitic nature (herchelite type). However, although the matrix subjected to the acid attack is more porous, the most mentionable difference is not the micro structure but the chemical composition of the gel, as well as the composition of the zeolites. In samples that have been immersed in HCI dissolution, the aluminium content in the gel and the zeolites has decreased, and dealumination has been produced (see Figure 9, Point 3 and 4). Similar results were obtained when N dissolution was used as activator.

Figure 9. Micrograph and analysis by EDX on M-FA-W material at 90 days (a) without HCI (b) immersed in HCI (dynamic test). P1=N-A-S-H gel; P2=Zeotile, type herchelite; P3= N-A-S-H gel with a low Al content; P4= Zeolite with low Al content

3.3 Behaviour at high temperatures

The tests at high temperatures were made in alkali activated fly ash pastes and cement was used again as reference system. The used dosages, as well as the curing conditions appear in Table 5. There were used 1x1x6 prismatic specimens with these pastes. These specimens, once demolded, were stored in the curing chamber until the 28 days of age, when they were subject to a strength test at high temperatures.

Table 5. Nomenclature, dosage and curing conditions of the pastes

The strength test at high temperatures consists in putting the specimens in an oven for an hour at 200, 400, 600, 800 y 1000 °C temperatures. After this time the specimens are removed from the oven and abruptly cooled down at room temperature. Next, we proceeded to evalúate residual mechanical strength at room temperature through flexure and compression tests. These tests were made in an Ibertest press, at a 2,4KN/S load speed according to standard EN 196-1.

In Figure 10 residual strength valúes are presented at room temperature for tested materials at different temperatures (from 200 °C to 1000 °C) thermal post-treatment tests. These results show that Portland cement presents a constant reduction in its strength valúes at both flexure and compression. For flexural strength this reduction is 33% at 400 °C. From 600°°C this cement (P-CE-A) presents negligible flexural strength. In contrast, in cement obtained from fly ashes (P-FA-N y P-FA-W) flexural strength remains constant after the thermal treatment up to 400° C, at higher temperatures residual flexure strength drops at the third part of the initial one. However, this valué, on equal test terms is very superior to that obtained for conventional cement (P-CE-A).

With regard to compressive strength valúes, compressive strength is practically nuil in Portland cement from 600-800 C° where also spalling were registered in the oven. However for ash based materials, the residual compressive strength keeps practically constant (in fact a light increasing between 800 and 1000°°C is registered), independently of the temperature of the thermal treatment.

Figure 10. Residual (a) Flexure (b) Compressive strength valúes. Tests after one hour of exposure to different temperatures

Figure 11 shows the physical aspect of specimens inmediately after suffering the corresponding thermal treatments. While cement specimens at 600°°C appear cracked and completely destroyed at 1000°°C. The presence of cracks is not observed in ash elaborated specimens, though it is observed a plástic deformation of the material from 600°°C. This deformation increases with exposition temperature, is more intense for P-FA-N materials and it is considered the main cause of flexure strength losses.

Figure 11. The physical aspect of the paste specimens after thermal treatment, one hour at different temperatures for (a) P-CE-A; (b) P-FA-N and (c) P-FA-W materials

Additional studies (Fernández-Jiménez et al., 2008; Krivenko and Kovalchuk 2007) have shown that the drop on flexure strengths observed in cemente obtained by alkali activation of fly ash (P-FA-N and P-FA-W) at temperatures that exceed 600°°C (from this valué the mechanical properties of commercial Portland cement demotion quickly) these materials present a partial sinterization point that affects its dimensional stability. However once the material has cooled down, the solidification of the cast produces a more compact matrix and thus increases compressive mechanical strengths at the same time that detects the formation of the phases in the cementitious material (Fernández-Jiménez et al 2008; Gourley y Johnson 2005).

3.4 Uses

Considering the good mechanical properties as well as its extraordinary matrix-steel bonding its dimensional stability and its fire resistance, alkali activated fly ash mortars and concretes can be used in the precast industry for the manufacture of different kinds of less demanding elements from the technological view point such as light posts, blocks for building or paving.

Thus the idea of using alkali activated fly ash elaborated concretes, as binder component, to produce precast elements for use on construction derivates comes from the following confirmed facts:

i) Most of the precast concrete elements are pieces of a size that is easy to handle, but with an extraordinary technological complexity. Its design and production only can be approached with materials that guarantee the development of resistant, durable properties, with reinforcement bond and volume stability, etc. Activated fly ash concretes fulfil all these demands.

ii) The conventional production processes used in precast demand an accelerated thermal curing of the concrete. Alkali activation of fly ashes optimizes in similar thermal curing conditions. Production processes, consequently, are not modified significantly by the fact of changing one raw material (Portland cement) for another (fly ashes and alkali activators).

iii) In the case of pretensioned elements, the pretensing operation of the metal lie reinforcement is made before pouring the concrete in the mould. Considering that these alkali activated fly ash concretes can develop very high initial strengths (during the first 12-20 hours) as well as they show a very good reinforcement bond, the time needed for demoulding could be reduced. This could cause a significant increase in the production of these elements at the precast pants.

iv) There is not an easy solution for the problems that entails the traditional concrete curing accelerated thermal process, quality and durability of finished producís, because the recommended measures in the codes of good practice (to limit curing temperature in 60° C, select aggregates that keep its inert condition at high temperatures, etc.) are generally insufficient and almost always incompatible or difficult to conciliate with the needs of a massive production system. The use of activated ashes could change radically the current scenario. In this land of durability a good behaviour from alkali activated fly ash concrete can be predicted, given the existing chemical-mineralogical similarity between new material and some natural zeolites, extraordinarily stable materials.

Finally, we can not leave unmentioned the economic and ecologic aspeets that come into play. We must remember that the material proposed as substitute for Portland cement in the elaboration of concretes is an industrial sub product that is widespread through the 5 continents and that is mainly accumulated in large expanses of land because of the impossibility of consuming it.

Figure 12 shows some pictures of the elaboration process of railroad cross ties with an H-FA-W dosage (see Table 2) at industrial level. The manufacture of sleepers for railroads was made without the need of modifying significantly the usual production line that was used on the factory, The most relevant modification was the increase of the curing temperature from 50°C to 85°C- 90°C, that is why a tunnel through which water steam was injected (see Figure 12). The increase of curing temperature in these materials do not because the durability problems that appear in Portland cement when is cured at temperatures above 60°°C.

The manufactured railroad cross ties were subject to the static and dynamic tests requires by the standards. The achieved results fulfilled both the the requirements of Spanish and the European standard. The static test consists in applying a static load over the section to study, and in this way determine the load that produces the appearance of the first crack; the load that produces a remaining crack of a certain width (0.05 y 0.10 mm); rupture load. The dynamic test consists in applying an increasing pulsatory load, simulating exceptional shock loads, and determining how loads produce the typical cracks during the static test. The achieved results of both tests are shown in Table 6.

Figure 12. Pictures about the manufacturing process of railroads cross ties, at industrial level, with alkali activated fly ash concretes (without OPC)

Table 6. Results achieved railroads cross ties made with alkali activated fly ash concretes

4. Conclusions

The main conclusions that can be extracted from this paper are:

In regard with their technologic properties, alkali activated fly ash concretes, mortars and pastes generally present best technologic properties than conventional Portland cement concrete, standing out the fast development of initial mechanical strength, low drying shrinkages, and excellent matrix-to-steel bonding.

In relation with their durability, mineralogical and microstructural phases of alkali activated fly ash pastes, mortars and concretes are different from those of an OPC, that is why deterioration processes are different. So they present an acceptable behaviour when exposed to sulfate and sea water attacks. In the presence of sulfate as demotion product, a sodium sulfate precipitation is formed. On sea water an ionic exchange of Na by Mg is produced, and that slightly modifies the morphology of the gel making it more porous. In highly acidic environments alkali activated fly ash pastes and concretes suffer a dealumination process. This implies a strength drop and mass loss. However, their behaviour is better than the observed in OPC pastes and mortars in equal conditions.

As for their strength at high temperatures, their mechanical properties are kept or improve between 25 and 600°C, interval in which the mechanical properties of commercial cement are quickly reduced. Nonetheless, the appearance of glass-like phases form 600°C produces the manifestation of local plasticity phenomena, which get generalized for higher temperatures. This causes a very quick material deterioration, restricting third use only to those cases in which the material does not need to bear loads at temperatures near or higher to 600°C.

Apart from good technologic performance of these materials it is very important to remark the easy adaptability of these kind of materials to the existing facilities in current industry. This makes you think that this new concrete can be used to manufacture any type of constructive element of precast industry, besides sleepers. Definitely, these results give place to think that in a near future this material could have an important development on the construction industry.

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E-mail: anafj@ietcc.csic.es

Contribución. Mejor artículo Conpat 2009

X Congreso Latinoamericano de patología de la Construcción y XII Congreso de Control de Calidad en la Construcción. Congreso Internacional de Patología, Control de Calidad y Rehabilitación de Estructuras y Construcción. 29 de Septiembre al 2 de Octubre de 2009. Valparaíso-Chile.