Scholarly article on topic 'The Influence of Gaize Addition on Sulphate Corrosion of CEM II/A and CEM II/B Cements'

The Influence of Gaize Addition on Sulphate Corrosion of CEM II/A and CEM II/B Cements Academic research paper on "Materials engineering"

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{"calcalerous gaize" / "sulfate attack" / expansion / "sulfate resistant cements"}

Abstract of research paper on Materials engineering, author of scientific article — Jan Małolepszy, Piotr Stępień

Abstract The paper presents and discusses the results of resistance to sulfate aggression cement mortars containing calcalerous gaize. Were tested cements containing 15-35% gaize (CEM II A, B) and the reference cement CEM I (no additive). Using DTA/TG, XRD investigated the effect of gaize on the hydration of cement. Linear change mortars were tested according to the procedure in PN-B-19707, complement the research was to determine the strength of the mortar as a result of following long-term storage of samples in corrosive environments. Mortar microstructure studies were performed using mercury porosimetry and SEM/EDS. The results of the study allow us to conclude that already 15% of the content of Gaize making cement resistant to corrosive environments rich in sulfate ions.

Academic research paper on topic "The Influence of Gaize Addition on Sulphate Corrosion of CEM II/A and CEM II/B Cements"

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Procedia Engineering 108 (2015) 270 - 276

Procedía Engineering

www.elsevier.com/loeate/proeedia

7th Scientific-Technical Conference Material Problems in Civil Engineering (MATBUD'2015)

The influence of gaize addition on sulphate corrosion of CEM II/A and CEM II/B cements

Jan Malolepszya, Piotr St^piena*

aAGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland

Abstract

The paper presents and discusses the results of resistance to sulfate aggression cement mortars containing calcalerous gaize. Were tested cements containing 15-35% gaize (CEM II A, B) and the reference cement CEM I (no additive). Using DTA/TG, XRD investigated the effect of gaize on the hydration of cement. Linear change mortars were tested according to the procedure in PN-B-19707, complement the research was to determine the strength of the mortar as a result of following long-term storage of samples in corrosive environments. Mortar micro structure studies were performed using mercury porosimetry and SEM/EDS. The results of the study allow us to conclude that already 15% of the content of Gaize making cement resistant to corrosive environments rich in sulfate ions.

© 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-reviewunder responsibilityof organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering Keywords: calcalerous gaize; sulfate attack; expansion; sulfate resistant cements

1. Introduction

The available range of cements acceptable by PN-EN 197-1 standard creates possibilities of choosing a binder for a specific construction element, considering its purpose. The development of construction engineering: hydraulic infrastructure, sewage treatment plants, large scale foundations, bridge supports or underground mining cause tremendous demand for cements of untypical characteristics, since the application of ordinary portland

* Corresponding author.

E-mail address: pstepien@agh.edu.pl

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 .2015.06.147

cements in such directions may cause difficulties in the execution of construction or lower concrete durability. Cements, that due to its chemical or phase composition exhibit favourable application properties of exceptional usefulness, are accounted as special cements and defined in PN-B 19707 standard [1]. The standard divides cements into 2 groups:

• high sulphate resistance cements (HSR)

• low alkali content cements (NA)

The standard considers cements sulphate resistant, if they pass the expansion test (maximum acceptable expansion of 0.5%) and have desired phase composition. For CEM I cement C3A content should be < 3% and Al2O3 content should be < 5%. Fly ash portland cement should have less than 10% C3A in clinker and at least 25% of silicaeous fly ash addition. In slag cements, the content of granulated blast furnace slag shall exceed 55%. For pozzolanic cements, the content of tricalcium aluminate in portland clinker should be less than 10%, and total content of silica fume and fly ash should exceed 25%. Authors [2,3] indicate the existence of additional group of "special" cements, characterized with low heat of hydration.

Sulphate corrosion is one of the factors deteriorating properties of concrete. Hydrated calcium aluminate and portlandite, both incorporated in hardened cement binders, can react with SO42- ions yielding ettringite (secondary, delayed), gypsum [4,5] or thaumasite [6]. New phase formation is accompanied by increase in volume (by 227% in case of ettringite and by 124% in case of gypsum, in respect to reagent [7]), introducing internal strain, resulting in gradual concrete deterioration. Many authors [8-17] widely discusses sulphate corrosion, indicating methods of improving resistance of cements on SO42- rich environment. Authors [8,9] suggest low C3A content cement application, limiting the amount of reagents available. The decrease of permeability of hardened matrix also leads to increased sulphate resistance by hindering the penetration of corrosive factor into the matrix. Another practical hints [10] are: lower w/c ratio (lesser amount of water or greater amount of cement), intensified compaction, prolonged curing and ageing period. Interesting concept of increasing sulphate resistance is stated by [11], introducing Ba2+ ions into the system, which cause instant sulphate ions precipitation in form of low-soluble BaSO4, which imposes the reaction with portlandite or tricalcium aluminate. The presence of calcite [12] also positively influences sulphate resistance. CaCO3 reacts with C3A, creating hydrated calcium carboaluminates (the reaction occurs from the very beginning of hydration process) which denses microstructure, and results in decrease of total porosity. The addition of pozzolana increases corrosion resistance in two ways [13]. Firstly, it dilutes the amount of C3A and secondly it reacts with Ca(OH)2, yielding additional amounts of C-S-H. The effect is the decrease in the amount of capilary pores and the increase in the amount of gel pores. This is especially demanded in the paste-aggregate contact zone, increasing adhesion of the matrix to the aggregate.

Calcareous gaize, being a natural mix of calcite and pozzolanicly active silica, increases corrosion resistance as a cement additive [14,15]. Authors have assessed the sulfate resistance of cement containing gaize, based on visual observation, XRD examination and strength changes after 3 years of immersion in corrosive solution. The paper brings the discussion of improving the corrosion resistance by adding gaize, by analysing the system by the means of SEM/EDS, porosimetry, DTA analysis and expansion in accordance with PN-B 19707.

2. Experimental

On purpose to define the influence of calcaraeous gaize on the corrosion resistance of cements, four cements were made, containing 0-35% of addition (table 1). Cement without additive was used as a reference cement (CEM 0), numeric symbols of other cements indicate the amount of gaize incorporated. Chemical composition of raw materials used is shown in the table 2. The amount of phases [% of mass], calculated by Bogue formulas, is following: C3S = 69.6; yS-C2S = 10.2; C3A = 9.5; C4AF = 6.7. Physical properties of cements, determined in accordance with PN-EN 196 standards, were subject to previous research [18,19].

Table 1. Composition of cements [% mass.].

Component CEM 0 CEM 15 CEM 25 CEM 35

Portland clinkier 95 80 70 60

Gaize - 15 25 35

Gypsum 5 5 5 5

Table 2. Chemical composition of raw materials [% mass].

Oxides CaO MgO SiO2 Fe2O 3 Al2Os SO3 Na2Oe L.O.I.

Gaize 33.5 1.1 32.1 1.0 1.7 0.2 0.1 30.1

PC 66.8 2.2 21.8 2.2 5.0 0.6 0.7 -

Gypsum 33.8 0.5 3.2 0.1 0.5 42.1 0.1 19.4

2.1. Examination of chemical corrosion resistance

The resistance on chemical corrosion was examined on cement mortars using two methods. The first one was based on measuring the length change of specimens immersed in Na2SO4 solution in respect to samples immersed in water, in accordance with PN-B 19707 standard. Cuboid-shape samples of dimensions 20x20x160 mm were prepared in accordance with PN-EN 196-1 standard, with decreased number of shake-ups to 10. Brass pins were mounted into the mortars, allowing for measurements using Graff-Kaufman apparatus. The specimens were stored in moulds for the first 24 hours, and were immersed in water for the next 27 days after demolding. After 28 days their length was measured, and 3 samples of each cement were placed in a solution of sodium sulphate of concentration of 16.0±0.5 grams per litre. Remaining samples were stored in water. Length measurements were made every 4 weeks for the period of one year. Corrosive solution was changed every 4 weeks. The results of the measurements are shown in the figure 1.

S O si 1

& * 0,6

-CEM O --CEM 15

---CEM 35

8 12 16

20 24 28 32 Time, weeks

36 40 44 48 52

Fig. 1. Expansion of mortars.

Obtained results show, that cement without the addition of gaize shows the biggest expansion, significantly exceeding the limit set by the standard of 0.5%. Cements incorporating gaize were showing no linear changes throughout the entire period of examination. The photograph of samples after one year of immersion in corrosive solution is shown in the figure 2. The second method of assessing the sulphate corrosion resistance of cements is based on their strength examination [20], and it is described as long-term or classical method. The method is based on placing mortar specimens in corrosive solution and in water, and comparing their strength after long periods of time. Specimens prepared in accordance with PN-EN 196-1 standard were stored in water for the first 28 days,

after which half of the samples were placed in 5% solution of sodium sulphate. After 180 and 360 days of immersion in corrosive solution (or in water) both series compressive strength was tested, and the results are shown in table 3.

Table 3. Results of sulfate corrosion resistance of cements [MPa].

Days Indeks

Sample 28 28 + 180 28 + 360 R co™ sive / R water

water water Na2SO4 water Na2SO4 180 360

CEM 0 53.9 55.7 48.1 55.5 37.8 0.86 0.68

CEM 15 53.2 55.2 57.7 56.9 53.6 1.12 0.94

CEM 25 46.8 54.1 56.3 55.3 55.0 1.04 0.99

CEM 35 43.3 48.7 51.9 51.7 50.1 1.06 0.96

The evaluation of sulphate resistance by the examination of compressive strength confirms the lack of resistance of mortars made of portland cement without gaize addition on long-term exposure to corrosive environment. Both after 180 and 360 days decrease in compressive strength was recorded. Mortars made of cement without gaize were characterised by large result scattering, which was caused by inconsistent progress of corrosion process. Gaize addition (of any amount) effectively eliminated corrosive effects. In case of these samples, no deterioration of properties were observed. Specimen sample of CEM 0 cement after one year of immersion in corrosive solution is shown in figure 3. Samples prepared from other cements did not show any geometrical changes or cracks.

Fig. 2. Specimens after one year in Na2SO4 solution.

Fig. 3. Sample CEM 0 after one year in Na2SO4 solution.

2.2. Course of hydration process

Changes in portlandite content shown above reflect the pozzolanic reaction taking place in the hardening mortar. Changes in the content of calcium hydroxide for reference sample and samples containing gaize were measured by the means of thermogravimetry, for the range of 450-600°C. Results are shown in respect to dry cement. Reference cement CEM I was used, with addition of ground gaize. For the measurement purposes the content of gaize was increased, on purpose to observe the reaction more distinctly. The amount of gaize, expressed as percentage of mass, is given on the chart. The examination was carried out on pastes, with constant w/c ratio of 0.35. Before the examination, the hydration was stopped by placing ground samples in vacuum dryer. Results indicate decreasing amount of portlandite over time in pastes containing gaize. The reaction with active silica creates additional amounts of hardly soluble C-S-H phase (of low C/S ratio). Products of this reaction positively modify the microstructure of the paste by decreasing porosity and improving pore structure.

8 7,5 7 6,5 6 5,5 5 4,5 4 3,5 3

♦ CEM I (B) CEM B (10) --a- CEM B (30) — X- CEM B (50)

50 100

Time [days]

Fig. 4. Evolution of the CH content (determined from TG mass loss) normalized to the PC content in paste.

2.3. Investigation ofmicrostructure of cement mortars

Microporosity and pore size distribution was examined by the means of mercury porosimetry in standardized mortars for all examined cements. Results for mortars after 360 days of curing in water are shown in figure 5.

0,09 ] 0,08 I 0,07

£ 0,05

H 0,03

JS 0,02

^ 0,01

CEM 0 CEM 15 CEM 25 CEM 35

Pore diameter [nm]

Fig. 5. Cumulative curves of pore volume for mortars after one year storage in water.

Presented curves show correlations of pore distributions in cements containing different amounts of gaize against reference cement. Content of gel pores of diameter less than 50 nm is increased with the increase of gaize addition, and it is highest for cement containing 35% of gaize. The situation is reversed in case of capillary pores (diameter of about 100 nm), which content is highest for reference cement. These pores are responsible for cement matrix permeability, and they are mainly located in the contact zone of paste and aggregate. Mortar samples cured in the corrosive environment (one year in Na2SO4 solution) were examined by the means of scanning microscope. Figure 6 shows the microstructure of a mortar prepared from a reference cement. Products of sulphate corrosion such as gypsum (point 1) and ettringite (point 2) can be observed (their presence is confirmed by the EDS analysis

carried out in the marked areas). Large and well developed crystals of gypsum are visible, evenly distributed on the surface of the picture. This confirms advanced corrosion of the sample.

Fig. 6. SEM micrographs of mortar CEM 0. Fig. 7. EDS point 1 in Fig. 6. Fig. 8. EDS point 2 in Fig. 6.

SEM photographs presented below show the breaking of mortar prepared with CEM 15 cement (figure 9-10). In the figure 9 the contact zone between cement paste and aggregate is visible. A very close bonding of paste with the aggregate is visible, the whole zone is densely packed and creates a massive and impermeable microstructure. Figure 10 shows the same area of the sample with 10 times greater magnification in respect to figure 9. EDS analysis shows the presence of C-S-H phase in a dense form. There are no corrosion products visible.

Fig. 9. SEM micrographs of mortar CEM 15 (1000x). Fig. 10. SEM micrographs of mortar CEM 15 (10000x).

O Ka CK i

111.., l.Ll

.1 iiill Jili. L. LI

Fig. 11. EDS point 1 in Fig. 10.

3. Conclusions

Conducted research clearly indicate the positive influence of calcaraeous gaize addition on the hydration of cement paste, giving sulphate resistant properties. Linear changes of binders containing calcaraeous gaize meet the requirements of a standard in terms of constant linear size of specimens. The addition of gaize causes that mortars retain their properties even after one year of being exposed to aggressive environment. Gaize incorporated in cement immunes the cement matrix in two ways against corrosive factors:

• by reacting with Ca(OH)2, which is easily corrosive due to its relatively high solubility,

• by changing the pore structure, which hinder transport of gases and liquids containing corrosive agents. The results show that the addition of 15% of gaize makes cement sulphate resistant.

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