Over the last years great effort has been made in order to substitute clinker for less energy and low CO2 demanding materials. In many countries, it is popular to use GGBS as an additive material to meet these requirements and to improve the durability of concrete. The construction industry needs durable materials with improved properties.
Following this objective, this work is a part of an ongoing project developed in the materials laboratory of Casey Concrete, Co Wexford, in co-operation with Warsaw University of Technology, Poland, Toulouse University, France.
This paper presents data on engineering properties such as compressive strength, water absorption, expansion and visual change of mortar specimens incorporating GGBS addition, subjected to severe sulfate attack and cured at 20◦C. Specimens with 50 per cent replacement level of GGBS addition from two different sources, were immersed in magnesium sulfate solutions with 10 per cent of concentration for periods of up to 150 days.
The experimental results show that addition of 50 per cent GGBS to concrete structures improves the strength, and resistance against aggressive acid and sulphate environments. In order to identify the products formed by sulfate attack, microstructural analyses, such as XRD and SEM, were also performed on paste and mortar samples with similar replacement levels of GGBS addition.
The test results demonstrated that mortar and paste samples incorporating GGBS with higher content of Al2O3 were more susceptible to sulfate attack. On top of that, the deterioration was strongly associated with ettringite, gypsum and formation in the magnesium sulfate solution.
1. Introduction
The problem with concrete durability was and remains in the field of scientific interest as well as construction engineers, currently developing concrete technology develops in the direction of improving and obtaining concrete to ensure higher stability at low production costs.
From long time, the development of concrete technology has been attributed for a high strength and we are at present conscious that high strength is not always equivalent to high durability.
The foundation of essential the durability of concrete structure is European standard I.S-EN 206-1: "Concrete-Part 1:Requirement, properties, productions and conformity” [1], were specifies the minimum cement content and maximum w/c ratio for concrete threatened to chemical aggression, depending on the concrete exposure classes (XA)
The farm yard is very exacting on concrete, exposure to silage effluent, slurry, cleaning fluids and mechanical abrasion from farm vehicles often results in concrete damage, requiring concrete to be repaired or replaced. The main aggressive agents on the farm are acids and sulphates: lactic and acetic acid are present where silage is stored; sulphates are present in different effluents and manures.
The greatest amount of concrete damaging to occur where is a combination of chemical attack and physical abrasion, such as silage pit aprons where tractors are handling silage, animals are trafficking slabs or where power-hosing washes acids onto concrete floors.
Concrete is a porous material and exposed to various damaging environments results in a change in its properties. With greater degree of permeability of concrete can lead to sulphate corrosion in the process gives rise to expansive gypsum, ettringite, eventually weakening and destruction of concrete. The hydration of GGBS produces increases level of Calcium Silicate Hydrate (CSH) which reduces the permeability of concrete.
Sulphate ions can be in surrounding in acidic rain, sulphurous groundwater and other sources on the farm and can migrate into the concrete by diffusion and then react with the active chemical components of the concrete.
The influence of magnesium sulphate in the soil and groundwater may impact and reduce the life of the concrete structure; these sulphates are capable of expansion, cracking, spalling and loss of strength in the concrete structure. The attack occurs when sulphate in the water reacts with the Tri-calcium aluminate (C
3A) present in cement and phase formed during hydration.
Tri-calcium aluminate (C
3A) is a main component of the cement which is reacted with sulphates in the sulphate corrosion process. GGBS (Ground Granulated Blast Furnace Slag) does not contain tri-calcium (C
3A) and its inclusion in concrete reduces the susceptible material to sulphate attack. Both the reduced C
3A content and permeability of concrete made with GGBS make it ideal for the farm environment.
BS 8500 [2] places a restriction on cement combinations where the C
3A content of the Portland cement fraction shall not exceed 10 per cent if the alumina content of the slag exceeds 14 per cent. For this reason two sources of GGBS are considered in this study, one with an alumina content > 14 per cent and one with an alumina content < 14 per cent. The C
3A content of cement used in this study is 4.8 per cent.
1.1 Background
Problems with durability of the concrete in contact with soil or groundwater containing sulphate leads to degradation caused by the phenomenon of concrete expansion in the cement paste.
Durability problems can occur with concrete in contact with soils or groundwater containing sulphates which cause expansion forces in the cement paste leading to degradation.
Many studies was done to determine the effect and performance of blast furnace slag on the concrete structures were based on the chemical composition and mineralogy [3-4].
Mantel [5] lead and tested different mixes and found there is no relationship between the common ratios and performance. With this, there are still generally strong dependence on such ratios, and they are incorporated into standards even; for example, it is required by BSI 197-1:2000 that (CaO + MgO)/SiO2 should exceed 1 [6].
By the European standard EN 197-1 one of the main mineral additives used in the production of cement, it is granulated blast furnace slag. The chemical slag composition consists mainly of oxides of CaO, MgO, SiO2 and Al2O3 [6].
A study examining the role of alumina content in GGBS found that richest in alumina, favoured the formation of calcium hemicarboaluminate, even at later stages of hydration. The composition of the slag also had an effect on the hydrotalcite-like phase seen in all blended systems [7]. For this reason two GGBS sources with different alumina contents were considered in this study.
Ground granulated blast furnace slag is an addition inactive product, whose slow hydration which requires activation, produce analogous hydration products to cement, mainly a C-S-H, albeit with a lower ratio of Ca/Si and increased the ratio of Al/Si [8,9].
Also, a change in content of aluminate, carbonate and sulfate can affect on the hydrate assemblage [10]. The quantity and composition of the hydrates formed are dependent on the composition of the starting materials that comprise the blend. The slow hydration of the slag, leading to a lower degree of hydration at very early ages, slag blends can outperform neat cement systems at later ages with respect to strength [11,12].
Increasing the performance is dependent on the hydrate assemblage phase and cement composition microstructure. The hydration of slags in ordinary portland cement blends can be accompanied with some consumption of CH [13].
The work defines the phenomenon on the change in hydrate phase assemblages as cement is blended with blast furnace slag, and the correlation and effect this to mechanical performance. Also, the impact of the alumina content of the slag is were studied [14], and confirmed that, the effect of sulfate level,is an important additive to regulate setting time.
Richardson confirmed that the acids can cause ‘dissolution of the cement paste and certain types of aggregate’ [15].
The attack of sea water on the concrete occurs via several factors such as freezing and thawing attack, the reaction of sulphates and erosion. Chemical reactions occur when the cementitious material is in contact with a sulphate solution and can include processes of alkali-aggregate and dissolution of hydrates. Chemicals of the sulphate solution, in terms of the type and concentration of actions are those triggers reactions that can be very serious in the particular environment.
Cao confirmed, that, sulphate solution react with C
3A in hydrating Portland cement, and the resulting compounds are about double the size of the original compounds [16].
Perkins confirmed, that there are two main mechanisms of attack on concrete that result due to the presence of sulphate ions. The first of these reactions is the production of gypsum and the second is ettringite formation, as shown in the equations below. In most cases, the formation of gypsum is related with loss of strength, mass and expansion [17].
Ca(OH)2 + SO4 + 2H2O ® CaSO4 . 2H2O + 2OH
2(3CaO. Al2O3 . 12H2O) + 3(Na2SO4 . 10H2O) ®
® 3CaO. Al2O3 . 3CaSO4 . 32H2O + 2Al(OH)3 + 6NaOH +16H2O
3CaO . 2SiO2 . 3H2O + 3MgSO4 + (6 + 2n)H2O ®
® 3CaSO4 . 2H2O + 3Mg(OH)2 + 2(SiO2 . nH2O).
2Na + MgSO4 + H2O ® Na2SO4 + Mg + H2O
Durability is the ability of concrete to resist abrasion, weathering action and chemical attack. Different concretes require different degrees of durability depending on the exposure environment Fig. (1).
1.2 Concrete with GGBS improved durability
GGBS as an economic, technically superior and environmentally friendly additive cement can improve the durability of a concrete by reducing the water permeability, GGBS concrete as compared with ordinary Portland cement concrete has greater long-term strength, limit the temperature rise in large concrete pours, a low permeability resulting in reduced chloride penetration, increasing resistance to sulphate attack, corrosion and alkali silica reaction. GGBS can extend the service life of concrete structures and reduce the overall maintenance costs [18].
Generally GGBS in concrete can improve the durability. GGBS concrete has a low permeability resulting in reduced chloride penetration, enhanced resistance to sulphate attack and alkali silica reaction as compared with ordinary Portland cement concrete [19]. It has been reported that a higher calcium hydrate (CH) content will in general produce concrete of poor durability due to an inhomogeneous mix with poor bonding between calcium silicate hydrate (CSH) and CH. Higher CH contents will lead to a greater permeability and a lower durability.
The GGBS particles are retained in CSH form resulting in a hardened paste of greater density and smaller pore size as compared to Portland cement paste. Smaller pore size gives rise to a lower permeability and from this a higher durability [20].
Laboratory testing results and field experience have shown that properly proportioned slag Portland cement concretes have the following properties compared to ordinary Portland mixes: higher strengths, higher ratio of flexural to compressive strengths, improved resistance to sulphates, also slag Portland cement has lowered expansions from magnesium sulphate reactions.
2. Materials and test methods
The mortar mixes were prepared and tested to investigate the relative resistance of the cementations binders to the aggressive sulphate environments. Mortar prisms of dimensions 40 × 40 × 160 mm, were made and tested after [28, 56, 90, 120, 150, 180] days for (compressive, flexural) strength and water absorption.
Mortar prisms of dimensions 20 × 20 × 160 mm, were made and tested after [4, 8, 12, 16, 20] weeks for mass loss and prism expansion. The microstructural analysis and X-ray analysis (XRD) was taken in (Department of Building Materials Engineering Toulouse University, France) for the prisms which were immersed in 10 per cent Magnesium sulphate after a five-cycle period.
All the mortar prisms were prepared through use of the standard mix in EN 196-1 for cement conformity testing. Each mix contained 450 kg of cement, 1350 kg of sand and 225 ml of water.
The following locally available materials were used for this research:
- Sand: (Casey Enterprises- Sand Pit)
- Cement: (CEM I, 42.5R, Ordinary Portland Cement - O Brien Cement), Tab. (1)
- GGBS: (Ground Granulated Blast-Furnace Slag), Tab. (1)
The freshly cast specimens were placed in a moist air cabinet at 20 ˚C and de-moulded after 24 hours. After this they were immersed in a water bath at 20 ± 2˚C to cure for period 28 days.
After 28 days the prism were divided in two parts, one part were placed in a curing tank at 20 ˚C for control test (as reference, witness) for (compressive, flexural) strength and water absorption, and the reference samples for (mass loss and expansion) were stored in distilled water, and the second part of the prisms were immersed in 10% Magnesium sulphate solution.
Durability tests by salt crystallisation were carried out in three mixes: 100 per cent OPC, 50 per cent GGBS-A, 50 per cent GGBS-B. Five cycles, (one cycle=28-day) of 10 per cent magnesium sulphate solution were repeated over a five-month period. In order to determine the rate of deterioration, following each cycle, the mortars were tested for water absorption, compressive strength, mass loss, prism expansion, Microstructural analysis and XRD.
The length of each sample exposed to magnesium sulphate environment was weekly measured at the first month and later was monthly by using a vertical comparator.
Table (1) Chemical characterisation of cement: CEM I 42.5R & GGBS
3. Experimental results
Laboratory results of the mortar prisms for sulphate magnesium resisting test for all binder combinations were taken for a period of five months. The results are shown in the Table [2, 3, 4, 5, 6].
3.1.Visual inspection following exposure to magnesium sulphate solution:
3.1.1. For 100 per cent OPC mortars samples, visual degradation was obvious on corners, edges, longitudinal crack indicated also we observed a lot of white stains salt on the mortars surface. See Figure (2).
[caption id="attachment_24607" align="alignright" width="226"]
Figure (2). 100 per cent OPC after immersion in 10 per cent MgSO4 solution[/caption]
3.1.2. For 50 per cent GGBS-A mortar samples visually fared better on exposure to 10 per cent magnesium sulphate solution. The degradation took very little place, there was very slight rounding on the corners and no cracking observed. See Figure (3).
[caption id="attachment_24609" align="alignright" width="300"]
Figure (3). 50 per cent GGBS-A with Al2O3 = 10.16 after immersion in 10 per cent MgSO4 solution[/caption]
3.1.3. For 50 per cent GGBS-B mortar samples visually observed little surface degradation took place, there was slight rounding in the corners and small longitudinal crack indicated. See Figure (4).
[caption id="attachment_24611" align="alignright" width="300"]
Figure (4). 50 per cent GGBS-B with Al2O3 = 14.76 after immersion in 10 per cent MgSO4 solution[/caption]
[caption id="attachment_24612" align="alignright" width="300"]
Figure (4). 50 per cent GGBS-B with Al2O3 = 14.76 after immersion in 10 per cent MgSO4 solution[/caption]
3.2. Samples test results
3.2.1. Loss of Compression strength:
From mortar samples test results Tab. 2 and Figure (2.1, 2.2), the samples containing 50 per cent GGBS showed a greater compression strength than the 100 per cent OPC after immersed in 10 per cent magnesium sulphate solution.
Tab.2. Loss of compression strength as a result of exposure to 10 per cent MgSO4 solution
3.2.2. Loss of flexural strength:
From Tab. 3 and Figures (3.1, 3.2), Mortar samples containing GGBS showed a greater flexural strength than the 100 per cent OPC after immersed in 10 per cent magnesium sulphate solution.
3.2.3. Mass loss as a result of salt crystallisation:
From mortar samples test results Tab. 4, and Figures (4.1, 4.2, 4.3), showed us that, the 100 per cent OPC samples lost mass at a faster rate than the GGBS samples after immersed in 10 per cent magnesium sulphate solution.
[caption id="attachment_24621" align="alignright" width="300"]
Fig 4.2[/caption]
[caption id="attachment_24622" align="alignright" width="300"]
Fig 4.3[/caption]
3.2.4. Expansion as a result of exposed to magnesium sulphate solution
The mortar samples results Tab. 5 and Figure 5-1, showed the high percentage change in length for the 100 per cent OPC samples in comparing to GGBS samples after immersed in 10 per cent magnesium sulphate solution.
Tab. 5. Expansion as a result of exposure to 10 per cent MgSO4 solution
[caption id="attachment_24624" align="alignright" width="300"]
Fig 5.1. Expansion as a result of salt crystallisation (10 per cent MgSO4). mm on the left; Day on the right[/caption]
3.2.5. Water absorption of OPC and GGBS samples:
From the Tab. 6 and Figure 6.1 showed the amount of water absorbed by the 50 per cent GGBS-A samples is lower than 50 per cent GGBS-B and that absorbed by the OPC samples.
[caption id="attachment_24626" align="alignright" width="300"]
Fig 6.1. Absorption (percentage) as a result of salt crystallisation in (10 per cent MgSO4). Percentage is on the left, day on the bottom[/caption]
End of Article Part I. Part II will be published on November 10
Authors: Professor Pawel Lukowski, head of the Department of Building Materials Engineering, Warsaw University of Technology, Poland; Professor Martin Cyr, head of the Department of Building Materials Engineering, Toulouse University, France; Dr Joanna Julia Sokolowska, Department of Building Materials Engineering. Warsaw University of Technology, Poland; Ali Salih, MSc Eng, technical and quality manager, Casey Concrete, Co Wexford