Part II: role of aluminium oxide content of GGBS on concrete in exposure to sulphate and acid environments
10 November 2015
Ballynancoran Wind Farm, Co Wexford
This, the final part of a two-part 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.
3.3. Microstructural analysis:
XRD Analysis The main purpose for this test was to examine the behaviour of reaction between the various cementitious binders and the sulphate solution. For this purpose samples were taken from those mortar prisms which were used in the sulphate expansion test at the end of the expansion test. The samples were then powdered using grinding or polishing machine. This powder was then examined using X-ray diffractometer. The results have been scanned and shown in figure 20.
3.3.1. Degradation depth measurement
Many pictures were taken with camera and SEM then they were treated with Image J program.
3.3.2. SEM observations and mapping
3.3.3. EDS chemical profiles:
Chemical profiles with EDS analysis were done in order to appreciate the chemical composition evolution of the cement paste in the samples
Form visual observation, the OPC mortar samples had degraded significantly more than the 50 per cent GGBS specimens, and the 100 per cent OPC mortars had greater crack of surface than that of the 50 per cent, GGBS mortars. It was also clear that GGBS A (Al2O3 = 10.16) had suffered less damage than GGBS B (Al2O3 = 14.76), GGBS had developed no cracking. The visual inspection clearly demonstrated concrete deterioration in aggressive environment due to sulphate attack.
Loss of compressive and flexural strength as a result of exposure to 10 per cent magnesium sulphate solution:
GGBS mixes showed a low loss in compressive and flexural strength than the 100% OPC mix, this is probably due to the hydraulic cements produced by GGBS.
The percentage loss in compressive strength of the 100 per cent OPC samples was over twice (11.09 per cent) as much as that of the 50 per cent GGBS-A samples (4.78 per cent) and 50 per cent GGBS-B samples (6.05 per cent).
The percentage loss in flexural strength of the 100 per cent OPC samples was near twice (4.8 per cent) as much as that of the 50 per cent GGBS-A samples (2.6 per cent) and for 50 per cent GGBS-B samples was (4.0 per cent).
Higher compressive and flexural strengths losses were evident for GGBS B compared the GGBS A.
Mass loss as a result of exposure to 10 per cent magnesium sulphate solution
The 100 per cent OPC samples showing a faster rate and greatest total loss of mass than the GGBS samples. The loss mass of the 50 per cent GGBS mortar was very slow.
The decrease in mass of the OPC mortar was quite small (4.65 per cent), however, it was relatively large when compared to the percentage mass lost by the 50 per cent GGBS-A (1.62 per cent) and 50 per cent GGBS-B (2.18). There was a higher rate of mass loss for GGBS B compared to GGBS A.
Length change of samples exposed to 10 per cent magnesium sulphate solution:
The changes in length resulting from shrinkage and thermal effects of the sulphate solution. The effects of replacement materials on the volume expansion of cement paste are shown in Table 5. The results indicate that the replacement of GGBS reduces expansion compared to control cement paste without GGBS.
The length of mortar tests results have been expressed as the difference in the percentage change in length. The percentage change in length for the 100% per cent OPC samples was (0.043 per cent) comparing to 50 per cent GGBS-A samples was (0.023 per cent) and to the 50 per cent GGBS-B samples was (0.027 per cent).
There was minimal percentage change in length between the GGBS A and B specimens.
Water absorption of OPC and GGBS samples:
The results showing in Tab. 6, that the OPC mortars initially absorbed more water than the GGBS samples indicating that is more porous. All samples showed a progressive increase in water absorption.
The amount of water absorbed by the 50 per cent GGBS-A samples was (4.6 per cent) is lower than (5.2 per cent) for 50 per cent GGBS-B samples and (7.8 per cent) that absorbed by the OPC samples. GGBS A was slightly less permeable than GGBS B.
XRD patterns for the powdered samples from the paste specimens exposed to magnesium sulphate solution, these patterns indicated the presence of ettringite, gypsum, portlandite, calcite, bassantle and vanessa, which are the expected products formed by cement hydration and magnesium sulphate attack.
XRD analyses shows that, the surface deterioration was observed in magnesium sulphate solution for GGBS as well as OPC prisms. The expansion of surface clearly shows the formation of ettringite in case of OPC.
The results on the graphs of the XRD are largely in agreement with the results which were obtained in the case of expansion tests. In the figures 18, 19, 20, the peaks are large and high which are indicating that gypsum and ettringite had been formed. in case of CEM I and with GGBS samples. So the results obtained from this test are indicating that GGBS containing samples perform better in the sulphate solutions than the OPC sample.
Important concentration of sulfur in all samples was observed due to gypsum formation. Calcium concentration is lower at the surface than in the core of the samples (calcium lixiviation phenomenon due to magnesium sulfate). For 50 per cent GGBS-B cement mortar, calcium concentration decreases in the core as magnesium concentration increases up to mass percentage close to 30 per cent. A large amount of gypsum crystals were observed in GGBS B specimens as evidenced in Figure 14 and Figure 16 whereas no crystals were evident in GGBS A.
OPC composites incorporating GGBS are more durable than those made with OPC alone in aggressive environments under the action of sulphates such as those acids produced by silage.
Durability of GGBS A was superior to GGBS B in testing and GGBS A specimens showed the smallest loss in compressive, flexural strength, water absorption and loss mass as a result of sulphate magnesium solution.
GGBS A and B have similar physical properties but have different Al2O3 contents, GGBS A – Al2O3 = 10.16; GGBS B – Al2O3.
Durability for agricultural concrete can be specified by incorporating lower Al2O3 content of GGBS as a partial substitute for OPC.
Lower Al2O3 content of GGBS concrete has better water permeability, improved resistance to corrosion and sulphate attack. As a result, the service life of a structure is enhanced and the maintenance cost reduced.
The C3A content of cement used in this study is 4.8 per cent which is below BS 8500 consideration for alumina content of GGBS. The difference in durability performance of source A and source B GGBS is attributed to Al2O3 content > 14 per cent.
Increasing the Al2O3 content in GGBS would greatly affect the strength and in results the concrete durability.
The laboratory programme has highlighted a concrete specification that is capable of withstanding aggressive substances. The range of aggressive environments associated with sulphate phenomenon and other forms of durability attack needs to be quantified and used as an input for future research work.
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
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