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Concrete In Australia : December 2011
Concrete in Australia Vol 37 No 4 21 Review of sugar attack of concrete and its prevention Marita L. Berndt† AECOM and Centre for Sustainable Infrastructure, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria Sugar solutions and acid reaction products are aggressive towards hardened concrete. e chemistry of reactions causing deterioration is not well researched and appears to be complex. is paper reviews sugar attack mechanisms, provides examples of deterioration and discusses means of preventing attack. Concrete mix design and the use of protective overlays are considered. In addition, recommendations for successful installation of overlays and avoidance of common defects are provided. 1.0 INTRODUCTION e use of sugar to retard the set of concrete is well known. Sugar solutions and sugar reaction products can also cause degradation of hardened concrete. Dry sugar in itself is not chemically aggressive towards concrete. However, if the sugar goes into solution through contact with water, deterioration of concrete can be expected. Examples of situations where sugar attack may occur include sugar refineries, sugar storage facilities and beverage and food processing plants. Concrete exposed to sugar solutions undergoes degradation of the binding phases in hardened cement paste thereby causing surface loss and exposure of aggregate particles. In practical terms this can eventually lead to uneven and rough surfaces and reduction in surface strength of the concrete. ese consequences can impact operational aspects as well as safety. Examples of sugar attack on concrete are shown in Figures 1 to 4. All photos were taken of floor slabs exposed to dry and occasionally wet sugar or sugar solutions. e floors are also subjected to front end loader traffic and abrasion by particulate sugar. e original concrete mix design for the slabs was unknown but the design strength was 3000 psi (21 MPa). e objective of this review is to present a summary of relevant literature on sugar attack of concrete and how it might be prevented. 2.0 SUGAR ATTACK MECHANISMS Both sugar solutions and sugar reaction products are implicated in degradation of concrete. e exact mechanism of sugar attack is not well researched. Biczók (1972) provided detail on sugar degradation and suggested that sugar solutions combine with calcium hydroxide in hardened cement paste to form water soluble lime salts which are readily leached from concrete. Caution against direct exposure to sugar solutions was given by Biczók (1972) due to the likely additional presence of organic acids. e most detailed published study of sugar degradation of concrete appears to be that of Skenderovic et al (1991). e authors conducted a series of experiments in which samples of hardened cement paste were exposed to different concentrations of sugar solutions varying from 5% to 20%. It was determined that the reactions between sugar solutions and hardened cement paste were more complex than reaction with Ca(OH)2 alone. Magnesium, silicon, aluminium and iron ions, in addition to calcium ions, were leached from the paste. ese ions probably originated from calcium silicate hydrate, calcium aluminate hydrate and calcium alumina-ferrite hydrate phases in cement paste. e results indicated that ferrites are readily dissolved in sugar solutions, followed by aluminates and silicates. Mass loss increased with time and increasing the sugar concentration from 10% to 20% did not significantly influence the rate or total mass loss. Porosity increased on exposure to sugar solutions. Although Skenderovic et al (1991) did not measure changes in strength, the observed dissolution of Ca2+, Mg2+, Al3+ and Si4+ and associated decomposition of hydrate phases responsible for cementitious binding can be expected to cause significant loss of strength. e increased solubility of calcium ions in cementitious materials exposed to sugar solutions is used in studies of thaumasite formation. Research by Martinez-Ramirez et al (2011) investigated the effect of sugar solution concentration and temperature on formation of thaumasite. e study involved reactions between sodium silicate, sodium carbonate, sodium sulphate and calcium oxide sugary solutions rather than actual cement. However, the production of a sucrose- calcium adduct (CaO in sucrose) was found to increase the calcium solubility. is adduct is a precursor to thaumasite and its formation is favoured at higher sucrose concentrations. us, the reactions between hydrated cement paste and sugar solutions appear complex and dependent on several factors including concentration and temperature. 3.0 ORGANIC ACID ATTACK As discussed above, sugar reaction products can include organic acids. Depending on conditions, other reactants and Corresponding author Marita Berndt can be contacted by email at: firstname.lastname@example.org.