Silica fume concrete in hot and temperate environments
his investigation deals with the influence of hot and temperate curing environments on the hardened properties of concrete and mortar mixes. Condensed silica fume was blended with OPC as a potential alternative cementitious material to plain OPC for use in the hot Iraqi climate, in an attempt to find a cement combination that would overcome some of the durability problems experienced when using a plain OPC concrete in such an environment. Throughout the investigation two curing environments were used: the first simulating the UK temperate climate and the second simulating the hot Iraqi climate. Temperature and humidity were varied to simulate day and night time. The first stage of the experimental study was the development of a mix design method capable of producing an OPC-CSF cement concrete of a medium workability and a specific 28-days compressive strength ranging between 25 to 55 MPa, both with and without superplasticizer. Three grades of concrete strength were chosen (25,40 and 55 MPa) and the effect of four cement replacement levels (5,10,15 and 20%) of silica fume on concrete compressive strength was assessed. Test results showed that CSF was relatively more effective in lean mixes than in rich ones. Compressive strength of CSF concrete increased with increasing CSF percentages for both normal and superplasticized mixes up to an optimum levels of 10-15% and 15-20%, respectively. The amount of OPC (kg/m`3) necessary to bring a change in compressive strength of 1MPa was also determined and the theoritical blend proportions of OPC-CSF necessary to produce 28-day compressive strength equivalent to the plain OPC mixes were determined from the produced data above. The theoretical blend proportions were examined experimentally and the data were used to establish the relationship between strength and water/cementitious ratio for the blend mixes with and, without superplasticizer. Results showed that this basic relationship had changed quantitatively but not qualitatively when CSF was used. A cost study using current OPC and CSF material costs -was performed in an attempt to determine'the- most economic blend proportions. A total of eleven different concrete -mixes were selected to study the effect of curing environment (hot and temperate), initial curing time (0,1,3 and`-7 days) and curing method (water and polythene sheeting) on the compressive strength, permeability and absorption properties of the CSF concretes. Tests were carried out at 3,7, '14, 28,56,90 and 180 days of age. In addition five different mortar, mixes were used to examine the effect of curing environment (temperate and hot) an the *permeability, pore size distribution and durability to magnesium sulphate attack. Test results showed that hot Iraqi curing environment was favourable to the early-age strength, absorption and permeability of plain OPC mixes. However, - the later-age properties were significantly lower than those obtained for concretes cured in a temperate UK environment. For plain OPC mixes a critical curing period of 3 days was found under both temperate and hot environment. For the CSF blend mixes critical curing periods for the temperate and hot environment were found to be 3 and 1 day respectively. Results also reveal the importance of curing specimens immediately after casting for one day. Research work has also confirmed the superiority of water curing over polythene sheeting in a temperate environment for the rich plain OPC and CSF mixes. However, there was no significant difference between water and polythene for lean mixes. The reduction in , permeability and absorption properties of CSF mixes cured in a both temperate and hot environments is thought to be due to the changes in the pore structure brought about by the use of silica fume. Combining CSF with OPC was found to increase the percentages and volume of fine pores at the expense-of coarse pores. This effect may be described as a "refining" effect. Finally, the performance of CSF mortar mixes cured in a temperate and hot environment and their resistance to magnesium sulphate attack was significantly better than the plain OPC ones.