Cement and Concrete Research 29 (1999) 497–501 0008-8846/99/$–see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-8846(98)00197-5 Effect of curing procedures on properties of silica fume concrete Houssam A. Toutanji b, *, Ziad Bayasi a a Department of Civil Engineering, San Diego State University, San Diego, CA 92182, USA b Department of Civil and Environmental Engineering, University of Alabama in Huntsville, Huntsville, AL 35899, USA Manuscript received 24 September 1997; accepted manuscript 19 October 1998 Abstract The effect of curing procedure on hardened silica fume concrete is reported. The silica fume contents were 10%, 15%, 20%, and 30% by weight of cement. The aggregate to cementitious material ratios (cement + silica fume) ranged from 1.0 to 3.8. Three different curing meth- ods were used: steam, moist, and air curing. Mechanical properties such as compressive strength, permeability, and permeable voids were de- termined. Steam curing was found to enhance the properties of silica fume concrete, whereas air curing exhibited adverse effects as com- pared to moist curing. Enhancement in the mechanical properties of silica fume concrete caused by steam curing was manifested by strength increase and permeability and permeable void volume decrease. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Curing procedure; Moist curing; Steam curing; Air curing; Silica fume; Compressive strength; Flexural strength; Permeability It is well documented that the use of silica fume as a par- tial replacement for cement in combination with superplas- ticizers provides a significant increase in the strength of concrete [1–3]. The water reduction of fresh concrete and the formation of a more densely compacted matrix at the in- terfacial zone are believed to be the main causes for this im- proved strength, permeability, and durability. The action of silica fume in concrete is physiochemical. The physical phase of this action is in the refinement of the void system of cement paste and particularly the transition zone. The chemical phase consists of the pozzolanic reaction, which transforms the weak calcium hydroxide [Ca(OH) 2 ] crystals into the strong calcium silicate hydrate (CSH) gel. The results of these actions of silica fume provide signifi- cant improvements in compressive and flexural strengths, as well as dramatic improvements in durability and imperme- ability [4–7]. Utilization of methods of curing for silica fume concrete requires consideration. Curing affects the properties of plain and silica fume concrete by altering ce- ment hydration and pozzolanic reaction. Bentur et al. [8] reported that the strength of silica fume concrete is greater than that of silica fume paste. They at- tributed it to the change in the role of aggregate in the con- crete. In cement concrete, the aggregate functions as an in- ert filler, but, due to the presence of the weak interfacial zone, composite concrete is weaker than cement paste. A similar conclusion made by Toutanji and El-Korchi [9] was that the increase in compressive strength of mortar contain- ing silica fume, as a partial replacement for cement, greatly contributes to strengthening the bond between the cement paste and aggregate. They reported that partial replacement of cement by silica fume and the addition of superplasticizer increase the strength of mortar but have no influence on the strength of cement paste. Their results were verified by sta- tistical analysis using hypothesis testing at a 95% confi- dence level. On the other hand, some researchers have reported that the increase in concrete strength is due, in great part, to a higher quality of the cement paste matrix. Cong et al. [10] found that the replacement of cement by silica fume (up to 18%) and the addition of superplasticizer increase the strength of cement paste. Darwin et al. [11] found that the strengths of both cement paste and mortar increase when 15% of the cement is replaced by silica fume. However, the ratio of the strength of mortar to the strength of cement paste is lower for the materials containing silica fume, which indicates that using silica fume enhances the strength of mortar more than it does to that of cement paste. ACI Committee 234 [12] concluded that silica fume does not improve the 7-day compressive strength of concrete when cured at 50°F (10°C). However, when cured under temperatures exceeding 68 ° F (20 ° C), the 7-day compressive strength tends to improve significantly [13]. It was also in- dicated that the pozzolanic reaction is, in general, very tem- * Corresponding author. Tel.: 205-890-6370; Fax: 205-890-6724; E-mail: toutanji@ebs330.eb.uah.edu.