Rate of pozzolanic reaction of metakaolin in high-performance cement pastes

Abstract

This paper assesses the hydration progress in metakaolin (MK)-blended high-performance cement pastes with age from the measurements of compressive strength, porosity, and pore size distribution, the degree of pozzolanic reaction, and the Ca(OH)₂(CH) content of the MK-blended cement pastes at a water-to-binder ratio (w/b) of 0.3. Comparisons are also made with pastes containing silica fume (SF), fly ash (FA), and control Portland cement (PC). It is found that at early ages, the rates of pozzolanic reaction and CH consumption in the MK-blended cement pastes are higher than in the SF- or FA-blended cement pastes. The higher pozzolanic activity of MK results in a higher rate of strength development and pore structure refinement for the cement pastes at early ages. Although the rate of pozzolanic reaction of MK becomes slower after 28 days of curing, the pozzolanic reaction in the blended cement pastes with a w/b of 0.3 still continues at the age of 90 days. At this age, about half of the MK still are unreacted.

Introduction

The advances of concrete technology show that the use of mineral admixtures such as silica fume (SF) and fly ash (FA) is necessary and essential for producing high-performance concrete. In recent years, there has been a growing interest in the use of metakaolin (MK) for this purpose [1], [2], [3], [4], [5].

MK is a thermally activated aluminosilicate material obtained by calcining kaolin clay within the temperature range of 700–850°C [1], [3], [6]. It contains typically 50–55% SiO₂ and 40–45% Al₂O₃ and is highly reactive. It has been reported that the replacement of cement by 5–15% MK results in significant increases in compressive strength for high-performance concretes and mortars at ages of up to 28 days, particularly at early ages [2], [3], [4], [5]. The replacement also results in improved concrete durability properties, including the resistance to chloride penetration, freezing and thawing, and deicing salting scaling [2], [4].

While a number of studies have been conducted on high-performance MK concrete or mortars [2], [3], [4], [5], there has been few studies concerning the hydration progress of MK in high-performance pastes at low water-to-binder (w/b) ratios. The only study involving the hydration of MK-blended cement pastes with low w/b ratios seems to have been carried out by Ambroise et al. [6]. In their study, the control paste was prepared at a water-to-cement ratio (w/c) of 0.25. The MK pastes were prepared at varied w/b ratios from 0.28 to 0.54 when the cement was replaced by 10–50% MK to achieve the same consistency as the control paste. Most of the studies on the hydration reactions of MK-blended cement pastes were carried out with a w/b ratio higher than 0.5 [7], [8], [9], [10], [11]. Khatib and Wild [8] and Wild and Khatib [9] found that at a w/b ratio of 0.55, there is a decrease in the porosity and the CH content of MK-blended pastes and mortars at ages between 7 and 14 days. Subsequently, beyond 28 days, the porosity and CH levels increase. These phenomena were attributed to a critical change in the reaction between MK and CH at around 14 days, which involves the phase transformation of the reaction products, that inhibits the reaction of MK with CH [8], [9], [12]. Consequently, the strength enhancement produced by MK reaches a maximum within the first 14 days and then declines [12]. More recently, Frias and Cabrera [11] determined the pore size distribution and CH content of the MK-blended cement pastes with a w/b ratio of 0.55 at the ages of up to 365 days. They reported no increase in total porosity and CH content between the ages of 14 and 28 days, and suggested that the pozzolanic reactions in the pastes containing 10% and 15% MK completed at the ages of 56 and 90 days due to the total consumption of MK. It should, however, be noted that Murat [7], Khatib et al. [8], and Wild et al. [12] cured the samples in closed polythene containers, while Frias and Cabrera [11] cured the samples in airtight containers. Also, different types of cements were used for each study and the drying procedures varied widely.

The present study is concerned with the MK-blended cement pastes at lower w/b ratios. The hydration progress of MK-blended cement pastes at a w/b of 0.3 is assessed from measurements of compressive strength, porosity and pore size distribution, degree of pozzolanic reaction, and CH content. The degree of pozzolanic reaction of MK in these pastes was quantitatively determined using a selective dissolution method. This method has been developed for quantifying the degrees of pozzolanic reactions [13], [14], which is different from traditional methods for assessing the degree of cement hydration through the determination of nonevaporable water and CH contents. Although a number of studies [13], [14], [15], [16], [17] on the degree of pozzolanic reactions for FA-, SF-, and zeolite-blended cement pastes have been presented using this method, no similar study has been presented for MK-blended cement pastes. The results are also compared with those obtained for MK-blended cement pastes prepared at higher w/b ratios and those for SF and FA-blended cement pastes.