Influence of mineral admixtures on thermal conductivity and compressive strength of mortar

Abstract

Cement paste with natural sand content and the effect of silica fume (SF), class C fly ash (FA) and blast furnace slag (BFS) on the thermal conductivity and compressive strength of mortar was investigated. SF, FA and BFS were added as replacement for cement by decreasing the cement in the ratios of 10, 20 and 30% by weight.

The maximum thermal conductivity of 1.186 W/mK was observed with the samples containing plain cement. It decreased with the increase of SF, FA and BFS as replacement for cement. The reductions due to SF were 17, 31 and 40% for 10, 20 and 30% SF (replacement for cement), respectively. FA induced reductions of 14, 26 and 33% for 10, 20 and 30% FA, respectively. Both SF and FA had a decreasing effect on thermal conductivity. BFS effect on the thermal conductivity, was approximately the same at all percent (BFS replacement for Portland cement, PC) and the reduction values were between 12 and 14%. Ten percent SF and 10 and 20% FA and all level BFS increased compressive strength a little at 120 days. However, except 10% SF the other admixters at all level replacement decreased compressive strength at early ages, especially FA decreased compressive strength as function of replacement percent.

Introduction

In recent years, in countries of mild climate, more attention has been given than before to reducing energy consumption whilst to maintain or improve comfort conditions in buildings. To this end, effort has been concentrated on improving the efficiency of heating appliances and the thermal insulation of buildings. In this study, it was concerned with the effect of silica fume (SF), fly ash (FA) and blast furnace slag (BFS) on the properties of mortar, and in particular with the thermal conductivity of these mineral ingredients.

Thermal conductivity of concrete increases with increasing moisture content. Since water has a conductivity about 25 times that of air, it is clear that when the air in the pores has been partially displaced by water or moisture, the concrete must have greater conductivity [1], [2], [3], [4], [5]. Steiger and Hurd [6] reported that when unit weight of concrete increased 1% due to the water absorbsion, the thermal conductivity of these specimens increases 5%.

Thermal conductivity of concrete increases with increasing cement content [7], [8], and thermal conductivity of aggregate [3], [4]. SF causes a decrease in the thermal conductivity and an increase in the specific heat of mortar [9].

Since the thermal conductivity of crystalline silica is about 15 times that of amorphous [10], it is natural for the concretes with amorphous silica to have lower conductivity [11], [12]. The amorphous silica in the mortar, which is the continuous phase in concrete taken as a composite, may also contribute to lower the thermal conductivity.

Admixtures, such as SF, FA, and BFS are used replacement for cement for improving the mechanical properties, decreasing the rate of hydration, decreasing the alkali aggregate reactivity and decreasing the permeability of concrete. However, their effects on the thermal conductivity have received little attention [9], [12].

It has been demonstrated that SF increases the strength of mortar [13]. However, it is also reported that SF is more effective in concrete than in paste [14]. This increase in concrete strength containing SF is attributed to an aggregate-paste bond improvement, which is associated with formation of less porous transition zone in SF concrete [15].

The relative performance of the FAs in concrete depends on the brand of cement used. In addition, the age of the test is an important factor influencing the relative performance of the various cementing materials [16].

In view of the global sustainable development, it is imperative that supplementary cementing materials be used in place of cement in the concrete industry. The most worldwide available supplementary cementing materials are FA, a by-product of thermal power stations, and SF, a by-product of silicon metal. It is estimated that, approximately 600 million tonnes of FA is available worldwide now, but at present, the current worldwide utilization rate of FA in concrete is about 10% [17]. Due to the rapid economic development and the growth in the world population consumption of the energy over the world, the FA has significantly increased. Thus, air and environment pollution became a problem, then, the idea of using waste material has gained popularity. FA, SF and BFS are three of the most common concrete ingredients due to their pozzolanic properties [17], [18].