16Chloride ion penetration resistance (CPR) of concrete containing fly ash (FA)/silica 17 fume (SF) against combined freezing-thawing and chloride attack was studied. The 18 total charge passed, immersed in tap water and sodium chloride solution, subjected to 19 50 freezing-thawing cycles was evaluated. It was found that immersed in tap water, 20 SF had more evident improvement on concrete's resistance to combined effects than 21 * (Dezhi Wang).FA. Sodium chloride solution immersion for 41d prior to test was more aggressive 22 than tap water. After 50 freezing-thawing cycles, CPR of concrete with FA increased, 23 while that with SF decreased. Interaction between freezing-thawing and chloride 24 attack accelerated concrete deterioration. 25 26 fly ash; silica fume; freezing-thawing; chloride attack; sodium chloride concentration; 28 interaction between freezing-thawing and chloride attack 29 30 1. Introduction 31 Concrete is versatile and the most widely used construction material in the world. But 32 owning to aggressive marine exposure environment and the extensive use of de-icing 33 salts in many countries, chloride induced corrosion becomes one of the most common 34 causes of degradation of reinforced concrete structures [1-4]. The first effect of 35 chloride ions is physical salt attack leading to surface cracking and scaling which is 36 similar in appearance to freezing-and-thawing damage and total disintegration of 37 low-quality concrete [5]. Another effect is that chloride ions are the most important 38 cause of corrosion of embedded rebar. When chloride ions penetrate concrete cover 39 and arrive at reinforcement bars, as their amount accumulates, the passive film may 40 break down (i.e. de-passivation) and corrosion of embedded rebar can then initiate [6, 41 7]. The accumulation of corrosion products can build up the swelling pressure around 42 the rebar resulting in cracking or spalling of concrete[8], which in turn facilitates the 43 ingress of moisture, oxygen, and chlorides to the rebar and accelerates rebar corrosion 44 [9]. Pitting corrosion is another threat to RC structures in a chloride environment [10] 45 and is a type of more serious corrosion on structural safety than general corrosion[11, 46 12], since it has resulted in quite high loss of cross-sectional area of reinforcement 47 bars [13] and structural damages[14], or in extreme situations, the final collapse of the 48 structure. 49 Chloride penetration in concrete can be characterized by the chloride diffusion 50 coefficient and the binding ability of matrix-forming solids [15]. In concrete, 51 chlorides can be chemically bound with cement's C3A or C4AF phases (e.g., Friedel's 52 salt) [16], or physically hold to the surface of hydration products (e.g., adsorption on 53 C-S-H) [3, 17]. Chloride diffusion depends on pure diffusion for water-saturated 54 concrete and capillary absorption of salty water for non-saturated concrete [18]. 55 Recently, there are several studies reported in literature on the transport of chloride 56 ions in conc...

This research investigates the mechanism of metakaolin for mitigating the autogenous and drying shrinkages of alkali-activated slag with regard to the activator parameters, including concentration and modulus. The results indicate that the incorporation of metakaolin can decrease the initial viscosity and setting time. Increasing activator concentration can promote the reaction process and shorten the setting time. An increase in the metakaolin content induces a decrease in compressive strength due to reduced formation of reaction products. However, increasing activator dosage and modulus can improve the compressive strength of alkali-activated slag containing 30% metakaolin. The inclusion of metakaolin can mitigate the autogenous and drying shrinkage of alkali-activated slag by coarsening the pore structure. On the other hand, increases in activator concentration and modulus result in an increase in magnitude of the autogenous and drying shrinkage of alkali-activated slag containing metakaolin. The influence of the activator modulus on the shrinkage behavior of alkali-activated slag-metakaolin binary system should be further investigated.

Metakaolin-based geopolymers possess excellent corrosion and high-temperature resistance, which are advantageous compared to ordinary Portland cement. The addition of slag in metakaolin-based geopolymers is a promising approach to improve their mechanical properties. Thus, this study investigated the effect of slag content on the strength and shrinkage properties of metakaolin-based geopolymers. Increasing the slag content and Na2O content was beneficial to the reaction of alkali-activated metakaolin-based geopolymers, thereby improving their compressive strength and density. After 56 days of aging, a maximum compressive strength of 86.1 MPa was achieved for a metakaolin-based geopolymer with a slag content of 50 mass%. When the Na2O content was 12%, the compressive strength of the metakaolin geopolymers with a slag content of 30% was 42.36% higher than those with a Na2O content of 8%. However, as the slag and alkali contents increased, the reaction rate of the metakaolin-based geopolymers increased, which significantly decreased the porosity, increased the shrinkage, and decreased the volumetric stability of the system. In this paper, in-depth study of the volume stability of alkali-activated metakaolin-based geopolymers plays an important role in further understanding, controlling, and utilizing the deformation behavior of geopolymers.