Rice approximately covers of land area
Rice approximately covers 1% of land area of the earth and it is one of the main sources of food for billions of people . One of the main agriculture industry residues is rice husk, which contains high amount of amorphous silica up to 85–95%. Rice husk ash (RHA) is a solid residue of rice husk controlled combustion, which shows the highest pozzolanic behavior among all plant residues productions .
Many of the previous studies have concentrated on investigating the optimum conditions for rice husk controlled combustion such as burning time and temperature, oxidizing procedures and heating rate in order to produce reactive ash . In this regard, several studies ,  have concluded that incineration of rice husk at temperatures below 500°C is imperfect and appreciable amount of unburned carbon is not expelled yet in this circumstances, which can result in adverse effects on ash pozzolanic activities. On the other hand, burning at temperatures higher than 700°C can lead to producing RHA with crystalline pozzolanic activity instead of amorphous one which is not preferable due to the reduction in reactivity , , , .
Uniform clusters of calcium-silicate-hydrates (C-S-H) are formed through the chemical pozzolanic reactions between the calcium hydroxide produced by the cement hydration and amorphous silica of RHA that result in a higher densification of the matrix . The addition of well pulverized RHA to cement paste could provide an environment for RHA particles to fill the voids between cement grains both physically and chemically and result in the reduction of porosity and enhancement of bond between cement and guanabenz , .
Researchers , , , , , ,  have proven that the incorporation of RHA in mortars can enhance the compressive strength in the long-term. However, they concluded that this ameliorative effect is limited to optimum replacement level of 20% by weight of cement. On the other hand, few other studies have found that the durability properties of the RHA blended mortars could face great enhancement against chloride ions penetration with higher dosages of replacement in comparison with compressive strength , .
In the last few decades, by the advent of nanotechnology, several studies devoted to investigating the effects of mineral nanoparticles on cementitious concretes and mortars , , , , , , , , , , , , , , , , . The results showed that a large surface area provided by nanoparticles hasten the rate of pozzolanic reactions and cement hydration. In this regard, researchers have found that the binary blended mortars containing RHA and nano-SiO2 with extremely fine amorphous silica particles would lead to prodigious enhancement of the compressive strength , , , , . Results show that the increase in long-term strength is mainly dependent on RHA, whereas the nano-SiO2 particles can extraordinary enhance the high early age strength. The addition of nano-alumina helped the compressive strength of mortars to enhance even when 30% RHA was used . Unfortunately, only limited studies are concerned with durability properties of mortars containing RHA and nanoparticles such as nano-TiO2 or nano-SiO2 , , , . Results show that the incorporation of nanoparticles and RHA can contribute to chloride resistivity enhancement in early and long-term age, respectively. However, the study of nano-RHA addition into cementitious materials is still untouched.
Materials According to ASTM C150  requirements, type II-425 Portland cement was used for all mortar mixtures. Its chemical and physical properties are shown in Table 1. Rice husk was provided from Mazandaran rice fields and was burnt at a temperature of 650°C and time duration of 1h. After combustion, the rice husk ash was pulverized to nanoparticles by means of planetary ball mill in order to attain particle sizes smaller than 100nm. Based on former studies, particles with dimensions lesser than 100nm can be considered as nanoparticles . Using trial and error method, three attempts with different time durations of 7, 10 and 13h were performed to achieve optimum grounding time duration. In the final operation, RHA particles with the mean size of 35.86nm were achieved by grounding time of 13h with the speed of 400rpm and the zirconium-balls/RHA mass ratio of 10:1. A micronizer was used to pulverize the RHA to micro size with the mean size of 0.588μm. In each turn, total of 200g RHA was ground for 1min in order to reach MRHA with the approximate size of cement particles. The results of zetasizer analysis for NRHA and MRHA are shown in Fig. 1. The XRF and XRD results of the RHA are also given in Table 1 and Fig. 2, respectively. The mostly low curved peaks of XRD diagram are representative of RHA high amorphous silica content which contribute a lot to enhance the pozzolanic activities.