The evaluation of autoclave curing conditions of natural pozzolan blended lime binder

Pozzolan blended lime is a type of lime with hydraulic properties mainly consisting of air lime with added pozzolanic material. Addition of a pozzolan to air lime provides the binder high compressive strength, resistance to water and durability. Curing is defined as action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and pozzolanic reactions to occur so that the potential properties of the mixture may develop. Curing has a strong influence on the properties of mortars. Proper curing will increase the volume stability, strength, impermeability, durability, abrasion resistance and resistance to freezing and thawing. Compressive strength of lime-pozzolan mortars increases relatively slowly over longer period of time, depending on the type of the pozzolan. There are studies regarding with the elevated temperature curing in order to accelerate hardening rate and shorten the period of strength development of these mortars. However, accelerated pozzolanic reaction rate during the first day cause a decrease in the ultimate strength. On the other hand, the strength of pozzolan blended lime mortars develops significantly with water curing. For these reasons, low temperature and pressure steam curing conditions of pozzolan blended lime mortars should be investigated due to the combination of water and temperature effect. The objectives of the present work is: • to investigate low temperature and pressure steam curing conditions in order to accelerate hardening rate and shorten the period of strength development of the hyraulic pozzolan blended lime mortar produced from the mixture of slaked lime and Datça volcanic tuffs; • to minimize the energy expended in production; • to investigate the possibility of using this product as an alternative for fired clay bricks and lime-sandstone blocks. The research has three stages. At first, the pozzolanic activity of the Datça Earth was tested according to the Turkish Standard 25. At the second stage, preliminary tests were conducted related to low temperature and pressure steam curing of Datça pozzolan blended lime. At the third stage, low temperature and pressure steam curing conditions of this mortar was investigated. To determine the effects of the low temperature and pressure steam autoclave curing conditions on the physical and mechanical properties of this mortar, the other curing conditions (air curing, water curing and temperature curing) were also included in the experimental program. Mortar was made of slaked lime pozzolan and water. Sand was not added into the mixture. The maximum grain size of the pozzolan was 200 micron and the ratio of L/P was arranged as 35% by weight. The amount of mixing water was determined according to flow test of the ASTM C593–95. Three specimens were cast in 40x40x160 mm moulds and coded in (XX-XX-XX) format. The first group indicates the mixture proportion of L/P,the second shows the curing temperature (°C) and the third shows the curing time (days and hours). To determine the effect of curing duration, test specimens immediately upon completion of molding were left at 20±2°C and 95±2 % RH for 7 days. After 7 days, air curing strength was determined of the three specimens and the rest cured at constant ambient temperature (20±2°C ve 50±2 % RH) until 28th and 90th days. To determine the effect of water curing, test specimens immediately upon completion of molding were left at 20±2°C and 95±2 % RH for 7 days. After 7 days, all of the specimens immersed in water until the testing day at 7th, 28th, and 90th days. To determine the effect of elevated curing temperatures, test specimens were pre-cured 20±2°C and 95±2 % RH for 36 hours. After this initial curing, they were removed from the moulds and cured in the oven at 70 and 90°C 95±2 % RH for 6 days. To determine the effect of the both elevated temperatures and water vapur pressure, test specimens were pre-cured at 20±2°C and 95±2 % RH for 36 hours. After this initial curing, they were removed from the moulds and cured in the autoclave at 70 and 90°C under the 0.02 and 0.04 MPa pressure, respectively, for 6, 8 and 10 hours. The heating and cooling rate of the autoclave was 35°C/h. The specimens cured at ambient temperature, in the oven and autoclave were tested directly without dried. Specimens cured in water were dried for 24 hours at 50°C. Physical tests such as specific weight, unit weight, capillary water absorption, water absorption under the atmospheric pressure, ultrasound velocity/Young's modulus of elasticity and mechanical tests such as flexural and compressive strengths were performed on the samples. Research results indicate that curing at low relative humidity negatively impact the properties of lime mortar blended Datça pozzolan. Physical and mechanical characteristics of the lime based Datça pozzolan mortar develope significantly with water curing. When curing temperature in oven is increased from 70°C to 90°C, physical and mechanical properties of the mortar gradually decrease. When the mortar specimens are cured under the curing conditions of low temperature and pressure steam, physical and mechanical properties of the specimens gradually decrease depending on the increase both the temperature (above 70°C) and curing duration (above 6 hours). Thus, curing at 70°C for 6 hours is the ideal condition for low temperature and pressure steam autoclave curing to reach the mortar high strength in a short time. The compressive strengths of the specimens cured in water for 90 days and in autoclave at 70°C for 6 hours are similar to each other. In addition, the compressive strengths of the specimens are cured in oven and autoclave at the same temperature (90°C) are equal. Thus, it may be concluded that low temperature and pressure steam autoclave curing is an appropriate method in the interest of saving time. When producing building materials with similar physical and mechanical properties, electrical energy consumption of oven curing for 6 days is much higher than that of autoclave curing for 6 hours under the same temperature. This situation reveales that autoclave curing is more energy efficient method than the oven curing. The materials produced at 70°C for 6 hours under the low temperature and pressure steam curing conditions can be used instead of loadbearing fired clay bricks and non-loadbearing lime-sandstone blocks.