Experimental Materials and Methods

Earthen plasters were mixed using either clayey-silt soil or commercially available bagged clay, masonry sand, and water. The soil was excavated to build a foundation on a residential building site in Haliburton, Ontario and subsequently used to plaster the walls of a load-bearing straw-bale building. The bagged clay was Ball Clay #123, a fine grained hydrous aluminum silicate clay with high unflred strength produced by the HC Spinks Clay Company.

The soil was broken up by hand and a mechanical sieve analysis for particles larger than 0.075 mm in diameter was conducted. Hydrometer tests in accordance with ASTM D 422 (2002) were performed for particles less than 0.075 mm.

Table 1 is the testing matrix, which shows the preparation parameters of each batch of plaster. Standard cube and cylinder specimens were prepared for each batch. Soil and sand for each mixture had a volume ratio of 1 : 1.5 (1 : 2.4 by mass), typical proportions for straw bale applications. The exceptions to this were batches R1 with soil to sand ratio of 1 : 1 and batch R3 with soil to sand ratio 1 : 3. Table 2 shows the percent by mass clay contents obtained in batches R1, R2 and R3 and the corresponding plaster strengths.

Table 2. Clay content for batches R1, R2 and, R3.

For plasters M1 … M5, the initial moisture contents (M. C.) were varied from 0.126 to 0.146 by mass. These moisture contents represent the workable limits for straw bale application. The drying time was varied for plasters T1, T2, and T3. Ten days is the earliest that the plaster hardens. Straw – bale builders generally regard 14 days as the time to reach adequate strength. The third drying time, 18 days, was chosen for linearity. Moisture content at time of testing was varied with batches C1, C2 and, C3. These batches were allowed to dry, and then subjected to extreme heat, moisture, or laboratory air. This is intended to simulate plaster subjected to hot, dry weather or a heavy rainfall, with the laboratory air acting as the control environment. Batch C1 was placed in a moisture room with 100% relative humidity prior to testing. Batch C2 dried in a laboratory environment. Batch C3 was placed in a drying oven at 110°C prior to testing. Cubes were left in the oven or moisture room for 24 hours and cylinders for 48 hours to ensure complete drying or moisture penetration.

Batch S1 was made with packaged clay purchased from a local earth-brick manufacturer. Portland – cement and lime plaster cylinders and cubes (P1, P2 and, P3) were cast in accordance with ASTM C 39 (1996) and ASTM C 109 (1998) respectively. The proportions of masonry sand, hydrated lime and Portland-cement were equal to 4.5 : 1.25 : 0.25 for all three batches.

To prepare the earthen plasters, the soil or bagged clay (batch S1 only) was massed and mixed thoroughly with water. The contents of the mixing bucket were allowed to soak for approximately two hours. The sand was then added to the mix.

The cubes and cylinders were allowed to dry in a controlled temperature room with a low-speed fan to speed the drying process. The cubes and cylinders were removed from the molds after 4 days. All specimens continued to dry in the laboratory for 10 more days, except for batches T1, T3, C1 and, C3. Specimens from batch T1 were tested after only 10 days and specimens from batch T3 were tested after 18 days. After 14 days, specimens from batch C1 and C3 were placed in the moisture room and drying oven respectively. Cubes were tested 24 hours later and cylinders were tested 48 hours later.

The cubes and cylinders were loaded until failure at a rate of 0.485 mm/min using an Instron Testing Machine. Due to the relatively low strength of the plaster, soft cork pads were used to cap the cylinders.