How to Accurately Test Thermal Conductivity of 90×90×190mm Porous Hollow Clay Bricks (+Avoid 3 Critical Errors)

Why Accurate Thermal Conductivity Testing Matters for Hollow Clay Bricks

• Failed compliance with national energy-saving standards for buildings;
• Increased construction costs due to incorrect insulation layer design;
• Quality complaints from clients and damaged brand reputation.

Step-by-Step Guide to Accurate Testing

1. Pre-Treat Samples: Eliminate Moisture to Avoid False High Readings

1. Place 9 same-batch 90×90×190mm bricks into a constant-temperature drying oven (capacity: 50–100L, to ensure the 9 bricks can be placed in a single layer without stacking, guaranteeing uniform drying via sufficient ventilation) set to 105±2℃;
2. Weigh the bricks in batches every 2 hours using a precision electronic balance (accuracy ≥0.0001g) until the weight difference of each brick between two consecutive measurements is ≤0.1% — this indicates constant weight;
3. Immediately transfer the dried 9 bricks to a desiccator to cool to room temperature (23±2℃) to prevent reabsorption of moisture from the air. Ensure the bricks remain intact and free of hole damage during transfer.

2. Sample Preparation: Fit 90mm Bricks to 300mm Tester Chambers (Detailed Operation)

1. 3×3 Combined Stacking with Custom Fixture: Take 9 same-batch dried bricks (ensuring no cracks or hole damage) and place them on a flat marble platform. Use a custom 270×270mm positioning frame (inner size matching 3×3 bricks) to align the bricks tightly—each brick’s edges must be flush with the frame, and no gaps between adjacent bricks are allowed. After stacking, fix the combined sample with a custom clamp that features neat edges. The custom clamp not only prevents displacement during subsequent operations but also ensures the combined sample has uniform, neat edges, eliminating the need for additional trimming. This stacking method minimizes the gap with the 300mm chamber (only 15mm per side), reducing the difficulty of gap filling.
2. Gap Filling with Same-Material Mortar: First, prepare the filling mortar—mix the same clay as the brick with clean water at a mass ratio of 3:1 to make a paste (consistency: no flow when applied, able to adhere to the brick surface). Before filling, use masking tape to cover the outer edges of the brick holes to prevent mortar from entering the holes and changing the porous structure. Then, use a special joint filler to fill the gaps between bricks and between the combined sample and the tester chamber: fill the gaps layer by layer (each layer thickness ≤2mm) and compact with a small trowel to eliminate internal air bubbles. After filling, scrape the surface with a scraper to make it flush with the combined sample’s surface. Finally, remove the masking tape and place the sample in a room-temperature environment (23±2℃, humidity 50±5%) for 24 hours of natural curing to ensure the mortar is fully bonded.
3. Final Inspection Before Testing: After curing, check the combined sample for loose mortar, air bubbles, or surface unevenness. If any defects are found, repair them with a small amount of mortar and cure for another 2 hours. Then, use a digital caliper (accuracy 0.01mm) to measure the combined sample’s length, width, and thickness at 10 evenly distributed points—ensure the size deviation is ≤0.1mm, and the surface flatness (measured by a straightedge) is ≤0.05mm. The thickness value will be used as the core parameter for subsequent thermal conductivity calculation.

3. Testing with Steady-State Plate Tester (Gold Standard for Building Materials)

1. Calibrate the tester with a standard clay brick sample (known thermal conductivity) before testing — this eliminates system errors from the instrument;
2. Apply a thin layer of thermal conductive silicone (thickness ＜0.1mm) to both sides of the combined sample. This fills micro-gaps between the sample and the tester’s hot/cold plates, reducing contact thermal resistance;
3. Place the sample into the chamber and apply a light pressure of 0.05–0.1MPa. This ensures tight contact without crushing the brick’s holes;
4. Set test parameters: Temperature = 23±2℃ (room temperature, matching actual application conditions), temperature difference between hot and cold plates = 20℃ (optimized for porous hollow clay bricks, balancing testing efficiency and data stability);
5. Wait for thermal steady state: Monitor heat flux and temperature values — when fluctuations are ≤0.5% for 30 consecutive minutes, the sample reaches thermal equilibrium. Record the thermal conductivity reading.

Critical Errors to Avoid

1. Moisture Interference Error: This is the primary error in porous brick testing. Residual moisture in the brick holes has a much higher thermal conductivity (0.6 W/(m·K)) than dry brick (0.4–0.5 W/(m·K)), which can lead to a false high reading by more than 15%. → Avoidance Method: Strictly follow the pre-treatment process — dry the 9 bricks in a 105±2℃ oven (50–100L capacity, single-layer placement) until constant weight (consecutive weight difference of each brick ≤0.1%), then immediately transfer to a desiccator to cool to room temperature (23±2℃). Do not expose the dried bricks to air for a long time to prevent reabsorption of moisture.
2. Sample Preparation Structure Damage/Gap Error: Improper operations such as random stacking without positioning, loose fixing of the custom clamp (causing edge irregularity), uneven mortar filling, or mortar seeping into holes will change the brick’s porous structure or form air pockets (thermal conductivity of air: 0.023 W/(m·K)), resulting in an error exceeding 10%. → Avoidance Method: Use a 270×270mm positioning frame for 3×3 stacking of the 9 dried bricks to ensure alignment; fix the combined sample firmly with a custom clamp (with neat edges) to maintain edge regularity, eliminating trimming needs; prepare mortar at a clay-water ratio of 3:1, use masking tape to cover holes before filling, and fill layer by layer (each layer ≤2mm) to eliminate air bubbles. Conduct a final inspection after curing to ensure no loose mortar or surface unevenness.