Question 3 of 7: Brick Mass Loss on Dehydration; How Bricks Are Made

Question 3: Brick Mass Loss on Dehydration; How Bricks Are Made (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Brick mass $m_0=1.35$ kg; 85 wt% SiO$_2$ (inert on heating), 15 wt% Na$_2$SiO$_3$·9H$_2$O, which loses 6 of its 9 waters of hydration just above 100°C. Atomic masses (page 1): Na=23.0, Si=28.1, O=16.00, H=1.01.

Find. (a) Brick mass after heating. (b) Brick-making process and the factors governing final mechanical properties.

Approach

Only the hydrate loses mass; the sand fraction is unaffected. Find the molar mass of the hydrate, the mass fraction represented by exactly 6 of its 9 waters, apply that fraction to the hydrate's mass in the brick, and subtract from the total.

1. Molar masses. $$M_{\text{Na}_2\text{SiO}_3}=2(23.0)+28.1+3(16.00)=122.1\ \text{g/mol},\qquad M_{\text{H}_2\text{O}}=2(1.01)+16.00=18.02\ \text{g/mol},$$ $$M_{\text{hydrate}}=122.1+9(18.02)=284.28\ \text{g/mol}.$$
2. Mass fraction lost as 6H$_2$O. $$f_{\text{lost}}=\frac{6\times18.02}{284.28}=0.3803\ \ (38.03\%\text{ of the hydrate's own mass}).$$
3. Apply to the brick. Mass of hydrate in the brick: $0.15\times1.35=0.2025$ kg. Water driven off: $$\Delta m=0.2025\times0.3803=0.0770\ \text{kg}.$$ $$m_{\text{final}}=1.35-0.0770=\boxed{1.273\ \text{kg}}.$$
4. (b) How bricks are made. Clay (plus sand/additives such as this sodium-metasilicate binder) is mined, crushed, and blended to a workable, uniform particle-size mix; water is added and the mix is formed — typically by extrusion through a die (soft-mud process) or dry/semi-dry pressing into a mold; the green (unfired) brick is then dried slowly and uniformly to drive off free water without cracking, and finally fired in a kiln at a temperature high enough to partially vitrify the clay (bond the particles via a glassy silicate phase and some solid-state sintering) without full melting.
5. Factors governing final mechanical properties.
   • Firing temperature / degree of vitrification — more glassy bonding phase increases strength and reduces porosity, up to the point of overfiring (bloating, deformation).
   • Porosity — residual pores (from incomplete vitrification, trapped gas, or dehydration/burnout of organics) act as stress concentrators and reduce strength roughly exponentially with pore volume fraction.
   • Clay mineralogy and particle-size distribution — finer, more reactive clay fractions vitrify more readily and pack more densely.
   • Drying/firing shrinkage and rate — too-rapid drying or firing induces internal stress and microcracking that persists into the finished brick.
   • Cooling rate — controls residual thermal stress and, for crystallizable glassy phases, the final crystalline/glassy balance.