Question 3 of 7: Mass Loss on Heating a Sand–Sodium-Metasilicate Brick; Brick Manufacturing

Question 3: Mass Loss on Heating a Sand–Sodium-Metasilicate Brick; Brick Manufacturing (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 $1.35$ kg: $85\%$ sand (SiO$_2$, inert on heating to $100^{\circ}$C) and $15\%$ sodium metasilicate nonahydrate (Na$_2$SiO$_3\cdot9$H$_2$O), which loses $6$ of its $9$ waters of hydration at $100^{\circ}$C (becoming Na$_2$SiO$_3\cdot3$H$_2$O).

Find. (a) Total brick mass after heating slightly above $100^{\circ}$C. (b) The brick manufacturing process and the factors controlling the finished brick's mechanical properties.

Approach

(a) Split the brick into its two components; the sand mass is unchanged, while the metasilicate component loses a mass fraction equal to (moles of water lost × $M_{H_2O}$)/(molar mass of the hydrate). Sum the two post-heating masses. (b) is a descriptive ceramics-processing question answered from standard brick/whiteware manufacturing practice.

1. Molar masses (page-1 atomic-mass table). $$M_{Na} = 22.99,\quad M_{Si}=28.1,\quad M_O=16.00,\quad M_H=1.01\ \text{(g/mol)}$$ $$M_{Na_2SiO_3} = 2(22.99)+28.1+3(16.00) = 122.08\ \text{g/mol}, \qquad M_{H_2O}=2(1.01)+16.00=18.02\ \text{g/mol}$$ $$M_{\text{hydrate}} = M_{Na_2SiO_3}+9M_{H_2O} = 122.08+9(18.02) = 284.26\ \text{g/mol}$$
2. Split the brick into its two components. $$m_{sand} = 0.85(1.35) = 1.1475\ \text{kg (unchanged)}, \qquad m_{hydrate} = 0.15(1.35) = 0.2025\ \text{kg}$$
3. Mass of water lost from the metasilicate component. Losing $6$ of the $9$ waters removes a mass fraction $6M_{H_2O}/M_{\text{hydrate}}$ of that component: $$\Delta m_{water} = m_{hydrate}\times\frac{6(18.02)}{284.26} = 0.2025\times0.3804$$ $$\boxed{\Delta m_{water} \approx 0.0770\ \text{kg}}$$
4. Total brick mass after heating. The dehydrated metasilicate residue (Na$_2$SiO$_3\cdot3$H$_2$O) plus the unchanged sand: $$m_{brick,\,after} = m_{sand} + (m_{hydrate}-\Delta m_{water}) = 1.1475 + (0.2025-0.0770)$$ $$\boxed{m_{brick,\,after} \approx 1.273\ \text{kg}}$$ (a loss of about $5.7\%$ of the original $1.35$ kg, all of it water vapour driven off the metasilicate component.)
5. (b) How bricks are made. Conventional clay/shale brick manufacture follows four stages. Winning and preparation: clay or shale is mined, then crushed and ground to a controlled particle-size distribution, and blended with water (and sometimes grog, sand, or other fluxing/filler additions) to a workable, plastic consistency. Forming: the plastic body is shaped by one of the standard routes — stiff-mud extrusion through a die (the dominant modern method, producing wire-cut brick), soft-mud moulding (pressed into sanded or oiled moulds, historically common), or dry-pressing (a semi-dry, granular mix compacted at high pressure, giving the most precise dimensions and densest microstructure). Drying: green (unfired) bricks are dried slowly and uniformly in a controlled-humidity dryer to remove free water without inducing differential-shrinkage cracks. Firing: the dried bricks are fired in a kiln, typically in the range $900$–$1200^{\circ}$C, which burns out any residual organics, decomposes carbonates, and — critically — partially vitrifies the silicate/flux phases into a glassy bonding matrix that fuses the clay particles together and closes off much of the porosity.
6. (b) Factors controlling the finished brick's mechanical properties. Firing temperature and time control the degree of vitrification: more glassy bond and lower residual porosity raise strength (and lower water absorption) but risk warping, bloating, or over-vitrification if pushed too far. Porosity and pore-size distribution are the single dominant factor for a brittle ceramic — pores act as stress concentrators (Griffith-type flaws) that reduce the effective load-bearing cross-section, so strength falls sharply as porosity rises. Raw-material composition (clay mineralogy, flux/iron-oxide content, particle size) sets both the vitrification temperature and the fired colour/durability. Forming method and green density affect particle packing and hence the starting pore structure before firing. Drying and cooling rate govern whether internal thermal/shrinkage stresses produce microcracks. Together, these determine the brick's compressive strength, water absorption, freeze-thaw durability, and abrasion resistance.