Rock Hard Cement — Technical Specifications & Mix Ratios
Independent EN 197-1 batch testing. Reliable, repeatable performance in Caribbean site conditions.
Rock Hard Cement is Portland cement CEM I 42.5 N, manufactured to EN 197-1 and independently batch-tested for compressive strength, setting time, fineness, soundness and chemistry. This page is the working technical reference for engineers, quantity surveyors, contractors and specifiers designing with Rock Hard cement in Jamaica.
| Cement type | Portland cement CEM I / 42.5 N |
| Compressive strength (2 days) | ≥ 10 N/mm² |
| Compressive strength (28 days) | 42.5 – 62.5 N/mm² |
| Initial setting time | ≥ 60 min |
| Fineness (Blaine) | ≥ 280 m²/kg |
| Soundness (Le Chatelier) | ≤ 10 mm |
| Sulphate content (SO₃) | ≤ 3.5 % |
| Chloride content (Cl⁻) | ≤ 0.10 % |
| Blockwork mortar | 1 : 4 (cement : sand) |
| General concrete C20/25 | 1 : 2 : 4 (cement : sand : coarse aggregate) |
| Structural concrete C25/30 | 1 : 1.5 : 3 |
| Plastering mortar | 1 : 4 (cement : plaster sand) |
| Floor screed | 1 : 3 (cement : sharp sand) |
Reference values only. Final mix design should be verified by a qualified engineer or construction professional.
Storage & handling
- • Store bags on dry timber pallets, off the floor.
- • Keep under cover, away from moisture and direct rain.
- • Use oldest stock first — cement is best used within 3 months of manufacture.
- • Do not stack bags more than 8 high to prevent compaction.
- • Wear appropriate PPE — gloves, eye protection, dust mask when mixing.
- • Keep out of reach of children and away from waterways.
How to mix concrete with Rock Hard Cement
- Choose the mix ratio. Select the correct ratio for the pour — 1:2:4 for general C20/25 concrete slabs and foundations, 1:1.5:3 for structural C25/30 columns and beams, 1:4 for blockwork mortar and plastering, and 1:3 for floor screed.
- Measure the materials. Measure cement, sand and coarse aggregate by volume using consistent gauging boxes. Do not mix by shovel counts — accuracy is what gives Rock Hard cement its rated strength.
- Dry mix. Combine cement, sand and coarse aggregate on a clean, non-absorbent surface (or in a mixer) until the colour is uniform grey with no streaks.
- Add water. Add clean water gradually. Aim for a water-cement ratio around 0.5 for structural concrete — enough for workability, but no more. Excess water lowers strength.
- Place and compact. Place the concrete within the initial setting time (≥ 60 minutes at standard conditions). Compact with a vibrator or by rodding to remove air voids.
- Cure. Keep the pour moist for at least 7 days — cover with damp hessian, plastic sheeting or ponding. Correct curing is the single biggest factor in reaching design strength.
Applications across Caribbean construction
Rock Hard CEM I 42.5 N is used across Jamaica for residential slabs and foundations, commercial columns and beams, block laying and plastering mortar, floor screed, and precast elements. In coastal environments, its controlled chloride content (≤ 0.10 %) and low sulphate content (≤ 3.5 %) support long-term durability.
Certificates and specification support
Batch-specific certificates of analysis are available on request. For tender or green-building submissions, we also supply environmental product declaration (EPD) and carbon-footprint data on request from sales@rockhardcementja.com.